In this article, I want to look at a somewhat fundamental aspect of general nutrition; that is the distinction between calorie intake, nutrient intake, and food intake. This is relevant for the simple fact that the average person doesn’t think in terms of calorie intake, and they probably don’t think in terms of nutrient intake. Rather, they think in terms of eating food; thus it’s important to examine the distinction between those three ‘categories’ of intake.
Ignoring the fringe that claims that the calorie counting theory is invalid or doesn’t work, most diet books deal predominantly with calories (everywhere but the US, joules are used). They’ll discuss caloric intake (from food) or caloric expenditure (from activity) or compare the two. In discussing what is happening to the body (weight gain or loss), they’ll compare the energy balance equation which compares calories in to calories out.
So what, you ask, is a calorie? A calorie is a measure of heat. One calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. Fascinating stuff, this nutritional science.
You’ll also frequently see kilocalorie (kcal) used. Technically one kcal is equal to 1000 calories (kilo = 1000). In the US, most people use calories and kilocalories interchangeably, although it’s incorrect to do so. You’ll sometimes see ‘cal’ with a lower-case ‘c’ used to mean a calorie and ‘Cal’ with an upper-case ‘C’ to mean kilocalorie but that’s just too much to pay attention to.
Without boring you with some true minutiae, this simply has to do with keeping the numbers reasonable and easier to deal with.In the real world, people mean kilocalories when they say calories even if pedantic nutritional types get their panties all twisted over it. I use calories to mean kilocalories just because it’s easier and, let’s face it, everybody knows what I mean.
As noted above, in all countries but the United States, the energy value of foods are measured in joules which is just another measure of energy. If you really need to convert for some reason, 1 calorie is equal to 4.2 joules (or 1000 cal = 4,200 joules). I always hate having to convert when I read research studies. Let’s see, they gave them 5000 joules per day, divide by 4.2, carry the one, umm, where’s that damn calculator. Ok, it’s 1200 calories per day. The nice researchers list energy intake in both joules and calories, because they realize that most people in the US are too dumb to understand metric.
To give you an idea of some representative numbers, your average non-huge male at about 170 pounds has a daily maintenance energy need of about 2700 calories or so (11,340 joules or 11.3 megajoules), a female of 135 lbs about 2000 calories (8,400 joules or 8.4 megajoules). That number is subject to change and scales with weight (larger individuals having higher daily energy needs; smaller individuals lower). This is discussed in more detail in Metabolic Rate: Overview.
I’d note that you can Quick Estimate Maintenance Calories with the following values: 14-16 calories/lb current bodyweight for men and for 13-15 cal/lb for women. Keep in mind that these are only estimates.
So you know, the calorie values (with joule conversions for non-American readers) for the macronutrients are:
- Protein: 4 calories per gram (16.8 joules/gram)
- Carbohydrate: 4 calories per gram (16.8 joules/gram)
- Fat: 9 calories per gram (37.8 joules/gram)
- Alcohol: 7 calories per gram (29.4 joules/gram)
Note: these are average values and can vary very slightly (i.e. protein ranges from about 3.8 to 4.2 cal/gram depending on the specific type of protein). Worrying about slight difference in energy content between different sources of protein or carbs is taking anal-compulsion to a whole new level. I shant mention it again. That’s right…shant.
When researchers determine the calorie values of foods (i.e. what you see when you read the food labels), they do it by burning the foods in something called a bomb calorimeter and measuring how much heat is given off. A potential problem is that the human body is not a bomb calorimeter and that simple fact affects the energy balance equation. That is to say, just because a gram of protein burned in a bomb calorimeter has 4 calories doesn’t mean your body derives 4 calories from it energy wise.
I suppose I should mention fiber which is (or at least should be) consumed in gram amounts each day and is therefore considered a macronutrient (fiber is usually considered a subset of carbohydrates). Fiber has a number of important benefits but providing a lot of energy to the body isn’t one of them for the most part.
It’s generally stated that humans don’t derive any calories from fiber, since we lack the enzymes necessary to break it down like cows. This isn’t strictly speaking true, our lower intestine may break down fiber to short chain fatty acids which are used for energy by the micro-organisms living in our gut.
Although there is ongoing debate in this area, a caloric value of 1.5-2 cal/gram for fiber is the currently accepted value. Even then, since a fiber intake for most people these days probably doesn’t top 25 grams/day, that’s only about 35-50 calories total per day. A massive 100 grams of fiber/day (you’d have to eat a tree) would 150-200 calories which would be significant.
For all practical purposes, unless your fiber intake is ungodly high, we can ignore the caloric content of fiber. You can also ignore the fiber grams for carbohydrate count if you’re keeping track of those for some reason (such as a ketogenic diet).
Of course, we don’t eat calories which is why people often get confused when that’s all that’s being talked about. Rather, we consume the various macronutrients (protein, carbs, fat and alcohol) which provide calories to the body during their metabolism. As the chart above shows, the different nutrients provide the body with different caloric values; a gram of protein and a gram of fat will provide 4 and 9 calories/gram respectively.
