So last time, in the guise of addressing the emails of an Internet crazy person and his assertion (so far as I can tell) that since calories are not an actual physical thing, the calorie model of body weight is not valid. I addressed a ton of different issues related to the concepts of calories. This included what they are, how they are measured, some serious pedantry regarding vocabulary and what they ultimately represent. The overall summary of what I wrote there is that while it is true that calories are not a physical entity (i.e. I can’t hand you a bottle of calories), they are valid in that they represent a defined measurable quantity (the generation of heat) related to how something that is real (i.e. food or nutrients) is metabolized within the body.
Basically calories, like other concepts such as watts or horsepower, are a semantic proxy for something that does exist. And while our language is imprecise it is the height of semantic stupidity to dismiss the concept based on that imprecise language. No, we do not “eat” calories, we eat food. Said food being metabolized in such a way as to produce heat which can be measured and defined in terms of calories.
So what does this all mean in terms of the calorie or energy balance model of bodyweight? Well, that’s what I want to talk about today. Some of this will tread the same ground as my article on Energy Balance and I’ll try to only summarize some of those concepts since the details can be found there.
Calorie/Energy Balance: Part 1
At the most basic level, we often talk about calorie balance which represents the difference between calorie intake (from food) and calorie expenditure (from a variety of components such as Resting Metabolic Rate, Activity and others). If calorie intake exceeds calorie expenditure, we gain weight/fat (I’ll explain this momentarily). If calorie intake matches calorie expenditure, no change occurs. If calorie intake is lower than calorie expenditure we lose weight. More technically we state that
Calorie Expenditure – Calorie Intake = The Change in the Energy Stored in the Body
Please note that that last term doesn’t even use the term weight but rather the change in energy stored in the body. This is explained in detail in the energy balance equation article along with more briefly below. But conceptually what it says that is calorie intake exceeds calorie expenditure by say, 500 calories, those 500 calories will be stored in the body. By extension, if calorie expenditure exceeds intake by 500 calories, those 500 calories have to be lost from the body (or the body has to adjust metabolic rate to compensate).
Now in keeping with the topic of Part 1 of this article, clearly this isn’t true in the sense that calorie are not a physical entity that can be stored or lost from the body. It’s not like money. If I make 10 dollars more than I spend, I can stick those 10 dollars under my mattress so that the IRS can’t find it. If I suddenly need to spend 10 dollars, I can remove it from the mattress for use. It’s a physical entity that can be stored or removed and calories aren’t.
But as I’ll show in a bit more detail, that’s because, once again the idea that calories are stored or lost from the body is just a semantic shorthand and proxy for what is actually going on since it saves a lot of tedious writing.
A Primer on Digestion
Remember that at the end of the day we eat foods. Foods contain the macronutrients in some proportion and amount. So what happens when we eat those foods? Well they get chewed into smaller bits which starts digestion before getting to the stomach where enzymes break them down into their constituent molecules. Some of the food will be lost to digestion and just show up in poop a day or three later but we only care about the food that is actually digested and absorbed.
After a bunch of toing and froing amino acids, glucose and fructose get dumped into the portal vein where they go to the liver while fat metabolism is way more complex than I want to describe (simply: most but not all fats go into the lymphatic system after being repackaged and eventually end up in the bloodstream). But basically the foods we eat release nutrients that enter the bloodstream where they can be used by the body. In the most general sense, there are two fates of nutrients in the body once they make it through digestion, they can be burned for energy or they can be stored somewhere (somewhere meaning a variety of places such as the liver, muscles, fat cells or the nutrients just sit in the bloodstream in a holding pattern).
So imagine that you have eaten an apple containing 25 grams of digestible carbohydrate. You chew it up (yummm), it hits the stomach where it is digested and, when all is said and done, now you have say 25 grams worth of carbohydrate molecules floating around in the bloodstream. Some of them will be metabolized by the liver or muscle and used immediately for energy and some will be stored for later use in the liver (as glycogen), muscle (as glycogen or Intramuscular Triglyceride) or fat cells (as triglyercide). Please note that carbohydrates are rarely converted to fat for storage.
Now it should be obvious that even if all of the macronutrients (protein, carbohydrate, fat, alcohol) we might consume can generate heat during metabolism (measured in calories) to the body, they aren’t identical in many ways. They differ in their primary roles in the body (protein being primarily structural in terms of building new tissues with carbohydrate and fat being primarily there to provide energy to the body), how they are metabolized and even in their relatively propensity to be stored or burned for energy. And it is, quite factually, somewhat simplistic to talk only in terms of calories for this reason.
More accurate by far would be to talk in terms of nutrient balance rather than calorie balance. This is conceptually identical with the difference between the intake and burning of a given nutrient determining how the amount of that nutrient in the body changes. So consider carbohydrate for example.
