As discussed in Dr. Peat's latest newsletter, Directing epigenetic adaption, there is an assumption that, because we have polyunsaturated fats in our brains, and bacteria in our intestines, the presence of these substances is somehow an adaptive advantage, if not an essential requirement for proper function. As Dr. Peat points out, these assumptions may not be in our best interest.
As it happens, there is experimental evidence that both polyunsaturated fats and intestinal bacteria interfere with cellular metabolism, ultimately promoting the formation of nitric oxide, lactic acid, and inflammation in a self perpetuating vicious cycle. Research also shows that both germ free animals and animals fed a diet free of polyunsaturated fat tend to live longer, have a higher metabolic rate, less susceptibility to obesity, diabetes, and tumors, and heal more quickly than animals with "normal" levels of unsaturated fats in their tissues and bacteria in their intestines.
Besides avoiding polyunsaturated fats and bacteria-feeding starch, restriction of dietary arginine can also help to lower nitric oxide formation since arginine is readily converted into it.
References
Directing epigenetic adaptation, Ray Peat's Newsletter, January 2015
