I joined in a wave of the intermittent fasting trend about 10 years ago — before I was a dietitian. That’s when most of the writings on the topic were in the form of blog posts and self-published PDF e-books. Today, a perusal of the Internet turns up several best-selling books extolling the benefits of intermittent fasting for weight loss and improvements in the metabolic risk factors that contribute to Type 2 diabetes and heart disease.
Then, and even now, the intermittent fasting hype was way ahead of the science. Most early research data came from animal studies, with human data coming from observations of participants of religious-based fasts or from small, short clinical studies. A systematic review published last year in the journal Nutrients looked at studies of at least six months that assigned adults with overweight or obese BMIs to either intermittent fasting or daily calorie restriction, and found no evidence that intermittent fasting was superior. The authors cited the need for longer, larger studies to assess sustainability and effects on weight maintenance.
So I was eager to read the results of a study published in the May issue of JAMA Internal Medicine that was longer and larger, enrolling 100 participants for a year — six months of weight loss and six of weight maintenance. Researchers randomly assigned metabolically healthy adults ages 18 to 64 who had BMIs in the obese category to an alternate-day energy restriction group, a daily-calorie-restriction group or a control group whose members ate their usual diet.
Researchers found that the intermittent fasters had a harder time following their diets and were more likely to drop out than daily calorie restrictors. Weight loss and weight regain were similar between the dieting groups, as were changes to fat and lean tissue — which is significant, because one intermittent fasting claim is that it leads to less muscle loss than traditional calorie-restrictive diets. Reduction of cardiovascular risk factors, including blood pressure, cholesterol and triglycerides, were also similar between the two dieting groups. The conclusion? Intermittent fasting was no better, and no worse, than a standard, calorie-restrictive diet.
Although no one study should be taken as a be-all, end-all answer, the results add substance to what previous research studies have overwhelmingly found.
The JAMA study used alternate-day energy restriction for the fasting group, whose members ate one meal containing 25 percent of their usual daily intake on fasting days and “feasted” on 125 percent of their usual daily intake on the other days, for an average 25 percent calorie reduction. The calorie-restriction group reduced calories by 25 percent each day, spread over three meals. Participants started out sedentary, and researchers asked them not to increase activity.
The takeaway? Intermittent fasting may actually be less sustainable in the long term for most people than daily caloric restriction, which itself is not sustainable, as the majority of people who lose weight on calorie-restrictive diets regain the weight, sometimes repeatedly, as with yo-yo dieting. The authors questioned whether there was a difference in perceived hunger or actual levels of appetite-related hormones between intermittent fasters and calorie restrictors. The answer is no, according to a small study published in April in the journal Clinical Nutrition. Researchers found that neither method has an advantage for weight loss or for lessening the body’s means of compensating for perceived starvation, which include slowing the metabolism and increasing levels of the “hunger hormone” ghrelin.
I tolerated the hunger that ebbed and flowed on fasting days and wasn’t ravenous when it was time to eat, but not everyone has that experience. I stopped doing intermittent fasting when I went back to grad school to study nutrition because I had a hard time focusing on fasting days. My brain needed regular fuel! Today, I know that restrictive diets don’t work, regardless of the form. Despite the hype, intermittent fasting isn’t a magic bullet — it’s plain old calorie restriction in a new outfit.
Dennett is a registered dietitian nutritionist and owner of Nutrition by Carrie.
Despite the hype, intermittent fasting isn’t a magic weight-loss cure