Plant-based and vegan diets

• Lower LDL cholesterol and ApoB and reduce the risk of heart disease.
• Lose weight and improve blood sugar control in type 2 diabetes.
• Reduce the risk of colorectal and other cancers.
• Live longer by replacing processed and red meat with plant foods.
• Reduce the environmental footprint of eating and avoid harm to animals.

"Plant-based" is used loosely, so the first step is to say which version you mean:

Plant-based diet vs vegan

Vegan describes what is excluded, often for ethical reasons. Whole-food plant-based describes what is eaten and is a health framing. A vegan diet built on white bread, fries, sweets and soda is plant-based in name only. US cohorts made this distinction measurable: a healthful plant-based diet index scored whole grains, fruit, vegetables, nuts, legumes, oils and tea or coffee positively, while an unhealthful index scored refined grains, potatoes, sweets, juices and sugary drinks positively [12].

The two behave very differently. The healthful pattern was linked to 25% lower coronary heart disease risk, the unhealthful one to 32% higher risk [6]. Every rule below is written with that in mind.

Blood lipids. This is the best-established effect. A meta-analysis of 30 randomized trials found that vegetarian and vegan diets lowered total cholesterol by 0.34 mmol/L, LDL cholesterol by 0.30 mmol/L and ApoB by 12.9 mg/dL compared with omnivorous diets [1]. In the Stanford twins trial, the vegan twin's LDL fell 13.9 mg/dL more than the omnivore twin's in 8 weeks [2]. See LDL cholesterol and ApoB.

Heart disease. In 48,188 EPIC-Oxford participants followed for 18 years, vegetarians had 22% lower rates of ischaemic heart disease than meat eaters, about 10 fewer cases per 1,000 people over 10 years [3]. A meta-analysis of cohorts put the reduction at 25% [4].

Reversal claims. Dean Ornish's Lifestyle Heart Trial combined a 10% fat whole-food vegetarian diet with exercise, stress management, smoking cessation and group support. Over 5 years, average coronary stenosis improved by 3.1 percentage points in the intervention group and worsened by 11.8 points in controls, with 25 versus 45 cardiac events [5]. The trial was small (48 randomized, 35 completed) and tested a whole lifestyle package, so it cannot isolate the diet.

The stroke signal. The same EPIC-Oxford analysis found 20% higher rates of total stroke in vegetarians, about 3 more cases per 1,000 over 10 years, mostly haemorrhagic [3]. Other cohorts have not consistently shown this. It is one reason to plan a plant-based diet well rather than treat it as automatically protective.

Weight. A meta-analysis of 11 trials in people with overweight or type 2 diabetes found vegan diets reduced body weight by 4.1 kg and BMI by 1.38 kg/m2 compared with control diets [7]. In a 16-week trial of 244 adults, a low-fat vegan diet reduced weight by 5.9 kg [8]. In a tightly controlled inpatient study, people ate 689 kcal a day less on a minimally processed plant-based, low-fat diet than on an animal-based ketogenic diet [10].

Type 2 diabetes. In 99 adults with type 2 diabetes, a low-fat vegan diet reduced weight by 6.5 kg versus 3.1 kg on a standard diabetes diet over 22 weeks, and HbA1c fell 1.23 versus 0.38 points among those whose medication did not change [9]. In the Adventist Health Study 2, diabetes prevalence was 2.9% in vegans and 7.6% in nonvegetarians [11]. A healthful plant-based index was linked to 34% lower diabetes risk even after adjusting for BMI [12]. See HbA1c and fasting insulin.

How to lower triglycerides on a plant-based diet

Triglycerides are the exception: the trial meta-analysis found no average effect on them [7]. If they are high, the usual focus is on legumes, whole grains and nuts in place of refined starches, juices and sweets; see triglycerides.

Mortality. Two large cohorts of health-conscious people disagree:

• In the Adventist Health Study 2 (73,308 people), vegetarians combined had 12% lower all-cause mortality than nonvegetarians (HR 0.88). The estimate for vegans was similar (0.85) but less certain [13].
• In two pooled UK cohorts (60,310 people), vegetarians and comparable meat eaters had the same all-cause mortality (HR 1.02) [14].
• Across cohorts, vegetarian diets were not linked to lower all-cause mortality overall [4].

The UK comparison group was itself health-conscious, a likely reason the gap disappears. In other words, the advantage of a vegetarian diet may be smaller than the advantage of a generally healthy lifestyle.

Cancer. Vegetarian diets were linked to 8% lower total cancer incidence, and vegan diets to 15% lower in the few studies available [4]. In Adventists, vegetarians had 22% lower colorectal cancer risk [15]. These are observational findings, and no trial has tested cancer outcomes. The colorectal finding fits the evidence on processed and red meat; see avoiding red meat.

Vitamin B12. Plants do not supply reliable B12. In studies using sensitive markers, deficiency affected 62% of pregnant vegetarian women, 25% to 86% of children, 21% to 41% of adolescents and 11% to 90% of older adults, with higher rates in vegans [20]. The consequences can be serious: the CDC reported two breastfed children of vegetarian mothers who developed neurologic impairment from B12 deficiency [21]. The Academy of Nutrition and Dietetics states that vegans need a reliable B12 source such as fortified foods or supplements [22]. Low B12 raises homocysteine, a useful lab check.

