Nutrition

How Does The Body Gain And Burn Fat?

How Does The Body Gain And Burn Fat?

Contrary to popular belief, the key to fat loss is not a simple matter of calories in versus calories out. If you’ve been tracking your activity and logging your food down to the last gram, but still haven’t experienced any fat loss, then you may be starting to doubt this old way of thinking. Women over 35 can especially attest to this. They’re doing the same diet and exercise that once kept them in good shape, but no longer seeing the same results. Rather than merely considering caloric balance, it’s important to think of making exercise and nutrition choices that create an ideal hormonal environment for fat loss.


Why Doesn’t Calories In Versus Calories Out Work?

Once you understand the hormonal environment that allows fat loss to occur, you will realize that asking how many calories you are burning during your workout is not even the correct question. In other words, that 300-calorie slice of cake cannot be canceled out by 40 minutes on the treadmill.

Let’s consider the diet of our ancestors. Studies analyzing the amounts of macronutrients (carbohydrates, fats, proteins) in hunter-gatherer diets, discovered that the macronutrient obtained and consumed in the smallest amount was carbohydrate. This is important to consider because the body is going to predicate body fat storage on the macronutrient that is least abundant. In other words, if you consume the previously scarcest macronutrient in excess, then your body thinks that it is safe to store body fat.

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This happens because body fat storage is contingent upon the hormone insulin. Insulin’s job is to signal circulating blood glucose (sugar) to be taken up into cells. Blood glucose is primarily stored as glycogen in muscles and secondarily stored in the liver. When these storage areas are full, insulin receptors on the muscle and liver cells become less sensitive, which means no more sugar will be allowed in. Because glucose isn’t entering the cells, it will accumulate in the bloodstream, creating an even stronger signal for insulin production. However, because these storage areas are full, that sugar needs to be converted and stored as fat in adipose tissue. These complex hormonal metabolic processes and all of the factors that influence them are the very reasons why calories consumed aren’t negated by calories burned through physical activity.


How Does The Body Burn Fat?

Storing the fat is the easy part. The hard part is mobilizing fat to get it out of the adipose (fat) tissue. In order for the process of fat burning or hydrolysis to occur insulin levels must drop. Hormone sensitive lipase (HSL), the key enzyme involved, will be unable to mobilize body fat if insulin is too high even if you are in a calorie deficit!

Exercise can play a beneficial role in the quest to lose fat, and it’s not by simply increasing daily calorie burn as previously thought. Exercise creates the need for more glucose to be moved into the muscle cell. Over time, consistent exercise will trigger the need for more insulin receptors on the muscle cell, making you more insulin sensitive and decreasing circulating insulin. Decreased insulin in the blood means that HSL can do its job and promote fat loss!

how do i lose fat

The best type of training to promote fat loss is high intensity training (HIT – near maximal to supramaximal intensity training). Firstly, HIT requires the use of the glucose stored in your muscle, and triggers the release of the epinephrine and norepinephrine (fight or flight hormones) in contrast to low to moderate intensity training. Epinephrine and norepinephrine help trigger HSL. When epinephrine and norepinephrine act on HSL an amplification cascade occurs, activating hundreds of thousands of enzymes that promote more fat burning.


Take-Home Message

If this process of fat mobilization is functioning properly, then the body can autoregulate to being lean. However, this process of autoregulation goes awry when insulin sensitivity is impaired and high circulating insulin makes it difficult to mobilize body fat, even in a caloric deficit. That means your massive Independence Day cheat meal isn’t going to ruin your body composition goals. However, you should remember that your collective food choices and training patterns are going to help dictate your body’s ability to burn and utilize fat.


Cordain, L., Miller, J. B., Eaton, S. B., Mann, N., Holt, S. H., & Speth, J. D. (2000). Plant-animal subsistence ratios and macronutrient energy estimations in worldwide hunter-gatherer diets. The American journal of clinical nutrition71(3), 682-692.

Steinberg, D., Vaughan, M., & Margolis, S. (1960). Control of fatty acid release from adipose tissue through control of the rate of triglyceride synthesis. Journal of Biological Chemistry, 235(9), PC38-PC39.

Zimmermann, R., Strauss, J. G., Haemmerle, G., Schosiwohl, G., Birner-Gruenberger, R., Riederer, M., … & Hermetter, A. (2004). Fat mobilization in adipose tissue is promoted by adipose triglyceride lipase. Science.



Sarah Joseph

Sarah Joseph

Writer and expert


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