Macro-nutrition, Exercise & the Calorie

The Big Idea Here

It would be asking too much to expect you, gentle browser, to read this entire page without some clear idea of what you’re getting yourself in for.

I imagine myself limited to a text message or two to convey the Big Idea that is the simple essence of what follows. So here goes.

The Calorie is a perfectly stupid way to measure our food intake. To add up the Calories in carbohydrates, fat and protein in our diets suggests that they are substitutes for one another; and that idea is not just wrong but dangerous. If you were to take in nothing but protein, and expect your body to metabolize it for energy, you would likely kill yourself. Most low-carb high-protein diets actually give you your energy needs from fat, and even that is not without its risks. Protein and fat intake are, by and large, to replace protein and fat lost in cell death. Carbohydrate intake is to provide for energy needs. To measure protein and fat intake as if they were there to provide energy, and can substitute for that, is a dangerous point of view.

If you want to change your body, to add muscle, to lose fat, to become stronger, or what have you, then you need to consider each of the main components of diet as well as how your energy consumption in exercise impacts the ways in which your body will use protein, fat, and carbs.

Read on and learn more.

Macro-nutrition, exercise and the Calorie


Macro-nutrition means the bulk of what we eat; or, what we eat in bulk. This is often just referred to as diet, but I want to be a little picky here and draw a distinction between macro-nutrition and micro-nutrition. They are both components of what I’d call diet. That term, to me, includes everything that we consume; and not just those items that we eat in large amounts. What do I mean here? Well, it’s pretty simple. If I eat an orange, I’ll get 85 Calories. This is from roughly 21 grams of carbohydrates (4 gm as dietary fiber and 17 gm as sugars), as well as 2 grams of protein. That’s macro-nutrition from the orange. I’ll also get about 60 milligrams of Vitamin C and 326 milligrams of sodium. That’s micro-nutrition from the orange. I looked this all up on the Interwebs, so it has to be about right. If the units for the nutrients are in grams, then it’s macro-nutrition. If they’re in milligrams or micrograms, then it’s micro-nutrition. If we’re talking about “protein” or “carbohydrates” or “fats” or “fiber”, then we’re talking about macro-nutrtion. If we’re singling out a particular molecule as a nutrient, like a form of Vitamin C (ascorbic acid or a mineral ascorbate), then we’re talking about micro-nutrition. OK. Cool. We’ve got some terms out of the way.

Rules of thumb

Here are some reasonable rules of thumb concerning how much of each kind of macro-nutrient you need on a daily basis.  [By the way, I got this particular rule of thumb from Cardio Sucks! by Michael Matthews. It’s in the chapter on How to Eat Right.]




1 gram per pound of body weight

1.5 grams per pound of body weight

0.25 gram per pound of body weight

4 Calories per gram

4 Calories per gram

9 Calories per gram

What it works out to

So for a 200 pound man, this works out to 200 grams of protein, 300 grams of carbohydrates, and 50 grams of fat per day. That gives us all we need to work out the Calories in this suggested model of macro-nutrition: (200 gm protein x 4 Calories/gm protein) + (300 gm carb x 4 Calories/gm carb) + (50 gm fat x 9 Calories/gm fat) = 2,450 Calories This even makes some sense. Or does it? There’s something a little bit wonky here.


The calorie was originally devised as a unit for measuring heat, a process known as calorimetry. The word “calor” is Latin for heat; and calorimetry is all about measuring heat. Back in the late 1700s and early 1800s, it was not yet appreciated that heat is just another form of energy. What had been noticed was that there was a form of heat called “latent heat” that we now recognize as energy being stored as potential energy. The is certainly the case with the food we eat. It has latent heat. It’s not that an orange that we eat is hot; but the sugars inside that orange have stored energy. We can access that energy by metabolizing them; more specifically, by breaking them down into their component parts. There are really two words and two meanings when we talk about “calorie“. A calorie with a small “c” means a specific unit of energy in science; namely, the amount of energy needed to raise one gram of water by one degree Celsius at standard temperature and pressure. This is not the unit being applied when we talk about food. That unit is the “Calorie” with a large “C”. It is 1,000 calories. It is also sometimes called a “kilocalorie”. This “Calorie” is the commonly used unit of energy we get from our food.


