What is a calorie? In the world of physics, a calorie is the amount of energy required to heat 1 gram of water by 1 degree. In nutritional terms, calories mean food energy, energy in turn signifies how much we burn and how much we store. Somewhere in the equation is the first law of thermodynamics, which is generally interpreted as: calories in must equal calories out. If we apply this law to human metabolism we get the conventional wisdom: to lose weight you need to eat less and exercise more. No-brainer, right?
Let’s explore these concepts in a little more detail. I am no physicist (I can just see my physics lecturer nod fervently to this statement). If I can slowly walk my subtle-as-a-sledgehammer brain through these abstract concepts, so can you.
How is the caloric value of common foods determined?
The well-known 4,4,9 calorie values per 1 gram of carbs, protein and fat respectively, were first derived in the early 1900s by Wilbur Olin Atwater. He used a machine of his own invention, the respiration calorimeter, to measure the heat production of foods in the typical diet of the beginning of the 20th century. I would hazard a guess that our foods today are somewhat different to the ones Atwater happily incinerated in his lab. The unit “calorie” was used to measure the amount of heat energy released. Proteins, carbs, fats, alcohols, polyols and fibre were all allocated their average values and the rest is history. Apparently the use of Atwater system “has frequently been the cause of dispute, but no real alternatives have been proposed”.
Today most food manufacturers use the Atwater system to calculate the total calories in their product. Another less commonly used method is calorimetry, similar to Atwater’s experiments. The food in question is combusted with oxygen and the amount of heat energy generated is measured. Here is the first hiccup. Human bodies obviously do not “burn” food or generate that much heat. Getting really hot is a major problem for our brains as they tend to get delirious. Instead our bodies generate ATP, the carrier of energy required for all biochemical reactions in your body. Carbs, protein, fat, alcohol – all have the potential to generate ATP.
Why the potential? Because not every morsel that goes through your mouth is used for ATP production. Protein can be broken down to amino acids which can be used as energy. However, they are preferentially used up for cellular repair or the building of new proteins. So if your body is in the state of growth, illness, recovery from your latest unsuccessful attempt at a pull-up (guilty as charged), you might not be “burning” any of the protein that comes from your steak. In the same way, glucose can be used to replenish your glycogen stores in
the liver and skeletal muscles. So if your glycogen stores are empty after a workout, the glucose will be used preferentially to refill them. Fatty acids are used for building cellular membranes and cholesterol for hormones.
Basically, your body is not a furnace and your metabolism is not a ritual burning of a piece of cake. Biochemists have neatly summarised this process in the diagram below. I call it the WTF diagram.
But can’t we average all these processes and come up with a number of food calories required for everyday function? What about Basal Metabolic Rate, the BMR? Someone somewhere has worked it out. I can google the formula! And once I know it, I can just subtract 500 calories daily and it will result in half a kilo loss per week, right?
The BMR story
A nerd in me is very attracted to this simple idea. Calculating an easily predictable weight loss value is so elegant. And knowing your own special number, your Basal Metabolic Rate, is almost empowering.
The easiest way to do it is the Harris-Benedict equation suggested in 1919 (that’s before we knew about DNA). It estimates the strongest predictor of your BMR = your muscle mass, by taking into account your age (because older people always have less muscle), your sex (because women are always the weaker sex) and your weight (because your weight always indicates how muscle you carry). I don’t know about you but I have a few issues with some of these assumptions.
The next way to do it is to use the actual lean body mass measurement, either by DEXA scan or by the “fat scales”. I will not go in why the “fat scales” is a complete waste of your time and money. Let’s assume you do a DEXA, an x-ray imaging tool which will tell you your exact body composition. You also receive this charming picture. While it may tell you how much muscle you have (and diagnose osteoporosis, which was the initial application of this test), it won’t be able to tell if you are sick with pneumonia, if you are taking antidepressants, if you have overactive thyroid, insulin resistance, phaeochromocytoma and a host of other things which will determine your own fuel utilisation rate, a.k.a what your body does with your food.
Because your metabolism is not regulated by your muscle cells.
It is regulated by hormones.
Thankfully, in the research world there are two ways to actually measure the metabolic processes which happen in an individual organism: indirect and direct calorimetry.
Most energy-generating pathways in the body require oxygen and result in the production of carbon dioxide (CO2) as a by-product. Therefore if I fit you with a special gas mask, make you breathe (this shouldn’t be too hard) and then measure the amount of oxygen in and CO2 out, I can get an estimate of how much ATP you are generating, and also how much fat vs glucose you are using for its production. This “metabolic monitoring” is also used in Intensive Care Units to work out the energy requirements of critically ill patients. It is obviously a very involved process. Nevertheless, here is a study which address the potential pitfalls of interpreting measurements without taking into account the hormonal environment.
Are you serious? I am breathing through a machine and you are still not a 100% sure how many calories I need???
This is the final frontier, the golden standard. The description of this process can be summarised in just a few words: “secure chamber”, “constant temperature”, “air seal” and my favorite, “the panic button”. Yes, your whole body has to be inside, sometimes for several days. A couple of problems: first, it only measures your metabolic rate at that particular time. I hope we all agree that metabolism is not a static number, multitude of variables can affect it from day to day. The second problem is that it is a tad inconvenient.
So what are we left with? Helpful websites offer an easy formula to calculate your BMR measurement, your TDEE (Total Daily Energy Expenditure) measurement, your AUN (Another Useless Number) measurement. Then you subtract 500 calories (what a lovely even number) from your total, and that should give you a 1/2-1 kg loss each week. Conveniently, this rate of weight loss seems to be fairly similar no matter which diet/exercise regime you choose.
It might sound ridiculous that somebody might rely on a formula developed in 1919 to estimate the BMR of an average healthy person under normal conditions to apply in 2011 to an overweight individual. Followed by the advice to reduce their food intake by checking the caloric values, found by incinerating these foods under lab conditions. Finished by the recommendation to expend more energy by exercise while checking the number of calories supposedly burned on a treadmill machine where you input your weight and sex.
In other words, the absolute number of calories is about as useful as knowing how much the loose change in your wallet weighs in poods. The units are useless and how is it relevant to how much money is there anyway?