Are All Calories Created Equal?

In today’s weight-conscious world, the calorie is so ubiquitous that it is almost the first thing to jump to the foreground of one’s mind when they hear the dreaded word, “diet.” However, despite the widespread awareness of the calorie in North America, and its place in weight-loss, very few actually know just what a calorie really is.

A Calorie, put quite simply, is a unit of measurement for energy. In the calorie’s relationship to nutrition, food labeling and dieting, the term “Calorie” actually almost always refers to the kilogram calorie or the kilocalorie, denoted by the symbol kcal. Scientifically, in the quantitative sense, the kilocalorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.185 kJ. As food is the body’s source of energy, the Calorie is used to measure the amount of energy the body extracts from food through digestion. In order to achieve weight-loss, dieticians recommend increasing energy expenditure (exercise) and reducing energy intake (calories consumed through food) to reduce body weight. This creates a caloric deficit, which causes the body to turn to its fat stores to make up the difference, leading to fat-loss.

Traditionally, the prevailing belief has been, "a calorie, is a calorie, is a calorie" regardless of its macronutrient source: carbohydrate, protein or fat. As such, a calorie extracted from carbohydrate would fuel the body in the same way as a calorie extracted from protein or fat. However, a new understanding of the human body’s metabolism of the calories derived from one food versus another is emerging, with a great deal of support stemming from the impressive results of protein rich carbohydrate-restricted diets, such as the Atkins diet.

Ultimately, a calorie is a fixed unit of energy, regardless of its source, and as such, can be perceived, quantitatively, as immutable, and the same regardless of its source. However, the human body is many things but an ordered set of processes that comply with quantitative measurements of energy. As such, when considering the impact of the calorie within the human body, one must carefully consider the source, which leads to the necessity for a qualitative assessment of the calorie, based on the foods from whence it is derived.

There are fundamental differences between the calories extracted from foods, based on the foods from which they are derived. As an average, there are 4 kcal of energy per gram of carbohydrate and per gram of protein, and 9 kcal of energy per gram of fat. Fat, therefore, is obviously a very calorie-dense and energy-rich substance. The body’s metabolism of fat is so efficient that for every 100 calories of fat ingested, only 2 calories are required for digestion. That’s 98% efficiency! These averages are determined through the process of direct calorimetry, whereby the amount of food energy contained within a particular food is measured by completely burning a dried sample of the food in a bomb calorimeter.

Yet, moving beyond the measurements of calorimetry, the issue of the calorie and its impact on the body depending on its source is complicated further by the concept of the “thermic effect of food. ” Also known simply as the thermic effect, it is the increment in energy expenditure above resting metabolic rate due to the cost of processing food for storage and use. The thermic effect refers to the level of difficult the body encounters in trying to extract energy (calories) from food through digestion. This effect varies substantially between the digestion of different foods. All of the body’s manifold processes require energy, even those occuring in a resting state, from brain function to heart rate, to the process of digestion; the very means by which energy is brought into the body. Every time food is ingested, the body must expend energy simply to break it down and extract its energy. For some foods this is a very easy process, as in the case of fat with only 2% of the energy ingested from fat lost in the process of its digestion. Fat therefore has a very minimal thermic effect. In the case of carbohydrate, the thermic effect is around 8%. Although absolutely essential to the body for growth, tissue regeneration and supporting the immune system, protein is hard to process and has the highest thermic effect, at around 30%. That means that in a diet based largely on protein, such as a high-protein low-carb diet, the body burns 30% more calories a day, simply on digestion, translating to an elevated metabolic rate! Obviously the equation is more complicated than the simple calories in versus calories out understanding. This helps to explain the signifanct weight-loss achieved by individuals following a low-carb lifestyle, as high protein consumption will tend to increase metabolic rate through its high thermic effect, while simultaneously eliminating additional fat storage through the avoidance of insulin triggering foods (like refined sugar), and burning the body’s existing fat stores through ketosis.

Therefore, in response to the question, “are all calories created equal?” the answer is as convoluted as the various processes that go on within the human body, as there are two correct, albeit diametrically opposed answers. From a qualitative perspecitve, yes, a calorie is a fixed unit for the measurement of energy, a constant, and therefore the same as another calorie, just as a foot of string is as long as a foot of wood. But, from a qualitative perspective, which considers the source of the calories in question, because of the way the body processes different foods in order to extract their chemical energy and the subsequent thermic effect of these foods, all calories are not equal, based on their impact on the body and their effect on body composition.