Today I want to cover another fundamental aspect of nutrition that is somewhat easy to confuse. That concept is referred to as energy density. Energy density integrates, in a fashion, the concepts of calories, nutrients and food intake (a topic discussed in detail in the article Calories, Nutrients or Food?).
First I want to define energy density before looking at some examples that will hopefully make the concept a bit more clear. Finally, I’ll look at applications of the energy density concept in terms of dieting, weight gain, etc.
What is Energy Density?
Conceptually, energy density refers to how many calories are found in a given weight or volume or food. Ok, what does that mean. Let’s say that you have 1 gram of each of the three macronutrients which are protein, carbohydrates and fat. We know that these are given calorie values of 4 cal/g for protein and carbs and 9 cal/g for fat. Clearly, in this simple example, fat has over twice the energy density of either carbs or fat (9 cal in one gram vs. 4 cal in one gram).
This basic fact is generally interpreted one of two ways depending on whether a given author is pro- or anti-fat (and of course what the context is). Pro-fat authors will contend (usually in the context of exercise performance) that since fat contains twice the calories of carbohydrates, it provides more energy to the body on a gram per gram basis (again, the context is usually exercise performance). While there is an element of truth to this it leaves out some important information that I’m not going to get into in this article.
In contrast, anti-fat authors (usually coming at it from an obesity or weight gain standpoint) tend to blame high caloric intakes (and hence obesity) on a high fat intake because of it’s high energy density. That is, frequently diets higher in fat are also higher in calories because of the increased energy density.
Unfortunately, just looking at things as simply as above is misleading for the reasons I outlined in Calories, Nutrients or Food?. First and foremost, people don’t generally eat pure nutrients, they eat food. And, as discussed in that article, most foods are a combination of nutrients (e.g. milk contains protein, carbs and fat, meat contains protein and usually fat, most carb sources contain some protein). Even in those cases where folks (read: OCD athletes) may choose pure nutrients, most humans eat mixed meals containing multiple foods and that affects the overall energy density of the meal.
Of perhaps more relevance is that most foods also contain other non-nutrient compounds such as air, water, ash, fiber, etc. Put differently, 100 grams of say chicken won’t contain 100 grams of protein; rather, the amount of protein will be diluted by the presence of not only other nutrients but the other compounds that are present.
If this is difficult to understand, an easy example might be soup which is mostly water. So say you make a soup containing potatoes, vegetables, ground beef, some vegetable oil, spices and what have you. Each of the foods you put in will contain some amounts of carbohydrates, proteins and fat and each will fall somewhere in terms of their energy density. However, a cup of that soup will likely have a fairly low energy density because most of the volume is water.
Another example would be to compare two cups of pasta to one cup of pasta with one cup of steamed vegetables. Both meals are 2 cups (in terms of the total volume) but the second meal would contain far less calories due to the vegetables; and since the total volume of both meals is the same, the energy density (calories per unit weight/volume) would also be lower. In this specific case, the low energy density food dilutes the higher energy density food and brings the overall energy density of the meal/dish down.
Alternately, let’s take a typical 100 grams baked potato which contains about 46 grams of digestible carbohydrate; hopefully readers can sort of imagine about how big that would be (this is just a standard size baked potato). Now contrast that to 100 grams of pure table sugar which will contain very nearly 100 grams of carbohydrate; I bet you can imagine just how little sugar that would be.
In both cases we’re looking at 100 grams of total food but the caloric content of each is drastically different (about 200 calories vs. 400) as is the energy density. The potato will have an energy density of 200 calories/100 grams food compared to the 400 calories/100 grams food in the table sugar.
Of course, 100 grams of pure fat would contain nearly 900 calories and still have the highest energy density of all.
Here’s an example that will help to illustrate just how large a role water content plays in all of this. Two cups of grapes will contain about 100 calories while 2 cups of raisins (dried grapes) might contain nearly 800 calories. This is because the raisins have had most of the water and air content removed during the drying process. Same volume, same food, but drastically different energy densities.
One thing I would note is that, although fat is often blamed for the high-energy density of the diet, this isn’t always true. Food companies have come up with amazingly creative ways to make low-fat high-carb foods be exceedingly energy dense by removing water, air and fiber.
Most dietetics types seem to assume that a high-carb diet will be non-energy dense but they are assuming that people are eating naturally occurring carbohydrates such as potatoes; in the real world this is rarely the case. When you start moving into heavily processed carbohydrates, the energy densities can get up there pretty quickly.
As a general rule, foods high in water and fiber tend to be pretty low on the energy density scale and food that lack either tend to be much higher. Pure oils tend to have the highest energy densities (which is why I strongly recommend measuring them out, even if you don’t measure anything else in most of my books) and vegetables tend to have the lowest, everything else is somewhere in the middle.
Why Does This Matter?
Ok, enough explanation, why is this important. Energy density starts to become important when you are looking at food intake in the real world, both in terms of dieting and gaining weight (for athletes). A lot of this has to do with fullness and satiety, a topic I discussed somewhat in 9 Ways to Deal with Hunger on a Diet.
Some research suggests that humans eat a fixed weight of food each day (other research says this is not the case and I’ll let the scientists argue it out for the time being); thus people who eat higher energy density foods invariably end up eating more total calories. Basically, if you are going to eat 2 pounds of food per day (I’m pulling this number out of thin air), you will end up consuming more calories if you eat high energy density foods vs. low energy density foods.
