By Emmy Bawden
While the practice of fasting has existed throughout history, intermittent fasting (IF) has emerged as a popular strategy for improving health and losing weight. Experts believe this is due to its simplicity compared to other ways of eating, as well as research touting its benefits. Commonly studied IF methods include alternate-day fasting (eating less than 25% of your baseline food intake every other day), 5:2 fasting (fasting two days each week and eating normally five days per week), and daily time-restricted feeding (eating only during a certain time window, such as between 11 a.m. and 7 p.m.).
ARE THERE BENEFITS TO IF?
Although not fully understood, experts believe the beneficial effects of IF involve adaptive cellular responses that improve organ function and resistance to stress. The majority of IF studies have been conducted on animals, and results have shown a slowing or reversing of aging and disease processes, including diabetes, cardiovascular disease, cancers and neurodegenerative brain diseases. However, the results have been mixed and scientists are unsure of the influence that alternative variables such as genetics, sex, diet and age have on IF’s effects.
Human studies offer less convincing evidence of IF’s benefits. Of the human studies conducted, some demonstrate improvements in insulin resistance, inflammation and high cholesterol. However, these human studies haven’t provided sufficient evidence of long-term health effects (including longevity) or safety, and have only looked at short-term interventions ranging from several days to months. This is compared to animal studies that assess IF’s benefits throughout the lifespan. Experts also argue there is significant variation in how different mammals respond to calorie restriction, and that it’s misguided to assume human responses to IF will mirror the life-extending results seen in small animals (such as mice).
Weight-loss results are also mixed. While some human studies show short-term weight loss, other studies (including a recent one in the Journal of the American Medical Association), show no statistically significant benefit from IF with either weight loss or other health outcomes. That same study shows a significant loss of muscle mass with IF (65% of total weight lost, exceeding the expected 20-30%), which is positively correlated with weakness and weight regain.
With such conflicting data, it’s clear just how complex nutrition science is!
IS IT SAFE?
IF has downsides, including a significant increase in the stress hormone cortisol, shown in a review of 30 studies on caloric restriction and fasting. Moreover, such restriction has been studied for its ability to induce binge eating and weight regain. One possible culprit here is the increased release of neuropeptide Y, a neurochemical that enhances the drive to eat while reducing energy expenditure in an effort to restore balance.
IF in real life is likely significantly different than in a controlled study, and we can’t ignore the negative physical and mental health outcomes of such restrictive diets. In general, restrictive dieting is a significant predictor of eating disorders and weight cycling, which is associated with increased risk for heart disease, chronic inflammation, insulin resistance and increased abdominal fat. Because research shows the majority of restrictive diets fail within 1-5 years, in part due to a metabolic “slowing” and accelerated weight gain, it further supports the need for long-term IF studies.
WHO WOULD IF NOT WORK FOR?
Anyone considering intermittent fasting should first discuss it with their healthcare provider; however, IF is not recommended if you:
- Have diabetes
- Are pregnant or breastfeeding
- Use medications that require food intake
- Have a history of an eating disorder/disordered eating