Does time-restricted eating confer clinically significant health benefits for adults?
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Time-restricted eating is beneficial for the prevention and treatment of obesity, diabetes and other chronic diseases.
There are many benefits of time-restricted eating and many mechanisms that may play a role. It has been shown that time-restricted eating affects the gut microbiome. When we restrict our eating window to only a certain period during the day, it increases the diversity of the gut microbiome and also affects the composition. We know that our gut microbiome may affect our insulin sensitivity and our immune function, and it may have beneficial effects on our cardiovascular system.
Another mechanism is aligning the body clock with circadian rhythms. We have “clock genes” that are aligned with the central body clock in the hypothalamus. The central body clock is aligned with light and darkness. The peripheral body clock, the clock genes, are aligned with the central body clock through nutritional cues, through periods of fasting and eating. The disruption of the body clock and synchronization of the peripheral and central body clocks is present in all of the chronic diseases: in people who are overweight, who have diabetes, who have CVD and cancer. If we can make sure that our peripheral body clocks are being aligned with the central body clock through nutritional cues, that helps with the prevention and treatment of all the chronic diseases.
One review has looked at the time window that would be most beneficial for weight loss. Those researchers found that the sweet spot is a 6-hour eating window. If you limit the eating window even more, to only 4 hours, that does not bring additional benefits.
In terms of timing, there was a head-to-head comparison recently published that showed, compared with eating for 12 hours per day, early and late time-restricted eating have benefits. But at the same time, compared with late time-restricted eating, early time-restricted eating had more benefits in terms of insulin sensitivity, body weight management and other cardiometabolic outcomes.
We know that time-restricted eating works, but the mechanisms are still to be explored. We know that the gut microbiome composition changes. We know about the body clocks. But how it works all together is still a mystery to a certain extent. Also, there are so many studies, but they use different designs. The next phase of research should be to optimize the study protocol and see what an optimized protocol can do.
- For more information:
- Hana Kahleova, MD, PhD, is the director of clinical research for Physicians Committee for Responsible Medicine in Washington, D.C.
Time-restricted eating alone does not meet the generally recognized clinical significance threshold of 5% weight loss.
Time-restricted eating provides a useful case study for two principles in interpreting medical research. One is the challenges and limitations inherent in translating preclinical rodent studies to humans, and two is the difference between statistically significant and clinically significant results.
Time-restricted eating is a special case where extra care needs to be taken in applying the results of rodent studies to humans. Mice have a very different fasting physiology than humans. An overnight fasted mouse that fasts for 12 to 16 hours can lose 10% of its body weight and can develop physiologic torpor. We know that this doesn’t happen in people, fortunately, but it highlights the difference between a 12- to 16-hour fast in a mouse and a person. When looking at the remarkable potency of time-restricted eating interventions in mice, which can reverse obesity, hepatic steatosis, glucose intolerance and cause dramatic multi-organ system effects, it’s important to be cautious in extrapolating those results to people.
Regarding the second point, of the difference between statistically significant and clinically significant results, we now have enough randomized controlled trial data in people to have a decent idea of the effect of time-restricted eating interventions on body weight in people with obesity. For the most common time-restricted eating approach, which is an 8-hour time-restricted eating window, the average weight loss in randomized controlled trials is about 2% more weight loss than a control group. For the average person with obesity, who weighs perhaps 100 kg, or 220 lb, that amounts to 4 lb to 5 lb, which is, in general, not a clinically significant amount of weight loss.
If someone comes to me and is already following a time-restricted eating regimen or is interested in doing so, I don’t discourage that. One of the nice things about time-restricted eating is that it can be layered on to other interventions. There’s no reason that somebody can’t apply a calorie-restriction intervention, macronutrient restriction intervention or an obesity pharmacotherapy on top of time-restricted eating. But it’s important that people have realistic expectations about what time-restricted eating is likely to do for their body weight. It’s important for us as clinicians to provide realistic assessments of the likely therapeutic efficacy of the interventions we recommend.
- For more information:
- Max Petersen, MD, PhD, is a fellow in the division of endocrinology, metabolism and lipid research and the Center for Human Nutrition at Washington University School of Medicine in St. Louis.
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