Every week, Chef Martin Oswald and I go live on Substack to dig into one concept and build a meal around it. This week, Martin did something I hadn’t seen before. He set out five items on his counter. A pumpkin, a jar of miso, a handful of blueberries, a potato, and a single leek. Then he asked a question that reframed how I think about plant diversity.

You’ve heard the recommendation. Eat 30 different plants a week. It’s in every health book, every magazine, every wellness feed. And it’s a solid starting point. But Martin’s visual made something click for me as a physician. If you fill those 30 slots with iceberg lettuce, cucumber, celery, and watermelon, you can technically hit the number and still miss entire categories of fiber your gut bacteria depend on.

Not all plant fibers do the same job.

Three fibers, three jobs

I like to break fiber into three functional categories. Soluble fiber, the kind you find in oats, barley, apples, and beans, works like a net. It traps glucose and slows its absorption, which helps keep blood sugar from spiking after a meal. Insoluble fiber, found in the skins of fruits and vegetables, brown rice, and wheat bran, acts more like a bulldozer. It adds bulk and keeps things moving through the digestive tract.

Then there’s a third category that doesn’t get nearly enough attention. Fermentable fiber.

Resistant starch and inulin are two of the most well-studied fermentable fibers, and they behave differently from the first two types. They pass through the small intestine without being digested at all. They travel intact to the large intestine, where your gut bacteria ferment them and produce byproducts called short-chain fatty acids, particularly butyrate, propionate, and acetate. These short-chain fatty acids are sometimes referred to as postbiotics, and they feed the cells lining your colon, help maintain the intestinal barrier, and play a role in blood sugar regulation.

Research consistently shows that fermentable fiber consumption increases short-chain fatty acid production, supports the growth of beneficial bacterial populations like Bifidobacteria and Lactobacilli, and may improve insulin sensitivity. One study in healthy adults found that inulin increased GLP-1 (a satiety hormone) within 30 minutes and reduced ghrelin (a hunger hormone) several hours later. A large study of 174 young adults found that resistant starch from potatoes produced the greatest increase in fecal butyrate compared to resistant starch from maize or inulin from chicory root.

If you’re only eating colorful plants and never touching the foods that contain these fermentable fibers, you’re leaving a major category of gut support on the table.

The smarter 30

Martin’s five items on the counter represented five different ways to feed your gut. Colorful plants for phytonutrient diversity, fermented foods like miso and kefir for live bacterial cultures, polyphenol-rich foods like berries and green tea, resistant starch from cooked and cooled potatoes, and inulin-rich foods like leeks, onions, garlic, and sunchokes.

His argument, and I agree with it, is that 30 plants a week is a great framework, but the smarter version is choosing your 30 from across all of these categories. That’s what moves the needle from “eating a lot of plants” to actually nourishing the microbial ecosystem that regulates your metabolism.

The sunchoke advantage

This is where Martin’s demonstration got really interesting. He held up a single small sunchoke, roughly 50 grams, maybe the size of your thumb. That one piece contains approximately four to six grams of inulin, enough to be physiologically effective based on the clinical literature showing benefits at five grams daily.

Then he showed the comparison. To get the same amount of inulin from leeks, you’d need about five bulbs. From onions, roughly one entire onion. From garlic, an entire head. And from potatoes (as resistant starch rather than inulin), approximately 10 ounces of cooked and cooled potato.

Sunchokes, also called Jerusalem artichokes, store their carbohydrates primarily as inulin rather than starch. Research shows they contain 10 to 22 grams of inulin per 100 grams of fresh weight, making them one of the most concentrated whole-food sources available. Chicory root is technically higher, but you don’t cook with chicory root.

This doesn’t mean you need to eat sunchokes exclusively. Martin’s point was about understanding relative concentrations so you can make strategic choices. A little leek in your risotto, some raw onion on your salad, garlic in your stew, and sunchokes when you can find them. Those additions compound across meals.

Resistant starch, the cook-cool-repeat trick

Martin’s first dish was an Austrian potato salad called Kartoffelsalat. He grew up eating it. Everyone in Austria cooks it. And nobody there thinks of it as a gut health intervention, but that’s exactly what it is.

When you cook a starchy food like a potato and then let it cool, the starch molecules reorganize into a crystalline structure that resists digestion by human enzymes. This process is called retrogradation, and it converts digestible starch into resistant starch type 3 (RS3). The cooled potato now behaves more like a fermentable fiber than a simple carbohydrate. Your gut bacteria can ferment it, producing butyrate and other short-chain fatty acids. Research confirms that even when you gently rewarm these foods, a significant portion of the resistant starch remains intact.