This notion of metabolizing our food brings us to a fundamental aspect of just about everything our bodies do. There are two key processes going on in parallel within us at all times: we are either breaking things down or we are building things up. Metabolism refers to these two parallel processes. Breaking things down is called catabolism. Building things up is called anabolism. Metabolism means any change in the chemistry of our food or our bodies. Catabolism is that aspect of digestion and respiration (and our immune system, more of this later) where macro-nutrients are broken down into smaller component elements usually with a release of energy. Anabolism is that aspect of digestion and respiration and growth and immunity in which our bodies take component elements and use them to construct more complex molecules and tissues; for example, building new proteins up out of amino acids. Anabolic processes usually require energy in order to proceed. Let’s think about this just a little. If our bodies consume some starches, which are forms of carbohydrates, then our digestive tract will catabolize those into glucose, the most basic sugar. Starches are just long chains of glucose. Another name for these long chains of simple sugars (saccharides) is “polysaccharides”. Glucose is a monosaccharide. Table sugar is a disaccharide, meaning that it is built out of two simple sugars. Any polysaccharide can be catabolized into monosaccharides.

The process of digestion is pretty much one for one; that is, 1 gram of carbohydrates in will put 1 gram of glucose out into the blood stream. Based on how much glucose is in the blood stream, the pancreas releases insulin, which opens up cells to receive those glucose molecules.

In a similar way, proteins taken in can be catabolized into their component amino acids. Complex fats can be broken down into their component essential fatty acids.

Cellular respiration

Now things get a little complicated inside the machinery of the cell. I am going to gloss over the details for now, and give those to you in another page. The key idea I want to give you is that the glucose that I’ve carried as far as getting it into a cell in your body, is now going to go through the process of cellular respiration. That means, in essence, that it is going to be combined with oxygen in order to deliver energy to the cell; it’s going to be “burned” to release its latent heat. Of course, it’s not really going into a fire. It is going to be manipulated in some very complex chemical processes. One of the most significant of these is called the Citric Acid Cycle (aka the Krebs Cycle). Another is called the Electron Transport Chain. Here’s a schematic representation of what’s going on here:

Catabolism and the Citric Acid Cycle

Catabolism and the Citric Acid Cycle


Catabolism schematic” by Tim Vickers, vectorized by Fvasconcellosw:Image:Catabolism.png. Licensed under Public domain via Wikimedia Commons.

What this schematic shows is that proteins, polysaccharides (also known as carbohydrates) and fats can all be converted into something called “AcetylCoA”, which then goes into the Citric Acid Cycle, and which (after some other mysteries) takes ADP (adenosine diphosphate) and turns it into ATP (adenosine triphosphate). Voila. Simple, huh?

These processes occur within a part of the cell called the mitochondrion; and I’ll have more to say about all of this later. For now, it is sufficient to recognize that these incoming glucose molecules are oxidized and the output of the process is a basic “coin” of energy that is used in all sorts of reactions in the body, from making new proteins up out of amino acid building blocks to powering the muscles. This “coin” of energy is called adenosine triphospate or ATP. For each molecule of glucose coming in, the process of cellular respiration could produce, in theory, 38 molecules of ATP. In practice, it takes some energy (for which read other molecules of ATP) to get this process running; this consumption of ATP to produce ATP from glucose knocks the number back to 30. Actual measurements show that a practical number averages out to around 29.85 molecules of ATP for each molecule of glucose.

Energy in ATP

Well, this is interesting because we can find the energy released by ATP. This is a very simple case of anabolism and catabolism. In cellular respiration, there is an anabolic process of adding a single phosphate group to adenosine diphosphate (ADP) to make adenosine triphosphate. This consumes energy and it is how the energy from oxidizing the original glucose molecule is stored. Later, out in the apparatus of the cell, this same energy is released by catabolizing the ATP to produce one ATP and one phosphate group. So, what is this ATP coin worth? The answer is that it varies but a good round average number is a really small number. Of course, we are talking about molecules here and so we have to add them up to get any appreciable values. The number is, in scientific notation, 2.3x10-23 Calories, and that’s not much. [It’s 23 divided by 1 million billion billion, if you’re not familiar with scientific notation.] But that’s OK because we’ve already seen the answer in terms of the energy we get from one gram of carbohydrates; namely, 4 Calories (well, actually closer to 3.75, but let that go). OK. So there’s this cellular respiration machinery in the cell that takes in glucose that comes from the digestion of carbohydrates and turns it into all of these little ATP molecules that float around carrying some fraction of the energy from the glucose. [It might be worth noting here that if we actually totally burned up glucose like we could in a fire, we’d get a lot more energy by about 175 times. That is, our bodies extract only a small fraction of the total chemical energy available in carbohydrates. But maybe catching fire would be a bad side effect.] But is that what happens with proteins and fats too?

