Low-carbohydrate diets help many folks, but not all, lose excess fat weight. When low-carb diets help, it may be related to Total Energy Expenditure (TEE). When you read “energy,” think calories. TEE is a combination of calories needed for 1) basic life processes (i.e., basal metabolic rate, as needed to maintain heart beats, breathing, steady body heat, growth and repair of tissues, etc.), 2) processing of ingested food (dietary thermogenesis), and 3) physical exercise.
Here’s the abstract of an article in The Journal of Nutrition that examines the headline question. It’s complicated and I haven’t read the full study yet.
Many obesity experts believe that to lose excess fat weight, you have to ingest fewer calories than you burn on a daily basis for physical exercise and basal metabolic rate. This creates a calorie (energy) deficit. Your body satisfies that deficit by converting fat tissue to weightless energy. The authors of the study at hand are essentially saying that, after 2-3 weeks, a low-carb diet “revs up your metabolism” to burn more calories. That can help you lose weight or maintain weight loss, unless you over-eat.
Here you go, nutrition nerds:
The effect of macronutrient composition on total energy expenditure (TEE) remains controversial, with divergent findings among studies. One source of heterogeneity may be study duration, as physiological adaptation to lower carbohydrate intake may require 2 to 3 wk.
We tested the hypothesis that the effects of carbohydrate [expressed as % of energy intake (EI)] on TEE vary with time.
The sample included trials from a previous meta-analysis and new trials identified in a PubMed search through 9 March 2020 comparing lower- and higher-carbohydrate diets, controlled for EI or body weight. Three reviewers independently extracted data and reconciled discrepancies. Effects on TEE were pooled using inverse-variance-weighted meta-analysis, with between-study heterogeneity assessed using the I2 statistic. Meta-regression was used to quantify the influence of study duration, dichotomized at 2.5 wk.ResultsThe 29 trials ranged in duration from 1 to 140 d (median: 4 d) and included 617 participants. Difference in carbohydrate between intervention arms ranged from 8% to 77% EI (median: 30%). Compared with reported findings in the prior analysis (I2 = 32.2%), we found greater heterogeneity (I2 = 90.9% in the reanalysis, 81.6% in the updated analysis). Study duration modified the diet effect on TEE (P < 0.001). Among 23 shorter trials, TEE was reduced on lower-carbohydrate diets (−50.0 kcal/d; 95% CI: −77.4, −22.6 kcal/d) with substantial heterogeneity (I2 = 69.8). Among 6 longer trials, TEE was increased on low-carbohydrate diets (135.4 kcal/d; 95% CI: 72.0, 198.7 kcal/d) with low heterogeneity (I2 = 26.4). Expressed per 10% decrease in carbohydrate as %EI, the TEE effects in shorter and longer trials were −14.5 kcal/d and 50.4 kcal/d, respectively. Findings were materially unchanged in sensitivity analyses.
Lower-carbohydrate diets transiently reduce TEE, with a larger increase after ∼2.5 wk. These findings highlight the importance of longer trials to understand chronic macronutrient effects and suggest a mechanism whereby lower-carbohydrate diets may facilitate weight loss.
In the Discussion section the authors write:
This finding supports a prediction of the carbohydrate-insulin model and suggests a mechanism whereby dietary carbohydrate reduction could aid in the prevention and treatment of obesity. According to this model, the high insulin-to-glucagon ratio with a diet high in glycemic load (mathematical product of glycemic index and carbohydrate amount) shifts the partitioning of metabolic fuels from oxidation in lean tissue to storage in adipose tissue. If the effects observed here persist over the long term, then reducing dietary carbohydrate intake by half from 60% of energy intake (a typical level for low-fat diets) would increase energy expenditure by ∼150 kcal/d, counterbalancing (if not compensated for by other factors) much of the secular increase in energy intake thought by some to underlie the obesity epidemic.
Steve Parker, M.D.