Brain

Plant-based diets go with lower dementia and Parkinson’s risk

By WhyStopHarmingAnimals.com

Meat, especially processed meat, tracks with a higher risk of dementia[1][3], and healthful plant-based diets track with lower risks of both Alzheimer’s disease[2] and Parkinson’s[7]. Most of the evidence comes from large cohorts, with one small randomized trial pointing the same way. The research on mood is split[14][15][16].

Dementia and Alzheimer’s

The largest recent study is a Harvard analysis of 133,771 people published in Neurology in 2025[1]. Eating at least a quarter serving of processed red meat a day, compared with less than a tenth, came with 13% higher dementia risk[1], and each extra daily serving came with about 1.6 years of faster cognitive aging[1]. Replacing a serving of processed red meat with nuts and legumes came with 19% lower dementia risk[1]. Unprocessed red meat tracked with self-reported cognitive decline as well[1].

A 2026 cohort of 92,849 people, also in Neurology, looked at the whole diet[2]. People with the highest scores on a plant-based diet index had 12% lower risk of Alzheimer’s and related dementias than those with the lowest[2], and the healthful plant index came with 7% lower risk[2]. Junk plant foods such as refined grain, juice and sugar raised risk[2], so the benefit belonged to whole plant foods[2].

The link is old. Among Adventists, Giem, Beeson and Fraser found in 1993 that matched meat-eaters, including those who ate poultry and fish, were more than twice as likely to become demented as their vegetarian counterparts[3], and later research on vegetarian diets and dementia found vegetarian patterns carried the lowest dementia risk of the diet groups compared[4]. Grant’s 2023 review in the Journal of Alzheimer’s Disease lists higher intake of saturated and total fat, meat and ultra-processed food, and lower intake of fruit, vegetables, legumes and nuts, among the identified risk factors for Alzheimer’s[5].

In 2024 Ornish and colleagues reported the first randomized trial of intensive lifestyle change in people with mild cognitive impairment or early Alzheimer’s (Alzheimer’s Research & Therapy)[6]. The intervention group got delivered whole-food plant-based meals, exercise, stress management and group support for 20 weeks[6]. Cognition and function improved or held steady in most of them[6], while the control group mostly got worse[6]. The trial was small, about 51 people[6], and diet was one part of a package[6], so it can’t isolate the food[6]. Its direction matches the cohorts.

Parkinson’s disease

In UK Biobank, over 11.8 years and 577 new cases, the healthiest plant-based eating came with 22% lower Parkinson’s risk[7] and unhealthy plant-based eating with 38% higher risk[7] (Movement Disorders, 2023). Vegetables, nuts and tea each tracked with 25–31% lower risk on their own[7]. The link held in people without a high genetic risk score[7] but not in those with one.

Dairy points the other way[8]. In two long cohorts reported in Neurology in 2017, three or more servings of low-fat dairy a day came with 34% higher Parkinson’s risk than none, driven by skim and low-fat milk[8]. Total dairy on its own was not significant[8].

All of the dementia and Parkinson’s findings above, except the Ornish trial, are observational[6]. They show which diets go with which risks, adjusted for the usual confounders, and can’t prove cause.

Mood goes both ways

Some studies favor plant-based eating[9][10][11][12]. In Beezhold and Johnston’s small pilot trial (2012), omnivores randomized to drop meat, fish and poultry improved on some short-term mood scores[9], and a 2015 survey found vegans reported less stress and anxiety than omnivores[10]. In a meta-analysis of 709,703 adults, the healthiest plant-based diets came with 23% lower depression risk[11], while unhealthy plant-based diets came with more depression and anxiety[11]. At ten corporate sites, 292 employees were randomized to weekly vegan-diet instruction or no change for 18 weeks, and the diet group improved on depression, anxiety and work-productivity scores[12].

Other studies point the opposite way[13][14]. Pooling 13 studies of 49,889 people, a 2021 meta-analysis found vegetarians scored higher on depression, with high heterogeneity between studies[13]. Iguacel and colleagues (Nutrition Reviews, 2021) found vegan and vegetarian diets associated with higher depression risk and lower anxiety[14]. A 2022 meta-analysis found no significant association between vegetarian eating and depression, anxiety or stress[15], and a 2025 umbrella review found that most reviews showed no association or conflicting results, with reverse causation plausible[16]. On mood, the evidence does not settle anything[14][15][16].

IQ comes first

Gale and colleagues followed 8,170 people from the 1970 British birth cohort (BMJ, 2007)[17]. Those who were vegetarian at 30 had scored about five IQ points higher at age 10[17]. The order matters: intelligence in childhood predicted vegetarianism later, so the study says nothing about the diet raising anyone’s IQ[17]. The link held after adjusting for social class and qualifications[17]. The vegan subsample was small and scored lower than the other vegetarians[17].