So That's Why

obody has ever been born with an allergy, so why do so many of us get them? Jen, Chris and Matt explain the moment it happens.

In this episode of So That’s Why, the team tackles a question that sounds simple but hides a lot of clever biology. Not reacting to pollen, dust mites or cat dander turns out to be an active decision your immune system makes thousands of times a day. Allergies happen when that decision goes the other way.

The team looks at what twin studies reveal about how much of allergy risk is inherited, why the gut bacteria a baby has at one month old can matter years later, and why the famous “we’re too clean” hygiene hypothesis is roughly right but not for the reason most people think. They also explain sensitisation, the silent first encounter that sets an allergy up, how food allergies can start on the skin, and what treatments like oral immunotherapy can and can’t do.

Episode timestamps

• 00:00 Nobody is born with an allergy
• 01:40 Immune tolerance and why most of us don’t react
• 03:48 Is it in your genes?
• 07:41 Gut bacteria and the first few months of life
• 11:27 The hygiene hypothesis
• 14:17 Sensitisation, the allergy “first date”
• 16:49 How food allergies can start on the skin
• 18:54 Can allergies be outgrown or treated?

Why Not Reacting Is the Real Achievement

Starts at 01:40

Every day you breathe in foreign protein from pollen, dust mite droppings and cat dander. The impressive thing isn’t that some people react. It’s that most of the time, nobody does.

• This is called immune tolerance, and it’s active rather than passive
• Regulatory T cells step in and hold the rest of the immune system back
• In someone with an allergy, the same encounter triggers a Type 2 response instead

Chris: “Your immune system isn’t just kind of ignoring pollen, it’s making decisions about it.”

So an allergy isn’t a weak immune system. It’s an immune system doing a full-on job on something harmless.

How Much of Your Allergy Risk Is Genetic?

Starts at 03:48

Allergic disease has been climbing globally for decades, and our genes haven’t changed in that time. Genetics still set the starting point, though. What’s inherited isn’t the allergy itself but a tendency towards a TH2 response, the branch of immunity involved in allergic reactions.

• A study from the Netherlands Twin Register involving more than 14,000 people estimated inheritance at about 75% for asthma and 66% for allergy overall
• A cohort of about 25,000 Swedish twins found 82% for childhood asthma and 60 to 80% for hay fever, eczema and food allergy
• Specific genes have been linked to risk, including gasdermin B, IL1RL1 and TLR4

Chris: “So the genes set you up, but they don’t settle it, which I think is the hopeful bit.”How Gut Bacteria Teach the Immune System Tolerance

Starts at 07:41

The gut microbiome is established in the first weeks and months of life, and it shapes how the immune system behaves for years.

• Babies whose gut is dominated by Bifidobacterium at one month old show lower rates of food sensitisation years later
• Less diverse microbiomes, or dominance by Bacteroides or Klebsiella, are linked to higher risk by the first birthday
• The mechanism: gut bacteria make short-chain fatty acids (propionate and butyrate), which boost regulatory T cells, which act as a brake on the allergic response

The team is clear that factors like birth mode and antibiotic use are associations, not causes, and that antibiotics in infancy are given for very important reasons.

The First Date That Creates an Allergy

Starts at 14:17

There’s a specific moment when an allergy starts. It’s called sensitisation, and Matt compares it to a first date. Nothing seems to happen, but a memory is made.

1. On first contact, TH2 cells produce IgE antibodies tuned to that allergen
2. The IgE sits on mast cells and basophils, the cells that hold histamine
3. On the second encounter, the mast cells release histamine in a fast, coordinated burst

Jen: “Well, he wasn’t fine. He was being sensitised.”

The team also explains how food allergies can begin through broken skin, which helps explain the link between eczema and later food allergy, and why introducing allergenic foods early now seems to help.

About So That’s Why

So That’s Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgement. Just genuine curiosity and proper research.

Website:

Why do we get heart palpitations when a heart monitor so often shows a perfectly normal rhythm? Jen, Chris and Matt unpack the science behind one of the most unsettling sensations a healthy person can have.

A palpitation isn’t a diagnosis. It’s simply the word for becoming aware of your own heartbeat, and it can come from two very different places: a genuine change in what the heart is doing, or a change in how much attention you’re paying to it. In this episode, the team explores how adrenaline turns up both the speed and the force of each beat, why a “skipped” beat is usually an early one, and why sodium, potassium and magnesium matter for the heart’s electrical signals. They also look at Harvard Health figures on palpitations during perimenopause and after menopause, everyday triggers from caffeine to decongestants, the warning signs that need prompt medical attention, and what the evidence actually supports, from vagal manoeuvres to a two-week trigger diary.

