How to Burn Stored Fat During Exercise: Insulin, Glucose, and Fasting

What you eat and when you eat it may have a bigger impact on burning fat, achieving lean muscle, and reaching goals that how you are actually exercising.

Our diets can be broken down into the consumption of proteins, fats and carbohydrates. Although consuming clean protein is a huge part of balancing a healthy diet while exercising, today’s discussion focuses on fats and carbohydrates.  

Did you know that your body uses both fat and carbohydrates as its main sources of fuel?

Studies show that exercising before a meal will tap into fat storage more efficiently rather than using sugars available from a fresh carbohydrate rich meal for fuel. Ideally most of us would like to burn fat storage instead of simply burning the sugars we ate right before a workout. 

How does all this work?

Do the words insulin and glucose sound familiar? Most of us have heard these terms related to such things as diabetes and the warnings to “stay away from sugar”, but many of us have rarely thought about how both insulin and glucose play a critical role in our eating habits and when we choose to exercise. 

Let’s start with the basics: Glucose is a product of carbohydrate breakdown and is responsible for the main source of “easy” energy used by the body. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen to be used later. Insulin is a hormone produced by your body. Insulin allows glucose to be absorbed into the cell. Think of insulin like the Uber ride for glucose. Ideally you would like your insulin to be as efficient at delivering glucose into the cell as possible. 

When the body detects an increase in blood sugar (glucose), insulin is released by an organ called the pancreas. The insulin then binds to the glucose molecules in the blood and signals the cell to absorb it, lowering your blood sugar. Once the cell absorbs the glucose bound to the insulin the glucose is broken down and used for “easy” energy. During exercise, if this glucose is readily available to be used for energy, your body will utilize it first instead of fat.

For example, when you eat a bagel for breakfast before your workout, the body breaks the carbohydrates of that bagel down into sugar (glucose). The pancreas detects an increase in the sugar running through your bloodstream and releases insulin. Insulin acts like an Uber, picks up the sugar and delivers it into your cells to be used as fuel.

So how does this relate to exercise?

If you choose to exercise prior to eating a meal full of carbohydrates, your chances of burning fat increases. Remember from above, your body utilizes two forms of fuel, the sugar from carbohydrate breakdown or fat. If one is not available it will burn the other. 

Exercising when you have not eaten in a few hours is referred to as “exercising in a fasted state”. Many athletes find this to be the most efficient way to reach their exercise goals. When working out in a fasted state or even after having a meal consisting of protein and/or fat, glucose will NOT be readily available as an “easy fuel source” and the body will be forced to utilize other forms of fuel such as fat storage.  

A study published in The Journal of Clinical Endocrinology and Metabolism in 2019 found men who exercised before eating breakfast actually burned two times the amount of fat compared to those who exercised after eating breakfast. Not only did fat burn double but the study found these men to have reduced spikes in insulin after eating and greater insulin sensitivity. Remember, we want efficient insulin! These men also showed an increase in one key protein called “GLUT4” throughout their muscles. This protein is a necessary part of the transportation process of glucose into the muscle cells. 

TAKEAWAY: Fast or eat a small snack rich in protein and healthy fats BEFORE exercise to increase potential fat burn. 

Next time you look at your exercise goals try to decide how both the timing and content of your meal is helping or hurting you. Exercise itself is only a small piece of the puzzle and you cannot out-train a poor diet! 

 Reference: Edinburgh, RM, HE Bradley, NF Abdullah, SL Robinson, OJ Chrzanowski-Smith, JP Walhin, S Joanisse, KN Manolopoulos, A Philp, A Hengish, A Chabowski, FM Brodsky, F Koumanov, JA Betts, D Thompson, GA Wallis, and JT Gonzalez. J Clin Endocrinol Metab (2019)