In recent years, interest in intermittent fasting—that is, not eating at all for a certain number of days per week—has been increasing. Intermittent fasting (IF) has been practiced worldwide based mostly on traditional, cultural, or religious grounds, but recent experimental data suggest it’s not only safe but also effective for achieving weight loss. What's more, evidence is accumulating that it can produce a myriad of other health benefits. In this article, I summarize the data supporting the use of IF and include my recommendations for who might want to try it and how they should do it.
One concern about reducing calorie intake on a daily basis (that is, dieting) to lose weight is that it lowers the basal metabolic rate (in one study as much as 504 cal/day!), making weight loss harder to achieve. Some studies have suggested this effect continues for a full four years after beginning a diet, and may continue longer or even indefinitely, explaining one of the reasons why it’s so easy to regain any weight that’s lost, as opposed to maintaining the weight you've always been.
In one study of IF, however, basal metabolic rate was actually found to increase after both 36 and 60 hours of fasting, likely due to an increase of norepinephrine (which was likely caused by lower blood sugar induced by the fast). Fat metabolism also increased (meaning fat was preferentially burned for energy over glycogen, or carbohydrate), likely caused by the increase in norepinephrine levels (rather than a decrease in insulin levels, which wasn’t seen). Muscle breakdown wasn’t seen until the end of the third day of fasting. Other studies have shown that prolonged fasting—that is, beyond three days or more—can reduce the basal metabolic rate by 20%.
After a 36-hour fast, when you start eating again you continue to burn fat in excess of glycogen (carbohydrate) for energy. Long-term use of IF may, in fact, shift the substrate the body uses for energy preferentially to fat, though it's not clear how often you need to fast to bring about this shift. One concern about every-other-day fasting, however, is that it might represent too great a metabolic challenge: in the long-term it seems to decrease glucose tolerance, which might lead to or predispose to the development of diabetes. More work needs to be done to sort out the ideal frequency with which you should fast to lose weight.
Interestingly, IF doesn’t appear to make you hungrier on days you eat. In fact, studies show people actually eat 20% less on eating days. This may be because eating less itself seems to reduce the amount of food that makes you feel full.
Studies suggest you’ll lose muscle if you only eat 0.9 g of protein/kg of body weight on non-fasting days when you're intermittently fasting. It seems you need to eat 1.2 g of protein/kg of body weight on non-fasting days to preserve fat-free mass. That is to say, IF can cause muscle loss if you don’t eat enough protein and exercise.
In comparing the weight-loss effects of IF to continuous calorie restriction (a standard diet) multiple studies (admittedly all small) testing multiple different IF regimens demonstrated comparable reductions in body weight (with perhaps a slight advantage seen in IF). A longer-term, larger study called the HELENA trial will conclude in May of 2017. It will compare IF, continuous calorie restriction, and control groups for a year and look not only at effects on weight loss, BP, quality of life, and self-image, but also 82 pre-defined genes that encode proteins involved in energy metabolism, inflammation, immune function, aging, and neurological function. I’ll report back on the results later this year once they're published, which should help guide us more definitively in how to make IF work for weight loss.
Time restricted feeding (TRF), in which all food is consumed in a narrower window than what’s traditional (8 hours instead of 10 or 12) may mimic the positive effects of a longer fast. In one study, TRF was found to increase fat loss without compromising muscle mass in men who lifted weights (though it reduced anabolic hormones like testosterone and IGF-1) compared to men who lifted weights but ate on a normal schedule (the same total number of calories were consumed by both groups). The notion that the timing and interval of eating affects weight loss finds evidence in another study of women who ate their largest meal in the morning rather than in the evening and had greater weight loss than women who ate their largest meal in the evening (same number of total calories consumed per day in both groups). This was thought to be due to decreased insulin sensitivity at the end of the day, leading to higher levels of insulin around the evening meal. Higher levels of insulin may contribute to weight gain because insulin works to promote fat storage, decrease fat burning, and increase glycogen (carbohydrate) burning.
In conclusion, current evidence suggests—though doesn't yet prove—that IF may have a multitude of health benefits in humans. The HELENA trial will more definitively answer many of the questions that remain and hopefully provide even clearer proof that long-term use of IF is safe. If you're interested in losing weight, in maintaining weight loss, or gaining some of the other possible benefits of IF, IF may indeed be something you want to try.