Is intermittent fasting, popularized by the top-selling book “The Fast Diet” and several recent documentaries, just the latest weight loss fad? Or, is the practice of taking periodic breaks from eating an effective and research-backed means of losing weight and improving health, as it’s being touted? Here’s what is currently known about intermittent fasting for weight loss and health promotion.

What is intermittent fasting?

Intermittent fasting is a type of diet in which you alternate periods of normal food intake with complete or partial restriction of calories. In the broadest sense, most of us already practice intermittent fasting every night between bedtime and breakfast. But advocates of intermittent fasting as a dietary strategy for weight loss and/or health improvement typically recommend fasting (or severe restriction) for longer periods—up to 24 hours once or twice a week. For example: 

• You might have dinner one night, skip breakfast, lunch and snacks the next day, and then eat a normal dinner.
• Instead of skipping meals and snacks altogether, you might severely restrict your food consumption one or two days a week, consuming a very low calorie diet (only 10% to 25% of your energy needs). The popular “5:2 Fast Diet” is based on this style of intermittent fasting: you eat normally five days a week and diet two days a week, cutting your calorie intake for those two days to one-quarter of their normal level (about 500 calories if you are a woman, 600 if you are a man).
• Another type of intermittent fasting involves fasting every day for up to 16 hours, consuming food only during a short “eating window.” This is the type of fasting practiced by Muslims during the month of Ramadan.

Intermittent fasting for weight loss—the research

It is important to realize that intermittent fasting does not necessarily reduce your overall caloric consumption or lower your body weight, since you may compensate for reduced intake during the restriction period by overeating during the period of unlimited consumption. Nevertheless, there is preliminary evidence from animal studies and a few short-term human studies that if you don’t compensate by overeating on non-fasting days, various types of diets incorporating intermittent fasting can be at least as effective as continuously restricting calories for weight loss.

Only one study has examined 5:2 intermittent fasting such as recommended in “The Fast Diet.” This clinical trial compared the effects of intermittent fasting and continuous calorie restriction on weight loss in young, overweight women.[1] Women were randomly assigned to six months of either continuous calorie restriction (25% below estimated requirements) or intermittent fasting consisting of a very low calorie diet two days out of the week with no restriction on the other five days. Intermittent fasting was as effective as continuous calorie restriction for weight loss, with an average loss of 13 pounds over the six months. Those in the intermittent fasting group lost an average of 7% of their body weight. This amount of weight loss is less than one pound per week promoted by Michael Mosley, author of “The Fast Diet.”[2]

Other studies investigating intermittent fasting for weight loss have used different fasting protocols. In a trial in overweight adults with asthma, for example, subjects consumed only 20% of their normal calorie intake on alternate days (320 calories for women and 380 calories for men), using a commercially available meal replacement shake.[3] On the other days, subjects ate whatever they normally ate and to the point of satisfaction but were instructed not to intentionally overeat. During the eight week study, those who adhered to the diet lost 8% of their initial body weight. Additionally, they had decreased markers of oxidative stress and inflammation, and improvement in asthma-related symptoms and quality-of-life.

Beyond weight loss—research on the health benefits of fasting

In addition to its potential role in promoting weight loss, intermittent fasting may help delay aging and help prevent and treat numerous diseases. Studies in rodents have found that intermittent fasting protects against diabetes, cancer, heart disease, and degenerative diseases of the nervous system.[3] In humans, fasting helps reduce hypertension, LDL cholesterol and triglycerides, asthma, and rheumatoid arthritis.[4]

Mark Mattson, senior investigator for the National Institute on Aging, part of the US National Institutes of Health, has investigated the health benefits of intermittent fasting in rodents. He found considerable similarity between how cells respond to the stress of exercise and how cells respond to intermittent fasting, including increasing heart rate variability while reducing resting heart rate and blood pressure—changes you would see in trained athletes.[5] He also found that intermittent fasting can improve blood sugar regulation, reduce oxidative stress and inflammation, and increase the production of ketones—compounds released into the blood when the body is burning fat rather than glucose for energy which have been shown to preserve learning and memory functions and slow disease progression in the brain.[5]

More research on the 5:2 Fast Diet and intermittent fasting is needed

The beneficial effects on health of limiting food intake for certain periods of time have been recognized for a long time. The current evidence suggests that intermittent fasting may be a safe method of weight loss, at least in the short term. It may also be beneficial for overall health and wellbeing. However, the lack of large clinical studies makes it challenging to know whether intermittent fasting is a healthy way to achieve and maintain weight loss.

[1] Int J Obes (Lond). 2011 May;35(5):714-27.
[2] 5:2 Fast Diet. Why fast?
[3] Free Radic Biol Med. Mar 1, 2007; 42(5): 665–674.
[4] Cell Metab. 2014 Feb 4;19(2):181-92.
[5] CMAJ. Jun 11, 2013; 185(9): E363–E364.