The roles of different macronutrients in regulation of appetite, energy intake and adiposity

Highlights

• Increasing fat intake (up to 60% by energy) is a major driver of energy consumption and adiposity.

• Indigestible carbohydrates and elevated protein appear to promote satiety.

• High glycaemic index carbohydrates may have the opposite effect.

• A combination of 50–60% fat, 10–30% carbohydrates and 10–20% protein stimulates our reward system, promoting overconsumption.

• This may be a legacy of the composition of milk.

Abstract

Over-consumption of calories rather than decreased expenditure is the most likely cause of the obesity epidemic. The reasons for this remain disputed, with all the main macro-nutrients (fat, protein and carbohydrate) being implicated by different authors. Stimulated intake by declining dietary protein (protein leverage) may be important. The carbohydrate insulin model pins the blame for the epidemic on intake of refined carbohydrates. Yet, others blame the high energy density of fat. In mice (and probably humans), a combination of around 50–60% fat, 10–30% carbohydrates and 10–20% protein (by energy) seems to maximally stimulate food intake and results in the greatest levels of adiposity. Humans find this combination most rewarding. Any diet which moves away from this combination are likely to promote weight loss. Why this combination stimulates intake so much is unclear because it does not correlate to any ancestral adult human diet, but it is similar to human milk.

Introduction

The world is currently in the middle of a pandemic of obesity and related disorders [1,2]. Although it is often stated that obesity stems from a reduction in physical activity, combined with elevated food intake, the clear declines in, for example, work-time physical activity levels [3] do not appear mirrored by similar changes in total energy expenditure [4]. Nor does modern day total expenditure differ from nomadic herders [5] or modern-day hunter-gatherers [6]. Hence, the main factor causing the obesity epidemic appears to be increased food intake rather than declining expenditure [1,7]. Why this tendency to increase adiposity occurs in some, but not others, may be because of genetically based individual variability in the location of an upper regulatory point that prevents excessive fat gain [8,9]. The macro nutrients in our food (fat, protein and carbohydrates) have all been implicated in this problem. This area of research is enormous and multifaceted. It is only possible to scrape the surface in a short format review like this. I have tried, therefore, to cover the main important themes as I see them.