"Science tells us that our body houses microbial organisms. Then how much our weight is really our weight? If I am overweight, is it because of my own body cells or excess microflora?" asks Ajay Mathur from Mumbai in India.

Adam bravely sends off a sample to the 'Map My Gut' project at St Thomas' Hospital to have his microbes mapped. Prof Tim Spector reveals the shocking results - a diet of fried breakfasts and fizzy drinks has left his guts in disarray. But help is at hand to makeover his bacterial lodgers.

Science writer Ed Yong, author of 'I Contain Multitudes', reveals how much our microbes weigh. We're just beginning to discover the vast array of vital functions they perform, from controlling our weight, immune system and perhaps even influencing our mood and behaviour.

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Presenters: Hannah Fry, Adam Rutherford