Read Time: 15 Minutes

Those of you who have followed my page long enough have heard me repeat the basics of energy balance ad nauseam, but for good reason.  It is the single most important factor to consider when it comes to making changes in body composition. Understanding the basics of this one concept can put you lightyears ahead of the curve when trying to make changes to your own body. However, those of you who have never had any formal education in nutrition sciences may find the topic overly technical.  It’s true, at its core the concept of energy balance can be an incredibly nuanced topic but we can save that knowledge for the scientists who study it. Simply scratching the surface of the topic can be all that you need in order to make better decisions in your everyday life. With this article I hope to clarify some of the key concepts behind the science and convince those of you who are skeptical that energy balance is the basis of all weight changes, not hormones or carbohydrates like some “experts” will claim. 

First I want to clarify some terms.  

Energy balance: calories in = calories out

Positive energy balance: calories in > calories out

Negative energy balance: calories in < calories out

What is a Calorie?

A calorie is simply a unit of heat.  It is not unique to food, you can measure the calories in lots of different things.  Scientifically speaking, a calorie is defined as the amount of energy it takes to heat one gram of water by one degree Celsius at standard atmospheric pressure. In colloquial terms, we use the word calorie interchangeably with kilocalorie (at least in the US) even though this is technically incorrect.  A kilocalorie is one thousand times greater than a calorie and the kilocalorie is the true unit of energy we use on our nutritional labels. For whatever reason we have abandoned standard scientific naming conventions when it comes to labeling the energy in our food (again, at least in the US). Technically speaking, the calories on the nutrition labels on your food represent the amount of energy it takes to heat one kilogram of water by one degree Celsius at one atmosphere of pressure.  For the sake of simplicity, when I use the term calorie for the remainder of this post, I am referring to kilocalories.

The calories in our food are measured using a tool called a calorimeter.  Any of you who have taken a college level chemistry course have surely encountered some form of a calorimeter.   The way we measure calories in our food is to take a small sample of the food and place it into a sealed container that is submerged into an insulated container of water.  That sample of food is then ignited to initiate a combustion reaction. The chemical energy within the food is transferred to heat energy via the reaction. This heats the water in the surrounding area and we can measure the temperature change of the water to calculate the energy that was in that sample of food.  The energy from your food is stored within the bonds that hold the atoms of the molecules together. When these bonds are broken, energy is released. More on how this fits into weight loss and weight gain later. Obviously we aren’t going to pull out a calorimeter in our kitchens to assist us in obtaining accurate values for the energy in our food.  You can measure calories in a human by measuring the amount of heat a person expels. Luckily there are databases available that make our lives exponentially easier. MyFitnessPal is my database of choice. I find it is easy to use and has a wide supply of food choices to select from. Are all of the values perfect? No. Are they close enough to use to estimate your intake? Yes. 

Where do we start?

Energy balance is a function of calories ingested and calories expelled.  Calories ingested is obviously the calories you take in with your eating and calories expelled refers to the energy you use to fuel your daily living.¹  Each and every one of you has a value for how many calories you must consume in order to stay alive at rest.  This is called your basal metabolic rate or BMR. It is the rate of energy expenditure per unit time, usually measured in calories per day for dieting purposes.  None of us are laying in bed all day though. We have to live our lives which requires energy. Movement isn’t free. In addition to your BMR you also require energy to fuel your daily tasks whether that be cooking, working, taking care of your kids or exercise.  Metabolizing the food you eat also requires energy, we call this the thermic effect of food. It all adds up. This value is known as your total daily energy expenditure or TDEE. 

TDEE = BMR + Exercise Activity + Non Exercise Activity + Thermic Effect of Food

You can see that this value won’t be the same every day.  Some days you exercise more or are running errands non stop.  Some days you sit on the couch all day. Your energy requirements are not static.  If you take anything from this article let it be this: Your body does not run on a 24 hour clock.  Your energy intake doesn’t reset when the clock strikes midnight. Your body’s energy requirements are dynamic and changing with the stress that you expose it to.  This is the very reason you can’t look at weight fluctuations on a day to day basis. That means jack shit to be blunt. What really matters is how your weight is trending over a period of time.  Now, lets talk about the first part of the equation, calories in.

Calories In

The calories you consume throughout the day contribute to your daily calorie intake.  These calories pass through your digestive tract and most of the nutrients are absorbed in the small intestine.  For the calories to be counted as “in” they must be absorbed and passed into circulation. Believe it or not the interior of your digestive tract from your mouth to your backdoor is one long tube that is one with the external world.  Even though it is held within your person, the contents it holds are technically not in your body until they pass through the lining of your intestines. This is an important distinction because it means that not all of the calories you ingest will be absorbed and be available for utilization by your body.  For the most part, unless you eat a lot of food/fat or have some diseases, the vast majority of nutrients will be absorbed but there are always some nutrients that pass by and will be excreted.  

The nutrients in your food have two different fates once they are “in-in” (eaten and absorbed).  They can be used or stored. What happens to those nutrients depends on the needs and demand of your body.  The carbohydrate, protein, and fat in your food are broken down into simple sugars, the constituent amino acids, and free fatty acids respectively during digestion.  If you don’t have a current need for energy it is likely that those macronutrients will be stored. Each of the macros have a different form and capacity for how they are stored depending on the current state of energy balance.  Carbohydrates are stored as glycogen in both the muscle and liver. Your body’s storage capacity of glycogen is quite small, only around 500-700 grams worth but your capacity increases the more muscle mass you have.²  Each gram of glycogen is accompanied by roughly 3 grams of water.

The amino acids from the protein in your diet cannot be stored within the body.  Those amino acids must be sequenced into new proteins to be used for either structure or function, muscles and enzymes for instance.