Energy in proteins and fats

It turns out that the same cellular respiration machinery that I just described can also take proteins and fats and use their chemical energy to produce ATP also. The schematic above shows these options. The precise details are a little different than for glucose. Fat and protein come into the citric acid cycle in slightly different places than glucose does. There are some other differences in terms of whether the processes are “reversible”; that is, can the cycle run backwards and produce glucose or fat or protein and use up energy instead of storing it. These are nice details to address later. For now, I just want to point out that just because there is energy available from catabolizing protein in cellular respiration, that’s probably not what your body does with its protein intake in most circumstances. In fact, if you exercise, most of the protein that you take in will go to the anabolic process of building new muscle tissue. Lack of sufficient protein in the diet is rare in developed countries, but it has a name, Kwashiorkor. If you read that article (and I suggest that you do), note the reference that

Protein should be supplied only for anabolic purposes. The catabolic needs should be satisfied with carbohydrate and fat. Protein catabolism involves the urea cycle, which is located in the liver and can easily overwhelm the capacity of an already damaged organ.

Hence, even in a sedentary person, let alone an athlete, the protein component of macro-nutrition is not intended for energy production (i.e., “catabolic needs”). To measure protein intake in grams makes sense. To measure it in Calories is simply confusing because it suggests that your body is going to employ that protein for energy production; and that is not only a bad idea, it can be fatal. It seems that more and more people are running marathons. I know because I ran my first marathon myself last summer (July 2014) and I researched the activity a lot. In almost every major marathon event these days, it seems that one or more participants succumbs to rhabdomyolysis, a form of liver failure brought on by extreme exercise in which the body depletes its usual energy resources of glucose, glycogen, and fats and then begins to consume its own protein stored in muscle tissue. This usually happens in poorly prepared individuals who have not conditioned themselves properly, who fail to consume enough water and food during the marathon, and who have also been using anti-inflammatory drugs like NSAIDs. All of these factors contribute to depletion of the real energy resources of the body and force it to begin to consume its own protein stores for energy. The metabolic waste elements of catabolizing protein accumulate in the liver and, in extreme cases, can be fatal.

The same is true of much of the fat that you consume. Fats will be used to produce the lipid walls of the cells of new tissue. They will be used to produce cholesterol and hormones. They are used to produce Vitamin D, with sufficient sun exposure. They are used to transport fat-soluble vitamins through the blood stream.

This is unbelievably critical to become aware of. Just because it is possible to write down the potential energy that could be produced from a gram of protein or fat, chances are your body will actually expend energy to use much of that protein or fat to build new tissue if you are doing a sufficient amount of exercise. This is because it will take more energy to anabolize these macro-nutrients that will be released by catabolizing them. In this regard, a Calorie is not a Calorie. When I worked out that rule of thumb for the daily diet of a 200 pound man, all I did was compute the energy in Calories that could be obtained if and only if all the carbohydrates, proteins, and fats went through cellular respiration. However, if the physical body consuming that diet puts the protein and fat to anabolic uses (rebuilding tissue), then the net energy taken may only be what comes from the carbohydrate part. And even some part of the energy in the carbohydrates will go to what’s needed to anabolize the protein and fat to produce new tissue. This is true even for the sedentary person, but is especially true of someone doing at least moderate exercise on a consistent basis.


Image above by Robeter at en.wikipedia [Public domain], from Wikimedia Commons


Why would the macro-nutrients of two, let’s say, 200 pound men be used in very different ways? Let’s say that one is a lean, mean fighting machine of an athletic 6’4″ male who works out 6 days a week. Let’s say that the other is a completely sedentary 5’7″ overweight office worker. The athlete will actually get a smaller net of energy (measured in Calories) from exactly the same diet. Why? Because the process of exercising at his level will cause his body to utilize a major fraction of the grams of protein and fat in that diet to build up new muscle tissue and to repair the small micro-tears in his muscles that happen while working out. It may seem that, for example, 200 grams of protein a day is more than enough, especially if even the most athletic person is just maintaining a stable weight. The sedentary office man will not have the same demands on the protein and fat components of the diet. His body is not under the same demands of building and rebuilding his cells as the athlete’s is. As a result, his body is much more likely to absorb only the protein needed for the more modest amount of tissue rebuilding that he requires, and waste the rest. His body will use whatever energy value of the carbs and fats he consumes for his immediate needs. The excess, if any, will go to into storage in adipose (fat) cells. If a molecule of ATP is a “penny” of energy, then a molecule of fat is a “dollar”.

