This 2017 narrative review compares the basis and implications of the traditional, calorie-centric understanding of obesity to an alternative concept that positions obesity as a hormonally regulated condition.
The authors refers to “Concept 1” as the model that views obesity as a disease of energy imbalance whereby individuals become fat by eating more calories than they burn. Dietary guidelines are broadly based on this concept and generally recommend obese individuals consume 500 to 750 fewer calories than they expend each day (1). Significantly, an excess of only 175 calories per day would be sufficient to drive 3 kg of annual weight gain (2).
Practically, this guideline is all but impossible to implement. First, we are unable to accurately and precisely assess how many calories an individual actually burns each day — the 175-calorie-per-day excess noted above is within the error bounds of even gold-standard methodologies to assess daily energy expenditure (3). Our ability to assess the number of additional calories burned through exercise and other forms of activity is just as bad, with estimated error of ~100 calories per day (4). And yet, even if we could assess energy expenditure accurately, it is impossible to accurately estimate calorie intake in real-world environments, as neither individuals nor digital tools can accurately estimate portion sizes (5), and data from food labels and restaurant nutrition listings is frequently inaccurate (6).