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. 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. Growth hormone is a hormone that can aid fat loss and muscle gain, among other things.
- 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.
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%. However, fasting periods that are longer can suppress metabolism.
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.
Intermittent Fasting Helps You Reduce Calories and Lose Weight
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 (1), 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.
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.
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.
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.