There are a few approaches, but the two most popular are 16:8, which calls for squeezing all the day’s meals into an eight-hour window and fasting for the remaining 16 hours, and 5:2, in which five days of the week are spent eating normally and two are spent fasting (usually defined as eating only 500 to 600 calories per day). Why would someone opt for this way of eating versus a standard diet, such as going low carb or low fat? Some people say fasting has loads of health benefits. “The research so far proves the benefits of IF to the extent that it is worthwhile as a method to lose weight, manage your blood sugar, and slow down the aging process,” says Sara Gottfried, MD, of Berkeley, California, author of Women, Food, and Hormones, The Hormone Cure, The Hormone Reset Diet, and Brain Body Diet. But not everyone’s on board. “From my standpoint and the standpoint of a lot of other people, it does tend to fall into the next fad diet category,” says Elizabeth Lowden, MD, a Warrenville, Illinois–based physician who is board-certified in obesity medicine; endocrinology, diabetes, and metabolism; and internal medicine. A lot of the data is conflicted, she says, and many studies done on animals have not yet been repeated in people. “For every study that shows there’s no change, there are some studies that show maybe there is improvement,” Dr. Lowden says. So rather than take popular claims about IF at face value, we decided to dive into them and explore whether 12 touted benefits of the approach are legit or the science doesn’t yet stack up. That’s probably welcome news if you’re hoping to fast for weight loss, but the fact that those studies were short term means it’s unclear if IF is sustainable and can help you keep off extra pounds in the long run. What’s more, not all studies have found IF resulted in weight loss. A study published in September 2020 in JAMA Internal Medicine involving 116 overweight or obese people who ate between the hours of 12 and 8 p.m. for 12 weeks found that they did not have significantly more weight loss than the control group. The other catch: The amount of weight lost doesn’t seem to be any more than what you’d expect from another calorie-restricted diet. A review published in October 2019 in Nutrients found that similar amounts of weight and fat loss were achieved through IF and continuous energy restriction, such as a low-calorie diet. And depending on how many calories you’re eating each day, you could even end up gaining weight. After all, the diet doesn’t restrict high-calorie foods or total calories — it only restricts when you can eat. When the diet is done properly, IF can be as effective as normal caloric restriction, Lowden says. Some people, especially busy people who don’t have time to devote to meal planning, might even find a time-restricted diet easier to follow than something like the keto diet or the paleo diet, she says. The link between lower systolic blood pressure and IF appears in both animal and human studies, according to a review published in March 2019 in Nutrients. And a study published in September 2020 in the European Journal of Nutrition found that IF led to even greater reductions in systolic blood pressure than another diet that didn’t involve defined eating times. Having a healthy blood pressure is important — unhealthy levels can hurt your heart, brain, kidneys, and eyes, according to the Centers for Disease Control and Prevention (CDC). But so far the research shows that these blood pressure benefits last only while someone is doing IF. Once the diet ended and people returned to eating as was normal for them, researchers found that blood pressure readings returned to their initial levels. LDL cholesterol can raise your risk of heart disease or stroke, according to the CDC. The researchers also noted that IF reduced the presence of triglycerides, which are fats found in the blood that can lead to stroke, heart attack, or heart disease, according to the Mayo Clinic. Yet not all researchers agree that IF significantly reduces cholesterol levels. A review published in the summer 2020 issue of the Journal of Patient-Centered Research and Reviews concluded that there wasn’t a difference in cholesterol levels between those who did IF and those who followed a low-calorie diet. A study published in May 2019 in Nutrients found that time-restricted feeding, which the researchers defined as eating between 8 a.m. and 2 p.m., increased the expression of the autophagy gene LC3A and the protein MTOR, which regulates cell growth. This study was small, involving only 11 participants for four days. The Nutrition and Healthy Aging study investigated this effect in humans, and while a 16:8 approach did result in reductions in insulin resistance, the results were not significantly different from the control group. And again, this study was small. Registered dietitians advise people with diabetes to approach intermittent fasting with caution. People on certain medications for type 2 diabetes or those on insulin (whether to manage blood sugar for type 2 or type 1 diabetes) may be at a greater risk for low blood sugar, which can be life-threatening. Check with your doctor before trying intermittent fasting if you have any type of diabetes, they advise. The Nutrients study noted that while there aren’t prospective human studies demonstrating this effect, observational studies have shown that IF may deliver both cardiovascular and metabolic benefits. Lowden suspects that changes to metabolic parameters, such as lower levels of triglycerides and a decrease in blood sugar levels, are the result of losing weight and would be achieved no matter how the weight was lost, whether through IF or a low-carb diet, for example. For instance, a review published in January 2021 in the Cochrane Database of Systematic Reviews found that there wasn’t a significant difference between IF and reduced calorie intake in lowering risk factors for cardiovascular disease. Why? One theory is that IF regulates circadian rhythm, which determines sleep patterns. A regulated circadian rhythm means you’ll fall asleep easily and wake up feeling refreshed, though research to support this theory is limited, according to an article published in 2018 in Nature and Science of Sleep. The other theory centers on the fact that having your last meal earlier in the evening means you’ll have digested the food by the time you hit the pillow. According to the Sleep Foundation, going to sleep with a full stomach (especially if that last meal involved heavy or spicy foods) can interfere with digestion or give you heartburn, which can make it hard to fall asleep.
