Intermittent Fasting: Hungry for a Change?

Many people come to their doctor with questions about intermittent fasting as a way to lose weight, and hopefully improve their health (those goals are often related, but not always. There are plenty of diets that can worsen your health).

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So what really is “intermittent fasting”?

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Well, in reality, what most people these days call “intermittent fasting” is really “time-restricted eating”. Time-restricted eating is a diet plan that allows for a certain amount of hours in the day in which the person allows himself/herself to eat, and then purposefully does not eat the other hours of the day. Common variations include 6, 8, or 10 hours of the day in which eating is allowed.

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True “intermittent fasting” refers to choosing several days of the week during which one fasts for 24 hours.

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Regardless of what we call it, are either of these dietary patterns healthy? The data is mixed.

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In one recent study in the Journal of the Academy of Nutrition and Dietetics, Sun and colleagues looked at a population of 24,011 adults (aged ≥40 years) who participated in the National Health and Nutrition Examination Survey 1999-2014 and evaluated outcomes over time of participants who routinely skipped meals or had a shorter time interval between meals. Now, this didn’t specifically look at people attempting time-restricted eating, but it approximates that population. Most people who attempt time-restricted eating skip breakfast, and likely have shorter time intervals between the meals that they actually do eat (because they have to fit them into a shorter period of the day). In this study, participants who skipped breakfast had a 40% higher risk for CVD mortality compared with those who did not. And among participants eating three meals per day, the all-cause mortality risk was 17% higher when the average interval between two adjacent meals was ≤4.5 hours.

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On the other hand, there is a lot of “pre-clinical” animal model data to support time-restricted eating, and some data in humans.

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In 2020, Dr. Pam Taub published a small, exploratory study of 19 individuals with metabolic abnormalities and found several benefits when restricting eating to 10 hours a day for 10 weeks. On average, the participants reduced their weight by 3%, waist circumference by 4%, systolic blood pressure by 4%, and diastolic blood pressure by 8%. Participants also experienced an 11% reduction in low-density lipoprotein cholesterol.

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A 2022 meta-analysis (which is a type of research that combines many smaller research studies into a larger analysis) found that while time-restricted eating was associated with weight loss, lower triglycerides, lower total cholesterol, and lower low-density lipoprotein cholesterol, it did not find significant differences in waist circumference, body mass index, glycosylated hemoglobin, or blood pressure.

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A study published in the New England Journal of Medicine in 2022 suggested that there was no benefit for time-restricted eating. This trial of 139 patients compared two groups: calorie restriction alone or calorie restriction plus an 8-hour eating window. It found no significant difference in weight loss between the 2 groups. It is important to note that these participants were already observing what was close to a 10-hour window before the study started, which may have limited the effects of further restricting the eating period.

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At this point, the results are mixed, but there is certainly some data that supports an “eating window” of 10 hours each day. It is very likely that one of the main reasons such a restricted-eating window works to improve health is that it prevents the mindless nighttime eating of high-calorie, low-nutrition late night snacks, with which we all struggle. So any diet that stops us from doing that is likely to show a health benefit.

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References:

Sun Y et al. JAND 123(3):417-26.

Wilkinson MJ, et al. Cell Metab. 2020; 31:92–104

Liu D, et al. N Engl J Med. 2022; 386:1495–1504

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