How to make a low-carbohydrate diet look good, even when it’s not
The paper “Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial” by Ebbeling et al. published in the BMJ (1) has suggested that a low-carbohydrate diet can increase energy expenditure during a weight loss maintenance phase. Unfortunately, several methodological issues make the interpretation of the results problematic and the conclusions not well justified.
First of all, the main study objective was to find out if carbohydrate quantity had any significant effect on energy expenditure. In order to answer this question, it is necessary to keep the carbohydrate quality constant across the diets. This may be easily accomplished by standardizing the relative contribution of dietary fiber to total carbohydrates. However, the authors were not only changing carbohydrate quantity, but also carbohydrate quality. More specifically, glycemic index was increasing with the increasing carbohydrate content (GI=30 for low, GI=46 for moderate, and GI=49 for high carbohydrate content). In other words, the higher the carbohydrate quantity, the poorer the carbohydrate quality. This was also mirrored by an increase in serum triglycerides in the high-carbohydrate group, which is another marker of poor carbohydrate quality. Changing carbohydrate quality together with carbohydrate quantity has introduced a serious confounder into the pursuit of the stated research objective.
Another significant confounding factor was the repeated manipulation of caloric intake when the researchers were adjusting the participants’ energy intake periodically to maintain weight loss within 2 kg of the level achieved before randomization. Even though this study is claiming to have looked explicitly at the metabolic effects of carbohydrates, the changes in caloric intake may have significantly confounded the findings.
Interestingly, the authors admit that “total physical activity, and moderate to vigorous intensity physical activity, were marginally higher in the group assigned to the low carbohydrate diet,” but do not report the exercise in detail. The difference in exercise may well explain, at least partially, the difference in energy expenditure between the diet groups.
Furthermore, since the researchers did not report on body composition, it is plausible that the low-carbohydrate diet may have increased energy expenditure by augmented utilization of body protein, as shown previously (2).
Finally, the practical implications of the study’s findings are questionable. Long-term adherence to low-carbohydrate diets has been shown to be fairly low (3), plus these diets do not provide any benefits for weight management compared with other diets, particularly in studies with a duration of at least 12 months (4). Since it has been suggested that restriction of fat is more effective for weight loss than restriction of carbohydrate (5) and low-carbohydrate diets have been associated with higher all-cause mortality (6), higher cardiovascular and cancer mortality (7), and higher risk of type 2 diabetes (8), low-carbohydrate diets cannot be recommended for weight loss or weight maintenance without attention to all of these matters, and careful attention to the dietary composition (9). By introducing several significant confounders, the recent paper has just created more confusion. While it is possible to consume a healthy low-carbohydrate diet, the question is whether the low-carbohydrate content has any additional health benefits compared with a high-carbohydrate diet with the same carbohydrate quality.
References:
1. Ebbeling CB, Feldman HA, Klein GL, Wong JMW, Bielak L, Steltz SK, et al. Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial. BMJ. 2018 Nov 14;363:k4583.
2. Hall KD, Chen KY, Guo J, Lam YY, Leibel RL, Mayer LE, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men12. Am J Clin Nutr. 2016 Aug;104(2):324–33.
3. Dansinger ML, Gleason JA, Griffith JL, Selker HP, Schaefer EJ. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial. JAMA. 2005 Jan 5;293(1):43–53.
4. Snorgaard O, Poulsen GM, Andersen HK, Astrup A. Systematic review and meta-analysis of dietary carbohydrate restriction in patients with type 2 diabetes. BMJ Open Diabetes Res Care. 2017;5(1):e000354.
5. Hall KD, Bemis T, Brychta R, Chen KY, Courville A, Crayner EJ, et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metab. 2015 Sep 1;22(3):427–36.
6. Noto H, Goto A, Tsujimoto T, Noda M. Low-Carbohydrate Diets and All-Cause Mortality: A Systematic Review and Meta-Analysis of Observational Studies. PLoS ONE [Internet]. 2013 Jan 25;8(1). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3555979/
7. Fung TT, van Dam RM, Hankinson SE, Stampfer M, Willett WC, Hu FB. Low-carbohydrate diets and all-cause and cause-specific mortality: Two cohort Studies. Ann Intern Med. 2010 Sep 7;153(5):289–98.
8. de Koning L, Fung TT, Liao X, Chiuve SE, Rimm EB, Willett WC, et al. Low-carbohydrate diet scores and risk of type 2 diabetes in men123. Am J Clin Nutr. 2011 Apr;93(4):844–50.
9. Jenkins DJA, Wong JMW, Kendall CWC, Esfahani A, Ng VWY, Leong TCK, et al. Effect of a 6-month vegan low-carbohydrate ('Eco-Atkins’) diet on cardiovascular risk factors and body weight in hyperlipidaemic adults: a randomised controlled trial. BMJ Open. 2014 Feb 5;4(2):e003505.
