double digestion

Fortunately, we have come a long way since Metchnikoff's musings (若有所思的) on the large intestine. We recognized decades ago that, at the very least, this organ also absorbed crucial vitamins, synthesized by its colony of microbes.

Some of the foods we eat are digested by the enzymes coded (for) by our own genome, and then absorbed in the small intestine. But many food molecules - mostly the "indigestible" ones - are left over. These go on to the large intestine, where they meet a large and eager crowd of microbes, ready to break them down using their own enzymes. In the process of feeding them themselves, the microbiota release another set of leftovers.

Prevotella and Xylanibacter harbor genes coding for enzymes that allow them to break down xylan and cellulose.

The plant-eaters saw a rapid increase in bacterial groups that break down plant cell walls, whereas the meat-eaters lost their plant-degrading bacteria and gained species that break down proteins.

If fiber bolsters (supports) microbes that strengthen the gut wall's defence, lipopolysaccharides cannot get into the blood, the immune system can keep calm, and fat cells will become more numerous rather than filling up.

Rather than coming down to simply balancing calories-in with calories-out, the link between diet (particularly fiber), microbes, short-chain fatty acids, gut permeability and chronic inflammation makes obesity seem rather more like a disease of energy regulation than a simple case of overeating.

Human's use of fire to cook food prompted our transition as a species into being big-bodied, and even bigger-brained. Cooking both animal and plant food changes the chemical structure and makes nutrients available to the body that were inaccessible when it was raw. The same is true of cooking's effect on the nutrients available to the microbiota.

Collen, A. (2015) 10% Human: How Your Body's Microbes Hold the Key to Health and Happiness. London: William Collins.