How do some people eat so much and stay thin

While others keep dieting and cannot lose weight? Well, some people are young and still growing. For a teen-ager or young adult, much of their protein and fat intake will be used just like that in the athlete; it will go to make new tissue. The question of how much protein is necessary for top athletes is a matter of active research. An exemplary paper on the matter is due to Tarnopolsky. [You’ll need a PDF viewer for that article. Oh, and some scientific background.] Let me summarize the bottom line for you: top male performance athletes need around 1.6 grams/kilogram of body weigh per day (females need less 1.2/gm/kg/day); the average human gets by nicely with 1 gm/kg/day. Factoring in the ratio between kilograms and pounds (2.2:1), we see that this current recommendation for protein for top athletes comes out to about 3.5 gm/pound/day, which is something less than that Cardio Sucks! rule of thumb. Speaking about the average Joe, protein requirements would go down to 2.2 gm/pound/day or nearly half that earlier rule of thumb. [I will say that the Cardio Sucks! book is targeted at readers who at least want to become athletic; so perhaps the author simplified his rule of thumb for people who were headed in that direction.] Read more of Tarnopolsky and some of the current research on diet for performance athletes, and you’ll find that this 1.6 gm/kg/day is to be around 10% to 15% of total daily energy intake. Of this total protein intake, perhaps no more than 6% goes into cellular respiration. This suggests that 95+% of protein consumed is being used for tissue building and rebuilding. That is exactly consistent with a concern about rhabdomyolysis in endurance athletes. One wants to supply their energy needs from carbs and fats and not from protein.

For a 200 pound (90 kg) athlete, we’d have a revised recommendation of 144 gm of protein (versus the earlier 200 gm) and this would be part of a total diet of around 4,000 Calories (much more than the 2,450 Calories we worked out earlier). What about the carbohydrate component of diet then? Take a look at the following chart, which I have created based on data here. [Click on the image for a larger version.]

Carbohydrate requirements

Carbohydrate requirements

This graph is pretty interesting since it gives you a pretty good idea of what the incremental carbohydrate requirements are to fuel any given level of exercise. We could also easily translate this graph into something more specific for our hypothetical 200 pound male. Here’s what we get:

Calories from carbs for 200 lb man

Calories from carbs for 200 lb man

What comes out of this graph is also pretty interesting. It shows that our 200 pound man needs 1,600 Calories from carbohydrates per day just to sit around and another 12 Calories per minute from carbs to work out. That would suggest that a maintenance diet for a 200 pound man already at an ideal weight (whatever that is) who wasn’t doing much exercise would include 1,600 Calories from carbs, which comes to around 400 grams of carbs. This is quite a bit more than what that Cardio Sucks! rule of thumb gave us (it was 300 grams from carbs if you don’t want to look back). This little exploration is giving us more intake from carbs and less from protein than that earlier rule of thumb.

What about fat?

Good question. What do we use fat for? Several things: energy storage and delivery; transport of fat soluble vitamins (D, E); synthesis of vitamin D, cholesterol and steroid hormones;  and cell membranes. If you actually look around, you will find a broad range of opinions on the fraction of diet that ought to be derived from fats. One fairly standard model says 60% from carbs, 15% from protein, and the rest, 25% from fats. A completely contrarian model is the “ketogenic” diet, the most well-known version of which is from Dr. Atkins. [Ketogenic means that the idea is to produce ketones. The idea is that on a low-carb diet, your body will produce ketones from fats and these will replace glucose in the blood. If you stay on the diet for a week or more, your body chemistry is supposed to change to more easily convert fat to ketones. And this is supposed to melt the fat away more effectively. I’ll address this and other diet models on other parts of this web site. Of course, you still don’t want to be using protein for energy because of the liver-damage issues associated with that. So, a ketogenic diet like Dr. Atkins implies getting almost all your energy from catabolizing fat rather than from carbs.] This suggests 10% from carbs, 60% or more from fats, and the rest from protein. Variations on this ketogenic diet have been tried on endurance athletes with differing results. The basic idea of using a diet high in fats and low in carbs for athletes is to drive up the storage of energy before “game day”. This is a performance strategy, and whether or not it works is a little besides the point for the average person. Even folks interested in an appropriate level of exercise are probably not too concerned about whether the optimum way to store energy for their next weekend run is by pushing up their fat consumption or by carbohydrate loading. If you want to consider some of these permutations and combinations, take a look at this article (requires MS PowerPoint).