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How to make a low-carbohydrate diet look good, even when it’s not
The paper “Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial” by Ebbeling et al. published in the BMJ (1) has suggested that a low-carbohydrate diet can increase energy expenditure during a weight loss maintenance phase. Unfortunately, several methodological issues make the interpretation of the results problematic and the conclusions not well justified.
First of all, the main study objective was to find out if carbohydrate quantity had any significant effect on energy expenditure. In order to answer this question, it is necessary to keep the carbohydrate quality constant across the diets. This may be easily accomplished by standardizing the relative contribution of dietary fiber to total carbohydrates. However, the authors were not only changing carbohydrate quantity, but also carbohydrate quality. More specifically, glycemic index was increasing with the increasing carbohydrate content (GI=30 for low, GI=46 for moderate, and GI=49 for high carbohydrate content). In other words, the higher the carbohydrate quantity, the poorer the carbohydrate quality. This was also mirrored by an increase in serum triglycerides in the high-carbohydrate group, which is another marker of poor carbohydrate quality. Changing carbohydrate quality together with carbohydrate quantity has introduced a serious confounder into the pursuit of the stated research objective.
Another significant confounding factor was the repeated manipulation of caloric intake when the researchers were adjusting the participants’ energy intake periodically to maintain weight loss within 2 kg of the level achieved before randomization. Even though this study is claiming to have looked explicitly at the metabolic effects of carbohydrates, the changes in caloric intake may have significantly confounded the findings.
Interestingly, the authors admit that “total physical activity, and moderate to vigorous intensity physical activity, were marginally higher in the group assigned to the low carbohydrate diet,” but do not report the exercise in detail. The difference in exercise may well explain, at least partially, the difference in energy expenditure between the diet groups.
Furthermore, since the researchers did not report on body composition, it is plausible that the low-carbohydrate diet may have increased energy expenditure by augmented utilization of body protein, as shown previously (2).
Finally, the practical implications of the study’s findings are questionable. Long-term adherence to low-carbohydrate diets has been shown to be fairly low (3), plus these diets do not provide any benefits for weight management compared with other diets, particularly in studies with a duration of at least 12 months (4). Since it has been suggested that restriction of fat is more effective for weight loss than restriction of carbohydrate (5) and low-carbohydrate diets have been associated with higher all-cause mortality (6), higher cardiovascular and cancer mortality (7), and higher risk of type 2 diabetes (8), low-carbohydrate diets cannot be recommended for weight loss or weight maintenance without attention to all of these matters, and careful attention to the dietary composition (9). By introducing several significant confounders, the recent paper has just created more confusion. While it is possible to consume a healthy low-carbohydrate diet, the question is whether the low-carbohydrate content has any additional health benefits compared with a high-carbohydrate diet with the same carbohydrate quality.
References:
1. Ebbeling CB, Feldman HA, Klein GL, Wong JMW, Bielak L, Steltz SK, et al. Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial. BMJ. 2018 Nov 14;363:k4583.
2. Hall KD, Chen KY, Guo J, Lam YY, Leibel RL, Mayer LE, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men12. Am J Clin Nutr. 2016 Aug;104(2):324–33.
3. Dansinger ML, Gleason JA, Griffith JL, Selker HP, Schaefer EJ. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial. JAMA. 2005 Jan 5;293(1):43–53.
4. Snorgaard O, Poulsen GM, Andersen HK, Astrup A. Systematic review and meta-analysis of dietary carbohydrate restriction in patients with type 2 diabetes. BMJ Open Diabetes Res Care. 2017;5(1):e000354.
5. Hall KD, Bemis T, Brychta R, Chen KY, Courville A, Crayner EJ, et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metab. 2015 Sep 1;22(3):427–36.
6. Noto H, Goto A, Tsujimoto T, Noda M. Low-Carbohydrate Diets and All-Cause Mortality: A Systematic Review and Meta-Analysis of Observational Studies. PLoS ONE [Internet]. 2013 Jan 25;8(1). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3555979/
7. Fung TT, van Dam RM, Hankinson SE, Stampfer M, Willett WC, Hu FB. Low-carbohydrate diets and all-cause and cause-specific mortality: Two cohort Studies. Ann Intern Med. 2010 Sep 7;153(5):289–98.
8. de Koning L, Fung TT, Liao X, Chiuve SE, Rimm EB, Willett WC, et al. Low-carbohydrate diet scores and risk of type 2 diabetes in men123. Am J Clin Nutr. 2011 Apr;93(4):844–50.
9. Jenkins DJA, Wong JMW, Kendall CWC, Esfahani A, Ng VWY, Leong TCK, et al. Effect of a 6-month vegan low-carbohydrate ('Eco-Atkins’) diet on cardiovascular risk factors and body weight in hyperlipidaemic adults: a randomised controlled trial. BMJ Open. 2014 Feb 5;4(2):e003505.
Competing interests: No competing interests