Iodine. Vegans had the lowest urinary iodine of any group and did not reach optimal status; a few who ate a lot of seaweed took in far too much [23]. Iodized salt or a modest supplement is the usual approach; seaweed is unreliable.

Omega-3. Plasma EPA and DHA were lower in vegetarian and vegan men than in meat eaters and did not change with years on the diet [24]. Whether lower levels matter for health outcomes is not known. Algae oil supplies EPA and DHA without fish.

Bone. Vegans in EPIC-Oxford had 2.31 times the risk of hip fracture and 43% higher risk of any fracture than meat eaters [16]. A meta-analysis found lower bone density in vegetarians and vegans and more fractures in vegans [17]. Lower BMI explained part of this, and adjusting for calcium and protein weakened it slightly. Calcium-set tofu, fortified plant milks, adequate protein and vitamin D are the planning points.

Iron. Plant iron is less well absorbed. Checking ferritin is sensible, especially for menstruating women.

Protein quality. Soy and wheat protein stimulate less muscle protein synthesis per gram than several animal proteins. The likely reasons are lower digestibility, more of the amino acids being used by the gut and liver, lower leucine and a less complete amino acid profile. Larger servings, blends of plant proteins and protein-rich foods such as soy can offset this [19].

What happens in practice. In a 12-week resistance training study, 19 habitual vegans and 19 omnivores had protein intake raised to 1.6 g per kg of body weight a day, using soy for vegans and whey for omnivores. Leg lean mass rose 1.2 kg in both groups, and muscle size and leg-press strength gains did not differ [18].

Practical targets.

• Put a legume or soy food in every meal: lentils, beans, chickpeas, tofu, tempeh, edamame or soy milk.
• Spread protein over 3 to 4 meals.
• Use a soy or pea protein powder if total intake falls short, especially for older adults or people training hard.
• Combine grains and legumes over the day to round out amino acids; they do not need to be in the same meal.

For protein-focused eating more broadly, see high-protein diets.

The case for. The Academy of Nutrition and Dietetics holds that appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate and suitable for pregnancy, childhood, older age and athletes [22]. The EAT-Lancet Commission proposed a mostly plant diet as healthy for people and the planet [25]. Randomized trials consistently show lower LDL, weight and HbA1c [1] [7].

The case against. Animal-based advocates such as physician Paul Saladino argue that vegan diets lead to frailty and that the cohort data favoring plant eaters reflect healthy user bias. Some of their points have support: vegans have more fractures [16], lower B12, iodine and omega-3 status [20] [23] [24], and plant protein is less anabolic per gram [19]. And UK vegetarians did not live longer than comparable meat eaters [14].

Where the evidence lands. Most of the claimed harms are nutrient gaps that supplements and food choices can close. Most of the claimed benefits apply to the healthful version of the diet, not to plant-based junk food [6]. A diet does not need to be fully vegan to capture much of the benefit: a Mediterranean diet or a diet that mainly cuts red and processed meat moves in the same direction.

Plant-based diet vs carnivore diet

No trial has compared the two on health outcomes. On LDL cholesterol, plant-based diets lower it in trials [1], while diets high in saturated animal fat tend to raise it. See carnivore diet for the other side.

Why it might work

The main mechanisms are lower saturated fat and higher fiber, which lower LDL, and lower energy density, which reduces calorie intake [10]. In mice prone to atherosclerosis, soy protein produced smaller arterial lesions than casein, even with similar blood lipids [26]; animal results support plausibility, not the human grade.

• Skipping vitamin B12, or relying on spirulina, seaweed or unwashed vegetables for it.
• Replacing meat with refined carbohydrates, vegan junk food and sugary drinks.
• Eating too little protein, especially for older adults and people training for strength.
• Ignoring iodine, calcium and vitamin D, which affects thyroid and bone health.
• Relying on large amounts of seaweed for iodine, which can deliver far too much.
• Assuming any plant-based label means healthy.

• You are pregnant or breastfeeding: a vegan diet can be adequate, but B12, iodine, iron and DHA need planning with a clinician or dietitian, because infant B12 deficiency can cause neurologic harm.
• You are planning a vegan diet for an infant or young child: work with a pediatrician or pediatric dietitian.
• You have osteoporosis, a history of fractures or a very low BMI: plan calcium, vitamin D and protein carefully and discuss it with your clinician.
• You have an eating disorder or a history of one: restrictive diets can mask or worsen it.
• You have chronic kidney disease: potassium and phosphorus from plant foods may need adjusting with your care team.
• You take insulin, sulfonylureas or blood pressure drugs: weight loss and better glucose control can require dose changes, so check with your prescriber.

• Gas and bloating as fiber and legumes increase; build up over a few weeks.
• Vitamin B12 deficiency without supplements, which can cause anemia, fatigue and nerve damage.
• Low iodine status, which affects thyroid function.
• Lower bone density and higher fracture risk, particularly in vegans.
• Iron deficiency, more common in menstruating women.
• Lower blood EPA and DHA, of uncertain clinical importance.