In this episode

• 00:00 Why do we get heart palpitations?
• 01:20 Two causes, your heart or your awareness
• 02:38 How adrenaline changes your heartbeat
• 04:58 The skipped beat that arrives early
• 07:28 When the rhythm is normal but you still feel it
• 09:29 Hormones, stimulants and medications
• 13:04 When to get palpitations checked
• 15:45 What the evidence actually supports
• 18:41 So that’s why

Why Adrenaline Makes Your Heartbeat Feel Louder (02:38)

The most frequent driver of palpitations is the sympathetic nervous system, better known as fight or flight. Anxiety, fear, excitement and fever all trigger the release of stress hormones such as adrenaline.

• Adrenaline speeds the heart up and increases the force of each contraction, which are two separate effects
• Your heart rate climbs normally when you jog for a bus, yet almost nobody calls that a palpitation
• It’s the stronger beat, not the faster one, that people tend to notice

Chris: “If you think of your heartbeat as music that’s been playing quietly all day, adrenaline turns up the tempo and the volume at the same time.”The Skipped Beat That Actually Arrives Early (04:58)

The skipped beat is the sensation that frightens people most, but there often isn’t a gap at all.

• Many “skipped” beats are premature contractions, where an early electrical impulse fires ahead of the normal rhythm
• These are called ectopic beats (“ectopic” means out of place) and they’re extremely common in structurally normal hearts
• Every heartbeat starts as an electrical signal that relies on minerals such as sodium, potassium and magnesium moving across cell membranes, which is why electrolyte imbalances can set early beats off

Chris: “Your heart is basically an overachiever, Matt.”Why a Normal Heart Rhythm Can Still Feel Wrong (07:28)

When people wear an ambulatory ECG (a wearable device that records the heart’s electrical activity over hours or days), clinicians frequently find a completely normal rhythm, even during a reported episode.

• Johns Hopkins University is clear that the sensation is real, even when there’s no rhythm disturbance
• Perceiving your own heartbeat is a genuine sensory process, and how much attention it receives varies enormously
• Noticing the beat can trigger alarm, which feeds the very response that makes the beat stronger

Jen: “You notice the beat. Noticing it is alarming. Alarm triggers a sympathetic response, and the sympathetic response makes the beat stronger and easier to notice.”When Heart Palpitations Are Worth Getting Checked (13:04)

Most palpitations settle on their own, but occasionally they signal an arrhythmia, a genuine electrical malfunction such as atrial fibrillation.

• Palpitations with chest pain or pressure, breathlessness, dizziness, fainting or unusual sweating need emergency assessment
• Palpitations that are frequent, getting worse or persistent through the day should prompt an ECG or extended wearable monitoring
• Monitoring is non-invasive, widely available, and replaces guessing with an answer

Matt: “So it’s the company that it keeps rather than the palpitation on its own, and do go.”About So That’s Why

So That’s Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment. Just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Ever been in a foul mood for no obvious reason and blamed your brain? A surprising amount of that feeling starts a good deal further south. This week Jen, Chris and Jamie unpack why your gut has more say over your mood than you'd think, and why the gut, not the brain, does most of the talking.

Your gut houses around 500 million neurons and 100 trillion microorganisms, and that community is chemically busy all the time. It makes over 90% of your body's serotonin, produces calming signals like GABA, and reports back to your brain along the vagus nerve in seconds. The team explores the gut-brain connection, the two-way stress loop that links anxiety and IBS, and the genuinely encouraging news that your microbiome, and your mood, can start shifting in as little as 72 hours.

No jargon, no judgment, just the science behind why trusting your gut turns out to be more literal than it sounds.

In this episode: 00:04 - Your gut has a direct line to your brain 01:30 - 500 million neurons and 100 trillion microorganisms 03:02 - The vagus nerve and why the gut does most of the talking 04:16 - Where your serotonin is actually made 06:11 - The stress loop and how anxiety damages your gut 07:15 - What you eat changes your microbiome in 72 hours 09:53 - What to actually do about it 13:39 - The takeaway

Timestamps are based on transcript section markers and should be checked against the final edit.

Your Gut Does 80% of the Talking

The gut and brain are wired together by the vagus nerve, a direct line with no switchboard in between. Around 80% of the traffic on that line travels upwards, from gut to brain, which means the brain spends most of its time listening rather than instructing.

• The gut contains around 500 million neurons, more than your entire spinal cord
• Roughly 100 trillion microorganisms make up the gut microbiome
• A 2023 study found gut dysbiosis in around 75% of people diagnosed with major depression

"It's not a conversation. The gut keeps leaving voicemails for the brain." JamieOver 90% of Your Serotonin Is Made in Your Gut

Serotonin is the chemical we all associate with mood, and the overwhelming majority of it is manufactured in the gut. But there's a subtlety: gut serotonin can't cross the blood-brain barrier, so it doesn't travel to the brain directly. Instead it stimulates the vagus nerve, and the nerve carries the message.

