Understanding the elusive calorie as a unit of measure is critical for personal trainers when programming for their personal training clients for any phase of periodization. The most basic and fundamental law that governs whether you gain weight or lose weight is the first law of thermodynamics, which states that energy cannot be created or destroyed, it can only be transformed from one type to another. For our purposes here, the most common unit of energy measure is the calorie.

Although thermodynamics itself is generally not a simple subject, calories are explained quite easily according to its principles. Ultimately, your body weight is dependent only on the “difference” between the number of calories that you consume versus the number of calories that you burn (this is known as your caloric balance).



What Is a Calorie?

A calorie, as commonly referred to on food nutrition labels, is a unit of energy. More specifically, a calorie is the amount of energy needed to raise the temperature of 1 liter of water by 1 degree Celsius.

A calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries, it remains in common use as a unit of food energy.

Definitions of this unit of measure fall into two classes:

  • The small or gram calorie (symbol: cal) approximates the energy needed to increase the temperature of 1 gram of water by 1 °C.
  • The large (or kilogram/food calorie) (symbol: Cal)  approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1000 small calories or about 4.2 kilojoules.

In an attempt to avoid confusion, the large calorie is sometimes written with a capital C. This convention, however, is not always followed (and is sometimes impossible). Whether referring to large or small often must be inferred from context. When used in scientific contexts, the term calorie refers to the small variety.

When you eat food, you are consuming the energy that is stored within the protein, carbohydrate, and fat molecules of that food. The total amount of the energy stored in the food you are eating is represented by the calorie content of the food as indicated on its nutritional label.

How Does Your Body Use Calories?

Our bodies are capable of doing only two things with the calorie energy that it absorbs; either burn it or store it. This is how the effect of calories on our bodies can be explained according to the first law of thermodynamics. When we eat food, the calories that are in it can never disappear or be destroyed, they can only change form.

In their food form calories are stored as chemical energy in the bonds of the food molecules. When we eat them, calories do not disappear but rather are eventually transformed, or “burned,” into the different types of energy that your body utilizes or produces each day, like heat energy, electrical energy, sound energy, and kinetic (movement) energy, or, if they aren’t burned, they are stored again as more chemical energy.

Therefore, according to the first law of thermodynamics, any calories that you consume and do not burn must be stored in your body. And, unfortunately, the primary storage mechanism for the excess calories you consume is fat, not muscle.

Calorie Rules for Weight Loss and Weight Gain

These facts so far lead us to some very simple rules about calories by which weight loss and weight gain can be explained. These rules are absolutely fundamental to determining how much you weigh, and it is impossible to contravene them. They are the following:

1. If you eat more calories than you burn you will gain weight.
2. If you burn more than you eat you will lose weight.
3. If you eat the same amount of calories that you burn your weight will not change unless you add muscle mass with weight training.

To keep itself alive your body is always burning at least some minimum amount of calories that are used to support the function of vital organs like your heart, brain, nervous system, lungs, kidneys, liver, muscles, and skin. This rate of calorie burn is called your basal metabolic rate (BMR).

If you want to accomplish anything beyond simply staying alive, such as moving your body, for example, you will have to burn extra calories to do it. Therefore, on any given day the total number of calories you burn is the sum of your BMR plus the calories burned from your additional activities.

Try this Total Daily Energy Expenditure Calculator to estimate how many calories you burn each day. Once you know how many calories you burn in a day, you need to figure out how many calories you consume in a day.

This calorie counter found in the MyFitnessPal app to track the calories in the foods that you eat. Once you know how many calories you burn in a day and how many calories you consume in a day you can calculate if you are likely losing, maintaining, or gaining weight, and how quickly.

As an example, let’s say that you’ve determined you burn 2,000 calories a day and eat 2,500 calories a day. Therefore, you are eating 500 calories more each day than you burn and you are gaining weight. How fast are you gaining weight? Well, since there are approximately 3,500 calories stored in one pound of fat, you are putting on fat at a rate of about 1 pound every seven days (since 3,500 calories/pound divided by 500 calories equals 7 days/pound).

For another example, let’s say that you’ve calculated that you burn 2,000 calories a day and eat 1,800 calories a day. Therefore, you are burning 200 calories more each day than you eat and you are losing weight. In this scenario, you would be losing fat at a rate of about one pound every 17.5 days (since 3,500 calories/pound divided by 200 calories equals 17.5 days/pound).

Is it Really That Simple?

Well, yes and no. These basic principles apply to most people with normally functioning metabolisms and hormonal balance. Things can get tricky, however, when we consider the role hormones play in weight loss and gain. Some people may indeed be restricting calories enough to lose weight and yet, the fat does not seem to come off.

This can be extremely frustrating for both trainer and client. If a client has consistently restricted calories below BMR, they may have damaged their body systems and metabolism to a degree that can absolutely send the body into a stress response that retains fat and instead, uses lean tissue for energy. This is an unfortunate loop to be caught in and the difficult answer is that the client will have to learn to eat more for awhile to reset the metabolism, which will inevitably result in an initial net weight gain.

Few people on a difficult weight loss journey will want to hear or even understand this notion, let alone apply it. So it is imperative that you educate clearly and share materials rooted in the science to help them integrate this knowledge and subsequently shift their mindset enough to accept the inevitable: they will have to first gain in order to lose.

Conclusions about Calories

Although the concept of calories explained in this article reduces weight gain and weight loss to a simple formula, it is important to apply this knowledge safely. If you’re trying to lose weight, the best approach is to exercise regularly, get your calories from nutritious sources, and maintain a healthy rate of weight loss.

A healthy rate of weight loss is about 1 or 2 pounds each week (depending on how much fat you have available to lose). This corresponds to burning about 500 to 1,000 calories more than you consume each day.

To enhance the “calorie-balancing act” add some resistance training three times per week, as growing muscle is a 24/7 fat-burner.

Lastly, I recommend my clients start out by cutting out 100 calories a day, instead of the American Heart Association’s suggestion of 500 calories a day. The reason is simple. In my experience, trying to cut out 500 calories a day sets people up for failure. One hundred calories — which is the equivalent of about seven teaspoons of sugar or a small glass of white wine — is very manageable.

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-American Heart Association.


About the Author

Bill McGinnis is an NFPT-certified Master Fitness Trainer, and trains exclusively at the University of Texas Medical Branch Alumni Field House on Galveston Island, TX. He has over 24 years in the Fitness Industry, including work as the Men’s Fitness Trainer at the Betty Ford Center and as a Fitness Manager in Southern California. He currently specializes in training older clients for balance, strength, endurance, golf, tennis, and an improved quality of life. 


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or info@nfpt.com with questions or for more information.