Dietary adherence and acceptability of five different diets, including vegan and vegetarian diets, for weight loss: The New DIETs study

• No differences for dietary adherence and acceptability across the five diets
• No differences in FAQ, TFEQ, or PFS scores across the five groups
• Preferring or disliking the assigned diet did not impact adherence at six months.
• Non-adherent vegans/veg lost more weight than non-adherent omnivores.
• Non-adherent vegans/veg had lower cholesterol intake than non-adherent omnivores.

The goal of the present study was to examine dietary adherence and acceptability among participants from the New DIETs study who were randomized to one of four plant-based diets (vegan, vegetarian, pesco-vegetarian, semi-vegetarian) or an omnivore diet. Primary outcomes at two- and six months included dietary adherence (24-hour dietary recalls), weight loss and changes in animal product intake (mg cholesterol) by adherence status, Three-Factor Eating Questionnaire (TFEQ), Power of Food Scale (PFS), dietary acceptability (Food Acceptability Questionnaire), and impact of diet preference on adherence. No differences were found in dietary adherence or changes in FAQ, TFEQ, or PFS among the groups. At six months, non-adherent vegan and vegetarian participants (n = 16) had a significantly greater decrease in cholesterol intake (− 190.2 ± 199.2 mg) than non-adherent pesco-vegetarian/semi-vegetarian (n = 15, − 2.3 ± 200.3 mg, P = 0.02) or omnivore participants (n = 7, 17.0 ± 36.0, P = 0.04). Non-adherent vegan/vegetarian participants lost significantly more weight at six months (− 6.0 ± 6.7%) than non-adherent omnivore participants (− 0.4 ± 0.6%, P = 0.04). Dietary preference had no impact on adherence at six months. Due to equal rates of adherence and acceptability among the diet groups, instructing participants to follow vegan or vegetarian diets may have a greater impact on weight loss and animal product intake than providing instruction in more moderate approaches even among non-adherent participants.