• Gut bacteria also produce GABA, your main calming signal
• Short-chain fatty acids lower inflammation and protect the gut lining
• A 2025 Duke University study identified a "neurobiotic sense" that detects specific bacteria within seconds

"So the gut rings ahead and tells the brain what to do, and the brain just does it." JamieThe Stress Loop Runs Both Ways

Sustained pressure raises cortisol, which weakens the gut lining and increases intestinal permeability, commonly known as leaky gut. Bacterial byproducts then slip into the bloodstream, triggering inflammation that feeds straight back into low mood. Stress damages the gut, and the damaged gut worsens the stress.

• This is why the link between IBS and anxiety isn't a coincidence
• Diets high in ultra-processed foods reduce lactobacillus and bifidobacterium species
• A 2022 study found measurable microbiome shifts after just 72 hours of dietary change

The People With the Most to Gain, Gain the Most

Fermented foods and specific probiotic strains have real evidence behind them. A 2020 study of over 1,000 adults found regular fermented food eaters had measurably lower stress scores across all ages, and the benefit was strongest in those whose microbiome diversity started off lower.

"Rules on their own don't stick, but the reasons do." ChrisAbout So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Your brain builds a convincing world every night, then clears nearly all of it before you wake. So why do we dream?

Most of us have between four and seven separate dreams a night and forget up to 95% of them within minutes. Jen, Chris and Matt work through what is actually happening in the brain during REM sleep, why the forgetting is a consequence of the mechanism rather than a memory failure, and what the best current evidence says dreaming is for. The episode covers the four regions that swing out of balance during REM, the 2024 Scientific Reports study on dreaming and emotional memory, why babies spend so much more time in REM than adults, the simple line that separates a bad dream from a nightmare, and whether what you eat can genuinely change what you dream about. By the end you should be able to explain the whole thing in one sentence.

Chapters

• 00:00 Why do we dream
• 01:34 How much we actually dream and why we forget it
• 02:26 What is happening in the brain during REM sleep
• 07:37 Dreaming as emotional processing
• 10:44 How dreaming changes with age and how you wake
• 12:46 What separates a bad dream from a nightmare
• 15:15 Can food actually change your dreams
• 17:22 The verdict on why we dream

Four Regions Swing Out of Balance During REM Sleep (02:26)

Most vivid dreams happen during rapid eye movement sleep, and what makes REM unusual is not simply that the brain is busy. Several regions shift at once.

• The prefrontal cortex, which handles logic and self awareness, powers down. That is why nothing in a dream seems questionable while it is happening.
• The amygdala, involved in processing emotions, becomes more active, which helps explain the emotional intensity.
• The visual cortex fires with no real input coming in, generating the images fresh rather than replaying old footage.
• The hippocampus keeps feeding the dream real material from recent memory.

Underneath it all, the brainstem fires bursts of signals and the rest of the brain builds a story around them, which is why a dream lurches from your kitchen to a beach with no join.

"My brain's improvising over random noise and pretty much hoping that I don't notice." MattThe Same Switch Explains the Strangeness and the Forgetting (06:46)

The prefrontal cortex is not only the region that would normally question a dream. It is also the part that bookmarks a memory as worth keeping. With it turned down, dreams barely get flagged, so they are hardly stored.

"The one night your brain does its most vivid work, it keeps no receipts." ChrisDreaming Looks Like Emotional Sorting (07:37)

The leading theory is that dreaming is emotional and memory processing. The brain holds onto the memory of something that upset you while turning down the emotional charge attached to it. A 2024 study published in Scientific Reports found people who dreamed kept their emotionally charged memories, let neutral material fade, and rated the charged memories as less upsetting by morning. Older work from sleep researcher Rosalind Cartwright found people going through a painful breakup who dreamed about it recovered their mood noticeably faster.

The hosts are careful with the limits here. This is still open science, and much of it shows a link rather than a nailed down cause.

A Nightmare Is the Same Process Running Hot (12:46)

The dividing line is simple. A nightmare wakes you. A bad dream is unpleasant and you sleep through it, whereas a nightmare pulls you out of REM with the fear still running. Roughly two thirds of dreams carry sadness, worry or anger, and only about a fifth are clearly happy.

"A nightmare isn't your mind failing. It's that same system working on something heavy and getting stuck partway through." Chris

Stress is the biggest trigger, and lost sleep does it too, because the brain packs in extra REM to catch up. Frequent nightmares are common after trauma, affecting up to 70% of those living with it. If nightmares become frequent and start wearing you down, it is worth speaking to a GP, as treatments such as imagery rehearsal therapy genuinely work.

