What Is Intermittent Fasting?

Humans have been temporarily going without food for millennia. Known as fasting, this deprivation is intentionally done for differing reasons. 

Fasting can be part of a religious practice, serve as a means of protest in a hunger strike, or work as a means of weight loss.

Over the last decade, a type of fasting—intermittent fasting—has become popular as an easy way to lose weight and potentially even extend one’s lifespan. 

The History of Intermittent Fasting

Intermittent fasting is a diet that alternates between periods of consistent eating and eating little to no food. As opposed to prolonged fasting, practitioners typically do not go without food for more than 48 hours or two days at a time. 

By reducing the time you spend eating during intermittent fasting, you will undergo calorie restriction—one of the major ways to achieve weight loss. Compared to other reduced-calorie diets, like Weight Watchers, which involves tracking the calories you consume, intermittent fasting provides an easier way to manage dieting by tracking time.

Intermittent fasting was first studied by scientists in the early 20th century. In 1917 scientists noticed that a reduction in caloric intake impacted the lifespan of rats. In these early studies, steps were not taken to ensure the animals had proper nutrition, and though on average they lived longer lives, there were also more premature deaths. In the 1930s and 1940s, the focus turned to temporary fasting, avoiding malnourishment.

Intermittent fasting didn’t take off as a diet option until 2012 when Dr. Michael Mosley released a television show, Eat Fast, Live Longer, and a book, The Fast Diet. The trend continued with the publication of The 5:2 Diet by Kate Harrison and The Obesity Code by Dr. Jason Fung.

Intermittent fasting is now regarded as a popular way to extend the lifespan and lose weight, though there have been mixed scientific results on the health benefits of calorie restriction. 

When the University of Wisconsin and the National Institute on Aging (NIA) both conducted long-term studies in rhesus monkeys, the university found that caloric restriction delayed the onset of aging and prolonged lives, whereas NIA found that it had no effect.

There is one possible explanation for the differing results. The University of Wisconsin study comprised monkeys with greater body mass, indicating the benefits of calorie restriction may apply to individuals who are obese. Because rhesus monkeys share 93% of their genome with humans, the results of this study are more useful than previous rat studies. 

Intermittent Fasting Schedule

Intermittent fasting is a complex practice with a variety of eating schedules. This includes:

• Religious Observance: Historical examples of intermittent fasting include not eating on holy days in Judaism and abstaining from food from dawn to dusk during the month of Ramadan in Islam.

• Alternate-day Fasting: Eating plans in which you eat as much as you want one day and then don’t eat the next.

• 5:2 Method: If you can’t go a full day without eating, the 5:2 method involves eating normally 5 days a week and then restricting your calories to 500 twice a week.

• Time-restricted Feeding: On this regimen, you only eat during a fixed period every day. An example of a typical window is 5 hours to eat to 19 hours of non-eating.

• Two Meals a Day (2MAD): This plan lets you eat two meals as it involves either dropping breakfast or dinner, users typically have a 14-18 hour window every day in which they are fasting, similar to time-restricted feeding.

The Dangers of Intermittent Fasting

While intermittent fasting has exploded in popularity, it comes with health risks. The dangers include:

• Nutritional deficiencies
• Decreased physical performance
• Muscle loss
• Straining bodily systems
• Developing eating disorders

When you are fasting, your body doesn’t receive enough nutrients to conduct its normal functions. Even if you consume an excess of nutrients before or after fasting, while fasting your body may be forced to break down its own muscle or other body parts for nutrition and calories.

Intermittent fasting has been associated with dizziness, leading to mental as well as physical impairment. For performance athletes, those fasting were found to have lower power output, ran out of energy sooner, had decreased muscle mass, and had less growth factor than their non-fasting cohorts.

When you diet, you aren’t only losing weight by losing fat. When you diet by calorie restriction, 20% to 30% of the weight you lose is lean mass comprised of your muscles and organs. You can lose 65% of your total lean mass! While touted as a means of decreasing fat, intermittent fasting seems to be better at burning protein. 

While some studies have found that adding an exercise routine to intermittent fasting has enabled individuals to retain their muscle mass, adopting the same exercise routine while not fasting gained participants 5 pounds of muscle mass on average! If seek to lose weight while building muscle, fasting wouldn’t be the right choice.

Intermittent fasting’s effect on the body is similar to keto diets. A study at the Radboud University in the Netherlands notes that ketogenic diets high in protein and low in carbohydrates increase ketones in the body.

Your liver and kidneys must work harder to metabolize these ketones as your body switches from burning carbohydrates to fats and proteins. Intermittent fasting simultaneously increases levels of the stress hormone cortisol. The strain that fasting puts on the body and electrolyte imbalances it causes make it inappropriate for people with diabetes, high blood pressure, or heart disease.

A feature of intermittent fasting is paying attention to when and what you eat. The Seeds of Hope eating disorder treatment center calls out the dangers of fasting: “Intermittent fasting specifically has been recognized as an eating disorder trigger.” For resources to help overcome eating disorders, visit the National Association of Anorexia Nervosa and Associated Disorders (ANAD).

Intermittent Fasting Pros and Cons

While intermittent fasting can help people lose weight, keep in mind that weight loss is a claimed positive result with strong evidence behind it. Since intermittent fasting is likely to cause muscle loss, a traditional dietary restriction plan seems preferable. If you choose to fast, shorten the timeframe to minimize potential ill health effects.