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. Macro tracking is not the same thing as flexible restraint, though, and hitting exact numbers every day can itself become rigid.

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. One small RCT (23 people, about 40% dropout) tested macro-based “IIFYM” dieting by name. Most other support comes from the related but distinct flexible-versus-rigid dietary restraint research, which is mostly correlational.

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.

Flexible Dieting (IIFYM)Not studied. The approach sets only protein, carb, fat, and calorie targets. It says nothing about fiber, vitamins, or minerals, so food quality is up to the individual.

Plant-Based & Vegan DietsVitamin B12 is the main one for vegans, and it needs fortified foods or a supplement. Iodine, calcium, zinc, selenium, vitamin D, and omega-3s also need planning. Planned well, vegan diets can meet adult nutrient needs.

Flexible Dieting (IIFYM)Not studied. The main reported challenge is psychological: hitting exact macro numbers every day can become rigid for some people.

Plant-Based & Vegan DietsMore beans often means more gas at first, though fewer than half of people report it in the first week. No adjustment timeline has been studied.

Flexible Dieting (IIFYM)Breakfast: a bagel with two eggs and cheese. Lunch: chicken burrito bowl with rice, black beans, and salsa. Dinner: pasta with lean beef tomato sauce and a side salad. Snack: Greek yogurt with granola, or ice cream if it fits the day’s numbers.

Plant-Based & Vegan DietsBreakfast: oatmeal made with fortified soy milk, ground flax, and berries. Lunch: chickpea and quinoa salad with vegetables, tahini, and pumpkin seeds. Dinner: tofu or tempeh stir-fry with broccoli over brown rice. Snack: hummus with carrots. Plus a B12 supplement or fortified foods.

Flexible Dieting (IIFYM)Low cooking burden, but weighing and logging food every day is the main time cost. Socially flexible, since nothing is banned, though restaurant meals are hard to log accurately.

Plant-Based & Vegan DietsModerate prep, with planning for protein variety and fortified foods. Plant-based eating is easy at restaurants. Strict vegan eating is harder, since animal ingredients hide in many dishes.

Flexible Dieting (IIFYM)People with a history of disordered eating should be cautious. Many people being treated for eating disorders say food-tracking apps made their condition worse, although trials in low-risk groups found no harm. People with kidney disease should check protein targets with a clinician.

Plant-Based & Vegan DietsChildren on vegan diets need careful planning, since studies show shorter height and lower iron and B12 status. Pregnant and breastfeeding people need a reliable B12 source and should ask about omega-3s. One large study found higher hip fracture risk in vegans. Warfarin users should keep leafy-green intake steady.

Flexible Dieting (IIFYM)Evidence is thin. In the only trial, about 40% of participants dropped out over 10 weeks, similar to a rigid meal plan. Research links flexible eating attitudes with less binge eating and better weight outcomes, but that is not the same as macro tracking. Not studied after people stop.

Plant-Based & Vegan DietsIn a six-month trial, people stuck with a vegan diet about as well as with other diets, and the vegan group lost the most weight. In a 74-week trial in type 2 diabetes, weight loss held up as well as on a standard diabetes diet. No trial has tracked what happens after people stop.