About So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website:

Pre-diabetes might be the only common diagnosis experts can't agree on, and a quarter of the people told they have it quietly recover on their own. In this episode, Jen, Chris and Jamie unpack why we develop pre-diabetes, why it builds in near silence for a decade or more, and why a warning that lands while you feel perfectly fine is still genuinely good news.

The team dig into the two changes at the heart of it: muscles that stop listening to insulin, and insulin-making cells that go quiet rather than die off. They explain the "fat in the wrong place" idea that pulls it all together, why even slim people can be affected, and why three different tests can disagree so wildly that the experts still argue about the word itself. Then comes the hopeful part, which is the unusually deep evidence that modest, sustainable changes can shift the whole picture. As Chris puts it, pre-diabetes is best thought of as a stage, not a disease.

In this episode

• Two Quiet Changes Drive Pre-Diabetes (03:14)
• It's Fat in the Wrong Place, Not Just How Much (05:41)
• Why Experts Still Argue Over the Diagnosis (07:01)
• What the Evidence Says Actually Works (10:12)

(Timestamps are based on the transcript's section markers. Please verify against the final edit before publishing.)

Two Quiet Changes Drive Pre-Diabetes

03:14

Pre-diabetes comes down to two things drifting slightly out of step over years, each making the other worse. The first is insulin resistance, where your cells stop listening to insulin as well as they used to. The cells that go deaf first are your muscles, the biggest customer for blood sugar, taking up about three-quarters of it after a meal. The second is in the pancreas, where the insulin-making beta cells gradually stop keeping up.

• Insulin tells your cells to take sugar in and use it. When they stop listening, sugar hangs around in the bloodstream.
• This can build for ten or even twenty years before anything shows on a test. It's a drift, not a breakdown.
• The reassuring twist: the beta cells largely don't die. They seem to change job and go quiet, so there is a route back.

Chris: "Pre-diabetes is a stage, not a disease."Jamie: "They've downed tools. They're still in the building, they're just not doing the skilled bit."It's Fat in the Wrong Place, Not Just How Much

05:41

The idea that pulls everything together is a domestic one. Picture your body's fat as something you store in a cupboard. Most of it is meant to live there, under the skin, safely. But every cupboard has a size, and that size is different for everyone. The trouble starts when it overflows into places it was never meant to go.

• This explains why you can be fairly slim and still develop pre-diabetes. Your personal storage cupboard might just be a small one.
• It's part of why people of South Asian heritage can be affected at lower body weights, and why screening thresholds are set lower for some groups.
• The cupboard hasn't shrunk. Many of us are simply filling it faster and emptying it less than a couple of generations ago.

Jamie: "So the bathroom scales are never really telling the interesting story."Why Experts Still Argue Over the Diagnosis

07:01

Whether you count as having pre-diabetes depends heavily on which test you happen to get, and that is where the debate comes from. There are three common ways to diagnose it, and the share of people who count swings wildly between them. The most common, the HbA1c test, adds a further wrinkle, because it reads a touch higher in Black patients for the very same blood sugar.

• One analysis found the proportion of a group counted as pre-diabetic ranged from about 4% to over 40%. Same people, different test.
• Over a few years, very roughly a quarter of people progress, half stay put, and a quarter drift back to normal.
• This busts the biggest myth, that pre-diabetes means diabetes is definitely coming and nothing can be done. It's a real signal, just drawn with a slightly blunt pencil.

What the Evidence Says Actually Works

10:12

After all the uncertainty, this is one of the most changeable situations in medicine, and the evidence is unusually deep. In a landmark prevention trial, people making modest changes cut their chance of developing diabetes by getting on for 60% over about three years, which was clearly better than the medication group in the same trial.

• The levers are reassuringly dull: roughly 7% weight loss and about 150 minutes of movement a week, the equivalent of a brisk 20 to 25-minute walk a day.
• Movement helps on its own. People who missed the weight target but kept moving still cut their risk by over 40%.
• A Mediterranean-style diet of veg, beans, whole grains and olive oil meaningfully cut new diabetes in one big trial, even when people barely lost weight.
• A responsible note from the team: the pre-diabetic range is typically an HbA1c of 42 to 47. If it's on your radar, that's a good reason for a chat with your doctor, not a cue to panic or self-diagnose.

Jamie: "So the walking and eating group beat the drug? That's not how these stories usually go."About So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Ask anyone what makes us cry and they'll say sadness. When researchers track real crying episodes, they find something else entirely.

