To get straight to the point, energy density becomes practically relevant as a function of its influence on the number of calories we consume, which itself is the strongest determinant of energy balance.

Research indicates most of us tend to eat similar quantities of food every day. In order to really grasp the significance of this, we need to understand what diets of different energy densities might actually look like, so let’s explore some hypothetical examples in a similar vein to what we did in the previous section.

To begin with a fundamental concept, consider first the fact it is possible to meet your energy needs with diets of different energy densities. For example, olive oil is pure fat, and fat contains 9 calories per gram. As such, with a hypothetical energy requirement of 2500 calories per day (in other words, if you needed to consume 2500 calories per day to avoid being in an energy deficit or energy surplus), you would only need to consume 278g (about 0.6 pounds) of olive oil per day to meet that need. The energy density of your diet in this context would be 9 kcal/g.

Importantly, a dietary energy density of 9kcal/g cannot be surpassed, as fat is the most energy dense macronutrient, and it has an energy density of 9kcal/g. Although this is of minimal practical significance, as no one consumes only fat, it does mean that you can know with certainty that any calculated energy density above this threshold is inaccurate.

On the other hand, dextrose is pure carbohydrate, and carbs contain 4 calories per gram. This means that with the same energy requirement, you could meet your energy needs consuming a diet 625g of food (625g of dextrose) – about 1 and ⅓ pounds of food. Once again, this is a big difference – both in a relative sense (625g is more than double 278g), and in an absolute, practical sense (you’d be eating more than 300g more food in the case of dextrose).

At this point one may object that no one actually consumes only oil, or only dextrose – and, as previously mentioned, they’d be absolutely right. However, these two (admittedly extreme and unrealistic) examples serve only to demonstrate that it is possible to meet one’s energy requirements with diets of different energy densities. Put another way: energy content does not entail energy density.

Although health-conscious people usually get most of their dietary fat from whole foods and only a small part from preparing food in oil, the majority of people are likely less concerned with both the quantity and type of fat they ingest on a daily basis. In this context, most fat is introduced externally by means of cooking (e.g. frying or searing) or simply adding fat (e.g. using an oil-based dressing or sauce), rather than naturally occuring in whole foods, and the remainder of fat is to be found in products that also contain added fats (e.g. baked goods). This pattern of food selection also holds true for carbohydrates, in which case health-conscious people presumably get most of their carbohydrates from whole foods, while others get them from a more even combination of whole foods and sources that contain carbohydrates not naturally present, such as added sugar.

One could legitimately distinguish between these food categories on the basis of their being processed and unprocessed, respectively, but if we seek a more technical analysis of their differences, we find one key difference: water content.

Water contains, to no one’s surprise, no energy. Thus, water has an energy density of 0 kcal/g. The significance of this becomes apparent when we consider that water, beyond being essential for hydration, is also one of the largest components of whole foods. For example, a cooked white potato is 79% water and 17% carbohydrate. Yes, this means that almost all of a potato is actually just water. On the other hand, nuts are roughly 4-6% water and 65-75% fat. Why mention this at all? If we recall our earlier example relating to the energy density of carbohydrates, we’ve already established that it’s possible to consume twice the amount of food (by weight) in the form of pure carbohydrate than pure fat, with calorie content remaining identical. But that’s not all! Not only is the energy density of pure carbohydrate lower than the energy density of pure fat, carbohydrates in particular are usually accompanied by a significant amount of water, further lowering the resulting energy density of the relevant food. Indeed, unless you’re eating pure sugar, you’re necessarily eating carbohydrate-containing foods with an energy density lower than 4kcal/g, as a result of their water content. This presents a stronger case against the consumption of fat than against the consumption of carbohydrates in general, not because fat is somehow inherently bad, but rather because the relative lack of water contained in fat-containing foods puts it in a further disadvantaged position compared to carbohydrates and protein.

As such, to meet the previously-mentioned 2500 calorie daily energy intake, we might end up with a total weight of food eaten of 1250 grams per day, yielding an energy density of 2 kcal/g for the entire diet. Now, imagine our hypothetical diet includes 30g of nuts per day (a large handful). At an energy density of approximately 6.5kcal/g, 30 grams of nuts yields about 195 calories. If this portion of nuts were left out, we could reserve those 195 calories for a different food – such as, say, boiled mashed potatoes, which contain approximately 75 calories per 100 grams. In practical terms, this means that in order to match the energy content of 30g of nuts we’ve removed, we can (or have to) consume 260g of boiled potatoes – nearly nine times more food.

If we keep this change in food selection in mind when calculating the total daily weight of our diet, we end up with a total weight of food eaten of 1480 grams (1250g – 30g + 260g). In terms of energy density, we’ve reduced the energy density of the diet from 2kcal/g to 1.69 kcal/g. More practically, it turns out this changes in overall dietary energy density allows for an extra 230g (about half a pound) of food per day. Once again: this is a big difference – especially if the target of the diet is to lose weight without suffering from persistent hunger.

The essential point to understand is that whenever we alter the energy density of our diet (whether purposefully or inadvertently) without changing the weight of food we eat, the number of calories we consume necessarily changes. If you can learn to use this as a tool to consciously decrease the energy density of your diet, you can reduce the number of calories you consume while eating the same amount of food, or even more food. This brings us to the key advantage to understanding and manipulating dietary energy density – appetite control.


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