Even if this isn’t entirely the case, high energy density foods tend to make it easier to overconsume calories compared to low energy-density equivalents. This is especially true when you’re looking at uncontrolled diets (e.g. where people are not tracking calories), a topic I keep harping on but one that people keep confusing with other issues of the diet.
Simply put, foods with a low energy density will contain a relatively small number of calories in a large bulk or volume of food. As noted above, vegetables, fruits and other unrefined carbohydrates typically have a low energy density although even that’s not always the case as mentioned above
Foods with a high energy density will contain a large number of calories in a relatively small amount of food. This generally includes high-fat foods but can also include highly refined carbohydrates. I mentioned this above but I want to look at that issue in a little more detail.
It looks at first glance that high fat foods will always be more energy dense than similar amounts of carbohydrates or protein and that fact has been used as part of the campaign to lower dietary fat to help with the obesity problem. But is this always the case, are high-fat foods always higher energy density than high-carbohydrate foods?
Let’s take our baked potato example, containing about 46 grams of carbohydrate in a 100 gram potato. So it contains about 200 calories/100 grams. Let’s say we add 10 grams of fat in the form of butter (making anti-fat crusaders cringe) to the potato which adds 90 calories. Our potato/butter combination contains 290 calories in 110 grams of food.
Now let’s contrast this 110 grams of candy and let’s say it contains ~100 grams of sugar, or 400 calories. So the baked potato/butter combination contains 290 calories/110 grams of food while the candy contains 400 calories/100 grams of food. Despite being ‘high-fat’, the potato-butter combination has a lower energy density than the zero-fat high-sugar food its being compared to.
Semi-tangentially, while the anti-fat crusaders tend to blame high-fat diets for every malady known to man, there are real-world examples, such as the classic Mediterranean diet (which often contains up to 40% fat as total calories), where a high-fat diet doesn’t cause problems. Much of this is that the overall diet is still fairly low energy density because there are a tremendous amount of vegetables being eaten.
Despite having a high fat content, the Mediterranean diet has an overall low energy density because of the other foods that are present in such abundance. This is in contrast to most Western diets that tend to not only be high in fat but low in foods such as fruits and vegetables that would dilute the energy density of the fat.
As I mentioned above, one of the reasons that fat is so often blamed for its role in causing obesity is its high energy density. This is obviously true if you’re talking about pure fat (which few people eat) and it’s generally true that high fat foods have a high energy density. This difference is especially prevalent if they are compared to low-fat unrefined carbohydrate foods likes fruits, vegetables and unrefined grains. You know, the foods that nobody really eats but that anti-fat crusaders think people are eating.
As I alluded to above, a low-fat diet isn’t automatically a low energy-density diet. Many highly refined high-carbohydrate foods have a high energy density because the water, air, fiber, etc. has been removed. More and more, food companies have brought low-fat/high-carbohydrate but also high energy density foods to market.
Even diet staples such as pasta can have a surprisingly large number of calories in a fairly small volume. Such foods may be low (or even no) fat but still have a very high energy density. This not only makes them easy to overconsume but also means that they can contribute a rather large number of calories to the diet without providing much bulk.
To really drive this into the ground, a high-fat diet isn’t automatically a high energy density diet any more than a low-fat diet is automatically a low-energy density diet. The types and amounts of other foods in the diet play a major role and, in my experience, what dietetics types think people are eating in the real world isn’t what people are actually eating in the real world.
Which isn’t to say that an excessive fat intake isn’t contributing to the energy density (or caloric content) of the modern diet; of course it is. Dietary fat simply isn’t the only factor contributing to the problem of caloric overconsumption.
So Which is Better?
You may be thinking after the above discussion that choosing a low energy-density diet is always superior. If you’re talking about weight loss or even general health, there is probably much truth to that. For people who won’t or aren’t going to count calories, choosing low energy density foods will tend to reduce caloric intake without doing anything else. For people on a diet, foods with a lower energy density tend to be more filling, helping to keep hunger at bay.
But there are exceptions to this. Often times, people want or need to gain weight (especially athletes). In that situation, when very high caloric intakes may be required, low energy density foods can actually make it impossible to get in sufficient calories. Admittedly, this probably isn’t a situation that applies to the majority but it does come up. Trying to get 6000 calories per day eating nothing but low energy density foods can be a near impossibility and consuming higher energy density foods may be necessary to get the job done.
A related issue are the high-carbohydrate refeeeds that I recommend in my books (such as A Guide to Flexible Dieting and The Rapid Fat Loss Handbook). When the goal is to deliberately overconsume a lot of carbohydrates, people invariably get into trouble trying to do low-energy density, high-fiber foods. Not only can’t they get sufficient calories in to make the refeed worthwhile, but the high-fiber tends to make them explode (if you get my meaning). Similarly, in the post-workout period, higher energy density foods may be preferred due to a faster rate of digestion and the hormonal response.
And, of course, for people who are actively tracking calories, some of the above becomes much less relevant. As I discuss in Is A Calorie A Calorie?, much of this type of stuff only really matters when people aren’t tracking calories (because of the impact of different food choices on spontaneous food intake). For people who do track daily calories, clearly some higher energy density foods can be worked in (if desired) because total calories will still be controlled.