Jen, Chris and Matt unpack why we cry when we're sad, and discover the answer isn't the sadness at all. It's powerlessness. The triggers people actually report are far smaller than anyone assumes, turning up as conflict, criticism, rejection and minor personal failures rather than bereavement and tragedy.

The team follows the signal from the limbic system to the brainstem to the tear glands, explains why tears spill over your eyelid at all, and looks honestly at the evidence behind the popular idea that crying flushes stress toxins out of the body. They also tackle the biggest contradiction in this research. Over 70% of people say crying makes them feel better, while controlled lab studies found the opposite. The resolution comes down to timing, and to who else is in the room.

Chapters

00:00 Why do we cry when we're sad 00:51 Powerlessness, not sadness 04:20 What happens in the brain and body 08:33 What emotional tears are made of 10:51 Why crying varies so much between people 16:02 Does crying actually make you feel better 19:03 When crying isn't good for you 21:29 The verdict on why we cry

The Real Trigger Isn't Sadness, It's Powerlessness (00:51)

Sadness is almost always present when we cry, but rarely on its own. It turns up blended with anger, fear, frustration or disappointment, and the common denominator underneath is not being able to do anything about the situation.

One psychologist frames the tipping point as the moment psychological demands exceed your sensory effective threshold. In other words, the emotion has outpaced whatever coping mechanism was available at the time.

"So it isn't then about how bad the thing is, it's the gap, I guess, between that and what you've got left in the tank." Matt

This is also why the same event can floor you one week and barely register the next. Sleep deprivation, alcohol, stress and mental health all lower the threshold.

Why Tears Actually Run Down Your Face (04:20)

The sequence starts in the limbic system. The amygdala helps determine how intense an emotion feels, while the anterior cingulate cortex helps you recognise and respond to it. From there the signal travels to the pons in the brainstem, which activates the tear glands through a facial nerve.

The overflow itself is simpler than most people expect. The glands produce fluid faster than the tear ducts can drain it, so the drainage isn't failing, it has just been outpaced.

"So think about it like a sink with the tap on harder than the plug hole can cope with." Jen

The team also covers the three tear types (basal, reflex and emotional), and why there's no rigorous evidence that any other species produces the third kind.

The Stress Toxin Idea Doesn't Hold Up (08:33)

Some studies have reported higher levels of prolactin and adrenocorticotropic hormone in emotional tears, alongside compounds including leucine enkephalin, potassium and manganese. Chris and Jen are careful to flag that these findings still need stronger replication.

That matters, because they're the foundation of the popular idea that tears flush stress toxins out of the body. One of the influential early studies supporting it was not successfully repeated by later researchers. The chemistry that is present, including oxytocin, vasopressin and endogenous opioids, points at stress regulation and social connection rather than detox.

Whether a Cry Helps Depends on Who's in the Room (16:02)

Over 70% of people say crying makes them feel better. Controlled lab studies found criers felt worse immediately afterwards. The resolution appears to be timing. One study tracked mood after a sad film and found criers felt worse straight away, recovered within 20 minutes, and by 90 minutes were better than before the film began.

Context does the rest. Social support is the biggest single factor. There's also a finding about numbers that will be familiar to anyone who has cried at work, because people tend to feel better crying alone or with one other person, and worse with two or more.

"There's a real permission in that, isn't there? You're not actually then doing crying the wrong way." Matt

The episode closes on what tears are actually for. Researchers photographed faces and digitally removed the tears, and the same faces with tears visible were judged as sadder and more in need of help.

"So a good cry isn't a system failing, it's a message going out, and whether you feel better depends far more on who's around to answer it." JenAbout So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Nearly all of us blame the calendar for wrinkles, but the science says the calendar is mostly innocent.

Estimates put at least 80% of visible facial ageing down to sun exposure, not birthdays. In this episode, Jen, Chris and Matt go one layer beneath the surface of the skin to find out what a wrinkle actually is. They cover the slow collagen decline that starts in the late twenties, the fibroblasts that build the skin's scaffolding, and the MMP enzymes that break it down again. They explain how UV light manages to speed up demolition and quieten repair in a single move, why not all wrinkles are the same thing, and the sharp collagen drop that arrives around menopause. There is also a straight answer on collagen drinks, and the cheap, unglamorous product that outperformed everything else in a four-year controlled trial.

Key Point Timestamps

• 00:04 The question and the 80% figure
• 01:51 The collagen timeline, decade by decade
• 03:15 Collagen, elastin, fibroblasts and MMPs
• 08:18 How sunlight causes most of what we see
• 10:06 Dynamic, static and sleep wrinkles
• 12:22 What the evidence says about collagen supplements
• 14:28 SPF, retinoids and vitamin C
• 18:02 Injectables, lasers and the lifestyle stuff

Wrinkles Form in a Layer You Cannot See (03:15)

Wrinkles do not start in the visible surface of the skin. They form in the dermis underneath, where three things do the structural work. Collagen provides firmness and tensile strength, elastin lets skin stretch and spring back, and hyaluronic acid holds water and keeps the layers plump.

