The energy balance model of obesity: beyond calories in, calories out

Abstract

A recent Perspective article described the "carbohydrate-insulin model (CIM)" of obesity, asserting that it "better reflects knowledge on the biology of weight control" as compared with what was described as the "dominant energy balance model (EBM)," which fails to consider "biological mechanisms that promote weight gain." Unfortunately, the Perspective conflated and confused the principle of energy balance, a law of physics that is agnostic as to obesity mechanisms, with the EBM as a theoretical model of obesity that is firmly based on biology. In doing so, the authors presented a false choice between the CIM and a caricature of the EBM that does not reflect modern obesity science. Here, we present a more accurate description of the EBM where the brain is the primary organ responsible for body weight regulation operating mainly below our conscious awareness via complex endocrine, metabolic, and nervous system signals to control food intake in response to the body's dynamic energy needs as well as environmental influences. We also describe the recent history of the CIM and show how the latest "most comprehensive formulation" abandons a formerly central feature that required fat accumulation in adipose tissue to be the primary driver of positive energy balance. As such, the new CIM can be considered a special case of the more comprehensive EBM but with a narrower focus on diets high in glycemic load as the primary factor responsible for common obesity. We review data from a wide variety of studies that address the validity of each model and demonstrate that the EBM is a more robust theory of obesity than the CIM.

Keywords: carbohydrates; energy balance; food intake; insulin; obesity.

FIGURE 1

The energy balance model of obesity posits that body weight is regulated by the brain in response to external signals from the food environment that are integrated with internal signals to control food intake below our conscious awareness. Increased prevalence of obesity has resulted from changes in the food environment leading to increased food intake and circulating fuels. Hormones, including insulin, respond to nutrient intake and absorption to direct the flow of metabolic fluxes into and out of various organs and provide signals to the brain that control food intake. Energy supply to organs such as liver and muscle increases, which supports their increased growth during the development of obesity and can result in ectopic lipid accumulation. Signals indicating the energy status of various organs are sensed by the brain to control food intake by mechanisms that remain to be fully elucidated. Oxidation of carbohydrate, fat, and protein provides the body with its energy needs, which increase as obesity develops. Adaptations of metabolic fuel selection as well as changes in the endocrine milieu ensure that partitioning of overall energy imbalances are primarily reflected as changes in adipose tissue triglyceride storage regardless of diet composition. Inherited variation in the operation of these processes, particularly those in the brain, are responsible for a substantial proportion of the interindividual difference in susceptibility or resistance to developing obesity in a particular environment. Thick blue arrows indicate the flow of energy. GI, gastrointestinal.