Understanding the Basics of Calories In and Calories Out

As we head into the holiday season of parties, treats and excessive indulging, many people become concerned with the impact on their weight and overall health. It’s helpful to understand the science of caloric intake and calorie output and how the balance impacts whether you lose weight, gain weight or maintain.

The term “calorie” is frequently tossed around, yet its significance often remains unclear. You’ve probably heard that weight loss is a simple equation of adjusting and balancing calories in and calories out, but it is far from simple. 

Defining Calories

At a basic level, a calorie is simply a measurement for energy. In the context of nutrition, it represents the amount of energy released when the body metabolizes food. The three primary macronutrients—carbohydrates, proteins, and fats—contribute varying amounts of calories per gram, with carbohydrates and proteins providing 4 calories per gram, and fats delivering 9 calories per gram.


You cannot change the science of thermodynamics. It is true and the Law of Conservation of Energy states that energy can neither be created nor destroyed – only converted from one form of energy to another.

So, the following equations are accurate. 

If your caloric input exceeds your caloric output, you gain weight.

If your caloric output exceeds your caloric input, you lose weight.

If your caloric output and input are balanced, you maintain your weight.

You may have also heard that not all calories are equal. That is not accurate. A calorie is a calorie and just a simple measurement of energy. However, and this is an important point, all calories are not the same in terms of the impact on your body and systems. 

Calories In

…..refer to the energy converted from the consumption of food and beverages. The body utilizes this energy to perform essential functions such as breathing, circulating blood, organ function, and supporting physical movement and exercise. Calculating calories in seems pretty simple to track, however, studies show that we often underestimate the number of calories we consume. Even when tracking, the FDA reported that products can contain as much as 20% more calories than what is printed on the nutrition label. 

Calories Out

…is a little more complicated and refers to the energy expended by the body through basal metabolic rate (BMR), physical activity, Non-exercise activity thermogenesis (NEAT) and the thermic effect of food. BMR represents the energy needed for basic bodily functions at rest. Physical activity includes intentional exercise and daily movements while NEAT refers to non-intentional movement like fidgeting. The thermic effect of food accounts for the energy used in digesting, absorbing, and processing nutrients from the diet.

The problem with the equation comparing calories in and calories out is that the type of calories you put into your body can dramatically affect your caloric output, even at the same total calories consumed. Likewise, caloric output can also impact caloric input.

Calorie Type Matters

Here are some examples to highlight this point:

Caloric Intake is Too Low: can affect your organ functions and slow down your BMR thereby decreasing your overall caloric output.

Consume More Protein in Your Diet: higher thermic effect and will require more calories to break it down than the same number of calories consumed from carbohydrates and/or fat.

More Fiber: can suppress your appetite and cause you to consume fewer total calories. 

Caloric Reduction: if intake is too low, it may cause your NEAT to decrease causing an overall lower caloric output.

Reduce Caloric Intake Too Low: lose muscle, that will decrease your BMR reducing your total caloric output.

Exercise: studies show that your appetite can be suppressed causing you to consume fewer overall calories. 

There are many other factors that can control your caloric input and output such as stress, lack of sleep and hormones. 

Achieving a healthy balance involves being aware of both calories in and calories out and how various behaviors can impact either side of the equation.

Whether the goal is weight maintenance, loss, or gain, understanding these fundamental principles empowers individuals to make informed dietary and lifestyle choices. Ideally, our goal should be to assure the calories into our body allow us to feel great, move well, have high energy, maintain our muscle, keep our metabolism revving, reduce the risk for overall disease, and improve longevity.

In terms of caloric output, we should strive to move our bodies every day, choosing activities that maintain our muscle mass and RMR such as strength training, minimize our stress such as yoga, and maintain the health of our heart and lungs such as cardio exercise. 

Yours in health & fitness,
Sherri McMillan

Sherri McMillan

Sherri McMillan

Sherri McMillan, holds a master's degree in exercise physiology and has been inspiring the world to adopt a fitness lifestyle for more than 33 years. She has received numerous industry awards including 2010 CanFitPro International Presenter of the Year, 2006 IDEA Fitness Director of the Year, 1998 IDEA Personal Trainer of the Year, 1998 CanFitPro Fitness Presenter of the Year and 2005/2006 ACE Fitness Educator of the Year - Runner up. She is a fitness trainer, fitness columnist for various magazines and newspapers, author of five books and manuals including "Go For Fit - the Winning Way to Fat Loss" and "Fit over Forty" and the featured presenter in various fitness DVDs. She has presented hundreds of workshops to thousands of fitness leaders throughout Canada, Australia, Mexico, Jamaica, New Zealand, Germany, England, Spain, South America, Asia and the U.S. She is the owner of Northwest Personal Training in downtown Vancouver, the founder of WHY Racing Events & WHY Community, participates in various community fundraisers and can be found running, biking, or hiking around the community. Find more information at nwpersonaltraining.com.

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