Martin’s technique matters here. He boiled the potatoes, then sliced them thin while still hot. The thin slicing exposes more surface area, which allows the starch to leach into the dressing (veggie stock, a teaspoon of Dijon mustard, apple cider vinegar, and about a teaspoon of olive oil). As the salad sits, especially overnight in the refrigerator, the starch thickens the sauce into something that looks almost like mayonnaise. No mayonnaise in it. Just the potato starch doing its job.

Then you bring it out the next day, warm it gently to just above body temperature (around 120°F), and eat it. You’ve built resistant starch overnight and preserved it through gentle rewarming.

The dose-dependent approach

Martin also made a sunchoke and potato soup, and this is where the practical wisdom came through. He split it 50/50 between sunchoke and potato, because a full sunchoke soup would deliver too much inulin at once for most people.

This is the piece I want everyone to hear. If you’ve never eaten significant amounts of fermentable fiber, you cannot start with a bowl of sunchoke soup and expect your gut to handle it gracefully. The bloating, gas, and discomfort people experience when they “go plant-based” or suddenly add beans to every meal isn’t a sign that the food is wrong for them. It’s a sign they increased the dose too fast for their current microbiome to handle.

Start with roughly five grams of inulin per day, the equivalent of that small sunchoke or a generous portion of leeks or onion in a meal. Give your gut two weeks to adjust before increasing. You can dilute high-inulin foods by combining them with milder ingredients. Sunchoke soup cut with potato or cauliflower works well, and distributing onion across lunch and dinner rather than eating it all in one sitting makes a difference too.

For his third demonstration, Martin braised raw sunchoke wedges with bell pepper and ramps (a wild garlic from the allium family that appears in spring). The whole thing took about three minutes. The point was to show that these foods don’t require complicated recipes. You can add sunchoke to virtually any braise, soup, or stir-fry, controlling the dose based on how much your system tolerates.

One important caveat for anyone managing blood sugar closely. Even with the resistant starch benefit from cooking and cooling, potatoes can still spike glucose in some people, particularly those with diabetes or significant insulin resistance. If that’s you, lean into the inulin-rich foods instead. Leeks, onions, sunchokes, and asparagus deliver the fermentable fiber without the glycemic load.

What Chef Martin cooked

Martin prepared three dishes during our live. The first was Austrian potato salad (Kartoffelsalat) with ramps and an apple cider vinegar dressing. The second was a 50/50 sunchoke and potato soup finished with orange zest and sesame seeds. And the third was braised sunchoke wedges with bell pepper and ramps. He’ll be publishing the full recipes on his Substack soon. I’ll link them here when they’re live. In the meantime, subscribe to Chef Martin’s Substack so you don’t miss them.

This week’s habit

Pick one meal this week and add a fermentable fiber source you don’t normally eat. Throw a leek into your soup, toss some cooled potato into a salad, or grab a sunchoke if your grocery store or farmers market carries them. Start small, pay attention to how your gut responds, and build from there.

You already know what to do. Eat better. Move more. Sleep longer. You’ve known for years. The problem was never information.

Habit Healers is a live weekly coaching community where I teach one small habit per week and Chef Martin handles the food.

Every Tuesday at 4 PM PT, the group gets on Zoom to talk about what actually happened when you tried it. Real adjustments, not theory.

The habits rotate through five pillars of metabolic health. Blood sugar, movement, stress, sleep, and connection. You join whenever, start wherever, and build from there.

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References

* Baxter NT, Schmidt AW, Venkataraman A, Kim KS, Waldron C, Schmidt TM. Dynamics of Human Gut Microbiota and Short-Chain Fatty Acids in Response to Dietary Interventions with Three Fermentable Fibers. mBio. 2019;10(1):e02566-18. Published 2019 Jan 29. doi:10.1128/mBio.02566-18

* DeMartino P, Johnston EA, Petersen KS, Kris-Etherton PM, Cockburn DW. Additional Resistant Starch from One Potato Side Dish per Day Alters the Gut Microbiota but Not Fecal Short-Chain Fatty Acid Concentrations. Nutrients. 2022;14(3):721. Published 2022 Feb 8. doi:10.3390/nu14030721

* Tarini J, Wolever TM. The fermentable fibre inulin increases postprandial serum short-chain fatty acids and reduces free-fatty acids and ghrelin in healthy subjects. Appl Physiol Nutr Metab. 2010;35(1):9-16. doi:10.1139/H09-119

* Chen Z, Liang N, Zhang H, et al. Resistant starch and the gut microbiome: Exploring beneficial interactions and dietary impacts. Food Chem X. 2024;21:101118. Published 2024 Jan 3. doi:10.1016/j.fochx.2024.101118

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