In recent years, interest in intermittent fastingthat is, not eating at all for a certain number of days per weekhas been increasing. Intermittent fasting (IF) has been practiced worldwide based mostly on traditional, cultural, or religious grounds, but recent experimental data suggest its 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 includemy 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 onestudyas 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 adiet, and may continue longer or even indefinitely, explaining one of the reasons why its so easy to regain any weight thats lost, as opposed to maintaining theweight you've always been.
In onestudyof IF, however, basal metabolic rate was actually found toincreaseafter both 36 and 60 hours of fasting, likely due to an increase of norepinephrine (which was likely caused by lowerblood sugarinduced by the fast). Fat metabolism also increased (meaning fat was preferentially burnedfor energy over glycogen, or carbohydrate), likely causedby the increase in norepinephrine levels (rather than a decrease in insulin levels, which wasnt seen). Muscle breakdown wasnt seen until the end of the third day of fasting. Other studies have shown that prolonged fastingthat is, beyond three days or morecanreducethe 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) forenergy. 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 ofdiabetes. More work needs to be done to sort out the ideal frequency with which you shouldfast to lose weight.
Interestingly, IF doesnt appear to make you hungrier on days you eat. In fact,studiesshow people actually eat 20% less on eating days. This may be because eating lessitself seems to reduce the amount of food that makes you feel full.
Studiessuggest youll 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-freemass. That is to say,IFcancause muscle loss if you dont eat enough protein andexercise.
In comparing the weight-loss effects of IF to continuous calorie restriction (a standard diet) multiplestudies(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, largerstudycalled 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. Ill 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 whats traditional (8 hours instead of 10 or 12) may mimic the positive effects of a longerfast. In onestudy, 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 anotherstudyof 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 suggeststhough doesn't yet provethat 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 provideeven clearer proof that long-term use of IF is safe. Ifyou'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.
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Intermittent Fasting for Weight Loss and Other Benefits - Psychology Today (blog)
