Global Study Supports Meat-Free Diets for Cancer Prevention

An international study of nearly 2 million people suggests that meat-free diets can help stave off several major cancers — but it also reached some unexpected conclusions.

In what researchers describe as the largest-ever meta-analysis of meatless diets and cancer risk, compared with meat-eaters, vegetarians showed reduced risks for five cancers, including breast, prostate, and pancreatic. That was independent of factors such as physical activity, body weight, smoking habits, alcohol intake, and medical history.

“This study is really good news for those that follow a vegetarian diet because they have a lower risk of five cancer sites, some of which are really prevalent in the population,” study lead author Yashvee Dunneram, PhD, of Newcastle University, Tyne, England, said at a press briefing on the findings.

The analysis, published in the British Journal of Cancer, looked at data from nine observational studies conducted in the UK, US, India, and Taiwan. In total, they included more than 1.8 million participants who completed detailed questionnaires on lifestyle and medical factors and were followed for a median of 16 years.

While most were omnivores, the population included over 63,000 vegetarians. And compared with their meat-eating counterparts, vegetarians had reduced risks for:

• Multiple myeloma, 31% lower (hazard ratio [HR], 0.69; 95% CI, 0.51-0.93);
• Kidney cancer, 28% lower (HR, 0.72; 95% CI, 0.57-0.92);
• Pancreatic cancer, 21% lower (HR, 0.79; 95% CI, 0.65-0.97);
• Prostate cancer, 12% lower (HR, 0.88; 95% CI, 0.79-0.97); and
• Breast cancer, 9% lower (HR, 0.91; 95% CI, 0.86-0.97).

On the other hand, vegetarians were no less likely to develop colorectal cancer than meat-eaters — which would seem to conflict with a large body of evidence linking high intake of red and processed meats to an increased risk for the disease and consumption of whole grains and fiber to a protective effect.

Dunneram said her team was, in fact, “quite surprised with this finding.”

But the researchers also stressed that the reported intake of processed meats in this global study was low, at a median of about 16 g/d. For comparison, the average intake in the UK general population is more than double that amount.

That point was echoed by Dagfinn Aune, PhD, a researcher at Imperial College London, London, England, who was not involved in the study.

“It’s possible that lumping all meat-eaters (regardless of how much or little meat they ate) together may have diluted any effects of vegetarian diets on cancer risk, particularly if meat intake was low in some studies,” Aune said in comments shared via Science Media Centre.

In another unexpected finding, vegetarians had nearly double the risk for esophageal squamous cell carcinoma compared with meat-eaters.

Senior author Aurora Perez-Cornago, PhD, a nutritional epidemiologist at the University of Oxford, Oxford, England, said she could only speculate on the reasons.

It’s possible, for example, that people who exclude meat from their diets are more likely to have certain nutritional deficiencies. Perez-Cornago noted that low intake of riboflavin (vitamin B2, largely found in meat) has been tied to esophageal cancer risk.

Perhaps most surprising of all, vegans — who eschew all animal products, including dairy foods — had a 40% greater risk for colorectal cancer than meat-eaters (HR, 1.40; 95% CI, 1.12-1.75).

Again, the reasons are unclear, but Dunneram said it could be related to a mineral lacking in some vegans’ diets: calcium. Research has tied higher intake of dairy products, and specifically calcium, to lower colorectal cancer risk.

However, the findings on vegan diets could also come down to numbers, the researchers pointed out: The analysis included 8849 vegans in total and found only 93 cases of colorectal cancer among vegans across seven studies from the US and UK.

Aune said that studies including a “much larger” number of vegans are needed. He also noted that based on prior cohort studies, vegans (and vegetarians) may have a lower overall cancer incidence than meat-eaters.

On balance, the study authors said, meat-free diets may help reduce cancer risk — but vegetarians and vegans might need to boost their intake of certain nutrients, from fortified foods or supplements.

The analysis did have several limitations, according to Anne McTiernan, MD, PhD, a professor at Fred Hutch Cancer Center in Seattle, who studies lifestyle factors and cancer risk.

Besides the relatively small number of vegans, the study lacked data on Black and Hispanic individuals, which limits its generalizability, McTiernan told Medscape Medical News.

And as with any observational research, confounders are an issue. People who follow meat-free diets tend to maintain a lower body weight over time, for example.

Still, McTiernan doubted that body weight fully accounts for the reduced cancer risks seen here as the researchers adjusted for BMI (with weight and height self-reported in some studies and measured in others).

As for the take-home message, McTiernan agreed that vegans, in particular, may want to be careful that they are getting enough of certain vitamins and minerals.

But overall, the findings support the types of plant-rich diets long endorsed by groups such as the World Cancer Research Fund/American Institute for Cancer Research, the experts said.

The analysis also hinted at benefits from cutting out red and processed meat alone.

Among nearly 43,000 pescatarians — people who eat fish but no meat or poultry — the risks for breast (HR, 0.93), colorectal (HR, 0.85), and kidney (HR, 0.73) cancers were reduced relative to meat-eaters. Meanwhile, men who reported eating poultry, but no red or processed meat, had a decreased risk for prostate cancer (HR, 0.93).

In sum, Aune said, “these findings provide further support for dietary recommendations that emphasize higher intakes of whole plant foods, such as whole grains, fruits, vegetables, nuts and legumes and less meat.” And, McTiernan noted, it’s never too late for people to change their dietary habits.

“Clinical trials have shown immediate benefits to vegetarian diets, like reductions in lipids and weight loss — things that can affect health across the board,” she said.

This study was funded by the World Cancer Research Fund, Cancer Research UK, the Medical Research Council and others. The authors declared having no competing interests.

Ernie Mundell is a freelance medical journalist based in Los Angeles. He has more than 30 years of experience, including editorial positions at Reuters Health and HealthDay.

A version of this article first appeared on Medscape.com.