Why do obese patients not lose more weight when treated with low-calorie diets? A mechanistic perspective

Abstract

Maximal weight loss observed in low-calorie diet (LCD) studies tends to be small, and the mechanisms leading to this low treatment efficacy have not been clarified. Less-than-expected weight loss with LCDs can arise from an increase in fractional energy absorption (FEA), adaptations in energy expenditure, or incomplete patient diet adherence. We systematically reviewed studies of FEA and total energy expenditure (TEE) in obese patients undergoing weight loss with LCDs and in patients with reduced obesity (RO), respectively. This information was used to support an energy balance model that was then applied to examine patient adherence to prescribed LCD treatment programs. In the limited available literature, FEA was unchanged from baseline in short-term (<12 wk) treatment studies with LCDs; no long-term (>or=26 wk) studies were found. Review of doubly labeled water and respiratory chamber studies identified 10 reports of TEE in RO patients (n = 150) with long-term weight loss. These patients, who were weight stable, had a TEE almost identical to measured or predicted values in never-obese subjects (weighted mean difference: 1.3%; range: -1.7-8.5%). Modeling of energy balance, as supported by reviewed FEA and TEE studies, suggests that obese subjects participating in LCD programs have a weight loss less than half of that predicted. The small maximal weight loss observed with LCD treatments thus is likely not due to gastrointestinal adaptations but may be attributed, by deduction, to difficulties with patient adherence or, to a lesser degree, to metabolic adaptations induced by negative energy balance that are not captured by the current models.

Does metabolic compensation explain the majority of less-than-expected weight loss in obese adults during a short-term severe diet and exercise intervention?

“…Previous studies that have considered the influence of changes in RMR on less-than-expected weight loss have relied on measurements taken in energy balance before and after energy restriction. [2][3][4] Consequently, the extent to which the reduced RMR during energy restriction may have accounted for the less-than-expected weight loss was likely underestimated. In the study from Corral et al, 2 a daily kilocalorie discrepancy was determined from averaging the total energy expenditure measured (using doubly-labelled water) in energy balance at baseline and after B12 kg (15.5%) weight loss, then subtracting the energy intake during energy restriction (800 kcal per day) to get the 'actual' EDef, and from this the 'expected' weight loss was determined.…”

“…1 However, baseline EDef calculations such as these commonly overestimate the actual weight loss achieved. 2,3 Although a lack of adherence is often cited as the primary reason for the shortfall in weight loss, [2][3][4] it is also recognised that biological compensatory responses are elicited when energy restriction is imposed, essentially acting to reduce energy expenditure, 5 which in turn reduces the EDef and can reduce the weight loss. [6][7][8][9][10][11] Furthermore, the energy density of weight loss is not uniform, and initial body fat, the magnitude of weight loss and use of resistance exercise or high-protein diets may influence the applicability of the Wishnofsky constant.…”

Dietary adherence and weight loss success among overweight women: results from the A TO Z weight loss study

“…For example, in a recent review on low-calorie diets, the authors stated that the lack of success of such diets was likely due to difficulties with participants adherence. 14 Overall, studies investigating dietary adherence level and its association with weight loss success in randomized clinical trials in a large sample are scarce. Even the few studies that have examined the associations between dietary adherence and weight loss were constrained by a number of limitations such as self-reported measure of adherence measure, small sample sizes, low retention rate and short study follow-up period.…”

What is the required energy deficit per unit weight loss?

“…6 The fact that weight loss typically slows over time for a prescribed constant diet 9,37 suggests that either the energy expenditure decreases with time, or the dietary intervention is relaxed over time, or both.…”