In the last installment of Fitness Myths I went over how small frequent meals have no impact on your metabolism and that overall calorie consumption is the most important thing when it comes to metabolism. This week I will debunk the myth that fasting causes your metabolism to slow down, when in reality, fasting will actually speed up your metabolism.


I know we have all heard it before,

“If you stop eating, your body will go into starvation mode and slow down your metabolism to conserve energy.”  

For most people, this couldn’t be further from the truth because the average person doesn’t do weekly or monthly fasts. In fact, most people think that a 24 hour fast will slow down your metabolism and wreak havoc on your body. However, there have been numerous studies to debunk this idea. A study done by the American Journal of Clinical Nutrition, found that individuals who fasted for 84 hours increased their resting energy expenditure significantly ( 3.97 +/- 0.9 kJ/min on day 1 to 4.53 +/- 0.9 kJ/min on day 3).  If we do some simple math, we find that the subject burned more calories on the third day versus the first day.  The scientists believe the reason for this increase in metabolism is due to the fight or flight response that was triggered by the fasting. In the study, the subjects had a decrease in blood glucose levels and  an increase in norepinephrine levels by day day four:

“norepinephrine concentration from 1716. +/- 574 pmol/L on day 1 to 3728 +/- 1636 pmol/L on day 4 (P < 0.05). Serum glucose decreased from 4.9 +/- 0.5 to 3.5 +/- 0.5 mmol/L (P < 0.05), whereas insulin did not change significantly.”(1)

Another study done on 11 male subjects who fasted for 48 hours found the same results, 

"Starvation led to considerable alterations in basal metabolism including a significant (mean 3.6%) increase in resting metabolic rate."(2)

It is important to realize that fasting (16+hrs), just like exercise, triggers a stress response. All of our bodies require energy to function. We obtain this energy through the food we eat, and we save this energy by storing it in our liver, fat and muscle cells. When our body goes a brief period time without food (24 to 72 hours), it will activate a stress response to break down our fat and muscle cells to release energy into the blood. It does this through the release of stress hormones. One of the main stress hormone is norepinephrine (noradrenaline). It plays a huge roll in fat loss by stimulating hormone sensitive lipase (HSL). When HSL is stimulated, it breaks down triglycerides (fat cells) and releases it into the blood as fatty acids. This is referred to as lipolysis, the breakdown of triglycerides (fat) into fatty acids to be used as energy. It addition, norepinephrine has been shown to turn your white fat into brown fat by creating more mitochondria in the fat cell. By having more mitochondria in the cell, it allows the cell to be more efficient and create more energy. The body does this so that you can adapt to the stress of fasting. The mitochondria dense fat cell can now create more energy than before. This increase in energy production allows you to go without food longer. So not only does fasting increase your metabolism, but it also makes your body more efficient at using energy.

Pretty cool, right?

So give fasting a try and reap the benefits of a faster metabolism. Start out with a 16 hour fast (7pm to 11am), and increase it by eight hours each week or every two weeks. During the fast you can drink water, tea or coffee. If you are doing vigorous resistance training during your fast I recommend taking 3 to 5 grams of the amino acid leucine post workout to stimulate muscle growth and prevent muscle protein breakdown.