Some research has hinted that eating within certain windows, aka intermittent fasting or IF, might align better with our circadian rhythms and may improve blood sugar and cholesterol and boost longevity. For this reason, IF has been having a moment over the last decade.
Celebrities from Jennifer Aniston to Jimmy Kimmel have given it a shot, and dozens of diet books have been released explaining how to follow one of the plans, be it the 5:2 method, 16/8, eat-stop-eat or the warrior method. (To the uninitiated, these are all different forms of IF that have varying rules about when and how much you eat.)
Last summer, in our report "Should you try intermittent fasting for weight loss?" we raised a red flag about several very real potential drawbacks of the trendy diet, including its impact on fertility and risk for binge-eating tendencies and other disordered eating patterns. Not to mention, for most of us it's just downright difficult to stick with for the long haul. What about those birthday dinners that involve reservations at 8 p.m. after we have a business lunch at noon? (It's important to note that there's been no definitive research that fasting actually leads to more weight loss, if that's the goal.)
A new study, published April 21 in the New England Journal of Medicine, clarified that last detail. Scientists at Southern Medical University in China found that over the course of a full year, people who only ate from 8 a.m. to 4 p.m. didn't lose significantly more weight than those who ate at any time throughout the day. Let's break down this new research.
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What This Intermittent Fasting Study Found
To determine this, lead researcher Deying Liu, M.D., and his team recruited 139 adults who had a body mass index (BMI) between 28 and 45 at the start of the study. All males were told to eat between 1,500 and 1,800 calories per day, and all women were advised to eat between 1,200 and 1,500 calories per day. (Worth noting: EatingWell definitely doesn't advise dipping below 1,200 calories each day, and that's far too low for most adults to meet their energy needs.)
These 139 people were randomly assigned to one of two groups and kept food diaries to log their bites:
Fasting: Eat all calories in an 8-hour window (the 16/8 structure of IF) from 8 a.m. to 4 p.m.
Non-Fasting: Eat all calories throughout the day as desired
At the start and at the 6-month and 12-month mark, the researchers measured weight, body fat, BMI, blood pressure, blood sugar, triglycerides and other metabolic biometrics of all participants. After one year, 118 participants completed the study. The fasting group lost 17.6 pounds on average, while the non-fasting group lost an average of 13.9 pounds; neither this difference nor any differences in those other health measures, however, were statistically significant. In other words, this could be the result of water weight, small fluctuations throughout the day or other factors rather than definitive proof that IF leads to more weight loss.
"The two weight-loss regimens that we evaluated had similar success in patients with obesity, regardless of whether they reduced their calorie consumption through time-restricted eating or through calorie restriction alone," Liu and colleagues explain. "These results indicate that caloric intake restriction explained most of the beneficial effects seen with the time-restricted eating regimen."
This holds true to what shorter-term and smaller previous studies have pointed out: most people naturally eat fewer calories if they have fewer hours when they are allowed to eat, and this likely explains most of the scale shifts related to any IF plan. It might be true that time-restricted eating can lead to weight loss. But as with any other restrictive diet, that weight loss will likely only last as long as you can stick with the program.
The Bottom Line
"Almost every type of diet out there works for some people," Liu tells The New York Times. "But the take-home supported by this new research is that when subjected to a properly designed and conducted study—scientific investigation—it is not any more helpful than simply reducing daily calorie intake for weight loss and health factors."
So if you enjoy the parameters of IF, feel better eating within a certain window and are aiming to lose weight, go for it! But people with a history of disordered eating, individuals with a so-called "normal" body weight and people with diabetes may not be the best candidates for IF. Before making any drastic diet adjustment, consult with a dietitian for personalized advice.