The Real Benefits Of Fasting

The Real Benefits Of Fasting
Jaymes Gleeson


May 19, 2024

“After reviewing the research, I have come to the conclusion that short-term intermittent fasting was not only an effective and easy way to lose unwanted body fat, but it was also associated with many amazing health benefits.” Dr. Brad Pilon

In dozens of peer-reviewed scientific studies, short-term intermittent fasting has been found to have the following health benefits:

  • Decreased body fat and body weight
  • Maintenance of skeletal muscle mass
  • Decreased blood glucose levels
  • Decreased insulin levels & Increased insulin sensitivity
  • Increased lipolysis (breakdown of fat for energy) and fat oxidation
  • Increased uncoupling Protein 3 mRNA
  • Increased norepinephrine and epinephrine levels
  • Increased glucagon levels
  • Increased growth hormone levels

Many of these benefits were found after as little as 24 hours fasting.

Let’s take a closer look at these claims individually.

Decreased Insulin Levels & Increased Insulin Sensitivity

“Improving insulin sensitivity and reducing fasting insulin levels have major ramifications for your health, longevity, and resistance to disease. And it’s not just because weight gain is unhealthy. Insulin itself, in excess, exerts seriously damaging effects” Mark Sisson

High levels of circulating insulin over the course of a lifetime are associated with increased mortality. We want to be as insulin sensitive as possible. The opposite end of the spectrum, insulin resistance, is one step from diabetes.

When you eat, the insulin levels in your body increase (some foods have a greater effect on insulin than others). One of its roles is the storage of nutrients. “In other words”, says Dr. Pilon, “insulin is the primary signal that tells your body to store the energy from your food as body fat and glycogen (stored glucose in muscles or liver). When insulin levels are high, you are in storage mode, plain and simple. What’s more, when insulin levels are elevated, you are unable to release fat from your fat stores. In other words, when insulin levels are high, your body fat isn’t going anywhere”.

STUDY: Halberg N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. Journal of Applied Physiology 2005; 99:2128-2136

Researchers found that fasting for as little as 24 hours has been shown to drastically reduce your insulin levels.

STUDY: Klein S, et al. Progressive alterations in lipid and glucose metabolism during short term fasting in young adult men. American Journal of Physiology 1993; 265 (Endocrinology and metabolism 28): E801-E806

In research conducted on people who fasted for 72 hours, plasma insulin dropped dramatically, reaching a level that was less than half of their initial levels. What’s more impressive is that 70% of this reduction happened during the first 25 hours of fasting.

“Insulin puts fat in fat cells. That’s what it does. And our insulin levels, for the most part, are determined by the carb-content of our diet — the quantity and quality of the carbohydrates consumed. The way to get fat out of fat cells and burn it, which is what we want to do with it, is to lower insulin. This has been known since the early 1960s.” Gary Taubes

fat burning

Increased Lipolysis and Fat Burning

There are a few very important steps in fat burning. Dr. Pilon continues, “First, your fat has to be ‘released’ from your fat stores. A process is known as ‘lipolysis’, and it involves the process of releasing the fatty acids that make up your fat, and moving these fatty acids into your bloodstream”.

After a series of steps that allow these fatty acids to get to the mitochondria (the energy producing factory of every cell in your body) these fatty acids go through a process called oxidation. Once this has happened your body fat has now been used for energy.

Let’s review that quickly

– fat must be released from its storage spot – then transported through your system – then get to a cellular energy producing factory where it will be burned.

While we are resting, our muscles are the major contributor to our metabolic rate. During fasting, our muscles start oxidizing fatty acids for fuel. In other words, says triathlete and researcher Mark Sisson, “when we fast, our muscles turn into fat burning machines”.

A lesser known protein that’s found in our muscles and associated with fat burning is Uncoupling Protein-3 (UCP3). When fat burning increases so does the amount of UCP3 in our muscles.

STUDY: Tunstall RJ, et al. Fasting activates the gene expression of UCP3 independent of genes necessary for lipid transport and oxidation in skeletal muscle. Biochemical and Biophysical Research Communications 2002; 294:301-308

This amazing research has shown that as little as 15 hours into a fast, the gene expression for UCP3 increases five-fold.

A 24 hour period of fasting shifts your body from the fed state to the fasted state, which causes increases in both lipolysis (fat release) and fat oxidation (burning). Simply put, Dr. Pilon explains, “fasting allows your body to take a break from storing fat, and start burning it!”.

Probably the most revealing information in Dr. Pilon’s research was found in studies published by a group of scientists from the University of Texas, Medical Branch at Galverston. It examined how short-term fasting affects fat and sugar metabolism.