The fatty acids from your food are converted into triglycerides and stored in adipose tissue or within your muscles throughout the body.  Where fat is stored is dependent on a number of different things such as genetics, sex, or hormone status. Your body’s storage capacity in adipose is virtually unlimited.  

Calories Out

Throughout every minute of every day that you are alive you are burning energy.  You are your own little furnace burning through fuel to generate heat. Sometimes, like when you are exercising, you are burning through that fuel at a rapid pace.  Other times, like when you are sleeping, your fuel efficiency is much higher. As I mentioned earlier, energy is held between the bonds of atoms of our food. Our cells have been tasked with the job of obtaining this energy from our food and converting it into a usable form, adenosine triphosphate (ATP).  ATP is the energy currency of the body. All of your movement and bodily processes are fueled by ATP. The reactions within the cell to generate energy create end products of ATP, water, carbon dioxide and heat. The ATP from these reactions is used to fuel activity and sustain life. The water and carbon dioxide exit your body during exhalation or in your urine.  This means that when you lose weight, it is primarily leaving your body as carbon dioxide when you breathe and less in water when you go to the bathroom. When you take a second to think about this you can make sense of it on a practical basis. When you exercise at higher intensities it costs more energy. What happens when you are exercising at high intensities? You breathe harder.  Your body is requiring more oxygen in order to shuttle those energy containing molecules through the reactions to obtain ATP to fuel the exercise. Those energy containing molecules, made from a collection atoms each with an atomic weight, are broken down into the end products including the CO2 and water that is exhaled and excreted. This is how weight loss occurs. 

How does it all fit together?

So you have the calories you consume through food and the calories you burn with exercise and daily living, but what does that mean in terms of your weight status?  Remember earlier when we discussed total daily energy expenditure? Imagine a rudimentary balance where a board is sitting atop a sharp point with equal weight distribution on each side so that it is perfectly flat.  If the calories you consume go on the left side of that balance, when you eat food it will push that side of the scale down and out of energy balance. That food takes energy to metabolize though and it also costs energy to cook and clean up.  You also require energy to keep your heart beating, your lungs working, and your brain to fire. This adds up throughout the day and when you add the sum of these values to the right side of your balance, you’ll find that it shifts back to equilibrium.  Energy balance is exactly as it sounds, a balance of energy in versus energy out. When the energy in exceeds the energy out that excess energy is stored. If you recall that energy can be stored in a few different ways, in glycogen, in adipose tissue, or in different kinds of proteins.  

For those of you who may still be struggling to visualize what is happening I like to use the analogy of a car to help simplify the complex process that is energy storage and use within our bodies.  Think about your car. It relies on gasoline to generate energy to move. Gasoline for your car is equivalent to food for your body. Your car has a chassis, motor, tires, and other components that contribute to the overall weight of the car similar to how your body has a skeleton, muscles, tissues and organs.  For our analogy let’s assume your car weighs 2,000 pounds with a full fuel tank and it is considered to be in energy balance when the tank is full. Driving the car burns fuel thus making the car lighter because it is burning through its energy stores. The energy held within the bonds of the gas is used to help make the car move.  As the fuel burns, the elemental molecules that make up the chemical structure of the gas and possess weight are expelled out of the exhaust pipe, thus reducing the car’s weight. This process is the same within our body. Our fuel tank is our carbohydrate and fat stores and our exhaust pipe is our mouth. As we move, our bodies generate energy from our carbohydrate and fat stores and the by products are expelled resulting in weight loss.  So, if we wanted our car to lose weight, we would need to create an energy deficit by driving it around to burn off the gas held in its fuel tank. To get back to energy balance we would need to top off the tank with fuel. If we wanted our car to gain weight, we would need to create an energy surplus by adding extra fuel to the car, making it gain weight. Because the fuel tank is full we have to find a new place to store the fuel. If we fill up some gas cans and keep them in the trunk of our car it increases both the weight and energy potential of the car similar to how if we consume more calories than our body requires, we store the excess energy in our own trunk (lol) and gain weight.

How do I apply this to my own life?

The principle of energy balance can be used as a guide to help you construct your nutritional strategies but energy balance is not simple.  There is much more nuance to the topic than just energy in versus energy out. Disturbing energy balance will cause other changes in your body than just losing or gaining weight.  Changes in other parts of the body will occur to try and return back to energy balance (e.g. hunger and satiety hormones).³  That being said, if you are in negative energy balance (calories in < calories out) over a defined period of time, you will absolutely lose body mass.  The same rings true for the reciprocal of that as well. If you want to make changes to your body composition, it is best if you understand where your own body fits into the whole context of energy balance.  Experimenting and finding the point where that balance occurs allows you to model your eating and training in a way to support your goals. If you would like more information on how I recommend obtaining an estimate on where energy balance is for you and how to use that information to improve your own diet and training strategies, check out this story highlight I have on the topic from Instagram.  Click here to watch.

1. Hill, J. O., Wyatt, H. R., & Peters, J. C. (2012). Energy Balance and Obesity. Circulation, 126(1), 126–132. doi: 10.1161/circulationaha.111.087213
2. Acheson, K. J., Schutz, Y., Bessard, T., Anantharaman, K., Flatt, J. P., & Jéquier, E. (1988). Glycogen storage capacity and de novo lipogenesis during massive carbohydrate overfeeding in man. The American Journal of Clinical Nutrition, 48(2), 240–247. doi: 10.1093/ajcn/48.2.240
3. Sandoval, D., Cota, D., & Seeley, R. J. (2008). The Integrative Role of CNS Fuel-Sensing Mechanisms in Energy Balance and Glucose Regulation. Annual Review of Physiology, 70(1), 513–535. doi: 10.1146/annurev.physiol.70.120806.095256