There is a growing interest in the role of microbial populations of the gut in aging and health, with evidence from recent years suggesting that their level of influence might approach that of exercise. Some fraction of the benefits to health and longevity that occur due to the practice of either calorie restriction or intermittent fasting are thought to be mediated by resulting changes in gut microbe populations. This seems a safe assumption, given the evidence to hand, but the still open question is just how large or small that fraction might be. The consensus view remains that benefits largely result from increased cellular housekeeping, and the fact that calorie restriction fails to work in animals with disabled autophagy is telling.

Complicating the matter, however, calorie restriction and intermittent fasting are not just two ways of achieving exactly the same result. They produce significantly different patterns of gene expression in animal studies. Intermittent fasting without reducing calorie intake still produces health and longevity benefits in rodents. Calorie restriction lasting for less than three days in humans fails to produce the significant benefits to immune cell populations that fasting for four or longer days achieves. One could argue that the point is time spent in a state of hunger, but that seems overly simplistic given what is known. A mammalian body and its microbial fellow travelers are collectively a complicated system, and that system has correspondingly complicated responses to environmental circumstances.

In the open access paper here, researchers focus on one specific set of interactions between gut microbes and the immune system. Age-related (and other) changes to the microbiome can contribute to chronic inflammation and autoimmunity - here, the autoimmune condition in question is multiple sclerosis, in which immune cells attack the myelin sheathing of nerves, with catastrophic consequences. Intermittent fasting can help in this situation by reducing the influence of problematic microbial populations.

As is the case in all such investigations, the highly varied and dynamic nature of the gut microbiome makes it hard to settle on definitive results that are true for everyone at all times. Even for a given individual, what turns out to be a beneficial influence one year might be more or less beneficial the next year, because the state of the gut mitobiome shifts over time. Of all the presently available ways to manipulate gut bacteria, forms of calorie restriction appear the most reliable, but the degree to which they work in this matter is greatly obscured by the other reliable benefits they achieve in the operation of cellular metabolism.

Intermittent Fasting Confers Protection in CNS Autoimmunity by Altering the Gut Microbiota