Do Plant-Based Diets Prevent Cancer?

When it comes to naming the dietary patterns offering the most protection against cancer, plant-based diets emerge as the winner for reducing the risk for several cancers sensitive to lifestyle factors.

Though most research into dietary patterns over the past few decades has compared the Mediterranean diet with the Western diet, meta-analyses in the past 5 years have brought more attention to the benefits of plant-based diets. One of the leading hypotheses for the association between plant-based diets and reduced risk for certain cancers is how eating diverse plants affects the microbiome.

Most recently, for example, a study published on January 6 in Nature Microbiology compared the microbiomes of vegans, vegetarians, and omnivores across five cohorts totaling 21,561 individuals. Omnivores had more bacteria linked to increased risk for colon cancer, the researchers found, and microbes with favorable cardiometabolic markers were particularly plentiful in vegans’ microbiomes. But those healthy microbes in vegans also appeared in greater amounts in the microbiomes of omnivores who ate more plant-based foods. That finding suggests eating a diet emphasizing plants may be more helpful for preventing cancer than cutting out meat.

Decades ago, the idea that changing lifestyle behaviors could have an impact on cancer risk was so radical that many dismissed it as “false hope” sold by “snake oil salesmen,” according to Nigel Brockton, PhD, the vice president of research at the American Institute for Cancer Research (AICR). Today, it’s far better understood that lifestyle factors play a major role in many different cancers, but researchers are still working to disentangle precisely how. Nowhere is that more complex than with diet, suggested experts.

“Particularly over the last 10 years, there’s been a push in the research for [investigating] dietary patterns rather than individual foods or macronutrients,” Brockton said. The problem is, “if you just tell someone to eat a healthy diet, that means different things to many different people.” But as the evidence accumulates, it increasingly points to one over-arching theme: More plants, less cancer.

Defining ‘Plant-Based Diets’

The AICR promotes a plant-based diet based on the totality of evidence in its most current expert report on lifestyle factors and cancer. But a major research challenge in determining how protective plant-based diets might be in preventing certain cancers is the nebulous definition of “plant-based.” 

“Plant-based diet is actually an umbrella term that covers many different diet patterns,” said Elizabeth A. Platz, ScD, a professor of cancer epidemiology at Johns Hopkins Bloomberg School of Public Health, Baltimore. “There’s not a checklist.” 

Indeed, the AICR notes the multiple types of diets that fall under the plant-based umbrella. Instead of a checklist, they promote the “New American Plate,” composed of at least two thirds of vegetables, fruits, whole grains, and beans, and no more than one third of animal protein.

“There is no one definition of ‘plant-based diet’,” Anne McTiernan, MD, PhD, a professor of epidemiology at Fred Hutch Cancer Center, said. “Diets with a lot of vegetables and fruits, low in refined carbohydrates, and relatively low in saturated fats are ‘good’ in terms of reducing risk for obesity and at reducing risk of some cancers,” she continued. “However, no one diet stands out as ‘the diet’ one should follow.”

Carrie Daniel-MacDougall, PhD, MPH, an associate professor of epidemiology at The University of Texas MD Anderson Cancer Center in Houston, said AICR’s definition is more of a “plant-forward” diet.

“It doesn’t have to be entirely vegan or vegetarian. It just means you’re eating more plants than anything else,” she said. Having plants provide most of a person’s daily calories is a part of healthy diets of many different names, including the Mediterranean diet.

“From a research perspective and a public health perspective, we are pushing the same things every time,” Daniel-MacDougall said. “We’re just trying to get people to grasp it and do it.” Hearing “Mediterranean diet” may lead some to think only of hummus and parsley salad, or food that’s exotic or unfamiliar, she suggested. “Maybe ‘plant-based diet’ sounds more flexible, and they can fit that within what they already know and understand.” 

Another way to think about a plant-based diet is to move the focus of a meal away from meat being the central item, Platz said. Instead of eating meat as the main course with sides, “consider other sources of protein and make sure there’s plentiful, colorful fruits and vegetables,” she said.

In research, assessment of these dietary patterns varies greatly. Some studies use existing, pre-specified indices, such as the overall plant-based dietary index vs the healthful plant-based dietary index. Other examples of formal tools include the pro plant-based dietary pattern, the provegetarian food pattern, the EAT-Lancet (or Planetary Health) diet, and the portfolio diet. Others compare vegetarian or vegan dietary patterns more broadly with those who eat animal-based foods.

“Although the term plant-based is used, these are not vegan or vegetarian diets,” Edward Giovannucci, MD, a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health, Boston, said. “Individuals scoring highly on the healthy plant-based index still tend to eat ample amounts of animal-based foods, though they tend to be healthier, especially lower fat versions of meat and dairy.” 

What Research Findings Show

The vast majority of research assessing cancer risk and diet patterns focuses primarily on the Mediterranean diet, particularly as compared with a Western diet. But in the past 5-10 years, more studies have been zeroing in on vegetarian or mostly-plant-based diets, and enough evidence has accrued to draw several broad conclusions.

Digestive system cancers, including cancers of the esophagus, stomach, colon, rectum, liver, and possibly pancreas, appear to be the cancer types where plant-based diets are most beneficial for reducing risk, Edward Giovannucci, MD, a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health, said. They also might be protective against breast and prostate cancer, but again, evidence is still accruing, Giovannucci said. One 2023 study, for example, found plant-based diets reduce risk for recurrence of prostate cancer.

