A new study out of UC San Francisco tested intermittent fasting and found that it's no more effective for losing weight than eating throughout the day.

That's bad enough news for practitioners of the 16:8 plan (where dieters restrict their eating to an 8-hour window), but things look even worse when you tease apart the results. It seems that the intermittent fasters in the study lost significantly more muscle mass than those in the control group.

What They Did and What They Found

The study involved 116 men and women between the ages of 18 and 64. Half of the subjects were allowed to eat whatever they wanted to and however much they wanted to for an 8-hour window (12 PM to 8 PM). The other half was instructed to eat 3 "structured meals" per day.

Twelve weeks later, the "time-restricted eating" (TRE) group lost an average of 2 pounds (0.94 kg). On its own, that seems pretty good, but then you notice that the "consistent meal time" (CMT) group also lost weight, an average of 1.5 pounds (0.68 kg).

The thing is, people involved in clinical weight-loss trials almost always lose weight, so yeah, the TRE group lost two pounds but it's really no better, by scientific standards at least, than the 1.5 pounds lost by the control group.

Furthermore, the scientists didn't detect any metabolic changes between the two groups after 12 weeks. Fasting insulin, fasting glucose, hemoglobin A1C levels, estimated energy intake, total energy expenditure, and resting energy expenditure were all virtually the same.

The only notable difference was, alas, in lean muscle mass. The TRE group lost significantly more muscle than the control group.

What to Make of These Results

This study had a couple of shortcomings that might have muddled the results a bit. For one, they didn't measure the calorie intake of the participants, so it's impossible to say whether the TRE group actually ate less than the control group, which is part of the reason behind doing intermittent fasting in the first place.

Of course, the classic drawback of IF in general is that participants feel that they're not bound by the constraints of good dietary choices. They think they're free to clean out the refrigerator and cupboards, including the Ritz cracker crumbs under the contact paper, as long as they do it in 8 hours.

This could well have been the case in the current study. Hence the lack of any stupendous results.

Secondly, the researchers didn't monitor macronutrient intake. There's no way of knowing if the people in the TRE group lost muscle mass because they weren't eating enough protein during their feeding window, or if limiting protein intake to a meager 8-hour window simply doesn't have enough spillover effect to preserve muscle mass the rest of the day.

The researchers themselves were also a little critical of their results. They wondered whether the 12 PM to 8 PM was the ideal metabolic timeframe; that maybe earlier and shorter windows – say, 9 AM to 3 PM – would work better.

Regardless, the scientist who instigated the study, a former practitioner of IF, was so disappointed (and so convinced) by the results that he himself quit practicing the diet.

Should You Quit Practicing IF?

It's difficult to say. While other studies have shown it to be effective, few of them used controls of any kind. Secondly, as pointed out earlier, virtually all diet studies lead to some degree of weight loss, controls included, so it's difficult to figure out what's working.

The confusion is, admittedly, a bummer. We've long assumed that IF would have multiple beneficial effects on us human types, including but not limited to fat loss, mimicking the life-extending effects of chronic calorie deprivation (via SIRT 1 activation), reducing blood sugar, increasing insulin sensitivity, and decreasing production of ghrelin while increasing production of leptin (thereby decreasing appetite while accelerating fat loss).

Of course, one study does not a scientific truth make. As always, we need further and better-designed studies before we can make any definitive conclusions. One thing we should definitely take from this study, though, is the need for sufficient protein during periods of IF.

That might not be enough, though. It's hard to believe that restricting protein intake – however many grams are ingested – to just an 8-hour window is a valid strategy for building or even maintaining lean muscle mass. The current study agrees, but beyond that, so does everything we know about muscle physiology and protein biochemistry.


Dylan Lowe, Nancy Wu, Linnea Rohdin-Bibby, "Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity," JAMA Intern Med.