Calorie is a calorie is a calorie. Part I

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.

Food calories

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.

A bit different from “fat=calorie in – calorie out”, don’t you think?

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.

Harris-Benedict equation
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.

DEXA scan
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.

Indirect 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???

Direct calorimetry
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?

In part II I will discuss the application of ‘calories in = calories out’ to weight loss.

17 thoughts on “Calorie is a calorie is a calorie. Part I

  1. Interested in part 2, have not read that many words in a row since I read Lances books 🙂

    • I had some pictures to break up all those letters. You should hear my other half pepper our conversations with biochemical terms nowadays. I feel so proud 🙂

  2. A great post Anastasia.
    The fitness industry lives by 4,4,9 and the advice based off it is put forward with such fact. I’m seeing/hearing 4,4,9 & BMR & GI & BMI all being sprooked as gospel. It’s becoming more and more obvious that these “rock solid” bits of science are less than perfect. Far less!
    Very much looking forward to part 2.
    Keep it up.

    • Thanks eljaes. I agree we rely on these numbers too much. “My TDEE is 2238, take away 500 calories for weight loss, minus 336 calories for the brownie which had a GI of 39, but I can burn it off on a treadmill where I spend 342 calories. And at the end of the day I press the button behind my right ear to shut the system down till the morning”.

  3. The thing that interests me is that the system has been around for 100 years, but has not been dis-proven (is that an incorrect hyphenation?), nor conclusive alternatives offered. So, as such all and any discussion about the value or validity of calorie measurement is purely that, discussion. Should we discount thermodynamics when ATP is part of the energy transfer between cells? Is this the evolutionary answer to overheating, i.e. we DO ‘burn’ or rather convert the food we eat, and thus thermodynamics are a valid proposition, but not in the way we currently comprehend the method in which we convert food to energy for our bodies….

    • It’s not that surprising that this model has been neither disproven nor an alternative system proposed, given the complexities involved and the realities of research funding. That doesn’t mean that the system is correct, despite what (I think?) you’re implying. All it means is that researchers have a hammer that they know how to use, and they keep finding lots of nails to bang with it.

      And as far as I know, no one is saying that thermodynamics should be ignored. What Anastasia is pointing out is that measuring food energy as heat does not accurately represent how our body disposes of food. When you think about it, it is rather sketchy to compare the conversion of matter to heat energy through one very simple chemical process, combustion, with what actually happens in our bodies when we digest food and turn it into usable energy.

  4. Love it!
    I now know my Basal Metabolic Rate for today, starting to feel empowered. What will it be tomorrow? My weight fluctuates a little every week so my clothes tell me.……
    excuse me while I enjoy my glass of full cream milk.

    • Enjoy your milk, Lyn. Don’t forget to correctly estimate 250 mls and check the calories for that particular brand 🙂

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  9. I’m just now about to read this, and eagerly so. But before I do . . . how might I get my hands on that metabolism diagram, say, as a poster?

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