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Are All Calories Equal?

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🕗 7 minute read


Quick Takeaways:


Although food labels consider all food calories equal, our bodies process energy very differently to the methods used in calorie calculation, so calorie-matched foods can affect our body weight variably.


The Atwater 4/4/9 method for assigning fixed calorie values to carbs, proteins, and fats isn't reliable due to foods structural differences and our bodie's complex metabolic processes. .


While calorie counting helps achieve body composition goals, food quality is crucial, and thus the approach of eating low quality foods that adhere only to a calorie goal is flawed. 



For a long time, I myself have adhered to the 'Calories in, Calories out' methodology, believing that as long as I consumed fewer calories than I burned, I would lose weight regardless of where the calories came from. This straightforward approach, often regarded as the ultimate rule for weight management, posits that all calories are created equal, regardless of their source. However, is this the truth, or does our body extract different amounts of energy from calorie equivalent foods?

Calorie definition:

Firstly, let's define a calorie. A calorie is a unit of energy. The calories on our food labels are actually kilocalories (Kcals), which is 1000 calories. A Kcal is equal to the energy required to heat 1 litre of water by 1 degree Celsius. These Kcals stated on our food labels were originally determined through a process called calorimetry, where a sample of the food is burned in a laboratory environment within a calorimeter. The heat released during combustion is measured, and based on the temperature change of the surrounding water, the energy content of the food is calculated. However, now they are calculated by the Atwater system which makes assumptions of the foods ‘usable’ energy based on the amount of carbohydrate, protein, and fats within the foods. The Atwater system assigns 4 calories per gram to proteins and carbohydrates, and 9 calories per gram to fats. However, the structure of foods are complex, and so are our body's metabolic processes, so are the calories between different foods actually equal when broken down in our body? 

Are all calories equal?

Most of us would agree that broccoli is a healthier choice than lard, but if you eat an equal caloric amount of both, is the effect on energy extraction, and therefore, our body weight the same? ‘Calories in Vs. Calories Out’ methodology would contend that an equal number of calories from both sources would have the same effect on our body weight, however recent research has put this idea under scrutiny. 

In a study comparing calorie restriction, two groups both reduced their caloric intake by 800 kcals, one group reduced dietary fat intake while another restricted carbohydrate intake, and both reduced their caloric intake by 800 kcals. Later, the same participants were used, but diets were swapped, so the group that previously reduced fat by 800 kcals, now reduced carbohydrate by 800 kcals and vice versa. The results found that despite equal caloric restriction, reducing the calories from fat yielded 41% more body weight loss over a 6 day period (1). While we don’t know if this would have been the case over a longer period, it does prove that the body processes calories differently depending on their source. 

In a two week twin study conducted by BBC panorama, two twins were assigned calorie matched diets, one consisting of whole foods, while the other was an ‘ultra processed diet’. The twin on the whole foods diet lost weight, while the twin on the ultrapro crossed diet actually gained weight, in addition to significantly elevated blood lipids (2)

This research sheds light on the complexity of metabolism, revealing that not all calories are processed equally by our bodies. 

A calorie of broccoli likely does not have the same metabolic effect as a calorie of lard. While both foods provide energy, the way our bodies metabolize them and the subsequent effects on our health are vastly different. 

While looking into this, I found a surprising and somewhat alarming lack of research comparing the effects of calorie matched diets between different foods. Potentially due to the power of the processed food industry in suppressing research of this nature. However, if more research does become available, it will likely become more apparent that the calories in these foods are not equal. 

Why are calories not equal? 

If we continue with the broccoli vs lard example, there are numerous reasons why their calories are not equal when metabolized in our body:

Digestive efficiency: Broccoli, for instance, is rich in fiber, vitamins, and phytonutrients. It’s fibrous nature means it takes time to digest. When consumed, it provides a steady release of energy and helps maintain stable blood sugar levels. Additionally, the nutrients in broccoli support various bodily functions, including improved digestion. On the other hand, lard is high in saturated fats and contains no fiber. Unlike the slow and steady energy release from broccoli as the body breaks down its fibrous structure, lard requires very little ‘work’ from our digestive system, and thus the calories within the fats may be extracted more easily and more rapidly, which may promote greater storage as fat.

Thermic effect of food (TEF): TEF plays a significant role in how we process calories. TEF refers to the energy required to digest, absorb, and metabolize food. Foods like broccoli have a higher TEF because they require more energy to break down, meaning fewer net calories are absorbed. In contrast, lard has a lower TEF, resulting in more net energy being extracted from the food, potentially increasing the amount stored as fat.

Hormonal responses: Hormone release varies in response to different types of food. Foods high in refined sugars lead to blood sugar spikes, promoting insulin resistance and affecting other hormone release, potentially impacting fat storage. Therefore, in the long run, hormonal factors could result in a significant difference in our body fat levels between calorie-matched diets.

Shortfall with the Atwater system: The Atwater 4/4/9 Kcals system itself has limitations in that it assigns equal calorie values to all carbohydrates, proteins, and fats, irrespective of their structural diversity and the composition of the foods they are part of. In reality, the caloric availability of these macronutrients varies depending on factors such as structural complexity and their source (3)


By the laws of thermodynamics, it's undeniable that consuming fewer calories than we burn leads to weight loss. However, determining precisely how much energy our bodies extract from the food we eat isn't as straightforward as a food label would suggest. The human body metabolizes food through a far more complex process than a label amount can assign. As a result, 100 calories from lard and 100 calories from broccoli may have very different effects on body weight. And for this reason, the 'Calories in, Calories out' model is somewhat of an oversimplification of human metabolism. While maintaining a calorie deficit is crucial for weight loss and is the best metric for tracking our food intake that we have, the type of calories consumed plays a critical role in determining overall health outcomes, and may have a significant impact on the actual energy extracted by the foods we eat labeled with 'equal calories'. By prioritizing whole, nutrient-dense foods like broccoli over calorie-dense, nutrient-poor options like lard, we can better support our bodies' metabolic processes and achieve more sustainable health goals.


  • Caloric Intake vs. Metabolic Response: Despite the 'Calories in, Calories out' model, recent studies suggest that we extract and store different amounts of energy from foods with equal calorie intake due to differences in digestion, thermogenesis, and hormonal responses.
  • Complexity of Food Metabolism: Assigning equal calorie amounts to all macronutrients overlooks the structural diversity of foods and the intricate metabolic processes in the body, leading to inaccuracies in caloric estimation.
  • Impact on weight and health: Research findings indicate that diets composed of whole, nutrient-dense foods promote weight loss and metabolic health, while ultra-processed diets may lead to weight gain and adverse metabolic effects despite being ‘calorie matched’, highlighting the importance of food quality over mere calorie counting alone.
  • Factors Influencing Caloric Extraction: Digestive efficiency, thermic effect of food, and hormonal responses vary between different types of foods, influencing the amount of energy extracted and stored by the body, emphasizing the need for nuanced dietary considerations beyond calorie counting.

About the Author:

Joe is a certified personal trainer, strength and conditioning coach, and nutrition coach. While studying sport and exercise science, Joe's main focus has been on human physiology and performance. Joe has helped numerous clients achieve their health and fitness goals by applying research into practice to support their physical and overall well-being.



1) Hall, Kevin D et al. “Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity.” Cell metabolism vol. 22,3 (2015): 427-36. doi:10.1016/j.cmet.2015.07.021

2) BBC Panorama "How harmful can ultra-processed foods be for us?" BBC News

3) Novotny, Janet A et al. “Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets.” The American journal of clinical nutrition vol. 96,2 (2012): 296-301. doi:10.3945/ajcn.112.035782


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