In this group of self-selected research volunteers, those assigned to a low-fat vegan diet reported major changes in nutrient intake that largely persisted for 74 weeks. Reductions in reported fat, saturated fat, and cholesterol intake and increases in carbohydrate and fiber intake were greater in the vegan group, compared with the ADA group. These changes are similar to those reported in earlier studies using low-fat vegan diets (13
). Neither diet was associated with increases in reported hunger, tendency to overeat, or cravings. Compared with the ADA diet, the vegan diet required slightly more initial effort, but was less likely to be described as constraining. These findings suggest that both a vegan diet and a diet following 2003 ADA guidelines meet a reasonable level of acceptability, at least among motivated individuals, although the vegan diet appears to elicit much more pronounced long-term nutritional changes.
Dietary changes are essential for management of body weight, glycemia, blood lipids, and blood pressure, yet the acceptability of therapeutic diets has been largely neglected in clinical research. In the current study, dietary acceptability was assessed using a variety of indices, including attrition, adherence, and nutritional changes, as well as instruments quantifying eating behavior, diet acceptability, and cravings.
To the extent that attrition serves as a crude indicator of diet acceptability, it is noteworthy that there was no attrition during the initial 22 weeks. In previous diet studies for diabetes, attrition has varied widely. Attrition was low in older individuals (6% in a 10-week study  and 8% in a 6-month study ), and higher in a California Health Maintenance Organization study (15% at 5 months) (15
), a Costa Rican study (19% at 12 weeks) (16
), two Minneapolis studies (18%  and 46% , both at 6 months), and an Urban Atlanta study (approximately 50% at 6 months) (19
Dietary adherence criteria were met by a somewhat higher percentage in the vegan group (67%, 33/49) at 22 weeks, compared with the ADA group (44%, 22/50). These criteria, however, were not identical for the two groups. Notably, the vegan group had no energy intake limit, but had a stricter criterion for saturated fat (5% vs. 10% of energy). These criteria were used solely for statistical purposes and were more permissive than the guidelines used in presenting the diets to the participants. It is of concern that saturated fat intake in the ADA group exceeded 10% of energy for 34% (17/50) of participants at 22 weeks. This may have been the result of a focus on carbohydrate, at the expense of attention to fat intake, on the part of many ADA–group participants. At 74 weeks, group-specific dietary adherence criteria were met by similar numbers within each group—48% (24/50) in the ADA group and 51% (25/49) in the vegan group (p = 0.75). It is possible that the change from weekly meetings to biweekly optional meetings may have adversely influenced adherence; prior studies have shown that continued nutrition education increases adherence (2
The increase in dietary restraint among ADA-group participants at 22 weeks suggests that the ADA diet may have been experienced as somewhat more demanding than the vegan diet. Although the difference was statistically significant, it was moderate in magnitude at 22 weeks and no longer significant at 74 weeks. In a 14-week trial of a diet based on National Cholesterol Education Program guidelines, which are similar in certain respects (reductions in saturated fat and cholesterol) to the ADA guidelines, in postmenopausal overweight women, disinhibition and hunger scores fell (improved), although reported dietary restraint increased, suggesting that participants felt somewhat perturbed by the diet change, but were less likely to overeat in response to food cues and were less hungry (20
The ADA guidelines require vigilance regarding portion sizes to limit fat and carbohydrate intake and set energy-intake limits for overweight individuals. In contrast, the vegan diet requires participants to forgo certain familiar foods and, in some cases, to learn new tastes and new cooking methods. However, it does not limit portion sizes, energy intake, or carbohydrate intake, or require counting or estimating quantities of foods or food constituents.
Despite a wide range of responses to the Food Acceptability Questionnaire items, there was no suggestion of a lack of acceptability of either diet. This is not surprising for the ADA diet, which is drawn from familiar foods. The vegan diet required marginally more initial effort, but this difference was no longer apparent at 74 weeks.
Neither group reported increased cravings for any foods, and the vegan group reported a small but significant reduction in craving for fatty foods at 22 weeks, compared with the ADA group. This finding contradicts the notion that individuals adopting vegan diets have continued cravings for excluded foods. On the contrary, the desire for fatty foods appeared to diminish.
Overall, these results accord with prior studies of the acceptability of vegetarian or vegan diets. In a study of a program of lifestyle changes for reversing coronary atherosclerosis, there were no identifiable differences in the acceptability of a low-fat, vegetarian diet, compared to the diets followed by control participants referred to their physicians for cardiac care (21
). At five-year follow-up, most patients continued to maintain reductions in plasma cholesterol concentration and body weight, suggesting that dietary modifications had been largely maintained (22
In a trial using a low-fat, vegan diet for dysmenorrhea, participants reported after five weeks that the vegan diet required somewhat more effort, compared to an unrestricted diet, but was otherwise no different in any measure of acceptability or enjoyment (23
). Among overweight, postmenopausal women randomly assigned to either a low-fat, vegan diet or a diet following the guidelines of the National Cholesterol Education Program, there were no between-group differences on any measure of acceptability. (24
). Similarly, in a study of 250 young women who had tried both energy-restricted weight-loss diets and vegetarian diets, the median duration on an energy-restricted diet was 4 months, while the median duration on a vegetarian diet was 24 months (25
Vegetarian and vegan diets have demonstrated efficacy in the management of cardiovascular disease (22
), weight problems (26
), and diabetes (4
). That they are also perceived as practical and acceptable to many patients, at least in the research setting, is clinically important.
The findings of the present study and previous investigations show that vegetarian and low-fat vegan diets should not be described as extreme, difficult, or unacceptable, at least among research volunteers. They are highly effective in adducing major nutrient intake changes and achieve levels of adherence and acceptability comparable to those of other therapeutic diets. Nonetheless, while vegan diets have the advantages of being lower in fat, saturated fat, and cholesterol, and higher in fiber and complex carbohydrate, compared with omnivorous diets, planning is important with regard to sources of vitamins B12 and D, calcium, and iodine. (27
The study’s strengths include a demographically varied population, a sample size that was adequate to show between-group differences in nutrient intake, the use of multiple independent quantitative measures of adherence and acceptability, and an appropriate follow-up period. The present study also has limitations. All participants had diabetes and were self-selected research volunteers. Many were well-educated. Hospital and community dietetic practices include many such individuals, but also include patients with less education and motivation. The randomization procedure produced groups that were dissimilar for occupational status. Fewer ADA participants were retirees; retirees were more likely than workers to be adherent at 22 weeks (although this difference was no longer present at 74 weeks). All participants also had access to group support throughout the study. Dietary intake was based on self-report, which is vulnerable to distortion based on participants’ tendency to provide socially desirable responses and to underreporting, especially in individuals with higher BMIs (28