After only 24 hours of fasting, the amount of fat being released from people’s fat stores (lipolysis) and the amount being burnt for fuel (oxidation) have been significantly increased by over 50% with the biggest change occurring between 18 and 24 hours.

Increased Glucagon Levels

The hormonal equivalent to fed and fasted could be thought of as insulin and glucagon.

Insulin is the dominant hormone in the fed state, which causes you to store food calories in the form of fat and glycogen. Glucagon is one of the dominant hormones in the fasted state that causes fat burning.

Insulin = Fat Storage Glucagon = Fat burning

The primary role of glucagon is to maintain your blood sugar levels while you fast. It does this by shifting your body into ‘burning’ mode.

Glucagon has some amazing effects on our body, including maintaining our blood sugar levels, increasing fat burning, decreasing the production of cholesterol, and increasing the release of extra fluids from the body.

Because of the way we eat, we spend most of our time in an ‘insulin dominant’ metabolism (i.e. fat storage mode). By adding fasting into your lifestyle, you allow your body to revert back to a natural balance between an ‘insulin dominant’ (i.e. fat storage mode) and a ‘glucagon dominant’ (i.e. fat burning) metabolism.

Increased Epinephrine and Norepinephrine Levels

Epinephrine and norepinephrine are both fight or flight hormones, often called adrenaline and noradrenaline. When they are released into your bloodstream they

  • trigger the release of glucose from energy stores
  • increase fat burning
  • make you feel awake and alert

Fasting increases the amounts of both of these hormones in your blood. This is your body’s way of maintaining your blood sugar levels and increasing your fuel supply by helping to release fatty acids from your fat stores.

Increased Growth Hormone Levels

To increase the amount of growth hormone released in your body all you have to do is fast.

STUDY: Sarri KO, et al. Greek Orthodox fasting rituals: a hidden characteristic of the Mediterranean diet of Crete. British Journal of Nutrition (2004), 92, 27-284

STUDY: Hartmann ML, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced CH secretion during a two day fast in normal men. Journal of Clinical Endocrinology and Metabolism 1992; 74(4): 757- 765

Researchers found that short-term fasting can increase growth hormone levels by nearly six-fold. Fasting can cause very large increases in the amount of circulating growth hormone.

STUDY: Norrelund H, et al. Modulation of basal glucose metabolism and insulin sensitivity by growth hormone and free fatty acids during short-term fasting. European Journal of Endocrinology 2004; 150: 779-787

The above research also shows that fasting-induced growth hormone release helps burn fat, build muscle and increases metabolism. Fasting triggers the “growth hormone response”, and this response is what prevents you from losing muscle while you fast. And, since your muscle is largely responsible for your metabolism, growth hormone also plays a large part in keeping your metabolism elevated while you are fasting.

STUDY: Norreland H, et al. The protein retaining effects of growth hormone during fasting involve inhibition of muscle protein breakdown. Diabetes 2001; 12: 198-207

STUDY: Norreland H, et al. Abstracts of PhD Dissertations – effects of growth hormone on protein metabolism during dietary restriction. Studies in Normal, GH- Deficient and Obese subjects. Danish Medical Bulletin 200; 47(5): 370

Not only does growth hormone prevent you from losing muscle while you fast, but it is also vitally important in the process of releasing your stored fat so it can be burnt for energy.

This “growth hormone response” to fasting is so important that some researchers have actually argued that it is actually growth hormone and not glucagon that is the dominant hormone in the fasted state because it causes fat burning and preserves your muscle mass.

Increased Fat Loss

Metabolically, fasting sets you up perfectly for fat loss; it prepares your body by increasing all of the hormones necessary to increase fat burning. Added to that, it creates a large energy deficit, so your body has no choice but to start burning fat for energy.


For healthy people wanting a simple and effective way to lose weight, the combination of short-term fasting and exercise is an easy way to create a caloric deficit and has no negative impact on our metabolism or our muscle.

J.A. Gleeson is a Personal Trainer in Sydney.


By Jaymes Gleeson


J. A Gleeson is a health writer for The Carousel and Personal Trainer at Tribe Social Fitness, in the Sutherland Shire, Sydney. He has over 25 years experience as an athlete, athletics coach, consultant, personal trainer, educator and independent researcher. Jaymes won an Athletics Scholarship and studied in the United States in 1991. - San Francisco State University (Psychology, Nutrition, Athletics) - American Collage of Sports Medicine (Personal Training) Throughout the 90s he worked as athletics coach and personal trainer in the US. In the early 2000s, he worked in Snow Sports throughout Japan and returned to Australia in 2008 to continue wellness research and personal training in high end health clubs in Sydney.



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