What Is a “Calorie?”

To understand the importance of fat, protein, and carbohydrates in terms of nutrition and fat loss, it is important to first understand that these three groups of nutrients are sources of energy for our bodies.  We measure this energy in terms of “calories”.  Technically, there are two “types” of calories; in physics and chemistry, there is the calorie (lower case “c”), which is the amount of energy needed to raise the temperature of one gram of water by 1 degree Celsius.  In nutrition, the kilocalorie (or “Calorie with an upper case “C”) is used, and this is the amount of energy needed to raise the temperature of one kilogram, or liter, of water by 1 degree Celsius.  To add a little bit of confusion, it is assumed when you are talking about food that you are using the kilocalorie measurement, so even though all food energy is labeled as measured in “calories” with a lower-case “c”, it is actually measured in “kilocalories”, or calories with an upper-case “C”.  So, in short, the calorie is a unit of energy, and it is the one we use to measure foods.

“Harvesting” the Energy From Our Food

I do not want to get to in-depth and technical with the nutrition and food science, particularly the biology and chemistry, which can get gruesome.  However, certain concepts are easier to understand with a little bit of background in the basic biology and chemistry of what our bodies do with the food we eat.  Molecules are held together by “bonds” between the different atoms.  There can be more than one bond between two atoms; a single bond is weaker than a double bond, which is weaker than a triple bond.  When these bonds are broken, they release energy.

The primary goal of your body’s metabolism, as you eat and digest food, is to create a form of “energy currency” which can store energy or release it as needed.  The “currency” used by the human body is a molecule called “ATP”, and it provides energy in the form of double bonds.  The main source of producing ATP is for the body to digest food into its most basic units, which are then absorbed by the intestines, and delivered throughout the body in the bloodstream.  The body’s organs and tissues will absorb these basic food molecules, which then go through a process called the “TCA cycle” (you may or may not also know this as the “Krebs cycle”), which finally produces ATP for the body to use.  In order for the cell to perform the TCA cycle, it must have a supply of both fats and carbohydrates, which are “building blocks” in the TCA cycle.  Without both fats and carbohydrates, the  TCA cycle cannot be carried out, and ATP must come from another source.  Every cell in the body carries out the TCA cycle, meaning that as long as the basic building blocks are there, every cell can produce and use its own energy.

Fat

Fat contributes a vital molecule, called “acetyl CoA”, to the TCA cycle.  Because of the way fat molecules are built, a single fat molecule can donate over a dozen molecules of acetyl CoA.  Because fat contributes so much to the TCA cycle, it is the richest source of energy (calories) in the diet.  1 gram of fat provides 9kilocalories worth of energy.  Fat is also used as the main site of energy storage.  Although ATP is the body’s main energy “currency”, it is meant to be used almost instantly.  Fat can be stored until it is needed, even for a full lifetime.

Carbohydrates

Carbohydrates, or “carbs”, provide the other important molecule in the TCA cycle: glucose.  Glucose is split into two molecules called “pyruvate”, and each pyruvate will go through one complete “turn” of the TCA cycle.  Carbs do not provide as much energy as fat, only 4 kilocalories per gram of carbohydrate.  Carbohydrates can also be used to store energy.  This energy is stored as “glycogen”.  Glycogen, though, is meant to be used in times of exertion (ie exercise), as it releases energy much quicker than fat.  The downside is it is not as efficient at storing energy as fat is – it takes almost 7 times less fat as compared to glycogen to store the same amount of energy.

You can see now why “low-carb” diets have so many negative side effects; once the body is cut off from carbs, the TCA cycle will stop, and then where does the energy come from?  Fortunately, there are other sources of energy, but these are not meant to be used long-term, and can produce high levels of toxins.

Protein

Protein is never a primary source of energy.  Although protein does, through a number of methods, provide a small amount of energy all the time, this is not its primary function.  Protein is used to build tissues in the body.  It is especially rich in muscles.  Protein can, however, provide energy if it is needed.  If the body is starving, or is short on carbohydrates for the TCA cycle, protein can be broken down to provide some energy.  Protein, like carbohydrates, can provide 4 kilocalories per gram of protein.

Overview and Review

So, you sit down to eat a meal.  Your body digests the protein, carbohydrate, and fat from your meal into basic units, which it then absorbs.  These “building blocks” are delivered through the bloodstream to all of the cells in your body.  In these cells, fats are further broken down to create acetyl CoA, while glucose from carbs is broken into pyruvate.  The pyruvate and acetyl CoA enter the TCA cycle to produce ATP, the basic currency of energy in your body.  If you do not have enough glucose, the TCA cycle cannot run, and the body will find less efficient, more dangerous ways to provide energy, and will begin to break down stored energy, including fat and protein.  If there is an excess of energy, it will be stored as glycogen or fat.

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