Let’s start with something somewhat surprising: contrary to popular belief, how satiated you feel and how long you stay full after a meal has far less to do with the macronutrient content of that meal than you might think. Sure, macronutrients are in large part responsible for food texture, but we’re going to argue that they shouldn’t be discussed on the level that they’re being discussed when talking about appetite regulation specifically. This is because in the context of appetite, macronutrients have a lesser influence on outcomes we care about, just as they are of lesser significance than total daily caloric intake in terms of what matters when losing or gaining weight.
So where does this put energy density?
Up to this point, we’ve focused on definitions and hypothetical scenarios, which, although important as a foundation for further understanding, can only get you so far. Thus, in this section we’re going to review the actual research which has been done investigating the effects of energy density on satiety, satiation, hunger, and ad libitum (unlimited) food intake.
Contextualizing Energy Density: The Macro Problem
When reviewing the science of appetite, it’s apparent there has been a predominant focus on the effects of the three macronutrients (protein, carbs, and fat). This is understandable, as macronutrient composition is arguably second only to energy content in terms of overall nutritional significance. Unfortunately, this macronutrient focus results in a tendency for studies on this topic to not control for energy density, and this can generate unreliable results. For example, if an increase in protein intake were shown to be associated with an increase in satiety, but energy density was (inadvertently) decreased at the same time, we wouldn’t be able to reasonably conclude that it was really the protein having the desired effect, because the reduction in energy density ultimately led to an increase in total weight of food consumed, yielding a confounding variable.
A good example of this phenomenon is seen in a study by Weigle en colleagues (2005), where the authors found that after a 12-week diet, a high-protein diet produced significantly more satiety and led to fewer calories consumed when compared to a high-fat diet. To determine the degree to which protein impacts energy intake and satiety, the researchers in this study kept carbohydrate intake the same in both conditions (at 50% of total energy intake), and reduced fat intake from 35% to 20% to allow for an increase in protein from 15% to 30% of total daily energy intake. To illustrate the magnitude of this difference in practice, consider the example of a male who requires 2500 calories per day to maintain weight, and who pursues a 500 kcal deficit, leaving him with a daily energy intake of 2000 calories. Instead of his regular protein intake of 15% of total energy intake (equivalent to 75 grams of protein), he would consume 30% of his energy from protein (equivalent to 150 grams of protein). To accommodate this, he would concurrently cut his fat intake from 35% to 20% (equal to a relative reduction of 43%, or from 77 to 44 grams of fat per day). These two simple changes (more protein, less fat) could mean that he could typically eat an additional 400 grams of any typical low-fat animal meat product like chicken breast or beef tartar, up to 700 grams of fat free greek yoghurt, or two (thick and creamy) 350ml protein shakes. This could translate to 1-2 extra meals because of a simple change in the distribution of macronutrients.
This example is just one of many, in which there is no means of discerning the role of changes in energy density between diets. When energy density is controlled, increasing protein intake actually has little effect on energy intake, as shown here, here, here and here. Although these studies were smaller in scale, collectively there considerable research pointing in this direction when acute studies are considered.
A real-world example of an acute study is the work performed by Douglas et al (2012). In this study, researchers investigated how the ingestion of yogurts varying in protein content (5-24g protein per yogurt) influenced appetite and satiety over several hours. If you’re familiar with the popular claim that protein is most satiating of the three macronutrients, you’ll likely find the results of this study unsurprising: eating the high-protein yogurt generated greater perceived fullness than did eating the low-protein or moderate-protein yogurts. This could certainly make a good case for the popularity of protein bars! However, if we take a closer look at Table 1 in the study, which presents the yogurts’ characteristics, we observe that although the three yogurts were isocaloric (i.e. they all contained the same amount of calories), only the low-protein and moderate-protein yogurts were approximately equivalent in volume (170 ml and 165 ml, respectively). Meanwhile, in the case of the high-protein yogurt, volume was 250 ml – more than 50% higher! This resulted in the high-protein yogurt’s energy density being significantly lower: 0.66 kcal/g, compared to 0.94 kcal/g for both the low-protein and moderate-protein yogurts. Thus, it cannot be reasonably concluded on the basis of this study that increasing a food’s protein content alone – with all other food characteristics staying the same – increases satiety; to properly investigate that question, energy density needs to be controlled. In the next installment of this series, that’s precisely what we’ll look at.