Ed Yong writes: There are tens of trillions of bacteria in my gut and they are different from those in yours. Why?

This is a really basic question about the human microbiome and, rather vexingly, we still don’t have a good answer. Sure, we know some of the things that influence the roll call of species — diet and antibiotics, to name a few — but their relative importance is unclear and the list is far from complete. That bodes poorly for any attempt to work out whether these microbes are involved in diseases, and whether they can be tweaked to improve our health.

Two new studies have tried to address the problem. They’re the largest microbiome studies thus far published, looking at 1,135 Dutch adults and 1,106 Belgians respectively. Both looked at how hundreds of factors affect the microbiome, including age, height, weight, sleep, medical history, smoking, allergies, blood levels of various molecules, and a long list of foods. Both found dozens of factors that affect either the overall diversity of microbial species, or the abundance of particular ones. And encouragingly, their respective lists overlap considerably.

But here’s the important thing: Collectively, the factors they identified explain a tiny proportion of the variation between people’s microbiomes — 19 percent in the Dutch study, and just 8 percent in the Belgian. Which means we’re still largely in the dark about what makes my microbiome different from yours, let alone whether one is healthier than the other. [Continue reading…]