Appetite and energy balancing.

The idea that food intake is motivated by (or in anticipation of) 'hunger' arising from energy depletion is apparent in both public and scientific discourse on eating behaviour. In contrast, our thesis is that eating is largely unrelated to short-term energy depletion. Energy requirements meal-to-meal are trivial compared with total body energy stores, and energy supply to the body's tissues is maintained if a meal or even several meals are missed. Complex and exquisite metabolic machinery ensures that this happens, but metabolic regulation is only loosely coupled with the control of energy intake. Instead, food intake needs to be controlled because the limited capacity of the gut means that processing a meal presents a significant physiological challenge and potentially hinders other activities. We illustrate the relationship between energy (food) intake and energy expenditure with a simple analogy in which: (1) water in a bathtub represents body energy content, (2) water in a saucepan represents food in the gut, and (3) the bathtub is filled via the saucepan. Furthermore, (4) it takes hours to process and pass the full energy (macronutrient) content of the saucepan to the bathtub, and (5) both the saucepan and bathtub resist filling, representing negative feedbacks on appetite (desire to eat). This model is consistent with the observations that appetite is reduced acutely by energy intake (a meal added to the limited capacity of the saucepan/gut), but not increased by an acute increase in energy expenditure (energy removed from the large store of energy in the bathtub/body). The existence of relatively very weak but chronic negative feedback on appetite proportional to body fatness is supported by observations on the dynamics of energy intake and weight gain in rat dietary obesity. (We use the term 'appetite' here because 'hunger' implies energy depletion.) In our model, appetite is motivated by the accessibility of food and the anticipated and experienced pleasure of eating it. The latter, which is similar to food reward, is determined primarily by the state of emptiness of the gut and food liking related to the food's sensory qualities and macronutrient value and the individual's dietary history. Importantly, energy density adds value because energy dense foods are less satiating kJ for kJ and satiation limits further intake. That is, energy dense foods promote energy intake by virtue (1) of being more attractive and (2) having low satiating capacity kJ for kJ, and (1) is partly a consequence of (2). Energy storage is adapted to feast and famine and that includes unevenness over time of the costs of obtaining and ingesting food compared with engaging in other activities. However, in very low-cost food environments with energy dense foods readily available, risk of obesity is high. This risk can be and is mitigated by dietary restraint, which in its simplest form could mean missing the occasional meal. Another strategy we discuss is the energy dilution achieved by replacing some sugar in the diet with low-calorie sweeteners. Perhaps as or more significant, though, is that belief in short-term energy balancing (the energy depletion model) may undermine attempts to eat less. Therefore, correcting narratives of eating to be consistent with biological reality could also assist with weight control.