There are many different ways to lose weight.
One that has become popular in recent years is called intermittent fasting (1).
This is a way of eating that involves regular short-term fasts.
Fasting for short periods helps people eat fewer calories, and also helps optimize some hormones related to weight control.
There are several different intermittent fasting methods. Three popular ones are:
- The 16/8 Method: Skip breakfast every day and eat during an 8-hour feeding window, such as from 12 noon to 8 pm.
- Eat-Stop-Eat: Do one or two 24-hour fasts each week, for example by not eating from dinner one day until dinner the next day.
- The 5:2 Diet: Only eat 500-600 calories on two days of the week, but eat normally the other 5 days.
As long as you don’t compensate by eating much more during the non-fasting periods, then these methods will lead to reduced calorie intake and help you lose weight and belly fat (
Body fat is the body’s way of storing energy (calories).
When we don’t eat anything, the body changes several things to make the stored energy more accessible.
This has to do with changes in nervous system activity, as well as a major change in several crucial hormones.
Here are some of the things that change in your metabolism when you fast:
- Insulin: Insulin increases when we eat. When we fast, insulin decreases dramatically (4). Lower levels of insulin facilitate fat burning.
- Human growth hormone (HGH): Levels of growth hormone may skyrocket during a fast, increasing as much as 5-fold (5, 6). Growth hormone is a hormone that can aid fat loss and muscle gain, among other things (7, 8, 9).
- Norepinephrine (noradrenaline): The nervous system sends norepinephrine to the fat cells, making them break down body fat into free fatty acids that can be burned for energy (10, 11).
Interestingly, despite what the 5-6 meals a day proponents would have you believe, short-term fasting may actually increase fat burning.
Two studies have found that fasting for about 48 hours boosts metabolism by 3.6-14% (12, 13). However, fasting periods that are longer can suppress metabolism (14).
Bottom Line:Short-term fasting leads to several changes in the body that make fat burning easier. This includes reduced insulin, increased growth hormone, enhanced epinephrine signalling and a small boost in metabolism.
The main reason that intermittent fasting works for weight loss, is that it helps you eat fewer calories.
All of the different protocols involve skipping meals during the fasting periods. Unless you compensate by eating much more during the eating periods, then you will be taking in fewer calories.
According to a recent 2014 review study, intermittent fasting can lead to significant weight loss. In this review, intermittent fasting was found to reduce body weight by 3-8% over a period of 3-24 weeks (2).
When examining the rate of weight loss, people lost about 0.55 pounds (0.25 kg) per week with intermittent fasting, but 1.65 pounds (0.75 kg) per week with alternate-day fasting (2).
People also lost 4-7% of their waist circumference, indicating that they lost belly fat.
These results are very impressive, and they do show that intermittent fasting can be a useful weight loss aid.
All that being said, the benefits of intermittent fasting go way beyond just weight loss. It also has numerous benefits for metabolic health, and may even help prevent chronic disease and expand lifespan (15, 16).
Although calorie counting is generally not required when doing intermittent fasting, the weight loss is mostly mediated by an overall reduction in calorie intake.
Studies comparing intermittent fasting and continuous calorie restriction show no difference in weight loss if calories are matched between groups.
Bottom Line: Intermittent fasting is a convenient way to restrict calories without consciously trying to eat less. Many studies show that it can help you lose weight and belly fat.
One of the worst side effects of dieting, is that the body tends to burn muscle as well as fat (17).
Interestingly, there are some studies showing that intermittent fasting may be beneficial for holding on to muscle while losing body fat.
In one review study, intermittent calorie restriction caused a similar amount of weight loss as continuous calorie restriction, but with a much smaller reduction in muscle mass (18).
In the calorie restriction studies, 25% of the weight lost was muscle mass, compared to only 10% in the intermittent calorie restriction studies (18).
One study had participants eat the same amount of calories as before, except in just one huge meal in the evening. They lost body fat and increased their muscle mass, along with a host of other beneficial changes in health markers (19).
However, there were some limitations to these studies, so take the findings with a grain of salt.
Bottom Line: There is some evidence that intermittent fasting can help you hold on to more muscle mass when dieting, compared to standard calorie restriction.
In my opinion, one of the main benefits of intermittent fasting is the simplicity of it.
I personally do the 16/8 method, where I only eat during a certain “feeding window” each day.
Instead of eating 3+ meals per day, I eat only 2, which makes it a lot easier and simpler to maintain my healthy lifestyle.
The single best “diet” for you is the one you can stick to in the long run. If intermittent fasting makes it easier for you to stick to a healthy diet, then this has obvious benefits for long-term health and weight maintenance.
Bottom Line:One of the main benefits of intermittent fasting is that it makes healthy eating simpler. This may make it easier to stick to a healthy diet in the long run.
There are several things you need to keep in mind if you want to lose weight with intermittent fasting:
- Food quality: The foods you eat are still important. Try to eat mostly whole, single ingredient foods.
- Calories: Calories still count. Try to eat “normally” during the non-fasting periods, not so much that you compensate for the calories you missed by fasting.
- Consistency: Same as with any other weight loss method, you need to stick with it for an extended period of time if you want it to work.
- Patience: It can take your body some time to adapt to an intermittent fasting protocol. Try to be consistent with your meal schedule and it will get easier.
Most of the popular intermittent fasting protocols also recommend strength training. This is very important if you want to burn mostly body fat while holding on to muscle.
In the beginning, calorie counting is generally not required with intermittent fasting. However, if your weight loss stalls then calorie counting can be a useful tool.
Bottom Line: With intermittent fasting, you still need to eat healthy and maintain a calorie deficit if you want to lose weight. Being consistent is absolutely crucial, and strength training is important.
At the end of the day, intermittent fasting can be a useful tool to lose weight.
This is caused primarily by a reduction in calorie intake, but there are also some beneficial effects on hormones that come into play.
Intermittent fasting is not for everyone, but may be highly beneficial for some people.
This article contains much more information about intermittent fasting: Intermittent Fasting 101 – The Ultimate Beginner’s Guide.