The most popular method of intermittent fasting is called the 16:8 method. It includes skipping breakfast and eating lunch and dinner. For example, if you eat lunch at 1PM and dinner at 7PM, you have created an 18-hour fasting window between the two meals.
Other methods include the 5:2 method, which recommends fasting for two days a week, and the eat-stop-eat diet, which calls for one to two fasting days per week, where you stop eating from one meal until the same meal next day.
For example, you would eat lunch on Friday and then fast until lunch on Saturday. This gives you the benefits of a 24-hour fast while still being able to eat every day.
The commonly heard advantages of this type of diet include that it is easier to lose fat with it, that you can retain muscle easier and that it is easier to stick with compared to other, more commonly used diets.
While there are also a host of other benefits, like cancer prevention and cancer treatment (1,2,3), increased lifespan (4,5), preventing certain neurological diseases like Alzheimer’s disease (6), lower risk of type 2 diabetes (7) and more, we’re only going to focus on intermittent fasting as it relates to losing fat and building muscle.
From an uninformed viewpoint, intermittent fasting may resemble a simple calorie restriction diet – after all, it’s just like skipping breakfast.
However, fasting does have additional benefits compared to other diets.
Besides the natural reduction in calorie intake due to fewer meals, fasting also results in a progressive increase in mean growth hormone pulse frequency and pulse amplitude (8).
That means that the levels of growth hormone in your blood can reach as high as five times the normal numbers, without the adverse effects that come with taking the synthesized version.
Fasting also decreases insulin levels (9), which leads to the body burning more body fat for fuel, something that is highly desirable for people who are looking to lose weight.
Considering that our body fat evolved mainly for surviving through periods of famine, It would make sense for the body to try and burn it to preserve as much hard-earned muscle mass as possible.
In addition, periods of fasting alter the function of some hormones in our body.
It increases release of a hormone called norepinephrine or noradrenaline, which is responsible for two relevant things: In brown adipose tissue, it creates an increase in calories burned to generate body heat, and in adipose tissue, more commonly known as fat cells, it causes an increase in lipolysis, which is conversion of fat to substances that can be used directly as energy sources by muscles and other tissues.
Combined with decreased insulin and increased growth hormone, fasting actually increases your metabolic rate by 3.6-14% in the short term (10, 11).
In essence, this is why intermittent fasting works so well: it approaches the problem of losing weight from two directions by both reducing calories and increasing energy expenditure.
Then what about muscle mass? Won’t I lose some because of fasting?
Many people believe that fasting for a period of time will cause them to lose muscle, but studies have shown that muscle loss is comparable with diets based solely on calorie restriction and that fasting causes no adverse effects (12). If you combine your diet with resistance training, there is no fear of losing too much muscle (13).
Another common fear is that the body will go into “starvation mode” and start storing more calories as fat. However, that doesn’t occur until after 60 hours of fasting, much longer than any protocol mentioned here, and even then by a mere 8 percent (14).
And now comes the final question: is intermittent fasting any better for sustained, long-term fat loss compared to other studies?
The science on this is still inconclusive: some studies show that IF is better, more research is needed.