For longer duration events, like the marathon, there is the issue of a ready supply of energy to feed the body as the event progresses at a high rate of energy consumption (10-12 Calories per minute).  As initial blood sugar is depleted, the next source is glycogen, which can rapidly be converted into glucose. After this comes fat stores in adipose cells. Only if all of these resources are consumed and if the marathon athlete has not provided any more energy during the event by consuming fruits or energy drinks or glucose tablets or other sources of fuel along the way, then and only then will the body begin to convert protein stores into energy. This is a last resource, and as I keep saying, a dangerous threshold. Clearly the training and diet of a marathon runner will be very different from that of a sprinter or weight-lifter. Each need significant energy stores, but the best strategies for achieving those stores will be quite distinct.

Take that 200 pound fellow once more. If we go with the 60% from carbs, and we have 1,600 Calories from carbs as his base & sedentary case, the total Calories he needs per day is now up at 2,667. He gets 1,600 Calories from 400 grams of carbs. He gets his protein requirement of 1 gm/kg/day, which comes in at around 90 grams; yielding 360 Calories net 13.5% actually (measuring it in that goofy way). The rest will come from fat and that turns out to be 707 Calories from just under 80 grams of fat. This is in contrast to the Cardio Sucks! suggestions of 300 grams of carbs, 200 grams of protein, and 50 grams of fat. Both of these look like maintenance diets for zero weight gain or loss. Yet they are so different in composition.

What if we amp up this fellow’s level of exercise quite a bit. Say he’s averaging 1.5 hours a day, 7 days a week. Of course, it would be more likely that a serious athlete would be modulating their daily exercise levels quite a bit. As well, depending on a training strategy targeting some specific performance day, the weekly levels would be going up and down as well.  But say this is just an average. Now the Calories from carbohydrates goes up to 2,680, which means 670 grams of carbs. We’ve got ourselves an athlete so we want to feed him 1.6 gm protein/kg/day and that’s 144 grams, yielding 576 Calories as protein. That’s 3,256 Calories so far. If we add in fat at 25% of total, we get a grand total of 4,341 Calories. Of that, 1,085 Calories will be from about 120 grams of fat. Well, if we really had an athlete, we’d be modulating his macro-nutrition to go along with his varying levels of exercise. The main observation about our athlete is that the actual energy he’s using at these higher levels of performance will mainly be coming from carbohydrates. The higher protein is going to building and rebuilding tissue; that is, anabolic uses. The same is true of a large fraction of the fat intake. Some of that fat will be stored, but your typical athlete does not carry around a lot of excess adipose tissue. Fat storage will be driven up on purpose strategically as a means of energy storage before “game day” especially for endurance sports. A marathon runner doesn’t want to “hit the wall” at the 20th mile; a boxer doesn’t want to fade in the 10th round.

After all, what is a Calorie?

I hope you’re confused about now. I want you to be confused because this is confusing. You likely started reading this with a pretty solid idea of what a Calorie was from common usage of the term. I want you now to be questioning that it has any solid meaning at all in the context of a significant measure of macro-nutrients. If we can feed exactly the same diet to two individuals of the same sex and weight, and they each extract very different amounts of energy from the same food, not based on the food but based on their behavior, what can we say about the Calories that they’ve consumed from that food? What this says is that Calories is not a measure of something intrinsic in food; rather, it’s a statement about the relationships between an individual, their lifestyle, and their macro-nutrition.

In only the most simplistic point of view would it be accurate to state that some combination of protein, carbs, and fats have x Calories. Perhaps we could say that any given combination has the potential to deliver x Calories. Even that simplistic view has to be qualified by the obvious dangers associated with extracting any significant amount of energy from the protein component of diet. Oddly enough, (and within limits) it seems that the more work an individual is doing, the fewer Calories they will extract from the same combination of macro-nutrients. The exact results are highly variable and they depend upon the composition of the diet relative to the real energy expenditures of the individual.

The take-away

What’s the take-away? After all of these flourishes and head feints and ducking and weaving, it’s easy. A Calorie is not a Calorie. I’m going to be saying that over and over again. In this case, we see that a Calorie of protein is not the same as a Calorie of carbohydrates because the body of someone doing consistent exercise will exploit protein in a different way than that of a sedentary person. These two different individuals can each consume, say, 100 grams of protein, and the athlete will use Calories to anabolize that protein while the sedentary person may catabolize some into fat stored in adipose tissue, will use some to repair and rebuild cells and will likely waste the excess.

Sometimes people call this having a “higher metabolism”, meaning that the athlete is burning more energy. I suppose that’s true in a way, but the real impact of exercise is to reduce the net energy obtained from a given diet, although in practice that’s not quite what would happen. Someone who gradually becomes increasingly athletic sees their body composition changing even while they increase their total “Caloric” intake. Why? Because a smaller fraction of the athlete’s total “Caloric” intake is “Caloric”.

Again, a Calorie is not a Calorie.