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Calories-in, calories-out. Keep it simple

Updated: Jan 7, 2023


The calories-in, calories out model of weight change is based on the premise that changes in an individual's body weight will be driven by net calorie balance (calories consumed minus calories burned). Under this model, if more calories are consumed than are burned, an individual would be in a calorie surplus, and these extra calories will be stored in fat or glycogen (carbohydrate) reserves. If more calories are burned than are consumed, the reverse would be true, and an individual would be in a calorie deficit. This deficit would force the body to use up some stored energy and result in weight loss. To demonstrate how this basic model would be applied we will use a weight loss example. Under the assumption that a pound of fat is about 3500 calories and your goal is to lose about a pound per week, you would need to eat about 500 (3500/7 days) fewer calories than you burn per day. While this basic model may be an oversimplification of how weight change occurs, it illustrates the overarching framework of the calories-in, calories-out model and we will add nuances to this model later in this article.

Calories In

The calories-in component of the model includes all of the food that you eat and calorie constraining beverages that you drink. While the calories from food usually represents the majority of calories, liquid calories such as alcohol and sugar containing juice or soda can also contribute a significant number of calories to your daily intake. It can also be quite difficult to determine how many calories you are actually consuming as our natural abilities to estimate how much we eat is highly lacking in modern society (Zegman, 1984). If we aren’t tracking our intake using tools like food logs, individuals significantly underestimate how much they eat. This lack of accuracy is likely partially attributable to the prevalence of high sugar, high sodium, high fat processed foods that dominate our daily intakes. These foods are often very calorie dense and are designed to be incredibly easy to eat.

Calories Out

Calories-out, or total energy expenditure, can be broken down into the following four main components: Basal Metabolic Rate (BMR), Non-Exercise Activity Thermogenesis (NEAT), Exercise Activity Thermogenesis, and the Thermic Effect of Food.

  • Basal Metabolic Rate (70%) represents the largest component of your total daily energy expenditure and represents the number of calories it takes to keep you body alive if you were lying completely still. This component includes breathing, circulating blood, cell production and repair, and other crucial functions.

  • Non-Exercise Activity Thermogenesis (15%) is the second largest component of energy expenditure and includes all of the activity you do throughout the day that is not included in structured exercise. This includes walking, cleaning, fidgeting and other movements.

  • Exercise Activity Thermogenesis (5%) includes structured cardio and resistance training.

  • The Thermic Effect of Food (10%) is the energy that it takes to break down and assimilate calories from food and beverages.

Metabolic Adaptation

One of the primary reasons that the model presented above is a bit of an oversimplification is due to the flexibility of both calories in and out. The human body has a natural inclination to try and remain at a certain bodyweight and defends this weight using a variety of compensation mechanisms. If an individual is in a calorie deficit the body will respond by slightly decreasing BMR, NEAT and may burn fewer calories during exercise sessions. It will also have a lower thermic effect of food because it is digesting less food. The result of these effects is a slight decrease in your total energy expenditure and therefore you may not lose as much weight as you would expect. The opposite effects are seen when you are in a calorie surplus and these minor adaptations create a maintenance calorie range in which you likely will not lose or gain weight. Therefore, your maintenance level of calories is not a single number but a range in which your weight will remain stable. In addition, the longer you remain in a surplus or deficit the more your body will adapt its compensation mechanisms to stabilize your weight. This isn’t an incredibly rapid process, but over time it may require you to modify your calorie target if you still wish to keep seeing results. The magnitude of metabolic adaptations are highly individual and some individuals may see very little compensation while others may see significant amounts.

Criticisms of the Model

Now that the general framework has been given a bit more detail I would like to address some of the criticisms against the model.

Some educators or practitioners in the fitness industry have cited the metabolic compensation discussed above as a major flaw of the calories-in, calories-out model. They believe that, given the interaction between calories-in and calories-out, the whole model is too complex for us to quantify and that numerical values should not be used in the discussion of weight change. While the model isn’t as clean cut as 1 + 1 = 2, it doesn’t have to be. When we are tracking bodyweight and food intake over time we can determine whether we are in a calorie deficit, surplus, or maintenance. Based on how weight changes across a 5-7 day average, we can determine whether calorie intake needs to be adjusted up, down, or left the same. We don’t have to understand every mechanism in the body, we simply have to make small changes to our intake or expenditure based on the data we collect.

