A randomized clinical trial of a standard versus vegetarian diet for weight loss: the impact of treatment preference

International Journal of Obesity (2008) 32, 166–176 & 2008 Nature Publishing Group All rights reserved 0307-0565/08 $30.00 www.nature.com/ijo ORIGINAL ARTICLE A randomized clinical trial of a standard versus vegetarian diet for weight loss: the impact of treatment preference LE Burke1, M Warziski2, MA Styn2, E Music2, AG Hudson3 and SM Sereika4 1 Department of Health and Community Systems and the Graduate School of Public Health, Department of Epidemiology, University of Pittsburgh School of Nursing, Pittsburgh, PA, USA; 2Department of Health and Community Systems, University of Pittsburgh School of Nursing, Pittsburgh, PA, USA; 3Department of Epidemiology, University of Pittsburgh Graduate School of Public Health, Pittsburgh, PA, USA and 4Department of Health and Community Systems and the Graduate School of Public Health, Department of Biostatistics and Department of Epidemiology, University of Pittsburgh School of Nursing, Pittsburgh, PA, USA Background: With obesity rampant, methods to achieve sustained weight loss remain elusive. Objective: To compare the long-term weight-loss efficacy of 2 cal and fat-restricted diets, standard (omnivorous) versus lacto– ovo–vegetarian, and to determine the effect of a chosen diet versus an assigned diet. Design, subjects: A randomized clinical trial was conducted with 176 adults who were sedentary and overweight (mean body mass index, 34.0 kg/m2). Participants were first randomly assigned to either receive their preferred diet or be assigned to a diet group and second, were given their diet of preference or randomly assigned to a standard weight-loss diet or a lacto–ovo– vegetarian diet. Participants underwent a university-based weight-control program consisting of daily dietary and exercise goals plus 12 months of behavioral counseling followed by a 6-month maintenance phase. Measurements: Percentage change in body weight, body mass index, waist circumference, low- and high-density lipoprotein, glucose, insulin and macronutrient intake. Results: The program was completed by 132 (75%) of the participants. At 18 months, mean percentage weight loss was greater (P ¼ 0.01) in the two groups that were assigned a diet (standard, 8.0% (s.d., 7.8%); vegetarian, 7.9% (s.d., 8.1%)) than in those provided the diet of their choice (standard, 3.9% (s.d., 6.1%); vegetarian, 5.3% (s.d., 6.2%)). No difference was observed in weight loss between the two types of diet. Over the 18-month program, all groups showed significant weight loss. Conclusions: Participants assigned to their dietary preference did not have enhanced treatment outcomes. However, all groups lost weight with losses ranging from 4 to 8% at 18 months. International Journal of Obesity (2008) 32, 166–176; doi:10.1038/sj.ijo.0803706; published online 14 August 2007 Keywords: treatment preference; lacto–ovo–vegetarian; behavioral weight-loss treatment; randomized clinical trial Introduction According to the most recent report by Ogden et al.,1 the prevalence of overweight and obesity has not declined. Concurrent to the increasing prevalence in overweight and obesity, marked improvements have occurred in behavioral weight-loss interventions.2 Behavioral counseling for lifestyle change and weight control combined with increased Correspondence: Dr LE Burke, University of Pittsburgh School of Nursing and the Graduate School of Public Health, 3500 Victoria St, 415 Victoria Building, Pittsburgh, PA 15261, USA. E-mail: Received 14 February 2007; revised 7 May 2007; accepted 24 June 2007; published online 14 August 2007 physical activity and reduced calorie and fat intake have proven effective for short-term weight loss.3,4 However, researchers continue to seek methods to maintain improved dietary patterns and a healthy weight for the long term. Adoption of a vegetarian diet has resulted in improved health outcomes. Research has shown that such diets can be followed for sustained periods5–7 and can produce lasting weight loss.8,9 Others’ work as well as our own have shown that individuals who chose a vegetarian diet for weight loss report staying on that particular eating plan longer than some of the well-known fad diets.6,7 In a recent observational study, women who followed a semivegetarian, lactovegetarian or vegan diet had a lower risk of overweight and obesity than omnivorous women.10 Earlier work, by our group found that a vegetarian meal plan combined with a Dietary treatment preference and weight loss LE Burke et al 167 behavioral weight-counseling intervention led to weight loss and that those who followed the plan had not regained weight at 18 months.11 Literature suggests that changing from a diet containing meat and fish to a vegetarian diet may result in reduced calorie and saturated-fat intake, an increase in energy from carbohydrates, resulting in a leaner body mass12 and less weight regain.13 While these findings are promising, they may have been partially attributable to a self-selection effect among those who adhered to the diet. Moreover, to our knowledge, only one study of vegetarian diets has targeted weight loss and that study used the more restrictive 10% fat and vegan diet.14,15 No randomizedcontrolled trials have examined the role of the less restrictive lacto–ovo–vegetarian diet (LOV-D) in weight loss. Participant preference is another factor in the success of many treatments but has rarely been examined in weightloss efficacy.16–19 Conceptual models such as behavioral choice theory20 postulate that those who are matched to treatment based on their personal choices will demonstrate better outcomes than those assigned to a treatment that does not match their personal preference. The few studies that have examined the effect of providing participants their preferred weight-loss treatment have produced inconsistent findings.16–18 Moreover, the longest study was only 6 months in duration, thus not permitting an evaluation of the long-term effects of receiving one’s preference. Our preliminary study7 did not take preference into account in the random assignment to diet, and we are unaware of any randomized clinical trials that have examined treatment preference using diets that include a vegetarian option. We designed the present study to test two hypotheses: (1) choice of either a standard calorie- and fat-restricted diet (STD-D) or a calorie- and fat-restricted LOV-D would result in greater weight loss compared to having one of these diets randomly assigned; and (2) an LOV-D would result in greater weight loss than a STD-D. We chose to test both of these hypotheses together to address the potential effect of selection bias in the successful adoption and long-term adherence to a LOV-D. We also evaluated the effect of the treatment assignment on waist circumference, low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C), glucose, insulin and selected macronutrients and adherence to the treatment protocol. 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