Intermittent Fasting

Are the touted benefits real?

Diet culture has fascinated the masses for centuries. People are always looking for the next trend to lose weight, improve health, or increase longevity, Indeed, as far back as 1558, the Immortality diet, which consisted of 12 ounces of food and 14 ounces of wine, was thought to expand one's life expectancy.

In 2012, one type of diet, called intermittent fasting (IF), gained traction following Michael Mosley’s BBC documentary Eat Fast, Live Longer. Since its debut, Yougov America polls indicate that 24% of Americans have tried the diet as their first choice for weight management. IF has grown in popularity over recent years, touted as a beneficial way to lose weight (and be healthier more generally). In fact it was the most popular diet in the US in 2018, according to a survey by the International Food Information Council.

This diet is so popular that we tackled it on a recent podcast episode:

Many diets focus on what to eat, but intermittent fasting is all about when you eat. With intermittent fasting, you only eat during a specific time. Fasting for a certain number of hours each day or eating just one meal a couple days a week, can help your body burn fat.

Johns Hopkins neuroscientist Mark Mattson who has studied intermittent fasting for 25 years says that our bodies have evolved to be able to go without food for many hours, or even several days or longer. In prehistoric times, before humans learned to farm, they were hunters and gatherers who evolved to survive — and thrive — for long periods without eating. They had to: It took a lot of time and energy to hunt game and gather nuts and berries.

Obviously, a lot of things have changed since then – Our lifestyle and dietary patterns are totally different - TV, the internet and other entertainment are available 24/7. We stay awake for longer hours to catch our favorite shows, play games and chat online. We’re sitting and snacking all day — and most of the night.

So, is this logic really valid?

There are four main types of intermittent fasting:

Periodic time restricted eating, where one eats during only a certain time period per day (the most popular is 16:8, where you are allowed to eat during 8 hours and fast for 16 hours)

Alternate day fasting, where one eats normally one day followed by practicing restrictive eating the next

5:2 fasting (sometimes called periodic day fasting), where eating is strongly restricted or withheld two days per week while normal eating continues throughout remaining five days. Typically, during those 2 days per week, either a full 24 hour fast or consuming only 25% of caloric needs is implemented. Most of the research about IF utilizes this method.

Fasting mimicking diet, a low caloric diet limited to five consecutive days once per month.

All diets achieve weight loss through the same equation — you take in less food energy each day than your body burns for normal activity. Intermittent fasting achieves this goal by severely limiting calories during certain days of the week or during specified hours during the day. The theory is that intermittent fasting will help decrease appetite by slowing the body's metabolism.

So, what are the purported benefits of IF compared to other diets? Aside from the obvious goal of weight loss, other claims suggest that IF can improve insulin sensitivity, reduce blood pressure, help to control blood glucose, and even improve longevity.

To investigate this, let’s dig into the available data. Is IF better than other dietary changes or overall caloric reduction? (spoiler alert…the data do not indicate that it is, and it might actually be dangerous for some people)

Intermittent fasting is promoted to promise weight loss by way of the metabolic change/exchange process called ketosis, which generates energy from fat instead of glucose. Unfortunately, there has been no strong evidence to support this theory; it has been largely indeterminable as to whether the weight loss was related to duration of eating or mere calorie restriction.

Indeed, a meta-analysis from Harvard T.H Chan School of Public Health involving forty different studies on intermittent fasting from both healthy and obese persons, reveals that while “...fasting was effective for weight loss, with a typical loss of 7-11 pounds over 10 weeks,” in the long term, “a randomized controlled trial that followed 100 obese individuals for one year did not find intermittent fasting to be more effective than daily calorie restriction.”