Authors' response to Kahleova, Katz and Barnard
Many of the concerns raised by Kahleova, Katz and Barnard were addressed in the manuscript, but we will elaborate here.
1. We aimed to control as many factors as possible between treatment groups within the context of real-life diets (not liquid research formulas) over the course of a full academic year, including: the proportion of sugar to total carbohydrate, the proportion of saturated fat to total fat, the protein source, and non-starchy vegetables. While some dietary factors differed, as acknowledged in the Discussion section, the resulting diets reflect healthful representations of their respective macronutrient compositions. The high carbohydrate diet was in fact substantially higher in fiber than the lower carbohydrate diet, and exceeded prevailing recommendations on whole grain consumption. The diet effect on biomarkers, such as triglycerides, reflects the observed associations with total carbohydrate intake among a general population. Furthermore, the glycemic index of the high carbohydrate diet was quite low for a diet that must, in practice, include significant amounts of grains. Indeed, we avoided extreme manipulations of all groups – such as using an uncharacteristically low amount of saturated fat in the low carbohydrate group. A methods paper provides comprehensive details of dietary design, including our approach to foster treatment differentiation with conservative food substitutions (1).
2. The adjustment of energy intake was done to maintain weight stability after weight loss to test the primary hypothesis, as extensively discussed in our methods. Failure to control weight loss, as Kahleova et al seem to prefer, would severely confound our primary outcome, total energy expenditure.
3. We agree that spontaneous increases in physical activity level may mediate, to some degree, the diet effect, as hypothesized by the carbohydrate-insulin model (2, 3). (We provided no instruction to change physical activity level.) With the greater observed availability of metabolic fuels (4), participants may have felt more energetic. Interestingly, evidence suggests that physical activity level, like hunger, is under biological control (5).
4. We disagree that fat restriction is more effective for weight control, as demonstrated by multiple meta-analyses (6, 7, 8, 9, 10). We disagree that a low fat diet has benefit for longevity, as shown in a prospective 3-cohort study (11) and elsewhere. High fat foods like nuts, avocado, olive oil, full fat yogurt, fatty fish and dark chocolate are associated with protection against diabetes, heart disease and perhaps neurodegenerative diseases – but that debate is beyond the scope of this venue. We address the issue of translation to public health in the manuscript.
1. Julia MW Wong, Lisa Bielak, Ralph G Eddy, Lauren Stone, Paul R Lakin, Megan Sandman, Courtenay Devlin, Linda Seger-Shippee, Dina Wiroll, Patricia K Luoto, Gloria L Klein, David S Ludwig, Cara B Ebbeling. An academia-industry partnership for planning and executing a community-based feeding study. Curr Dev Nutr 2018; 2(9): nzy060.
2. Ludwig DS, Ebbeling CB. The Carbohydrate-Insulin Model of Obesity: Beyond “Calories In, Calories Out”. JAMA Intern Med 2018;178:1098-103
3. Ludwig DS, Friedman MI. Increasing adiposity: consequence or cause of overeating? JAMA 2014;311:2167-8
4. Walsh CO, Ebbeling CB, Swain JF, Markowitz RL, Feldman HA, Ludwig DS. Effects of diet composition on postprandial energy availability during weight loss maintenance. PLoS One 2013;8(3):e58172
5. Richmond RC, Davey Smith G, Ness AR, den Hoed M, McMahon G, Timpson NJ. Assessing causality in the association between child adiposity and physical activity levels: a Mendelian randomization analysis. PLoS Med 2014;18;11:e1001618.
6. Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr 2013;110:1178-87
7. Mancini JG, Filion KB, Atallah R, Eisenberg MJ. Systematic review of the mediterranean diet for long-term weight loss. Am J Med 2016;129:407-415
8. Mansoor N, Vinknes KJ, Veierød MB, Retterstøl K. Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials. Br J Nutr 2016;115:466-79
9. Sackner-Bernstein J, Kanter D, Kaul S. Dietary intervention for overweight and obese adults: comparison of low-carbohydrate and low-fat diets. A meta-analysis. PLoS One 2015;10:e0139817
10. Tobias DK, Chen M, Manson JE, Ludwig DS, Willett W, Hu FB. Effect of low-fat diet interventions versus other diet interventions on long-term weight change in adults: a systematic review and meta-analysis. Lancet Diabetes Endocrinol 2015;3:968-79
11. Dong D. Wang, Yanping Li, Stephanie E. Chiuve, Meir J. Stampfer, JoAnn E. Manson, Eric B. Rimm, Walter C. Willett, Frank B. Hu. Association of specific dietary fats with total and cause-specific mortality. JAMA Intern Med 2016;176:1134-45
Competing interests: As detailed in the manuscript