Even then, just looking at the calories values are misleading because the different nutrients provide energy (calories) to the body with varying efficiency. In processing (and depending on how the processing occurs), the body may derive slightly different amounts of energy from different nutrients. Consuming 400 calories from carbohydrates may not have the same overall effect on metabolism as consuming 400 calories from fat or protein, even though the caloric value of all three is identical.
The main place this shows up is in something called the thermic effect of food (TEF), which refers to the amount of calories that are used by the body in processing. For example, in storage as glycogen, carbohydrates burn off 3-6% of the total energy ingested. Fat, in contrast, may only use 3% of its total energy content in storage.
Protein uses the most energy in processing, anywhere from 15-25% of its total energy value depending on the source. This can add up when very large scale substitutions of carbohydrate or fat with protein is made and this is at least part of why higher protein diets tend to outperform lower protein diets.
Even alcohol can lead to a ‘loss’ of energy by causing changes in liver function that serve to burn off excess calories; this seems to be more of a pathological adaptation than something people should seek out.
I want to mention here, and I’ll come back to this later, that the effect of whole body metabolism due to the above effects are generally fairly small, especially compared to all of the other variables that affect energy balance such as activity levels. For example, shuffling around carbs and fat might affect metabolic rate by a total of 3%. So for every 100 calories of fat you replaced with carbs, you’d burn 3 more calories per day.
Contrast that, for example, to the daily variance in food intake which may approach 23% (i.e. under uncontrolled conditions, someone’s caloric intake might vary by 23% from day to day). Protein has the largest potential to affect metabolic rate but there is generally a limit to how much protein can be consumed to this effect tends to be somewhat limited as well. Unless you’re looking at very large-scale replacement of carbs or fat with protein, the overall effect simply isn’t that large.
The distinction between calories and nutrients also becomes important when we start to consider the specific mechanisms which cause the body to lose or gain fat and muscle (I’m not talking about insulin by the way). Simply looking at caloric intake vs. expenditure may be misleading under certain circumstances. Looking at the actual nutrient balance (nutrient intake vs. nutrient burning) is more important in determining what’s really going on in the body. This will make more sense soon.
Now, of course, we don’t eat individual nutrients, at least not under most real-world conditions. That is, you don’t typically sit down and pick from a tub of protein, a container of fat and a container of carbohydrate; not unless you’re an obsessed athlete or bodybuilder. Rather, you probably derive your nutrients (and hence your calories) from food.
This is perhaps one of the most important distinctions that needs to be made because the source of a given nutrient (or calories) can affect things such as fullness and hunger, energy levels, overall health and body composition. Consuming 100 grams of digestible carbohydrates from a sweet potato and 100 grams of digestible carbohydrates from a candy bar may provide identical amounts of carbohydrate (100 grams) and calories (400 calories) to the body; but that doesn’t mean that they will have identical effects on bodily function.
Hormone levels, fullness and health can all be impacted by the source of nutrients/calories. Different sources of nutrients and calories may contain other substances (vitamins and minerals, phytonutrients, fiber) that may be of relevance. Individual taste preferences also come into play here: humans eat for reasons that may be wholly unrelated to the macronutrient composition of the food. We eat because things taste good or for social or cultural reasons; only obsessed athletes pick foods based on their macronutrient profile or caloric content.
All food contains nutrients in some proportion or another, and those nutrients provide energy in the form of calories to the body. From a very simplistic sense, this means that all that matters from a bodily energy standpoint is the caloric content: 3000 calories are always 3000 calories regardless of where they come from. And to a first approximation, and assuming a few other factors are taken into account, looking at calories does give us a good indication of what’s going on.
Unfortunately, this simplistic approach isn’t always sufficient for our needs and we have to get into more detail. The amounts of nutrients ingested affects whole body fuel metabolism, hormone levels and a whole host of other factors that may be relevant to body composition, health or overall function.
So even though carbs and protein both contain 4 calories/gram, that doesn’t automatically make them identical in terms of their actual physiological effect on the body. From a strictly physiological standpoint, looking at macronutrient intake tends to be sufficient, we can determine what’s going on in the body based on the amounts and proportions of the different nutrients being consumed.
But this is only correct when we consider strict physiology and the simple fact is that humans are more than just a collection of physiological processes. This is especially true when you start looking at real world eating behavior.
Even then, since we generally don’t eat individual nutrients, it become relevant to consider the food source of those nutrients in the discussion. Foods not only contain different amounts of the various macronutrients but may contain other compounds that are relevant to the overall discussion.
Understand me here? Issues such as hunger control, long-term adherence, individual variance, athletic performance, and a few others all go into the determination of what food might or might not be a better choice under a given set of circumstances.
Additionally, whether you’re looking at intake under controlled or uncontrolled conditions tends to vastly affect what conclusions are drawn; some foods are easier to overeat and their consumption under uncontrolled conditions may lead to increase caloric intake and fat gain. I discuss this topic in more detail in Is a Calorie a Calorie.