Carbohydrate balance = Carbohydrate Intake – Carbohydrate Oxidation
If carbohydrate intake is greater than oxidation, carbohydrate balance will be positive and carbohydrate will be stored in the body and the same holds for the other nutrients more or less. I say more or less as there are differences in how well or poorly the nutrients are stored or used for energy (i.e. there is no store of alcohol and it all has to be burned off) but just keep the general concept in mind. If intake is greater than burning, we get storage and if burning is greater than intake, we get removal. If they are equal, there is no change. The same holds true for the other nutrients
Protein balance = Protein Intake – Protein Oxidation
Fat balance = Fat Intake – Fat Oxidation
If you’re wondering about alcohol, well, alcohol is weird. There is no storehouse of alcohol in the body and it will always be burned off for energy. Read more here.
Calorie/Energy Balance: Part 2
So let’s put the above together. Let’s imagine that you are eating 101 grams of carbohydrate per day and burning/oxidizing 100 grams per day. By the nutrient balance concept, this means that 1 gram of carbohydrate will be stored somewhere in the body. And that’s what happens: since there is an excess of 1 grams of carbohydrate in the bloodstream that didn’t get burned off, it had to be stored. Now the body has one more gram of carbohydrate stored than it had a day ago.
Now, on the next day say you burn the same 100 grams of carbohydrate but only eat 99 grams of carbohydrate. The body will now pull that extra gram of carbohydrate from the day before put it through a whole bunch of metabolic processes that generate ATP which generate heat which can be measured in calories (alternately it may shift fuel metabolism to burn more of something else but I’m trying to keep this simplified here). We know that that during that process, enough heat will be produced from the metabolism/oxidation of the gram of carbohydrate to generate 4 calories (again, technically Kcalories)
Thus we can say, accurately now, that on the first day, the body stored 4 calories worth of carbohydrate. And on the second day it used 4 calories worth of carbohydrate. Again, this isn’t to mean that literally 4 calories were stored since calories are not a physical thing. Rather, just as with the overall term, speaking in terms of calories is just a proxy/semantic shorthand to represent what happened metabolically. The 1 gram of carbohydrate represents 4 calories of energy that can be generated when it is metabolized at some later point.
Store 10 grams of carbohydrate in the body and you have stored 40 calories worth of energy, if needed, the body can break them down and generate 40 calories of heat. It’s semantic stupidity, once again, to focus on the words ‘store 4 calories’ or ‘burn 4 calories’. No you’re not literally doing either but the storage of those 10 grams represents the 40 calories of potential heat energy which can be generated if they are oxidized metabolically in the body.
Calories are just a proxy for what’s going on.
Calorie/Energy Balance: Part 2
The same holds if you move up a level and talk about actual changes in tissue gain. Consider body fat. We usually say that an excess calorie intake of 3,500 calories is the equivalent of one pound weight gain. Now this is incorrect on a lot of levels mostly because people lost sight of what the original concept was meant to represent. Energetically, in terms of the potential heat energy that is represented, only one pound of pure fat contains ~3,500 calories.
The math works like this: 1 pound of body fat is 454 grams of tissue of which about 85-90% is actual stored fatty triglyceride (the other 10-15% is water and various cellular machinery but don’t get caught up on this). By math, 454 * 0.85-0.9 = 385-410 grams of stored triglyceride. Since each gram represents 9 calories of potential heat energy this is 3465-3672 calories, or about 3,500 calories. In contrast, one pound of glycogen is only about 1800 calories (454 grams * 4 calories per gram potential energy), one pound of muscle is about 600 calories of potential heat energy if it’s broken down for energy and one pound of water is zero calories.
And this is important because the proportion of glycogen, fat and muscle that is gained or lost during over- or under-eating can vary depending on a host of factors. If someone is gaining 100% fat and overeats 3,500 calories, they will gain one actual pound of weight. Muscle is way more complicated since, despite only containing about 600 calories of metabolizable energy, it takes ~2,700 calories to build that single pound.
Weight loss is a bit easier to explain this concept. Someone losing 100% fat with a 3,500 calorie deficit will lose about one pound of weight. But someone losing 100% muscle with a 3,500 calorie deficit will lose nearly 6 lbs of weight (3,500 calories / 600 calories metabolizable energy = just under 6 lbs). Someone losing a proportion of fat and muscle will fall somewhere between those extremes and you can lose or gain water with no deficit or surplus.
But once again, referring to this in terms of calories is just a semantic shorthand. We are storing nutrients or building tissue that is built from those nutrients. And those nutrients/tissue built from those nutrients represent a certain number of potential calories of heat energy when and if they are broken down for energy. It’s just faster and less tedious to write that “Someone has a 500 calorie surplus” than to write that “Someone has a nutrient surplus resulting in the storage of excess nutrients that represents the amount of stored energy that would raise a certain amount of water by a certain number of degrees C if the body were required to oxidize it for energy.”
It’s just semantic shorthand where the potential calorie generation of a given surplus is used as a proxy for what’s actually going on. We eat food which produced nutrients which can be burned or stored in the body and which represent the potential to generate an amount of heat that can be measured in calories if and when those nutrients are burned. So, as with the general concept, while calories are not technically a physical or actual entity, they are a proxy for something that is.
And while I in no way expect this to change the mind of the crazy Internet person that stimulated it (he tends to duck and dive when faced with facts he can’t ignore), hopefully everyone who read it burned lots of calories doing so.