One of the world’s most popular health (and even fitness trends) is intermittent fasting. Intermittent fasting is an eating pattern that cycles between periods of fasting and periods of eating. It focuses more on when you eat than on what or how much.

People use intermittent fasting to lose weight and improve their health, with several studies showing that it can have powerful effects on your body and brain.

It’s not a diet in the conventional sense, but more accurately described as an eating pattern.

Fasting has been a practice throughout much of human history and, as a result, we evolved to be able to function without food for extended periods of time. Using it from time to time is just as natural as eating three meals a day — if not more so.

Main methods

There’s more than one way of doing intermittent fasting. There are several methods that involve splitting the day or week into eating and fasting periods. Essentially, you introduce a daily or weekly fasting period.

These are the most popular methods:

• The 16/8 method: Also called the Leangains protocol, it involves skipping breakfast and restricting your daily eating period to 8 hours, such as 1–9 p.m. Then you fast for 16 hours in between.
• Eat-Stop-Eat: This involves fasting for 24 hours, once or twice a week, for example by not eating from dinner one day until dinner the next day.
• The 5:2 diet: With this method, you consume only 500–600 calories on two non-consecutive days of the week but eat normally the other 5 days.

During the fasting periods, you eat either very little or nothing at all (no drinks with calories, either). By reducing your calorie intake, all of these methods can cause weight loss as long as you don’t compensate by eating much when you do eat. Many people find the 16/8 method to be the simplest, most sustainable, and the easiest to stick to. It’s by far the most popular.

Effects on the cells and hormones

Fasting leads to a set of reactions in your body on the cellular and molecular level. Your body adjusts hormone levels to make stored body fat more accessible, while your cells also initiate important repair processes and change the expression of genes.

The levels of growth hormone can also skyrocket with intermittent fasting, increasing as much as 5-fold. This has benefits for fat loss and muscle gain, among other things. Insulin sensitivity improves and levels of insulin drop dramatically. Lower insulin levels make stored body fat more accessible.

When you fast, your cells can initiate or accelerate cellular repair processes. This includes autophagy, where cells digest and remove old and dysfunctional proteins that build up inside cells. Changes are also observed in the function of genes related to longevity and protection against disease.

It’s also been suggested that periods of intermittent fasting increase cellular stress resistance and defense, repairing important pathways that improve the resistance of cells to stressors, promoting cellular growth. However, the science around intermittent fasting is not entirely clear.

The science behind it

When thinking about fasting, weight loss is the most common reason given. By making you eat fewer meals, it can lead to an automatic reduction in calorie intake. Additionally, it changes hormone levels to facilitate weight loss.

Studies show that intermittent fasting can be a very powerful weight-loss tool. A 2014 review study found that this eating pattern can cause 3–8% weight loss over 3–24 weeks. According to the same study, people also lost 4–7% of their waist circumference.

It should be said that researchers studying fasting are calling for more human studies, especially large-scale human studies. Many of the purported benefits of intermittent fasting come from animal studies or small-scale animal studies and have not been confirmed in larger cohorts. In addition, while weight loss seems like the most appealing upside, other claims are debatable.

For instance, decades of studies on rodents have shown that intermittent fasting can help them remain lean, develop fewer aging-related diseases, and live 30-40% longer. A 2019 review of studies in the New England Journal of Medicine showed that intermittent fasting can lower blood pressure and cholesterol levels, improve brain health, reduce inflammation, and boost endurance. But studies on humans have been less conclusive. It seems to be good for losing extra pounds, but anything else is less clear.

However, even then, keep in mind that the main reason for its success is that intermittent fasting helps you eat fewer calories overall. If you binge and eat massive amounts during your eating periods, you may not lose any weight at all.

Not for everyone

Intermittent fasting is probably not for everyone. This is actually one of the reasons why it’s so hard to thoroughly confirm any nutritional study: there’s so much variety between different individuals, diets, and a million other factors that can have an influence. It may even be detrimental.

If you’re underweight or have a history of eating disorders, you should not fast without consulting with a health professional first. In these cases, it can be downright harmful.

There is also some evidence that intermittent fasting may not be as beneficial for women as it is for men. For example, one study showed that it improved insulin sensitivity in men, but worsened blood sugar control in women. Another study suggested that it can lead to shedding muscles, not fat, which would be counterproductive.

There are a number of anecdotal reports of women whose menstrual period stopped when they started doing fasting and went back to normal when they resumed their previous eating patterns. For these reasons, women should be especially careful with intermittent fasting. They should follow separate guidelines, like easing into the practice and stopping immediately if they have any problems like amenorrhea (absence of menstruation). This eating pattern is likely also a bad idea if you’re pregnant or breastfeeding. As always, consult with a professional before embarking on any diet.

Safety and side effects

Among the side effects, hunger is considered the main one — the bane of all diets. People also feel weak and their brains may not perform as well as they are used to. This may only be temporary, as it can take some time for the body to adapt to the new meal schedule. Most people seem to get on and adapt after a couple of weeks.

If you have a medical condition, you should consult with your doctor before trying intermittent fasting. This is particularly important if you have diabetes, problems with blood sugar regulation, low blood pressure, take medications, are underweight or have a history with eating disorders.

All that being said, intermittent fasting has a rather remarkable safety profile. There is nothing dangerous about not eating for a while if you’re healthy and well-nourished overall. Unlike other diets which can be extreme, it doesn’t seem to produce any major risks.

Getting started with intermittent fasting

It’s likely that at some point in your life, you’ve inadvertently done some intermittent fasting. If you’ve ever eaten dinner, then slept late and not eaten until lunch the next day, then you’ve probably already fasted for 16+ hours. Ever skipped breakfast or dinner? Same thing. Some people instinctively eat this way. They simply don’t feel hungry in the morning.

Many people consider the 16/8 method the simplest and most sustainable way of intermittent fasting, so this one might be the best one to try first. If you find it easy and feel good during the fast, then maybe try moving on to more advanced fasts like 24-hour fasts 1–2 times per week.

Another approach is to simply fast whenever it’s convenient — simply skip meals from time to time when you’re not hungry or don’t have time to cook. There is no need to follow a structured intermittent fasting plan to derive at least some of the benefits.

Don’t put pressure on yourself

Intermittent fasting is not something that anyone needs to do. It’s simply one of many lifestyle strategies that can improve your health. Eating quality food, exercising, and taking care of your sleep are still the most important factors to focus on.

If you don’t like the idea of fasting, then you can safely ignore this article and continue to do what works for you. At the end of the day, there is no one-size-fits-all solution when it comes to nutrition. The best diet for you is the one you can stick to in the long run.

Intermittent fasting is great for some people, not others. The only way to find out which group you belong to is to try it out. If you feel good when fasting and find it to be a sustainable way of eating, it can be a very powerful tool to lose weight and improve your health.