This article from the more scientific end of the popular science press covers recent research into the beneficial effects of calorie restriction and intermittent fasting in humans. These interventions have been shown to extend life and improve health in near all species tested to date, slowing measures of aging along the way. This area of the field has grown in recent years, with the addition of a fair amount of new human data. Fasting and low calorie diets have been tested as adjuvants for cancer treatment, for example, and as independent ways to improve metrics of health.
When external calories stop fueling an animal's metabolism, stores of triglycerides in fat cells are mobilized, and levels of ketones - chemicals that result from the burning of fat for fuel - rise. Decreases in body weight follow. Scientists are further detailing both the underlying metabolic dynamics and interesting physiological phenomena aside from weight loss as they study permutations of fasting in animal models and in humans. Data has recently emerged from research on several forms of so-called intermittent-fasting regimens, including alternate-day fasting, the so-called 5:2 diet, time-restricted feeding, and periodic fasting. Although these regimens vary, they all involve a rhythmic disruption in the typical flow of calories into the metabolic machinery. As the body of scientific literature around fasting has grown, results have been cherry-picked and molded into fad diets. But as books of dubious scientific merit extolling the virtues of fasting fill the shelves, serious researchers continue to probe the genetic, immunologic, and metabolic dynamics that occur in fasting animals to separate hype from reality.
For the majority of genus Homo's more than 2 million year evolution, hominins' access to nutrients and calories was spotty, at best. Perhaps our ancestors, and their digestive systems, evolved to endure periodic bouts of starvation. Oscillations between feast and famine may even have served as a selective pressure, tuning early human physiology to function optimally in an environment where resources were unpredictable. "Individuals whose brains and bodies and physical performance were optimal in a fasted state would be more likely to get food and compete with other individuals who were not able to function at quite as good a level. So the assumption then is that we evolved probably most of our organ systems to be able to function optimally in intermittent fasting-type conditions."
Results from studies in both animal models and humans point to distinct benefits of withholding food in one temporal pattern or another. In recent years, scientists have learned that fasting might trigger not only weight loss and life-span extension - benefits that have long been linked to caloric restriction - but also boost the performance of the brain, the immune system, and organs central to metabolism, such as the liver and pancreas. Fasting, some researchers claim, can even alter the course of some diseases, from cancer and multiple sclerosis to diabetes and Alzheimer's.
Fasting that involves longer periods of food deprivation can cause changes to the immune system and the hematopoietic stem cells that support it. Researchers are finding that periodic fasting, less frequent but longer bouts of severe calorie restriction, can reshape immune cell populations in the body. One research group employs the fasting-mimicking diet (FMD). Using a periodic three-day FMD regimen for 30 days in a mouse model of multiple sclerosis, the researchers showed that the fast-and-feed cycles pruned away populations of autoimmune T cells, replacing them with immune cells that were no longer bent on attacking neural tissue. Oligodendrocyte precursor cells regenerated and remyelinated axons, and the clinical severity of the autoimmune disorder declined. "One in five of the mice went to back to no symptoms at all. One in two of the mice went down to very low levels of the symptoms. The real benefit that we've shown in a number of papers is about killing damaged cells and then turning on stem cells. And then in the refeeding period, [stem cells are] replacing the dead cells with newly generated cells. I think that is where the real benefit is."