Calorie counting might appeal for all sorts of reasons. Some might want to use it to help with weight loss, whilst others might want to ensure they are obtaining sufficient energy for training sessions or competition. The following is an overview of the limitations of calorie-counting.

 

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Energy Expenditure

In order to exist and to perform its tasks, every cell in our body requires energy. The total sum of all this energy usage - of every cell in our body over the course of a day - is our daily metabolic rate. If calculated based upon our body mass, composition and gender alone, this is our basal metabolic rate. If taking into account our typical lifestyle and physical activity habits it is our resting metabolic rate.

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Metabolic rate is traditionally estimated based upon gender, body weight, composition, and lifestyle and physical activity habits. As estimates, they cannot be taken to be accurate to the kilojoule (KJ) or kilocalorie (Kcal), but serve as a guide. The most accurate way of measuring metabolic rate requires the use of a metabolic chamber, which is a room that measures gas exchange (oxygen and carbon dioxide) within the chamber, and can thereby measure energy usage directly.  However, this only represents energy usage during the time in the chamber. Direct measurements can also be made using gas measurements for a few minutes at a time, but these are necessarily less representative than longer sessions in metabolic chambers.

 

Energy expenditure will vary for an individual as their weight and lean mass changes, and as their physical activity habits change. There are daily fluctuations in metabolism due to variations in sleep patterns, dietary intake, activity and hormones. As a result, even if metabolism were calculated accurately on any given day, it will only represent that one day, not necessarily being indicative of metabolism from day-to-day or week-to-week. Any calculations of metabolic rate are a crude estimate at best, no matter how sophisticated the calculations.

 

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All Calories are not Equal

 

Calories (Kcals) are a common measure of both energy intake and energy expenditure. The calorie number used to represent daily energy expenditure is the sum total of all the energy reactions in every cell in the body over the course of a day. The calorie number used to represent the energy content of foods is estimated based upon the total amounts of protein, carbohydrate and fat within the meal.  Calculations made some decades ago determined the approximate energy value of carbohydrates and proteins at around 4 kcals per gram, and of fat at around 9 kcals per gram.

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The numbers of calories per gram of macronutrient were originally calculated using a process called bomb calorimetry, in which foods are burned and the energy released is measured.  Although this system is valid as a means of representing energy content of a substance, it is not valid to suppose that calories calculated through bomb calorimetry are the same as calories utilised within the body.

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The cells of our body do not 'burn' foods. Once food has been ingested it is subject to chemical and mechanical breakdown throughout the digestive tract until it is absorbed. The liver will process some foods further, following absorption, and reactions may also take place in the blood, as the substrates are transported to cells for use. Within the cells the substrates are broken down further via chemical reactions, none of which involve anything approaching bomb calorimetry. We therefore have a measure of energy expenditure and another of energy intake that are both given as calories, but what those calories represent is very different.

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Different cells of our body use substrates differently, and substrates would have been processed to different extents before being available for use. It is not sufficient to group all carbohydrates together, as some can be used by some cells and others not. There is still no evidence that fructose can be used by muscle cells, for example, whereas glucose and many other carbohydrates are. To suppose 200 kcals of fruit delivers the same amount of usable energy to body cells as 200 kcals of chicken, 200 kcals of chocolate, or 200 kcals of compost seems unlikely, especially as the supposition is based upon what happens when each is burned. 

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Proteins are made up of amino acids and fats are made up of fatty acids, all of which have their own uses and reactions within the body. To assume any gram of protein will deliver about 4 grams of energy, and any gram of fat will deliver 9 grams is not realistic. It is simplistic to a fault on many many levels of basic chemistry and biology. So, calorie numbers from different foods are not necessarily equivalent, because they will be processed and utilised differently.

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What If We Knew Precisely How Much Energy We Consumed?

So, let us imagine some very busy scientists had accurately calculated the precise number of calories contained within every food we might want to eat, and those calories were precisely representative of their different fates within the body. In other words, 1 kcal of energy intake would represent 1 kcal available for use by the body. Calorie counting still would not work, and for one simple reason.  We defecate. 

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That we defecate is the clearest possible indication there can be that we do not absorb 100% of the food we ingest. Now, not all of our faeces is food matter as much of it is bacteria from our guts, but it is nevertheless a considerable amount of energy that is lost. Food has been broken down throughout our digestive tracts, and most of it has been absorbed, but certainly not all of it.  Some potential nutrients cannot be absorbed because we lack the enzymes , bacteria or gut length to break the food down. There will be some nutrients we could absorb into the blood but do not, perhaps because we ate so much that it moved passed the region of the gut where it should have been absorbed, or insufficient amounts of gastrointestinal secretions were available to break it down. Whatever the reason, we do not absorb 100% of the nutrients we consume, and the clear and obvious indication of this is that we defecate.

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Although it is possible to calculate energy lost in faeces, and indeed there are scientists who measure such things, it is variable according to dietary, lifestyle and hormonal habits. So, even if we had a willing scientist ready to measure the energy content of our faeces it would still be estimates, not least because energy itself would still not be based upon actual substrate processing by cells.

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In addition, even nutrients absorbed into the blood are not utilised exclusively in energy reactions, as they are also required for building, rebuilding and repairing cells (all of which requires energy in addition to the structural components). In other words, will the fats you eat be utilised in energy reactions, in the building of cells or in healing processes? Will proteins be broken down for use as energy or for structural components of muscles, immune or other cells? Without knowing this, how much food do we need purely for energy reactions, and how much do we need for structural and other functional purposes? How can we possibly know how much we will need as our physical activity habits vary from one day to the next?

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Summary

If energy intake matches energy expenditure we maintain weight, with weight being lost when we undereat and gained when we overeat. Unfortunately it is not possible to calculate energy expenditure with any accuracy on a daily basis, and the means used to measure energy intake are hopeless (requiring expensive laboratory facilities). As energy intake is calculated based upon bomb calorimetry measurements of basic ingredients (not the whole processed mass), calorie counting is actually quite crude. It is fair to say that a meal apparently containing 1000 kcals will deliver more available energy than a meal of 100 kcals, but the numbers themselves are estimates not representative of what a calorie actually means to the cells of the body.  Overall, it is better to keep things simple: if body weight is being maintained, there is energy balance.  Weight gain and weight loss represent imbalances between energy needs and supply.  There really is not a benefit in making this issue more complicated, mostly because it becomes too complex to be meaningful.

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For a deeper understanding of nutrition from the perspective of our evolutionary past, I wrote:

Human Evolution, Diet and Health: The Case for Palaeolithic Nutrition

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For my interpretation of a healthy diet for anyone, I wrote:

Our Natural Diet: Optimal Nutrition for Health, Looks and Life

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