Quick Nutrition Lesson. What is the Calorie?

Everyone talks about “calories in versus calories out” when explaining how to lose or gain weight. However have they actually considered why this is the go to explanation for weight change? I deferred to the expertise of Greg Nuckols at MASS on the subject to delve into why exactly the Calorie is related to mass, and how the body actually processes food.

A Calorie is a unit of measurement, but one of heat, not of mass. One Calorie is the amount of energy needed to raise the temperature of 1kg of water by 1 degree C. To figure out how food turns into heat, let's start at the beginning. When we eat food, the food only truly enters the body when it crosses the intestinal lining in our gut. Our small intestine weeds through our food and pulls out all of the carbohydrates, protein, fat, vitamins, and minerals; the nutrients that are useful in the body. Unfortunately the small intestine doesn’t care if we really need those nutrients or not, it will draw almost all of them in no matter what, leaving only the fiber to pass through making up our poop.

Once inside the bloodstream, the nutrients are broken down; complex carbohydrates into simple carbs and eventually glucose, protein into amino acids, and fats (triglycerides) into free fatty acids. Next these simple nutrients need to find a place to be stored. Glucose is stored as glycogen in either the liver or inside the muscle cells for quick access for energy. Amino acids are either used for a plethora of uses in the body, including muscle synthesis if it is needed (like after rebuilding muscle after a lift). However, if it is not needed immediately, or if there is an abundance, it can be broken down to be used for energy as well. Fatty acids are either stored intracellularly in the muscle or in fatty deposits under the skin or around the organs. Fat storage in the muscle is the short term easy access energy while the subcutaneous fat deposits are the long term storage.

When the body needs to be active, or when the muscles are signaled to work, they need to get their energy from those nutrients that are in storage. The muscle grabs the closest stuff to them first, being the glycogen and fatty acids already within the muscle cell. Those storage levels drop as the body continues to exercise, so they are refilled either by new nutrients that are entering the body, or from the longer term storage deposits like glycogen in the liver or subcutaneous fat.

Now we are finally getting to the point of this article. How does the body create heat from mass (nutrients from food)? The key molecule is adenosine triphosphate (ATP). Via a number of metabolic systems, the muscle cells turn the base components of carbs, protein, and fat into ATP. The muscle cells use ATP directly for energy; to shorten the muscle fibers creating movement in the body. When ATP is used for energy it creates heat, which is measured in calories.

You still may be wondering how weight is lost when ATP is used since only heat is produced. The loss in weight actually comes from the metabolic breakdown of the macronutrients when ATP is created. The byproduct of these processes are CO2 and H2O, molecules with mass. We end up exhaling the CO2 out of our bodies from our lungs and excreting the H2O out in our urine. To sum it up, the food we eat is stored as macronutrients in the body, those nutrients are broken down into ATP, H2O, and CO2. ATP is the energy our muscles use, creating heat as a byproduct, measured in calories. The H2O and CO2 is the mass that our body gets rid of, and how we lose weight when active.

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