Even though interest in intermittent fasting is declining, it remains popular among people seeking weight loss or improved health. When I tried the trend back in the late 2000s, research on intermittent fasting was limited to rodent studies and a few small, low-quality human studies. Today, we have more human research on intermittent fasting, but what tends to hold the most sway is the anecdotal “evidence” of the “it worked for me, so it will work for everyone” variety.
But what does “work” mean? When someone says they lost weight or improved their health through intermittent fasting, that’s nice, but how long did their weight loss or lowered blood sugar last? It’s long-term evidence that’s truly important — in research studies and in life. Right now, it’s unclear what types of intermittent fasting may have benefits for certain health conditions or risk factors, and whether any benefits last beyond the short term.
There are four main types of intermittent fasting. The most restrictive is alternate day fasting, where you eat “normally” one day and skip eating the next day. With modified alternate day fasting, you eat normally one day, then eat one meal containing about one-fourth of your usual intake the next day. And 5:2 fasting is similar, except you eat normally five days per week, then eat one meal the other two days. Standing somewhat apart from the rest is time-restricted eating, which limits eating to a shortened window of time every day, usually of less than 10 hours.
Speaking at the recent annual meeting of the Academy of Nutrition and Dietetics (AND), Kristen Hoddy, a postdoctoral researcher at Pennington Biomedical Research Center in Baton Rouge, Louisiana, said some early intermittent fasting research was premature, because it didn’t directly compare fasting with daily calorie restriction (aka “regular” dieting).
Since then, she said, we’ve learned that when comparing intermittent fasting to daily calorie restriction, there’s no difference in weight loss, body composition or ability to stick with the diet. Current research has also found no differences in lowering total cholesterol, “bad” LDL cholesterol, triglycerides, blood pressure, blood sugar or A1c. Intermittent fasting does appear to have a very slight advantage for raising “good” HDL cholesterol and improving the body’s insulin responses, even when weight and body composition don’t change.
In a review of intermittent fasting research published in February 2021 in the Archives of Endocrinology and Metabolism, the authors point out that “its effects on weight are directly related to caloric restriction and not the metabolic effects of fasting” and that more research is needed to establish whether prolonged fasting can have the same benefits for insulin sensitivity and other metabolic health markers that have been observed in animals.
Similarly, in a study published in the journal Science Translational Medicine in June 2021, researchers concluded that intermittent fasting is nothing special, and certainly not a magic bullet for weight loss or health. Lead researcher professor James Betts said in a press release, “If you are following a fasting diet it is worth thinking about whether prolonged fasting periods is actually making it harder to maintain muscle mass and physical activity levels, which are known to be very important factors for long-term health.”
So, what about time-restricted eating? Results from a randomized controlled trial published in September 2020 in JAMA Internal Medicine found that eating all meals within an eight-hour window — then fasting for 16 hours — did not result in weight loss or improved health. The researchers thought that these participants would lose weight and see improvements in several cardiometabolic health markers. However, the group lost an average of less than 4 pounds in the 12 weeks of the study — about double lost by the control group participants, who ate three meals spaced throughout the day. The control group saw a small reduction in blood pressure, but neither group saw improvements in the other health markers.
Here’s where it gets scary. About 65% of the weight lost by the time-restricted eating group was from muscle. To put this in perspective, when someone loses weight through simple daily caloric restriction, about 20-30% of lost weight is from muscle. The more muscle someone loses during weight loss, the more likely they are to regain the weight, specifically in the form of body fat, leaving them with worse body composition than when they started. More alarmingly, much of the muscle loss in the time-restricted eating participants was in the arms and legs, which can lead to weakness, disability and impaired quality of life. The authors said this serves as a caution for people who are already at risk for sarcopenia (age-related muscle loss).
These results prompted the study’s lead author, Dr. Ethan Weiss, a cardiologist at the University of California, San Francisco, to end his six-year personal practice of time-restricted eating — and to stop recommending it to his patients. In interviews, he said that while he had found it personally beneficial, the results of his clinical trial highlighted the pitfalls of anecdotal evidence and personal biases.
Felicia Steger, a research assistant professor at University of Kansas Medical Center, has studied various types of intermittent fasting, including time-restricted eating. Also speaking at the AND annual meeting, she pointed out that the research in this area is “still really young,” and if someone’s diet quality isn’t great, that may be a better place to focus.
So what’s the moral of this story? If you’ve tried and benefited from intermittent fasting, don’t proselytize to others, because it may not benefit them. And if intermittent fasting doesn’t appeal to you, don’t try it — eating this way can be stressful, and can trigger disordered eating or eating disorders in some individuals. You may simply be better off focusing on eating enough fruits and vegetables, addressing habits like mindless grazing or overeating, and making tweaks to your food shopping and prep routine so you can consistently feed yourself with nutritious foods that also taste good to you.