It seems every few years a new weight loss fad arrives, and intermittent fasting (IF) has been garnering a lot of attention lately. The practice of intermittent fasting has actually been around for millennia, practiced around the world for various cultural and religious reasons. As is typical with Western appropriation, IF has now been commoditized and touted to enhance everything from weight loss to immune function to cognitive performance. While the mechanisms are not entirely defined IF may promote the use of fat stores as a primary fuel source, clean up old or dying cells, and improve the function of stress and immune pathways. What’s more, in animal studies, fasting has even been shown to improve longevity!
IF can be performed in a variety of ways including a 24 hour weekly fast, alternating day fasts consisting of very-low-calorie intake on fasting days, or extended hours of fasting over a single 24-hour period. All approaches include days with normal eating habits when not fasting. There have been a number of studies examining the health benefits of IF, particularly with respect to weight loss. So where do we stand?
A recently published review of 14 published studies of IF spanning over nearly 50 years attempts to summarize our current knowledge. The review included both animal (mice) and human subject studies and focused on an alternate day fasting. As you could imagine, the included studies varied widely in their protocols. However, the studies did generally demonstrate a link between IF and weight loss (albeit modest, typically < 10%), as well as reductions in body fat and markers of heart health. However, there were mixed results when comparing different fasting protocols and conventional caloric restriction, with some studies showing no difference with respect to weight loss. Furthermore, one study showed that subjects only lost weight when alternate-day fasting was combined with exercise. Interestingly, in one study, measures of fullness and satisfaction increased in the fasting group compared to a no-intervention control group. This has important implications for the feasibility of sustaining this practice.
Several prospective and randomized trials also showed improved insulin sensitivity associated with different types of IF compared with conventional caloric restriction. Insulin sensitivity is an important factor in the pathogenesis of diabetes (better insulin sensitivity means that less hormone is needed to deal with a glucose load). However, measured blood glucose levels only changed in animal studies and did not change in human trials.
Some of the most interesting pieces of evidence pointed to the positive effects of IF on delaying aging. Evidence includes improved biomarkers, reduced oxidative stress, and preserved memory shown in animal models. There are also studies showing that the microbiome, the microbial ecosystem in our gut, is altered by IF which may improve metabolic syndrome (obesity, fatty liver, diabetes). Finally, animal studies have suggested that IF may result in an anti-inflammatory state, though there is the paucity of data in humans.
Ultimately, this review shows us that intermittent fasting likely has beneficial effects on weight and fat mass. There are also intriguing benefits such as improving markers of inflammation, heart health, and insulin sensitivity. However, the included studies are quite heterogeneous, and further prospective studies are needed to draw more definitive conclusions about the specific effects on metabolic pathways as well as in specific patient populations. As with any dietary intervention, a big question is whether IF is sustainable. Ultimately that is up to the individual. In the end, I think it can be a viable strategy for motivated patients – but always consult your physician before embarking on any dietary strategy. For those motivated individuals who want to achieve their weight loss goals, combining weight-loss strategies, such as ESG and a supervised dietary plan is a good bet.