Updated: Mar 3, 2019
Ever wonder where fat goes when it gets burned? I mean, if you want to lose fat, shouldn’t you understand how it is burned in your body? Many people think you only burn fat during aerobic or cardiovascular exercise. Some people believe that fat is only used as a fuel for energy, especially when you lower your caloric intake to force the body into using those fat stores as fuel to replace the energy no longer eaten. Some believe it’s excreted through the urine or feces, while others think the fat is turned into muscle when you exercise and also that muscle turns into fat when you don’t exercise. My favorite one is that fat simply evaporates due to the heat created by muscles during exercise.
To be clear, some of these thoughts have some degree of truth to them. But also to be clear, none of them are entirely correct, and some are just out there, plain wrong.
No matter what you believe, I am going to set the record straight for you here, in this blog, or article as some like to call them (I have a long winded blog).
Anyway, not everyone burns fat the same way. It is estimated that at rest, the body uses about 50% of its energy from fat, and the other 50% from carbohydrate, but that’s only if you are unfit. If you are fit, the body undergoes something called glycogen sparing. This is where your metabolism will keep more sugar (glucose) in the muscle while burning more fat. A healthy or fit person will burn as much as 80% or more of every calorie from fat, and only 20% from glucose. That means that the best way to lose fat is to get yourself into the fit category rather than the unfit one. Don’t worry though, it’s easier than you think.
While the idea of the human metabolism is somewhat complex, we can break it down into some really simple language. When you begin to exercise you start initially by burning mostly sugar (in the form of glycogen). Lots of glycogen (sugar molecules linked together) can be stored in the muscle, liver and blood, more than enough to get you through the first minute or so of exercise. As you continue to exercise your body begins to convert fat for a fuel. Fat, or adipose tissue is made up of 29% water, 55-65% fat and 2-7% protein. The fatty material of adipose (fat) tissue is made up of a glycerol molecule and three chemically active –OH or hydroxyl groups called fatty acids. The result is three fatty acids attached to a glycerol molecule, or a tri-glyceride, the storage from of fat.
Now, obviously we measure triglyceride levels in the blood, and we want them as low as possible so they cannot contribute to hardening of the arteries, diabetes and strokes. Circulating free fatty acids (free because they are not attached to a glycerol molecule) are used for energy. As they enter the muscle cell to be converted to energy, they are broken down along with some sugar into smaller components such as carbon dioxide and water. In a study published in the journal BMJ, they postulated that if 10 kg of fat were oxidized (broken down), 8.4 kg would be converted and excreted as carbon dioxide (CO2) via the lungs, and 1.6 kg would be water (H20). So out of 10kg of fat we would excrete 10kg of by-product in the process of creating energy.
So, according to the studies calculations, you basically exhale 84 percent of your lost fat. The remaining 16 percent is metabolized into water, which is excreted through sweat and urine. So to put this into perspective, the fat is lost through increased activity (muscular) and respiration. The more muscle you use and the greater the increase in respiration the more calories are burned. So rather than cardiovascular exercise burning more fat, it is strength training that actually burns the lion’s share of fatty materials, which once converted to energy is breathed out as carbon dioxide.
Put into perspective, if you substituted one hour of sedentary lounging with one hour of moderate and sensitive strength training exercise which increases your respiratory rate, your metabolic rate would increase sevenfold. However, keep in mind that you can easily hamper any potential weight loss by eating too much food—and I would stress, by eating the wrong kinds of foods. No matter how hard you exercise you cannot out run your fork, keep that in mind the next time the doughnut display looks too good to pass up.
Ok, a few additional points. It’s important to recognize that most people who struggle with excess weight have some degree of insulin and leptin resistance (see the blog "How to Change Your Life" Part 1, 2, and 3 on this website). Leptin, as we have learned, is a hormone that helps you regulate your appetite. When your leptin levels rise, it signals your body that you're full, so you'll stop eating.
But, you can become resistant to leptin by eating too much fructose or overeating in general. As you become resistant to the effects of leptin, you end up overeating more, as your body gradually loses its ability to “hear” the signals leptin sends out. Research clearly shows that refined sugar (in particular processed fructose) is exceptionally effective at causing leptin resistance. Fructose also effectively blocks the burning of fat, a double whammy.
Basically, if you are insulin or leptin resistant, as long as you keep eating fructose and grains, you're programming your body to create and store fat not burn it. Add to that restricting calories too much, (see last week’s blog on calories in Vs calories out) and you further damage your metabolism while losing muscle (where you burn the fat). Thus you increase your body’s ability to store more fat and limit its ability to burn it off. This is one of the key reasons why, if you are overweight, you’d be wise to restrict your fructose consumption to about 15 to 25 grams of fructose per day from all sources and ensure that you engage in resistance training to increase fat burning muscle.
It is worth mentioning that strength training not only improves our metabolism and bone density, it also conditions your fat! Yes, really. Scientists have been studying a type of fat called "brown fat". This is a type of fat that generates heat that in turn burns energy instead of storing it. So-called “white fat” is the kind that is primarily stored, and because it’s also difficult to burn off, it tends to cause obesity. Research has shown that certain groups of people tend to have more brown fat than others, and there appears to be direct correlations between the activation of brown fat and metabolic measures of good health. For example:
Slender people naturally have more brown fat than obese people do
Younger people have more brown fat than elderly people
People with normal blood sugar levels have more brown fat than those with high blood sugar
People who exercise and strength train have more brown fat than non-exercisers
Newborns have a supply of brown fat to keep warm, but most of these stores are lost by the time adulthood is reached. However, although you have far less of it as an adult, research has shown that animals convert white fat into brown fat simply by exercising. Yes, yet another great benefit of exercise. It not only helps you lose the fat but it also helps you to keep it off too.
As you can imagine, the human metabolism is extremely complex. On the one hand, exercise helps convert unhealthy white fat into healthier, heat-producing and more metabolically active brown fat. Exercise also increases the oxidation of fat, which then leaves your body via your lungs, in the form of carbon dioxide, and your bodily fluids, in the form of water. What’s not so complex however, is how to optimize your metabolism—even if you don’t understand the exact mechanisms involved. Following simple basic exercise programming and food/diet modifications such as those taught at Revibe Fitness and Wellness will jump start your body’s ability to achieve your ideal weight and leanness.