First, thanks for being a ParentData reader. I really appreciate it; it means a lot to have your support. In return for this, you get to hear me rant about diet study data.
Back in January, I wrote about the “best” diet strategy for health. The conclusion was decidedly not that there is some great diet you’re missing. It was mostly that there is no single best diet for heart health or any other outcome, and that we do not know very much. It was pointed out to me after I sent it that I should also have been clearer that most diets fail and that diet culture is extremely problematic. Point taken.
Today, though, I want to do something that I think is slightly less fraught and just unpack one particular study, published in the New England Journal of Medicine a few weeks ago, that discusses intermittent fasting. The TL;DR is that it did not find that weight loss was greater with intermittent fasting than with simple calorie restriction. But let’s dive in.
This study is a randomized trial that enrolled 139 obese patients in China. The patients were divided into two groups. Both groups were instructed to follow a restricted-calorie diet (1,500 to 1,800 calories a day for men, 1,200 to 1,500 for women). One group, the fasting group, was told to eat only between 8 a.m. and 4 p.m. The other group was allowed to eat whenever, but with the same calorie restriction.
Participants were followed for 12 months. The main result on weight loss can be seen in the graph below. “DCR” is the calorie-restricted diet without time restriction, and “TRE” is the time-restricted version. As with virtually all diets: weight loss is initially steep, then plateaus before slightly rebounding. Although the time-restricted eating group loses slightly more weight in the first three months, there is no significant difference by 6 or 12 months in the amount of weight loss.
The authors look at all kinds of other outcomes, including blood pressure, fasting glucose, and body fat, and do not see any significant differences across the groups.
If you dig a bit deeper into the supplementary tables of the paper, it seems clear why we do not see differences across groups: there is no difference in their calorie intake. The graph below shows the measured calories for each group over the study period. (Note: Reported calorie intake is generally a large underestimate of actual calorie intake, so these numbers are likely too small, both before and after. However, the comparison across groups should capture differences.) Both groups show an initial large reduction in calorie intake, followed by a gradual increase back toward baseline.
The bottom line isn’t that intermittent fasting is worse than another diet option. It’s just that it’s not any better. Based on this follow-up, it shows the same profile as a standard approach: some weight loss, followed by some rebound.
Stepping back, there are really two reasons you might have thought intermittent fasting was a “better” option, both of which I think are rendered less plausible by these data.
One is the idea that intermittent fasting could work because it is easier to adhere to. I will confess to having had this instinct at various times. Diet rules are often complicated — calorie counting, nutrient comparison, etc. The rule “Don’t eat from 8 p.m. to noon” (or whatever it is) is at least easy to follow. Since adherence to diet is basically the whole thing in terms of long-term success, a diet that is easier to stick with could be better. In this study, we do not see evidence of that. The intermittent fasting diet does not have better adherence than with standard calorie restriction.
The other possibility is that there is something inherently health-promoting about going longer periods without eating. The idea would be that even with the same calorie intake, having periods of fasting improves heart health or some other outcome. That doesn’t appear to be the case based on these data. The calorie and weight reduction are similar, and there are no differential improvements in heart or metabolic health.
On the more positive side, both diets report the same number of adverse events, and these are generally mild (basically, people are hungry).
What do we take from this overall? I would say it’s simply that there isn’t something magic about intermittent fasting. If it is an approach to eating that works for you, fine. But there is still no “best” diet.