All three are made and maintained by one type of cell, the fibroblast. In young skin, fibroblasts lay down fresh collagen and elastin about as fast as the old material wears out.

Two things change that with age. A growing share of fibroblasts become senescent, meaning they largely stop producing and start giving off low-level inflammatory signals. At the same time, a family of enzymes called MMPs becomes more active. Their normal job is useful, clearing out worn collagen so fresh collagen can replace it, but the balance tips.

"Collagen and elastin get broken down and disorganised faster than the fibroblasts rebuild them." — ChrisSunlight Speeds Up Breakdown and Slows Repair (08:18)

When ultraviolet light hits the skin, it generates a burst of unstable molecules called free radicals. Within minutes, those free radicals switch on the MMPs, the very enzymes that break collagen down. Each unprotected sunny hour becomes a small spike of collagen loss.

Then it does a second thing simultaneously. It quietens the signal that tells fibroblasts to make new collagen.

That is why sunlight accounts for so much of what shows up on skin. It is not vague damage. It drives both ends of the same process, and the effect is cumulative rather than dramatic.

"The sun isn't ageing our skin in some vague way, it seems to be driving the two exact processes you've both just described at both ends, which kind of makes it feel less like fate and something that we can actually deal with." — MattThe Collagen Drop Nobody Talks About Enough (10:06)

Individual variation matters enormously here. How much protective pigment skin produces is largely down to genetics, and that pigment strongly affects how much UV gets through in the first place.

Hormones matter too, and this part is badly under-discussed. Around menopause, as oestrogen falls, women can lose up to about 30% of their skin's collagen in roughly the first five years. By the age of 50, nearly 50% can be gone. Set against the background rate of one to 1.5% a year, that is a very different order of change arriving in a very short window.

The Cheapest Bottle in the Shop Wins (14:28)

For collagen drinks and powders, a recent pooled analysis of 23 peer-reviewed trials found the strongest independent studies do not show a clear effect on wrinkles or elasticity. The gut cannot absorb collagen whole, so it breaks it into amino acids first.

What does have strong evidence is considerably less exciting. In a well-known controlled trial, people using a daily broad-spectrum sunscreen showed no measurable increase in skin ageing over four years, while the comparison group aged visibly. Retinoids and topical vitamin C form the next tier down.

"The cheapest and most ignored bottle in the shop beats the 40 quid miracle serum sitting right next to it." — Matt"It's never too late to start, because you're always preventing the next decade of damage even if you can't undo the last one." — JenAbout So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Your metabolism has an alibi. A study of over 6,000 people found metabolic rate is essentially flat from 20 to 60.

Most of us start blaming our metabolism somewhere in our thirties. The largest study ever done on human energy expenditure suggests we have been accusing the wrong party. Using doubly labelled water, the gold standard for measuring energy expenditure in real life, researchers tracked over 6,000 people aged from eight days old to 95 across 29 countries. Between 20 and 60, metabolic rate holds steady. No dip in the thirties, no crash in the forties, and no measurable effect of pregnancy or menopause once size and body composition are accounted for.

A real decline does arrive after 60, at around 0.7% a year, and Jen, Chris and Matt unpack the five things driving it. They also cover what actually holds it off, from resistance training and protein targets to where creatine, caffeine and green tea genuinely sit in the evidence.

Chapters

00:00 Why we all blame our metabolism 00:54 What the biggest study on energy expenditure found 04:58 So why do we gain weight in midlife 07:03 The five things that change after 60 13:07 Resistance training at any age 14:14 The protein numbers that have shifted 16:55 Creatine, aerobic activity, caffeine and green tea 22:07 The verdict on metabolism and age

Metabolic Rate Is Flat From 20 to 60 (00:54)

The doubly labelled water technique tracks heavy forms of hydrogen and oxygen as they leave the body, which tells researchers how much CO2 someone is producing and therefore how much energy they are burning. Crucially, it measures ordinary life rather than a treadmill test.

• Over 6,000 participants, aged eight days to 95, across 29 countries
• Energy expenditure peaks at around eight or nine months old, when babies burn roughly 50% faster than an adult relative to body size
• The rate settles at around 20 and then stays flat for four decades

"You've been blaming an innocent party, so it's like accusing a housemate who moved out years ago of finishing all the biscuits."JenWhy Midlife Weight Gain Happens Anyway (04:58)

If the rate is flat, the weight gain needs another explanation. Tom Sanders, professor of nutrition at King's College London, said findings like these support the view that rising obesity is driven by excess energy intake rather than falling energy expenditure. Alongside that, muscle mass tends to fall, and a body with less muscle burns less energy in total.