Individuals may also attribute the changes in weight to hormonal changes, “rebalancing/detoxifying the body”, or elimination of some negative substance (e.g carbs, gluten, meat, sugar, etc), while disregarding the role of calorie balance as a driving factor. In all of the these explanations the root cause of weight change is simplify calorie balance. The vast majority of hormonal changes do not significantly influence changes in bodyfat, unless an individual has a severe hormonal disorder, in which case they should inquire with an endocrinologist. Additionally, our abilities to directly target hormonal changes is incredibly limited and therefore nutritional approaches that attempt to “optimize” hormones are likely focusing on the wrong factors for weight loss. Additionally, detox protocols are unlikely to have any meaningful effect on weight loss, outside of the calorie deficit they create. A focus on toxins again focuses on micro factors instead of focusing on the big picture of weight loss through a calorie deficit. Elimination of certain foods or food categories is also highly misdirected. A certain food group will not stop you from losing weight and this has been proven in a variety of studies that show similar fat loss results between high-carb and high-fat diets (Sacks et al., 2009). When we consume the same number of calories, weight change will be the same whether we are consuming rice or bread, tofu or chicken, avocados or white sugar. While a focus on whole, fibrous, nutrient dense food is good for health and weight management, a diet that places excessive focus on targeting hormones, detoxifying the body, or restricting food groups can be quite harmful from a psychological perspective. If an individual believes they can only lose weight while drinking 4 detox teas a day or while avoiding all sugar, they may be highly restricted in their social life and may not enjoy how they have to eat. This places significant strain on one's ability to sustain the calorie deficit for long periods of time and doesn’t provide a framework on what to do after weight loss. In our opinion, the individuals who have proposed the discussed theories of weight-loss are creating overly-complex, highly restrictive diets to sound intelligent while sacrificing their client’s ability to sustain and enjoy their lifestyle. If a calories-in, calories-out approach is taken instead, an individual can still eat many of the foods they enjoy while remaining in a calorie deficit and losing weight.

The final criticism I will discuss centers around the following statements: “I lost weight and never tracked a single calorie, so calories-in, calories-out is unnecessary” or “I’m eating fewer calories than I burn and I’m still not losing weight so calories-in, calories-out is wrong”. As I will demonstrate, neither of these are actually a criticism of the model and are actually just a misunderstanding. In relation to the first of these statements, the individual is not completely incorrect but has misinterpreted part of the model. Whether someone used the model consciously or not doesn’t change the basic science of the situation. They were still in a calorie deficit, they simply didn’t achieve it by tracking. Stored energy doesn’t just vanish and conversely, it can’t be generated from nothing. This is the first law of thermodynamics and states that energy can not be created or destroyed in a closed system and can only change form (e.g fat into movement). The second statement is simply incorrect and can also be analyzed using the first law of thermodynamics. If they are burning more than they are eating and not losing weight, where is this extra energy coming from? Spontaneous generation of energy is impossible. If it were this would be a groundbreaking scientific achievement. The issue that this person is actually running into is that they aren’t truly in a deficit. While they may think they are, they likely aren’t. This may be due to the underestimation of calories consumed, overestimating of energy expenditure, or overestimating of maintenance calories. One thing to note here is that if the individual is relatively new to resistance training, they may lose weight slightly slower than expected. During the beginner stages of resistance training, individuals can gain fairly significant amounts of muscle that may slightly counter the fat lost.


Given the complexity of the human body it can often be overwhelming when trying to lose or gain weight. There are an endless number of diets that make the nutrition landscape very difficult to navigate but using the calories-in, calories-out model it is far easier to evaluate strategies and implement them in your life. If the goal is weight-loss but you eat more calories than you burn, you will not lose weight regardless of how few carbs you eat or the number of hours you fast. We would never deny that the human body is far more complex than we can understand so would never try to hack its mechanisms. Instead we take a large picture approach that focuses on the overall energy balance and allows people to integrate healthy nutritional strategies into their lifestyle in the long-run. The focus is on creating a principle based framework that supports individual preferences, opposed to a rule based tyranny that leaves little room for flexibility.

Connor Crouse, PN1, PTS

Head Nutrition Coach

KPCC Coaching


Sacks, F. M., Bray, G. A., Carey, V. J., Smith, S. R., Ryan, D. H., Anton, S. D., McManus, K., Champagne, C. M., Bishop, L. M., Laranjo, N., Leboff, M. S., Rood, J. C., de Jonge, L., Greenway, F. L., Loria, C. M., Obarzanek, E., & Williamson, D. A. (2009). Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. The New England journal of medicine, 360(9), 859–873.

Zegman M. A. (1984). Errors in food recording and calorie estimation: clinical and theoretical implications for obesity. Addictive behaviors, 9(4), 347–350.

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