Intermittent fasting: Does a new study show downsides — or not? – Harvard Health Blog
Intermittent Fasting (IF) is a timing-based approach to eating. The idea is that fasting long enough allows insulin levels to drop low enough that our bodies use fat for fuel. A growing body of evidence in animals and humans shows that this approach leads to significant weight loss. When combined with a nutritious, plant-based diet and regular physical activity, IF can be part of a weight loss or healthy maintenance plan, as I described in an article by previous blog.
Now a randomized controlled trial published in JAMA claims that IF has no significant weight loss benefit and a substantial negative effect on muscle mass. News organizations picked up the story and made headlines like A potential downside of intermittent fasting and an unintentional side effect of intermittent fasting.
But what did this study actually look and find?
In the study, 141 patients were randomly assigned to 12 weeks of a time-limited diet (TRE) that involved fasting for 16 hours and eating only for an eight-hour window of the day, or a consistent meal schedule (CMT) diet plan, with three structured meals a day and snacks.
Neither group received nutritional education or behavioral counseling, and no physical activity was recommended. There was no real control group (that is, a group that received no instruction on when to eat).
Interestingly, both groups lost weight. Considering the headlines, I had to read and reread the results several times as they show that the IF group lost a statistically significant amount of weight from start to finish – which was not true in the CMT group . The researchers reported: “There was a significant decrease in weight in the TRE group (−0.94 kg; 95% CI, −1.68 kg to −0.20 kg; P = 0.01) and a non-significant decrease in weight in the CMT group (- 0.68 kg; 95% CI, −1.41 kg to 0.05 kg; P = 0.07). “
Translated into plain English, the IF group lost more weight than what could have been chance: between half a pound and 4 pounds, or an average of 2 pounds. The structured meal group also lost weight, although the amounts lost could be due to chance: between 0.1 and 3 pounds, or an average of 1.5 pounds. The result was that there was no significant difference in the change in weight between the two groups. And the researchers found a loss of muscle mass in the IF group that did not occur in the CMT group.
Dive Deeper into the Study
By the way, maybe all of these people ate fried food or fast food, soda and sweet candy – we don’t know. The study does not mention the quality of the diet or physical activity. This is not how the FI is supposed to be done! And yet, the folks at IF still lost between half a pound and 4 pounds.
Importantly, the group of structured meals also lost weight. While this was not significant enough to prove that it was due to this intervention, for some participants it was enough to make the weight loss of a structured meal little different from the IF weight loss. But think about it: structured meals are an intervention. After all, some people eat more than three times a day, consuming several small meals throughout the day. Telling people to limit their diet to three meal times plus snacks may actually help some eat less.
The authors could well have concluded that the IF had indeed succeeded. They might also request a follow-up study with a true control group without intervention, as well as behavioral counseling, advice on healthy eating and recommended activity levels for the IF and CMT groups.
Does extra support make a difference?
Previous studies by FI that provided behavioral counseling and advice on nutrition and activity clearly showed positive results. For example, in a previous blog post I described a 2020 American Journal of Clinical Nutrition study in which 250 overweight or obese adults followed one of three diets for 12 months:
- SI on the 5: 2 protocol, which means drastically reducing food intake for two of the five days of the week (up to 500 calories for women and 700 calories for men)
- Mediterranean, which emphasizes fruits and vegetables, legumes, nuts, seeds, whole grains and olive oil with moderate fish, chicken, eggs and dairy, and with a allocation of one glass of wine per day for women and two per day for men
- Paleo, which emphasizes fruits and vegetables, animal protein, coconut products, butter and olive oil, as well as nuts, seeds, and legumes.
- And this is essential: all participants received training in behavioral strategies for weight loss, stress management, sleep and exercise.
Everyone has lost weight. The IF group lost more than anyone at an average of 8.8 pounds, the Mediterranean next at 6.2 pounds and Paleo last at 4 pounds. Compliance was better with the Mediterranean diet (57%) and IF (54%) than with the Paleo diet (35%), and better adherence resulted in weight loss of one to three pounds more. The Mediterranean and IF groups also experienced significant drops in blood pressure, another good result.
What about the loss of muscle mass that occurred in the IF group in the JAMA study? While this needs to be investigated further, it’s important to note that other research on IF that included advice on physical activity showed no loss of muscle mass.
The bottom line
What’s the takeaway here? A high-quality diet and plenty of physical activity – including resistance training – are essential to our good health, and there is no substitute for these recommendations. IF is just a tool, an approach that can be quite effective in losing weight for some people. Although this negative study adds to the body of literature on IF, it does not reverse it. We just need more high quality studies to better understand how to most effectively integrate IF into a healthy lifestyle.
Our sincere thanks to