The simple science of calories in vs. calories out

When talking about calories, we are talking about energy and energy balance. A calorie is a unit of measurement for energy in foods. The food we eat provides energy for our bodies to function daily ⁽⁷⁾. Energy balance refers to the relationship between the energy we are eating compared to the energy we use during the day (i.e., calories in versus calories out). When the calories we consume are equal to that which we burn during the day, we are said to be in a neutral energy balance. When we eat more than we use, this is a calorie surplus. Finally, when we use more than we eat, we are in a calorie deficit. So why the obsession with calorie deficits? Well, research shows that being in a calorie deficit is an effective tool for losing weight ^(2–4,7,8). 

Studies show that, depending on your daily energy expenditure, a deficit of around 500 calories a day can help you lose weight ^(2–4,9). If your daily energy expenditure is low, a smaller deficit is advised as having a very low-calorie diet is not recommended ^(2–4,9). Similarly, to increase weight a 200-500 calorie surplus seems to be effective ⁽⁷⁾. For example, if someone’s daily energy expenditure is 1800 calories, eating 1300 calories would likely lead to weight loss and eating 2000 to 2300 would likely cause weight gain. Similarly, eating around 1800 would help weight maintenance. We can clearly see that knowing how much energy we use during the day is important. How do we do this?

Upon starting your journey in achieving a calorie deficit, it’s important to highlight that fact that everyone’s body is different ⁽⁹⁾! This means that most of the online calorie calculators cannot be used for everyone ⁽⁵⁾. Therefore, we suggest going to a registered dietician to help you with the process, as its no easy task.

Total daily energy expenditure (TDEE) is made up of our basal (or resting) metabolic rate, physical activity, and food (no, that’s not a typo). Basal metabolic rate (or BMR) accounts for 60-70% of your TDEE and is the energy that the body uses to do all the necessary metabolic functions throughout the day ⁽⁷⁾. For example, the contractions of the heart that pump blood around the body or your immune system fighting foreign cells all require energy. BMR is related to several factors including your height, weight, age, gender, and muscle mass ⁽⁷⁾. There are several methods for estimating BMR. The most common method used in research is called indirect calorimetry. This measures your metabolic gases while resting which can be converted into energy expenditure. Apart from this method there are many equations that predict BMR ⁽⁹⁾. These equations use different variables from height and weight to fat-free mass. A commonly used technology is bioimpedance devices, which is a technology shown to be the most viable and affordable option for the general public ⁽⁹⁾, that can be found in most dieticians practice rooms. While these are easier to implement, they are specific to populations and do show a large degree of variability ⁽⁹⁾.  

Apart from BMR, physical activity also contributes toward our TDEE. This includes all our normal physical movements of our daily life, such as brushing teeth, walking around the house or washing dishes. The more active you are in your day-to-day life, the higher the number of calories you use throughout the day. Simple activities like walking up the stairs, washing your car, parking further away from the entrance and getting off your chair to walk around the office for 5 minutes every hour can drastically increase your daily calorie use. The addition to daily movements, is exercise ⁽⁷⁾. There are a lot of misconceptions to exercise for achieving weight loss. Studies show that very high intensity exercise, such as HITT interval type workouts may result in higher calorie usage, but the substrate (stored food group in our body that we use to provide energy) being used for those calories is mainly carbohydrates. Low to moderate intensity, steady state type exercise, such as walking, hiking or light jogging, is the best for breaking down fat ⁽¹⁰⁾. Therefore, when it comes to increasing your daily exercise expenditure, instead of killing yourself every day with very high intensity exercise, rather slow down a bit by incorporating daily 30 – 40 minutes fast paced walks.

The last thing that uses energy, is our food. The energy required to digest food is known as the thermic effect of food and contributes about 10% of our calorie expenditure per day ⁽⁷⁾. Studies show that protein has a higher thermic effect than carbohydrates and fats ⁽⁷⁾, so your body uses more energy to digest a chicken breast than it does an avocado.

Now that we understand a bit more about our TDEE, let’s talk about our energy intake!

Unfortunately, not all calories are equal. Calories come from the different foods that we eat, which are broken down into three macronutrients: protein, carbohydrates, and fats. Although they are all equally important, they each contain different amounts of calories per gram, as seen below ^(11–13).

Therefore, you can eat a larger amount of protein and carbohydrates (in grams) than you can fats, for the same number of calories. With regards to creating a calorie deficit, studies comparing the same calorie consumption, with different amounts of fats or carbs (for example high fat and low carb, or vice versa) found no difference, and concluded that the weight loss was due to a calorie deficit only ^(10,14). Any additional increases in weight loss between diets were commonly from water loss ⁽¹⁵⁾. Additionally, similar studies concluded that eating a diet high in fat does not result in an increase in fat oxidation (or breakdown) when compared to a diet high in carbohydrates ^(6,7). In addition to efficacy, traditional weight management programs have followed low fat, higher in carbohydrates options because of the positive effect carbohydrates have on feeling full and satisfied ⁽⁸⁾. The research clearly shows that calorie deficits are the most effective method of any weight loss program, irrespective of the distribution of macronutrients ^(6–8). The most important thing when changing your eating habits, is to adapt to an eating plan that is achievable for you that you can maintain over time ⁽⁸⁾.

** a tip for calorie counting, food products in South Africa are commonly quantified in kilojoules, which can be divided by 4.2 to equal calories. Eg: 800 kilojoules = 190 calories.

To bring you the most evidence-based and cutting information in the fields of sports and exercise science and health, SSISA works alongside the UCT Research Centre for Health through Physical Activity, Lifestyle and Sport (HPALS) to disseminate the latest research.  HPALS research focuses on optimizing human performance and promoting health and well-being through physical activity, sports participation, healthy eating and good sleep hygiene.  Their work begins at the DNA, to the human performance laboratory and ultimately to the community. To read more about the Health through Physical Activity, Lifestyle and Research Centre, Division of Physiological Sciences at the University of Cape Town, please see the HPALS website or email Ayesha Hendricks for more information about applications for MSc/PhD research programmes.

To get in touch with the Sports Science Institute of South Africa Group for Research Implementation and Translation (GRIT) Research Consultants, get in touch with Warren Lucas at research@ssisa.com or call 021 650 5728 for enquiries. Read more about the SSISA GRIT Team here.

Yours in Sports Science,

Sports Science Institute of South Africa

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