Plant-Based Diets Not Always Healthy; Quality Is Key

Diets consisting of high-quality — but not low-quality — plant-based foods and lower intakes of animal products may lower the risks for cancer, heart disease, and early death, new research suggests.

The prospective cohort study used data from more than 120,000 middle-aged adults followed for over 10 years in the UK Biobank. Those who consumed a healthful plant-based diet — with higher amounts of foods such as fruits, vegetables, legumes, whole grains, and nuts — and lower intakes of animal products, sugary drinks, and refined grains had a 16% lower risk of dying during follow-up compared to those with the lowest intakes of the healthful plant-based foods.

By contrast, an unhealthy plant-based diet was associated with a 23% higher total mortality risk.

“Not all plant-based diets are created equally. Our data provide evidence to support the notion that for health benefits the plant-based sources need to be whole grains, fruits and vegetables, legumes, nuts, etc, rather than processed plant-based foods,” study co-author Aedín Cassidy, PhD, of the School of Biological Sciences, Queen’s University, Belfast, Northern Ireland, told Medscape Medical News.

She added, “We do not necessarily need to radically shift diets to vegan or vegetarian regimens, but rather to switch proportions on the plate to incorporate more healthful plant-based foods, fish, and leaner cuts of meat into our habitual diet. This would have benefits for both individual health and planetary health.”  

The findings were published online March 28 in JAMA Network Open by Alysha S. Thompson, MSc, also at Queen’s University, and colleagues.

High- vs Low-Quality Plant-Based Diets Linked to Better Outcomes

The UK Biobank is a population-based, prospective study that included more than 500,000 participants aged 40-69 years at the time of recruitment between 2006 and 2010 at 22 centers in England, Scotland, and Wales. The current study included 126,395 individuals; slightly over half (55.9%) are women.

Food intake data were collected for at least two 24-hour periods to create both “healthful” and “unhealthful” plant-based diet indexes (PDIs). These included 17 food groups: whole grains, fruits, vegetables, nuts, legumes and vegetarian protein alternatives, tea and coffee, fruit juices, refined grains, potatoes, sugar-sweetened beverages, sweets and desserts, animal fat, dairy, eggs, fish or seafood, meat, and miscellaneous animal-derived foods. Data on oils weren’t available.

Higher scores on the healthful PDI (hPDI) and unhealthful PDI (uPDI) were scored positively or negatively based on quantities of those foods consumed.

Participants were then ranked in quartiles for portions of each food group and assigned scores between 2 (lowest-intake category) and 5 (highest).

During a follow-up of 10.6 to 12.2 years, there were 698 deaths attributed to cardiovascular disease, 3275 deaths due to cancer, 6890 individuals who experienced a cardiovascular incident, and 8939 with incident cancer.

Another 4751 experienced an incident fracture, which was evaluated due to the concern that diets low in animal protein might lead to insufficient vitamin B and calcium intake.

After adjustment for confounding factors, the hazard ratio (HR) for all-cause mortality in individuals with the highest healthful PDI score quartile compared with the lowest quartile was 0.84.

At the same time, the HR for all-cause mortality for those with the highest versus lowest unhealthful PDI scores was 1.23, and for cancer-related mortality was 1.19. All were statistically significant (P = .004).

Similarly, greater healthy plant-based diet adherence was associated with a significantly lower risk of being diagnosed with any cancer (HR, 0.93; P = .03), while higher unhealthful PDI scores yielded a higher risk (HR, 1.10; P = .004).

Moreover, higher healthy PDI scores were associated with lower risks for total cardiovascular incident risks (HR, 0.92; P = .007), as well as for the individual events of ischemic stroke (HR, 0.84; P = .08) and myocardial infarction (HR, 0.86; P = .004). Higher unhealthy PDI scores were similarly associated with greater risks for those outcomes, with an overall HR of 1.21 (P = .004).

No associations were found between either hPDI or uPDI and total or site-specific fracture risk.

And because 91.3% of the UK Biobank study population was White, “future studies among more racially, ethnically, and culturally diverse populations are needed to assess the risk of major chronic disease in relation to [plant-based diets],” the authors write.

Cassidy and Thompson reported no relevant financial relationships.

JAMA Netw Open. Published online March 28, 2023.

Miriam E. Tucker is a freelance journalist based in the Washington, DC, area. She is a regular contributor to Medscape, with other work appearing in The Washington Post, NPR’s Shots blog, and Diabetes Forecast magazine. She is on Twitter: @MiriamETucker.

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