High-quality plant diets protect against Alzheimer’s, even when started late at life

• Past studies show that eating a healthy, nutritious diet — such as a plant-based diet — may help in lowering a person’s risk of developing Alzheimer’s disease.
• While a plant-based diet can be healthy with whole grains, legumes, and vegetables, it can also be unhealthy if it contains ultra-processed foods, fried foods, or foods high in sugars and salt.
• A new study found that consuming an unhealthful plant-based diet may actually increase a person’s risk for Alzheimer’s disease and other types of dementia.

One such diet is a plant-based diet, which emphasizes eating whole foods like vegetables, fruits, whole grains, legumes, nuts, and seeds, with little or no animal-based protein.

While a plant-based diet can be healthy, it can also be unhealthy if you choose ultra-processed foods, fried foods, or foods high in sugars and salt.

Now, a new study published in Neurology, the medical journal of the American Academy of Neurology, has found that consuming an unhealthful plant-based diet may actually increase your risk for Alzheimer’s disease and other types of dementia.

For this study, researchers recruited almost 93,000 adults with an average age of 59. The study participant pool included people from several different ethnic groups, including Caucasian, African American, Japanese American, Native Hawaiian, and Latino.

Study participants were asked to fill out food questionnaires at the start of the study to measure the types of healthy and unhealthy foods they consumed, and were given scores based on how closely they followed a healthy or unhealthy plant-based diet.

“Plant foods have long been associated with broad health benefits, and plant-based diets have gained interest for environmental and other reasons,” Song-Yi Park, PhD, associate professor of population sciences in the Pacific Program at the University of Hawaii Cancer Center in Honolulu, Hawaii, and corresponding author of this study, told Medical News Today. “However, an earlier study by another group suggested that healthful and unhealthful plant foods may carry opposing effects on dementia risk.”

“Since this cohort has followed a large number of people from five racial and ethnic groups for (about) 30 years, we hoped to find out whether different quality plant foods are related to the subsequent ADRD risk differently. Especially, we wanted to see what happens to the related ADRD risk after people change their plant-diet quality over time,” she said.

Study participants were followed for an average of 11 years, and ranked into five subgroups based on their plant-based diet scores.

During that time, researchers found that participants in the best subgroup who ate the most plant foods had a 12% lower risk of dementia, compared to participants in the lowest subgroup.

When examining participants’ scores based on how healthy their plant-based diet was, the top subgroup lowered their dementia risk by 7%, while the lowest subgroup that consumed the most unhealthy plant-based foods increased their dementia risk by 6%.

“This finding confirmed the overall hypothesis that people consuming more plant foods and more healthful plant foods were less likely to develop dementia later on,” Park said.

Additionally, Park and her team analyzed a subset of their participant pool, who were followed for 10 years, to look at how participants’ diets changed over time.

Researchers discovered that the participants whose diets changed to a more unhealthy plant-based diet increased their dementia risk by 25%. And those who moved away from an unhealthy plant-based diet lowered their dementia risk by 11%.

“This finding was more interesting, more indicative of potential benefits from individual changes,” Park said.

“The fact that we found the strongest dementia relationship from changes in unhealthful plant foods, rather than all plant foods or even healthful plant foods, seems helpful to understand what changes we can start with. And these relationship patterns were similar in people who were younger or older than age 60 at baseline, suggesting it’s not (too) late to make the change at an older age.”
— Song-Yi Park, PhD

MNT spoke with Dung Trinh, MD, an internist for MemorialCare Medical Group and chief medical officer of the Healthy Brain Clinic in Irvine, CA, about this study, who commented that its results add meaningful evidence to something many clinicians have suspected for a long time: diet quality appears to matter for brain health, not just heart health.

“What stood out to me is that this was a very large, multiethnic cohort followed over many years, and the findings were not simply about ‘eating more plants,’ but about the difference between healthier and less healthy plant-based eating patterns,” Trinh explained.

“Just as important, the study found that changes in diet over time were linked to dementia risk, suggesting that improving eating habits later in life may still be worthwhile,” he said.

“At the same time, I would be careful not to overstate the results, because this was an observational study, so it shows an association rather than proving cause and effect,” he added.

As next steps for this research, Trinh said he would like to see intervention studies that test whether improving diet quality can directly reduce cognitive decline or dementia risk, rather than only being associated with it.

“I would also like researchers to better understand which specific dietary changes drive the greatest benefit, how diet interacts with other risk factors such as diabetes, hypertension, and genetics, and whether some populations benefit more than others,” he added.

MNT also spoke with Monique Richard, MS, RDN, LDN, a registered dietitian nutritionist and owner of Nutrition-In-Sight, about this research, who said that one of the most important distinctions to make is that while a plant-based diet can be healthy, much of its health benefits depend on the types of food they select.

“A plant-based diet can be incredibly health-promoting or it can also be highly processed and low in essential nutrients if convenient, manufactured food-like items are prioritized, or the only products available,” Richard detailed.

“Dietary patterns for an individual need to be adequate, varied, balanced, high in quality, nutrient-rich, and based on whole, real food curated by Mother Nature,” she continued.

“Research consistently shows that whole, minimally processed plant foods such as vegetables, fruits, legumes, whole grains, nuts, and seeds, are associated with better cognitive and cardiometabolic outcomes. In contrast, plant-based diets high in refined grains, added sugars, and ultra-processed foods may not offer the same protection and can even contribute to inflammation, metabolic dysfunction, and systemic deterioration.”
— Monique Richard, MS, RDN, LDN