Throughout history, a number of cultures have undertaken periods of fasting, for both religious and non-religious reasons. While the reasons for fasting in the past have often been associated with the virtues of temporarily depriving oneself, scientists have recently taken an interest in the potential medical benefits to fasting. Not surprisingly, this has led to a number of claims far and wide about the benefits of fasting diets for weight loss and improving metabolic health. What’s the deal with fasting?

Fasting is typically thought of as going some period of time without consuming food. Scientists have studied several different types of fasting, including intermittent fasting (IF), alternate day fasting (ADF), and fasting for 2 days per week (5-2). IF is one of the most common forms of fasting, and is actively promoted by several celebrities; intermittent fasting relies on restricting food intake to a specific window of time for the day (e.g. eating 8am-4pm only) and repeating this several times throughout the week. ADF, as the name suggests, involves having fasting days through the week and alternating, simulating fasting and refeeding cycles. The 5-2 Diet, popularized in the UK after a BBC documentary titled “Eat Fast and Live Longer”, instructs dieters to eat unrestricted for 5 days out of the week, and significantly reduce calorie intake (typically <400 calories) 2 days of the week.

All methods of fasting attempt to capitalize on the science, largely studied in animal models, that suggests that restricting calorie intake has unique benefits for slow aging and reduce the risk of age-related disease; these benefits are often linked to hormonal changes that occur when calories are regularly restricted as occurs with intermittent fasting.Whereas many are familiar with continuous calorie restriction to lose weight, many proponents of fasting and claims surround fasting highlight beneficial effects of fasting, independent of weight loss. Thus, fasting diets are promoted not just for individuals hoping to lose additional weight, but also for individuals hoping to manage inflammatory diseases, limit the effects of aging, and potentially extend their lives.

But is there anything special about fasting? Registered dietitians typically think about fasting methods in 2 ways: 1) is there anything unique about fasting that makes it have a better effect on our bodies from continuous calorie restriction?; 2) is fasting easier to adhere to than continuous calorie restriction?. The evidence, to date, is still pretty preliminary. When taking a look at all of the available data, most reviews of the literature concludes that there isn’t much evidence in humans that fasting is uniquely better than cutting calories continuously - they both induce weight loss. Few studies have measured other markers of health, such as inflammation and hormonal measures, and shown a unique effect of fasting techniques in humans. The equivocance of fasting diets with continuous calorie restriction on weight, body fat distribution, and circulating hormones was confirmed in a recent 50 week long study in individuals with overweight and obesity trying out the 5:2 diet. As far as whether fasting regimens are easier for people to adhere to, the evidence is quite mixed. Some short term studies have suggested that fasting methods are easier to stick to if you can make it through the first two weeks; however, several recent longer studies have suggested that individuals are less adherent to some fasting regimens. It’s likely that whether fasting would work for an individual depends on many aspects of that person’s life and their preferences, explaining why the data is so mixed. If fasting regimens sound like something that you’d prefer over cutting back on calorie intake every day, they might be a better option - but not a superior option. This has one big exception though - fasting diets are not recommended for individuals with diabetes- going long periods clearly increases the risk of experiencing hypoglycemia and its associated symptoms. Individuals interested in fasting with diabetes should work with their physicians and a registered dietitian to find an eating pattern that works best for their condition and its current management.

While we don’t know of many unique risks of intermittent fasting (apart from hypoglycemia risk in diabetes), it’s worth noting that it still shares the same side effects that typical calorie restricted diets have, including persistently increased hunger, feeling cold, and having a reduced libido. Other potential adverse effects should be discussed with your healthcare provider to ensure that any calorie restricted diet is appropriate for you. The Slice Plan: An Integrative Approach to a Healthier Lifestyle and a Better You - is a great tool for weight loss without restricting the foods you love. 

Happy Eating!

Kevin Klatt, PhD/RD Candidate