Your body has about ten times the number of microbial cells as it does human cells. Collectively this bacteria weighs about three pounds, about the same as your brain. The evolution of our understanding of these bacteria has been evolving rapidly over the past twenty years, and I expect its findings to be the most impactful on our health and wellness in the coming decades.
The New Yorker has a fantastic article by Michael Specter that starts and ends with our changing perception of heliobacter pylori, linked to gastritis and peptic ulcers, but whose absence in children (it used to be universal, now fewer than 5% of children in the US carry it in their guts) is linked to asthma. It also has consequences for weight:
There is equally convincing evidence that destroying H. pylori could alter metabolism in ways that increase the risk of obesity. Several research groups, including Blaser’s, have found a strong relationship in humans between the bacterium and two stomach hormones, ghrelin and leptin, both of which play central roles in regulating our appetites [...] The more ghrelin you have in your bloodstream, the more likely you are to overeat. Leptin functions in the opposite way, suppressing appetite and increasing energy levels. For people whose stomachs are infected by H. pylori, ghrelin became far less detectable after a meal. For the others, levels of the hormone remained high, and the effects are evident. [...]
That finding was not a complete surprise. Roughly three-quarters of the antibiotics consumed in the United States are fed to poultry, cows, and pigs, not to treat illness but as dietary supplements to promote faster growth. [...] Until recently, the biochemical reasons for that weight gain, and its unsettling implications or humans, were murky. [...] “A lot of things are happening at once,” he said. “The rise in obesity, celiac disease, asthma, allergy syndromes, and Type 1 diabetes. Bad eating habits are not sufficient to explain the world-wide explosion in obesity.
The environment inside our body is as complex and varied as the one outside our body and responds just as unpredictably to wholesale changes to its ecosystem. I’d recommend reading the entire article, unfortunately it’s not available in its entirety on the New Yorker’s site, however here’s a PDF of the entire thing.