Intermittent Fasting–What’s the Key to Success?

Intermittent Fasting–What’s the Key to Success?
  • Watch video here!

    Welcome to Impact Factor, your intermittent dose of commentary on a new medical study. I’m Dr F. Perry Wilson.

    I want you to think about the first calorie you consumed yesterday. Mine was probably the sugar in my coffee around 6 AM.

    Now think about the last calorie you consumed yesterday. Mine would have been some sugar in my tea around 9:30 PM.

    Most adults in the United States are like me, consuming calories over an approximately 15-hour period.

    But if you haven’t been living under a pizza lately, you will have heard of intermittent fasting, a dietary plan that extols the virtue of prolonged fasts to reset the metabolism. The details on any individual plan vary, but the central idea revolves around time-restricted eating—limiting caloric consumption to specific hours on the clock. And now, thanks to this paper appearing in Cell Metabolism, we have some evidence that a relatively modest time-restricted eating plan can significantly improve blood parameters among individuals with the metabolic syndrome.

    This is a small but nicely done study. Nineteen individuals with metabolic syndrome who had a daily eating interval of about 15 hours were followed for 3 months, during which they were asked to restrict their eating to a 10-hour window—think 8 AM to 6 PM.

    Other than that, there were no particular requirements. Participants could eat whatever they wanted and however much they wanted, provided it was in that timeframe.

    By and large, this was a compliant bunch, reducing their eating window to just over 10 hours. Detailed dietary profiling found that they weren’t skipping meals but compressing them—eating breakfast a bit later and dinner a bit earlier.

    And in that process, they ended up taking in fewer calories, about 200 fewer calories a day than during the baseline period. That reduction in caloric intake led to a fair amount of weight loss: around 7 pounds over the 3-month study.

    Several metabolic parameters improved. Body fat and systolic blood pressure decreased, LDL cholesterol went down, and the average participant lost about 4 cm of waist circumference.


    But not everything changed so dramatically. Fasting blood sugar and hemoglobin A1c got a bit lower, but not to the point of statistical significance, for example.

    There were a lot of measurements done in this study; 32 are reported in the outcome table, so we need to be a bit worried about false positives. But that’s not really the main limitation here.

    The main limitation is that these patients were enrolled in a study. See, without a control group, we don’t know if the beneficial changes seen were due to the effects of intermittent fasting or just because the patients knew they were being “watched.” They had to log in to an app, go to study visits, and so on. That alone may be enough to change behaviors in a beneficial direction.

    In other words, we don’t have great support here for particularly large, unique effects of intermittent fasting compared with other diets that lead to calorie restriction.

    And this leads to one of my central theories of diet studies: Any diet that makes it harder to eat, whether you are limiting certain types of foods or certain times of day, will probably [lead to weight loss]. One of the central drivers of the obesity epidemic is our ad libitum access to food. We often see promising results like this when we simply limit that free access.

    What I like about time-restricted eating is that it’s pretty easy to explain: Eat inside these hours, don’t eat outside of these hours. That’s a bit easier than explaining how, for example, ketosis works. But in the end, the key to any diet plan is adherence. Researchers contacted these participants 3 months after the study ended. At that point, only five were still adherent to the calorie window.

    Future studies examining novel dietary interventions would do well to prove that participants not only understand the diet but can stick with it.

    F. Perry Wilson, MD, MSCE, is an associate professor of medicine and director of Yale’s Program of Applied Translational Research. His science communication work can be found in the Huffington Post, on NPR, and here on Medscape. He tweets @methodsmanmd and hosts a repository of his communication work at