Are all Calories Created Equal?

Are 100 calories from chocolate the same as 100 calories from *insert any perceived healthy food here* Yes, those 100 calories are the exact same, however the foods constituents are different.


Let me explain.


The human body consists of many types of energy consuming cells within a variety of organs that work in systems to ensure we remain alive and function as required. These cells need energy to function thus we need to obtain energy from food to remain alive and function. Even though our body has natural reserves of energy (for example fat stores, muscle glycogen, liver glycogen) most of our energy comes from our nutrition. If we do not consume additional energy through our diet, the natural stores will deplete and we would ultimately die.


During exercise, energy requirements increase and energy provision can become critical to limiting fatigue. Different sports and different activities have different energy requirements and thus have different fueling needs and requirements. Energy expenditure refers to the energy expended (kilocalorie) per unit of time to produce power. Energy cannot be destroyed or created in an isolated system, it is transferred from one form to another. This is the first law of thermodynamics (law of conservation of energy).


When we expend energy for physical activity, we are not destroying it, the chemical energy from the breakdown of carbs and fats is converted into ATP (the energy currency of the cell) and then this produces mechanical energy (movement) and heat energy (body temperature rises). One calorie is the quantity of energy (heat) needed to raise the temperature of 1g or ml of water by 1 degree. Given that a calorie is relatively small unit, the term kilocalorie or Calorie is more often used to measure energy.


A kcal/Calorie is equal to 1,000 calories and refers to the heat required to raise 1kg of water by 1 degree. Thus if a slice of bread has 80 kcal, the 80 kcal would be enough energy to heat 80 kg of water by 1 degree. The food we eat contains energy in the form of macronutrients, of which there are three (protein carbohydrates and fats). The energy contained in these foods is stored in the chemical bonds of the molecules.


When these bonds are broken, energy is released and converted into another form of energy. For example, chemical energy and then mechanical energy.


Using a bomb calorimeter, it is possible to measure the energy content of foods. From studies using this method and after accounting for the process of digestion and absorption, the energy available from 1 gram of substrate of carbohydrate and protein is equal to 4 kilocalories or Calories and 9 calories per gram of substrate for fat.


In absolute terms and in scientific theory, a calorie is a calorie, period.

The first law of thermodynamics states that energy can neither be created or destroyed, it can only be transformed from one form to another. When we eat, we combust food to produce heat and for the body’s metabolic processes. The human body is constantly transforming energy.


Like when we record the temperature of the sun or measure the length of a nail, calories are just units of measurement! We can measure intakes through weighing and tracking food. Estimates of expenditures can be easily done using estimate equation. Measuring expenditures exactly is more complex process requiring expensive equipment.


Energy expenditure can be confirmed by tracking energy intakes and monitoring body weight change over time. The human body is very complex and while all foods contain energy (calories), they are metabolized in different ways due to their varying roles and constituents.


Different foods have differences in terms of the nutrients present, molecular structure, digestion, absorption, metabolism and so on. Different foods also have different effects on the central and peripheral regulation of energy intakes.


Below are 4 reasons to argue that calorie cannot always be seen as a just a calorie. In absolute scientific terms a calorie is a calorie, however the nuances associated with them makes it a complex topic.


Food quality is important, never forget that.


Metabolic Pathways

The metabolic pathways for protein are less efficient than the metabolic pathways for carbohydrate and fat. Protein contains 4 calories (kcal) per gram but a large portion of the protein calories are lost as heat when it is metabolized. Diet induced thermogenesis (DIT)/thermic effect of food (TEF) is a measure of the rise in metabolic rate when a food is ingested. It is the energy cost of eating. For fat this is 1-3%, for carbohydrate is 5% to 10% but for protein it is much larger, between 20% and 30%!


Protein, therefore, requires more energy to metabolize than fat and carbohydrate. This would mean that 100 kcal of protein consumed would end up as 70 to 80 kcal of stored energy. Compare this to fat and 100 kcal of fat would end up as 97 to 99 kcal of stored energy.


For this reason, calories from protein are often more favored during a weight loss phase. In calorie-controlled studies, where identical diets in terms calories but varying amounts of protein are given, the groups with the higher amount of protein tend to see better results for fat loss. This is put down to the high thermic effect of protein and the role of protein in maintaining lean muscle mass.



The desire to eat can change depending on the composition of a meal/food. Protein rich foods reduce appetite more effectively which may reduce overall energy intakes. Alternatively, it helps control eating behaviors and help maintain body weight and composition. If you were to try and overeat calories from meat compared to junk food, you would have an extremely hard time to continue eating beyond a certain volume of food. When foods are easier digested and have poor satiety, it is easier to overeat.


Different sugars are also metabolized differently and can have varying impacts on appetite. Glucose and fructose are two of the main simple sugars but are metabolized differently. Glucose can be metabolized by all tissues in the body but fructose can only be metabolized in the liver. After eating, fructose does not reduce levels of appetite-stimulating hormone ghrelin as much as glucose thus increasing opportunistic eating. Additionally, fructose does not stimulate satiety centers in the brain in the same way as glucose does.



Refined carbohydrates lead to faster and bigger spikes in blood sugar which can lead to cravings and increased food intake. This increased tendency to eat can be an indirect cause of weight gain. This is due to the subsequent increase in energy intake. It is not that carbohydrates are inherently fattening. This is why demonizing foods is not a good idea.


Refined carbohydrates are typically higher on the glycemic index and lower in fiber. This means more rapid digestion and absorption and quicker and larger spikes in blood sugar. When this rise rapidly declines, this stimulates the appetite center in the brain and results in cravings for more high carb foods to keep blood glucose levels stable. This is a major reason why food quality matters when dieting. Calories may dictate the weight loss but other factors impact on the journey. You always have unconditional permission to eat what you like but it must always be in your mind that food quality ultimately is essential is dietary success, regardless of the dietary goal.


Some foods affect satiety more than others. The satiety index is a measure of a foods ability to reduce hunger, increase feelings of fullness and reduce energy intake in the hours after a meal.


If you consume foods that are lower on the index, then you consume more in one sitting and have shorter times between eating episodes, again indirectly effecting total energy intakes.The opposite is true if you eat foods that are higher in the index. Meaning these foods would be more appropriate for weight loss when calories are controlled. Examples of high satiety foods include potatoes, eggs, meat, beans/legumes and fruits. Foods that are low on the satiety index are typically junk foods.



While all calories are equal, not all foods are created equal. Food quality is an unpinning pillar to dietary success. A diet with adequate energy, fiber, high in protein and with a lower intake of refined carbohydrates with respect to total energy intake will set you up for better hunger regulation and thus dietary control


Ronan O’Brien


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