Objective: A systems dynamics computer model was developed to examine how the interactions between carbohydrate and fat metabolism influence body weight regulation. It reflects the operation of a two reservoir-system: one representing the body's limited glycogen, and the other, its large fat reserves. The outflows from the reservoirs correspond to the oxidation of glucose and fat, whose relative contributions are affected by the size of the prevailing glycogen and fat reserves. Together, they meet the body's energy expenditure. Replenishments occur three times per day, in portions restoring total glycogen content to specific levels. A parameter mimicking the action of insulin is necessary to create realistic responses.
Research methods and procedures: The model was run for 125-day periods to establish the degree of adiposity for which rates of fat oxidation become commensurate with fat intake and the influence thereon of various dietary, environmental, lifestyle, and inherited variables.
Results: Equivalent degrees of adiposity can be sustained under a variety of conditions. For instance, the impact on steady-state body fat contents of a 10% increase or decrease in the energy provided by dietary fat is offset by a 26-gram decrease or increase in mean glycogen levels.
Discussion: Environmental factors such as food diversity, palatability, and availability can be expected to raise the range within which glycogen levels are habitually maintained. This restrains fat oxidation, until expansion of the fat mass is sufficient to promote fat oxidation to a rate commensurate with dietary fat intake. This metabolic leverage can explain why increased food offerings tend to raise the prevalence of obesity.