Note: Intermittent fasting or time-restricted eating is not appropriate or safe for everyone. It may be dangerous if you have diabetes, take medications that require food, and/or during cancer treatments when your body needs extra nutrition. Always check with your doctor before making any changes to what or how you eat.
Many people find that it is difficult to lose weight. As comedian Totie Fields said, “I have been on a diet for two weeks, and all I have lost is 14 days.” In 2021, US citizens spent more than 72 billion dollars on weight loss strategies, yet success at sustainable weight loss is limited. It is understandable then that consumers are anxious to try any novel approach to lose weight and keep it off; intermittent fasting is an example. Some people with a cancer diagnosis want to improve their eating habits and feel intermittent fasting is a good approach—but is it? Unlike diets that dictate what you eat, intermittent fasting focuses on when you eat. It can take several forms, but the most common version of intermittent fasting is time-restricted eating where food intake is limited to a certain number of hours each day. This is typically eight hours of eating, for example, during the hours of 11:00 am to 7:00 pm, with a fast of 16 hours afterward. For some people, this type of schedule will reduce the number of calories they eat in a day; this is especially true if it can curtail evening snacking.
Evidence of the benefits of intermittent fasting (also called time-restricted eating) was first seen in animal studies. More recently, several human trials have tried to answer two important questions:
- Is time-restricted eating effective for weight loss?
- If so, is it because subjects are eating fewer calories or that longer periods of fasting can support weight loss?
Researchers theorized that limiting food intake to a finite time each day can increase the breakdown of stored fat and/or cause changes in certain hormones that make losing weight easier. A 2021 study looked at the effect of a reduced calorie diet both with and without a time restriction for eating; both groups lost weight and the authors credited the weight loss to the reduced calories and not the time restriction1. Another experiment looked at 90 obese individuals who were following a low-calorie diet. One group ate all their meals within an 8-hour period, and the other group had no time restrictions. The study found that time-restricted eating was more effective for weight loss. The authors concluded that only eating during an 8-hour time period was roughly the same as reducing a person’s calorie intake by an additional 214 calories per day2. A third trial found that the use of time-restricted eating, in combination with a commercial weight loss program, resulted in more weight loss3. These studies were small, and the results are mixed. This often happens in science and highlights the need for careful consideration of how the study was designed and the need for larger trials (more participants). Whether you are limiting calories, the hours that you eat, or both, it is essential that you make good food choices in order to meet the recommended amount of nutrient-dense foods, such as fruits and vegetables, and protein to maintain muscle mass.
Not only is intermittent fasting a potential strategy for weight loss, but it may also improve other markers of health. For example, in the studies mentioned above, participants saw improvements in blood pressure2 and fasting blood glucose3.These changes can reduce the risk of chronic diseases, such as diabetes and cardiovascular disease.
The jury may still be out on the benefits of time-restricted eating for people trying to lose weight. For people who tend to snack too much, time-restricted eating may help limit calorie intake, and therefore, it may be a useful strategy for some to control weight. Additional studies with more participants and longer follow-up are needed to clarify the hype from the facts.
1 Liu D, Huang Y, Huang C, Yang S, Wei X, Zhang P, Guo D, Lin J, Xu B, Li C, He HHI J, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engle J Med 2022; 386:1495-1504.
2 Jamshed H, Steger FL, Bryan DR, et al. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med.2022;182(9):953–962. doi:10.1001/jamainternmed.2022.3050
3 Peeked, P.M., Greenway, F.L., Biles, S.K. et al. Effect of time restricted eating on body weight and fasting glucose in participants with obesity: results of a randomized, controlled, virtual clinical trial. Nutr. Diabetes 11, 6 (2021).