The body has many parts devoted to the processing of food. As a result, food processing is subject to multiple regulatory feedback loops. Working together, these feedback loops regulate body weight. Malfunctioning of any body part responsible for processing food or malfunction of any of the feedback loops can throw off body weight on the high side or the low side. For this reason, there are multiple causes of obesity and emaciation. I’ll devote several columns to exploring this complex system.

We live in a world with abundant food. When hunger occurs, food is easily obtained and people typically eat in response until a natural feedback loop kicks in to repress the desire to eat. The feeling of fullness that stops eating is a vital component of the body’s strategy for maintaining a set weight. It is not, however, the only strategy. Once eaten, the ingestion of food into the body is not inevitable. A second vital component to regulate weight is control over the absorption vs. rejection of food once it has been eaten. All food is not absorbed. A substantial fraction is excreted as solids. This is not because the excreted solids are indigestible. Ordinary food is nearly 100% digestible. Starving people do not excrete solids. Even fiber is absorbed. The body colonizes bacteria which can break down fiber. People can digest fiber just like cows. For people, however, the digestion of fiber is inefficient, and requires a long residence time in the digestive tract. The body only allows food to reside that long in the colon when it is starving.

The digestive process is not under conscious control. The regulatory loops that control the numerous fluids secreted into various parts of the digestive tracts and that control the mechanical forces that propel food along the digestive tract are entirely under the control of the subconscious. This remarkably complex system has some ability to sort through the food we eat and absorb more of what we need most while rejecting more of what we need the least. As people age, problems inevitably arise with one or more aspects of the body’s complex weight regulation systems. The problems commonly manifest themselves as obesity or emaciation. Look for yourself inside a nursing home. How many of the residents appear to be at a healthy weight?

If you want to maintain a healthy weight, it helps to grow up healthy and strong, and to efficiently heal any damage life brings to the digestive tract. The digestive tract is regularly attacked by microbes, so this is no easy task. I’ve argued in past columns that vitamin C and niacin have proven power to facilitate wound healing, and I’ll make the same argument again here. Vitamin C, niacin, and multivitamins are especially important for children. In addition to helping to heal wounds from any injuries that toxins or pathogens cause, extra vitamins help build a healthier digestive tract and immune system in the first place. With luck, increasing use of vitamin supplements by children will lead to reduced rates of childhood obesity.