"So your machinery isn't getting worse with age. There may simply be less of the expensive machinery in your body."JenThe Five Things That Change After 60 (07:03)

After 60, total energy expenditure falls by around 0.7% a year, which reaches roughly 26% lower by the age of 90. Five mechanisms run at once.

1. Sarcopenia. Inactive adults can lose 3 to 5% of their muscle per decade from their thirties. Muscle is metabolically active tissue, so it costs energy even at rest. Jen compares it to the standing charge on an energy bill, the bit you pay whether or not you switch anything on.
2. Mitochondria. Mitochondrial DNA accumulates mutations and oxidative damage, so ATP production becomes less efficient. Mitophagy, the process that clears out damaged mitochondria, also slows.
3. Hormones. Growth hormone, IGF-1 and testosterone decline alongside changes in thyroid function and insulin sensitivity.
4. Reduced output per kilo. The remaining tissue does less per kilogram than it used to.
5. Anabolic resistance. Ageing muscle responds less strongly to protein and to being used.

"So it's wear and tear, plus the maintenance team that's also slowing down."MattWhat Actually Holds It Off (13:07)

Resistance training is the single most evidence-backed intervention because it targets the biggest modifiable driver. Muscle protein synthesis machinery stays responsive to training into very old age, so there is no point at which starting stops being worthwhile.

• Protein targets for older adults now sit at roughly 1 to 1.2 grams per kilogram of body weight, rising to 1.2 to 1.5 for those training
• Around 25 to 40 grams of good quality protein per sitting, spread across three meals, does more than one large evening portion
• A 2025 systematic review and meta-analysis found creatine plus resistance training improved lower limb strength and lean muscle mass more than training alone
• Caffeine and green tea have real but small and short-lived effects
• If you have kidney disease or are on a medically restricted diet, protein targets are a conversation for your doctor

"Keep the muscle you've got and feed it properly. Your body stays responsive to training and diet far longer into life than people assume."ChrisAbout So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Two people, the same beach, the same sun. One turns golden, the other turns red. Why?

That is the question Jen, Chris and Jamie set out to answer this week, and the answer surprises almost everyone. Tanning isn't a skill you build over a summer. Whether you can tan at all is settled by your genes, and it was decided before you were born. The team unpack what a tan really is (a repair job, not a sign of health), why everyone has roughly the same number of pigment cells but wildly different results, and the two pigments that decide whether you go bronze or go pink. Along the way they bust the base tan myth, put sunbeds firmly in their place, and explain why the person who moves abroad and looks "permanently tanned" is actually showing years of damage. Chris, a formulation scientist who has spent decades working in sun protection, also lays out what genuinely protects your skin.

Key Point timestamps:

• 00:34 What a tan actually is
• 06:59 Why some people tan and others only burn
• 10:50 The Fitzpatrick scale and what it means
• 13:18 What actually protects you (and the truth about sunbeds)

(Timestamps are based on the transcript section markers and should be verified against the final edit.)

A Tan Is a Repair Job, Not a Sign of Health

(02:34)

Whatever your skin tone, you carry roughly the same density of pigment-making cells, about 1,200 per square millimetre. The pigment they make is melanin, the skin's own sunscreen. The catch is that skin doesn't make more melanin because the sun is good for it. It makes more because ultraviolet light has already caused damage. UVB harms a skin cell, a protein called p53 (nicknamed the guardian of the genome) raises the alarm, and that signal tells your pigment cells to start producing melanin. The damage always comes first.

"A tan is a repair job. It's not a glow of good health." (Chris)Why Some People Tan and Others Only Burn

(06:59)

There isn't one melanin; there are two. Eumelanin is the dark, more protective pigment that absorbs and safely disperses UV. Pheomelanin is the reddish one that offers little protection and can even trigger oxidative stress. Chris compares your pigment cells to little printing shops. The question was never how many you have, since everyone has about the same number. It's which melanin cartridges are loaded inside them, and that is largely genetic. Around 1 in 20 people, the classic redheads, make almost entirely pheomelanin and barely any eumelanin, so they burn and freckle however careful they are.

The Fitzpatrick Scale and What It Means

(10:50)

Dermatologists grade skin from 1 to 6 on how readily it burns and tans, using the Fitzpatrick scale, developed in the 1970s. Even the darkest, type 6 skin offers natural protection roughly equivalent to an SPF of only 10 to 13, and that is a rough comparison. Every skin type can still burn and be damaged. An older version of the scale wrongly suggested darker skin was safe, and that myth causes real harm because skin cancers can get spotted later.

