What Are Fasting Diets and Do They Work?

For the last few years, fasting diets, which restrict calories through periods of abstinence, have become very popular. They fall into different categories (alternate day fasting, modified fasting regimens, intermittent fasting, etc.), but all contain phases where participants do not eat for a certain number of hours.

If you haven’t taken part in a fasting diet, you may be wondering why people would purposely sign up to go hungry.

The main reason is weight management. Some studies have shown adherents to fasting diets lose weight. Others practice fasting because it may help with insulin resistance, inflammation, heart health, cancer prevention, brain health and longevity. And some practice fasting for religious reasons (Lent for Catholics; Ramadan for Muslims; Seventh-day Adventists also fast).

On some levels, fasting makes sense. As a species, humans have historically faced periods of feast and famine. Oscillating between hunger and plenty is probably more natural than a lifetime of eating three square meals a day. Our bodies, for example, store fat that can be converted to energy in times of want.

But that doesn’t mean it’s been proven to be beneficial. And we’ve been studying fasting for a long time. For example, a study of fasting research from 2015 concluded:

“There are insufficient data to determine the optimal fasting regimen, including the length of the fasting interval, the number of ‘fasting’ days per week, degree of energy restriction needed on fasting days, and recommendations for dietary behavior on non-fasting days,” the researchers wrote. The reason the data is so insufficient: Most fasting studies have been extrapolated from animal studies or from studies with very small numbers of participants.

Still, that hasn’t kept fasting supporters from trumpeting its benefits. And fasting may indeed have all the benefits attributed to it — from weight loss to longevity — researchers just haven’t been able to demonstrate those benefits with strong evidence.

So, should you try fasting? Before you answer that question, talk to your MDVIP-affiliated physician. There can be serious risks for patients who are elderly, frail, have several conditions that require multiple medications and pregnant women. Chances are your doctor will have a point of view as well as how to best manage fasting based on your medical history and condition.

What does a fasting diet look like?
The most popular fasting diet – indeed, one of the most popular fad diets — is intermittent fasting, where adherents cycle between periods of fasting and eating. There are four popular approaches:

• 5:2 diet: Two days of restricted calories every seven days (in some plans, these days are consecutive).
• 16:8 diet: Daily 16-hour fasts. Participants may eat whatever they want between noon and 8 p.m. But between 8 p.m. and the following day at noon, they do not consume caloric drinks or food.
• Eat stop eat: People who practice this type of fasting typically stop eating for 24 hours once or twice a week.
• Alternate day fasting: This involves, as its name suggests, not eating every other day. This may mean not eating at solid foods at all or eating a calorie-restricted diet on the fasting days.

Each approach has its pros and cons, just as fasting in general does. For most people, the biggest challenge of fasting is sticking to it, whether it's alternating days or some other approach. That’s one of the reasons nutritionists and doctors recommend finding a diet that works for you — and not following a fad.