The impacts of different macronutrients on body weight regulation remain unresolved, with different studies suggesting increased dietary fat, increased carbohydrates (particularly sugars), or reduced protein may all stimulate overconsumption and drive obesity. We exposed C57BL/6 mice to 29 different diets varying from 8.3% to 80% fat, 10% to 80% carbohydrate, 5% to 30% protein, and 5% to 30% sucrose. Only increased dietary fat content was associated with elevated energy intake and adiposity. This response was associated with increased gene expression in the 5-HT receptors, and the dopamine and opioid signaling pathways in the hypothalamus. We replicated the core findings in four other mouse strains (DBA/2, BALB/c, FVB, and C3H). Mice regulate their food consumption primarily to meet an energy rather than a protein target, but this system can be over-ridden by hedonic factors linked to fat, but not sucrose, consumption.

Introduction

These simple predictive models belie some of the complexities that are involved in modeling the impacts of macronutrient composition of the food on resultant body composition. First, we assumed that energy expenditure is a fixed variable that is independent of the dietary composition. This is unlikely to correct. For example, after food is ingested there is a period of elevated metabolism (variously known as the thermic effect of food, heat increment of feeding, or the specific dynamic action [SDA]). It is well established that different macronutrients have a hierarchy of impacts on SDA, with protein having the greatest and fat the smallest effect. Thus, we might anticipate post-ingestive energy demands would change in relation to the dietary composition. An additional complexity, however, is to understand how such changes in the post-ingestive period translate into total daily energy expenditure. For example, it is known that heat production from SDA and exercise may substitute for the energy demands of thermoregulation, and hence for mice that are housed below thermoneutrality there may be no net impact of SDA on total energy requirements, as our models assume. A second assumption is that the different models are appropriate under all conditions. Hence, we assume that the mice respond to a protein target or to an energy target. Yet it is conceivable that animals may move between different targets at different stages of their lives. In particular, these “stages” may be reflected in the existing levels of fat storage. Hence, it is possible that animals normally respond to dietary composition in a manner consistent with protein leverage ( Figure 1 A), but as those on lower protein diets become obese they may change their regulation toward regulation of energy intake ( Figure 1 B). Finally, we assume that the responses to the different dietary compositions are linear. Again, this is somewhat simplistic. In particular, for example, as protein contents of the diet approach zero, in theory to achieve these requirements intake would need to expand exponentially (in the limit to infinity). This would then be constrained by the practical limitations on feeding time and intestinal absorption capacity. Over the range we are considering, however, we assume that these non-linearities are relatively small.

Since the mouse is a widespread model used to understand human obesity, we aimed to explore how changes in macronutrient composition of the diet impact food intake. In particular, how changes in protein, carbohydrate (sugar), and fat contents of the diet leverage intake and cause adiposity. In total, we used 29 different diets varying orthogonally in their macronutrient composition to allow separation of the different macronutrient effects. We combined these observations with gene expression profiling of the hypothalamus and adipose tissue to assess if FGF pathways were stimulated, and measurements of aminopeptidase activity in the alimentary tract, to explore the underlying mechanisms by which protein, carbohydrate, or fat may exert its effects on appetite. The most extensive work was performed in C57BL/6 mice, and then the core observations were repeated in four other strains (BALB/c, C3H, DBA/2, and FVB), which include strains classically regarded as “resistant” to obesity.