"So it's protection that turns up late and leaves early." (Jamie)What Actually Protects You (and the Truth About Sunbeds)

(13:18)

If the tan isn't doing the protecting, something else has to. Chris recommends a broad-spectrum sunscreen (covering UVA and UVB) at SPF 25 or higher, reapplied through the day, plus clothing, shade and avoiding the midday sun. Sunbeds are not a gentler option. They deliver very concentrated UV, most of the damage is invisible, and starting young sharply raises the risk of serious skin cancer. And the pale person who moves somewhere sunny and looks bronzed years later? That is accumulated UV injury, not a built-up tan.

"There's no such thing as a safe tan, and you sadly can't train your skin into making one either." (Jen)About So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast

Cortisol has become the hormone everyone wants to get rid of. It is also one of the reasons you can get out of bed at all.

Jen, Chris and Matt unpack what cortisol is actually doing while you go about your day. They cover how the HPA axis works as a self-correcting chain rather than a tap left running, why cortisol climbs before you wake and jumps again in the first half hour after you open your eyes, and how one hormone reaches nearly every tissue in the body by slipping into the nucleus of your cells and changing which genes are switched on.

They also look at where the frightening reputation genuinely comes from. Chronic elevation is a real problem, the cortisol belly has more behind it than you might expect, and a flattened daily rhythm is one of the most consistent biological findings in depression. Adrenal fatigue, on the other hand, is not a recognised diagnosis.

In this episode

• 00:00 Introduction
• 01:13 What cortisol actually is
• 02:49 How your body decides how much you get
• 04:10 The daily rhythm and the cortisol awakening response
• 08:46 When cortisol goes wrong
• 11:09 Adrenal fatigue and the myths the internet loves
• 14:54 What actually helps
• 19:03 Well-timed cortisol, not low cortisol

Cortisol Is Talking to Almost Every Cell You Own (01:13)

Cortisol is a glucocorticoid, a steroid hormone built from cholesterol by the adrenal glands that sit on top of your kidneys. Nearly every tissue in the body carries glucocorticoid receptors, the docking sites cortisol uses to pass instructions into a cell.

The reason its reach is so wide comes down to one property. Cortisol is fat soluble, so it slips straight through the cell membrane and into the nucleus, where it changes which genes are switched on.

"So it's not tucked away doing one small job then, it's talking to more or less all of me at the same time." MattThe System Was Built to Turn Itself Down (02:49)

The HPA axis is a three-part chain. The hypothalamus releases CRH, which tells the pituitary to release ACTH, which tells the adrenals to release cortisol. Once cortisol rises, it signals back up the chain and tells the top to ease off.

"It's not a tap left running. It turns itself down once the messages seem to have landed." Matt

Almost everything that goes wrong with cortisol is a version of that tap failing to turn itself down.

Cortisol Clocks On Before You Do (04:10)

Cortisol runs highest in the early morning and lowest around midnight. On top of that sits the cortisol awakening response, named by the Endocrine Society. In the first half hour after waking, cortisol jumps a further 38 to 75 per cent on top of the level it was already climbing to, in roughly 77 per cent of people.

That rise is not a side effect of waking up. It is part of the signal that wakes you, and current thinking is that it is anticipatory. The jump tends to be bigger before a workday and it goes flat in burnout.

"So the snooze button then is me arguing with my own biology, and it seems losing." MattAdrenal Fatigue Is Not the Problem to Solve (11:09)

Adrenal fatigue, the idea that the glands get exhausted and stop making cortisol, is not a recognised diagnosis and the evidence does not support it. What is real is HPA axis dysregulation, where the signalling between brain and glands drifts out of rhythm.

"The glands aren't worn out, the timing's off, and that's a very different thing to put right." Chris

The team are also clear that a genuine cortisol problem is a blood test and a conversation with a doctor, not a hunch from an internet forum. And on supplements, they are upfront. Vegetology sells them, and for cortisol specifically the evidence is thin.

What Actually Helps (14:54)

Sleep is the single strongest regulator of the rhythm, by a wide margin. Gentle to moderate movement nudges the baseline down over time, while constant over-training keeps cortisol stuck high. A sharp blood sugar crash is one of the strongest triggers there is for a cortisol spike, so regular meals with a bit of protein help. Easing back on the big afternoon coffee and late alcohol helps the evening dip arrive on time.

"You were never meant to have low cortisol, you're meant to have well-timed cortisol." JenAbout So That's Why

So That's Why is a weekly podcast where Jen, Chris and Matt unpack the science behind everyday health questions. No jargon, no judgment, just genuine curiosity and proper research.

Website: https://www.vegetology.com/so-thats-why-podcast