Does Fasting Slow Down Your Metabolism?

In a past article (click here), I went over how small frequent meals won't increase your metabolism when calorie intake is the same. The overall calorie consumption is the primary factor when it comes to boosting metabolism. This week I will debunk the myth that fasting causes your metabolism to slow down. I know at one time or another we have all had someone tell us this:  “If you stop eating, your body will go into starvation mode and store fat to conserve energy.” But for most people, this couldn’t be further from the truth.

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 without food (24 to 72 hours), it will activate an acute stress response. The lack of food triggers a release of stress hormones that help break down our fat cells to release fat into the blood for energy. Remember, hormones, organs and cellular function require a tremendous amount of energy to function. So, your body must find a way to supply power if it is not receiving any in the form of food or liquids.

One of the primary stress hormones is norepinephrine (noradrenaline). Generally speaking, it triggers fat loss by stimulating hormone-sensitive lipase (HSL). When HSL stimulates, it breaks down triglycerides (fat cells) and releases it into the blood as fatty acids for energy (lipolysis). Also, 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 generate 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 power 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.

A study done by the American Journal of Clinical Nutrition found that individuals who fasted for 84 hours increased their resting energy expenditure significantly from the first day to the last day. (1) The scientists believe the increase in metabolism was due to the fight or flight response that I mentioned above (1). In the study, the subjects had a decrease in blood glucose levels and an increase in norepinephrine levels which proves fasting doesn't trigger fat storage. 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 increase in resting metabolic rate."(2) Pretty cool, right? Unfortatenely, the majority of people are missing out on the benefits of fasting. In fact, most have never gone through a whole day without food. So, I challenge you to reverse the trend and give fasting a try. Use the guide below to get started.

Fasting 101

  • Consume 0 calories for an extended period of time
  • Start by consuming only coffee, water, tea but eventually just do water
  • Start at 16 hours and work your way up to 72 hours
  • Increase your fast by 8 hours per week or month
  • Most fasts last 1 to 7 days
  • Intermittent fasting lasts 16 to 24 hours
  • The longest fast ever was 382 days (1)
  • Fasting has been a common practice in humans for thousands of years


  1. Zauner C, Schneeweiss B, Kranz A, Madl C, Ratheiser K, Kramer L, Roth E, Schneider B, Lenz K. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000 Jun;71(6):1511-5. PubMed PMID: 10837292.
  2. Mansell PI, Fellows IW, Macdonald IA. Enhanced thermogenic response to epinephrine after 48-h starvation in humans. Am J Physiol. 1990 Jan;258(1 Pt 2):R87-93. PubMed PMID: 2405717.
erik rokiskyComment