Flexible Dieting (IIFYM) vs Plant-Based & Vegan Diets

Flexible Dieting (IIFYM)Any foods that fit the day’s remaining macro and calorie targets — no specific food list.

Plant-Based & Vegan DietsLegumes, tofu, tempeh, seitan, whole grains, nuts and seeds; the vegan version fully excludes meat, poultry, seafood, dairy, eggs, and honey.

Flexible Dieting (IIFYM)None categorically banned — the only requirement is hitting daily macro/calorie targets.

Plant-Based & Vegan DietsVegan: all animal-derived foods and ingredients. Plant-based (looser): minimizes but doesn’t require full exclusion of meat, dairy, or eggs.

Flexible Dieting (IIFYM)No specific eating window — organized around daily macro/calorie totals, not timing.

Plant-Based & Vegan DietsNo specific eating window.

Flexible Dieting (IIFYM)Hit daily protein/carb/fat and calorie targets with maximum food flexibility.

Plant-Based & Vegan DietsCardiometabolic health, often alongside environmental or ethical motivations.

Flexible Dieting (IIFYM)People who do well with structure-light, numbers-based tracking and want the social and travel flexibility of no off-limits foods.

Plant-Based & Vegan DietsPeople motivated by environmental or ethical concerns, or by the cardiometabolic risk reduction the research points to, who are willing to plan meals rather than eat on autopilot.

Flexible Dieting (IIFYM)Defined entirely by protein/carb/fat gram targets and usually total calories — the core organizing principle of the diet.

Plant-Based & Vegan DietsNone specified — food-group based, not macro-ratio based.

Flexible Dieting (IIFYM)Research comparing flexible and rigid approaches to dietary restraint has generally linked flexible, non-black-and-white eating rules to lower rates of binge eating and disordered-eating symptoms than strict, all-or-nothing dieting, though findings aren't fully consistent across studies. The approach is also widely used in physique and bodybuilding contest prep, valued as a way to hit macro targets without the psychological rigidity of a fixed 'clean eating' list.

Plant-Based & Vegan DietsLower BMI, lower rates of type 2 diabetes and heart disease, and lower all-cause mortality in large cohort studies — concentrated in “healthful” plant-based patterns, not ones built on refined carbs and sugar.

Flexible Dieting (IIFYM)Limited/indirect — no peer-reviewed study has evaluated “IIFYM” by that name; supporting evidence comes from the related but distinct “flexible vs. rigid dietary restraint” literature, mostly correlational, with one small RCT (n=23) on macro-based tracking.

Plant-Based & Vegan DietsStrong cohort evidence (e.g. Adventist Health Study-2, ~73,000 people) though observational rather than randomized for the core mortality/disease claims.

Flexible Dieting (IIFYM)Low — no food is off-limits, but requires consistent daily tracking to work.

Plant-Based & Vegan DietsVariable — plant-based is low restrictiveness (a spectrum, no full exclusion); vegan is high (full animal-product exclusion).

Flexible Dieting (IIFYM)Flexible and affordable in principle since no specialty foods are required, though the diet says nothing about fiber, vitamins, minerals, or protein quality.

Plant-Based & Vegan DietsGenerally less expensive overall than a meat- and dairy-centered diet, since savings from cutting meat and dairy tend to outweigh the added cost of vegetables, whole grains, and plant-based meat substitutes, though reliable access to fresh produce remains a real barrier in areas with limited grocery options.