The most recent research uses different indices to assess dietary patterns without specifying the amount of animal-based foods that participants consumed. A 2023 meta-analysis included assessment of 22 prospective cohort studies on cancer with 57,759 participants and found greater adherence to a plant-based diet was associated with a 12% reduced risk for cancer, driven primarily by associations in reduction of breast, pancreatic, and prostate cancer. The benefit increased slightly to a 14% reduced risk when the dietary pattern emphasized healthy plant-based foods in particular, including vegetables, fruits, whole grains, and legumes. Those were studies that used the “healthful plant-based dietary index” instead of the “overall plant-based dietary index.” In fact, higher adherence to an “unhealthful plant-based dietary pattern” was linked to a 7% increased risk for cancer.

For specific cancers, plant-based diets were associated with a 9% reduced risk for breast cancer, a 13% reduced risk for prostate cancer, an 18% reduced risk for digestive system cancer, and 32% reduced risk for pancreatic cancer. Meanwhile, associations for colorectal, liver, lung, and stomach cancer did not reach significance. But with fewer studies looking at these cancers, there may simply not have been enough statistical power to reach significance. And, when researchers looked only at a healthful plant-based dietary pattern, colorectal cancer risk fell by 15%.

In exploring potential mechanisms for the associations, the authors noted the link between obesity and cancer and the fact that healthful plant-based dietary patterns tend to be low in energy density and saturated fat but high in fiber, which helps with both weight loss and maintenance. Plant-based diets also involve little to no consumption of red and processed meats, which are linked to higher risk for breast, colorectal, and lung cancers. That link is hypothesized to result from inflammation, oxidative stress, and dysfunctional insulin signaling from nitrates, heme iron, and other inflammatory components, according to a research review of the mechanisms that may explain the association. Plants, on the other hand, tend to have more anti-inflammatory and anti-oxidative effects that can interfere with cancer development.

Another 2023 meta-analysis of eight studies with 686,691 participants investigated potential links between vegetarian diets and gastrointestinal cancers. Vegetarians had a 23% lower risk for gastrointestinal cancers than non-vegetarians. More specifically, gastric cancer risk was 58% lower and colorectal cancer risk was 15% lower, but upper gastrointestinal cancer risk, except stomach, was not statistically different between the groups. A gender difference also emerged, with men showing a 43% reduced risk for overall gastrointestinal cancers but women showing no significant correlation. The risk reduction was also greater in Asian populations (57% lower) than in North American ones (24% lower).

Though still modest, the literature on different subtypes of plant-based diets or on specific cancers is growing. One 2023 meta-analysis of 10 studies narrowed the focus to plant- vs animal-based low carbohydrate diets. Cancer mortality was 14% higher with overall low carb diets and 16% higher with animal-based low carb diets. The plant-based low carb diets were associated with a lower risk for all-cause mortality, but they were not associated in either direction with cancer risk.

A 2022 meta-analysis of 49 studies and a little over 3 million participants looked only at digestive cancers and found an 18% lower risk for cancer among the cohort studies and a 30% lower risk among the case control studies with plant-based diets. Pancreatic cancer was 29% lower, colorectal cancer was 24% lower, rectal cancer was 16% lower, and colon cancer was 12% lower. There was no difference between risks with plant-based diets and vegan diets.

Earlier studies had focused specifically on vegetarian and vegan diets, which implied that participants following these patterns consumed no meat at all. A 2012 meta-analysis of seven studies found that vegetarians had an 18% lower risk for cancer than non-vegetarians. Similarly, a 2017 meta-analysis of 86 cross-sectional and 10 prospective cohort studies found an 8% reduced risk for cancer among vegans and vegetarians compared with omnivores, though no associations showed up for specific types of cancers.

But a shift appears to have occurred in the late 2010s, when researchers began looking not solely at vegetarian and vegan diets but at diets that were predominantly comprised of plants. An early meta-analysis in 2017, for example, did not specify “vegetarian” in comparing “plant-based” patterns to others. The analysis of one prospective cohort (492,306 participants) and six case control (10,558 participants) studies included four studies on colorectal cancer, two on gastric and esophageal cancers, and one on breast and ovarian cancer. Compared with high plant consumption diets, high meat consumption diets had a 64% increased risk for cancer. Compared with a mixed dietary pattern, plant-based diets had a 12% lower risk for cancer.

Identifying the Mechanisms

To some extent, the mechanisms for how plant-based diets can reduce risk for certain cancers isn’t a mystery, Platz said.

“It turns out, this strategy for eating, whether you follow more of a vegan version or an ad hoc version cobbling together things that are more plant-oriented, is just healthier in general,” she said. Plant-based diets “tend not to have ultra-processed foods, and they tend to be better for blood sugar because they have more fiber. They’re healthier ways of living that could reduce the risk of many chronic diseases, including diseases that themselves are risk factors for cancer, so ultimately they’re also useful for avoiding cancer.”

But there are some specific mechanisms that may play a role, she said. For example, H. pylori is an established risk factor for stomach cancer, and high sodium levels encourage the growth of the bacteria, thereby exacerbating the risk for gastric cancer. Salt is commonly used to preserve meat, so eating less meat can result in lower sodium intake.

Plant-based diets also tend to keep blood glucose low, and many plants are known to be anti-inflammatory, whereas high-sugar diets increase insulin, a growth factor, Platz said. Growth factors have long been implicated in cancer development.

Perhaps the simplest mechanism is the fact that plant-based diets are associated with lower overall caloric intake, thereby reducing risk for obesity, Brockton said. “Probably in the next 10 years, obesity will overtake smoking as the predominant risk factor for cancer,” he said.

Another likely mechanism that has been accruing more evidence is the role of a diverse plant-filled diet in promoting a healthy microbiome, both from increased fiber and from various phytochemicals.

“The microbiome has an impact on the immune system, so a healthier, more diverse microbiome leads to a healthier, more diverse immune system,” Brockton said. “They’re even showing that people with higher fiber intakes have better responses to immunotherapy in melanoma.”

While there is not enough evidence to link greater diversity of plant intake with reduced cancer risk, evidence does show that “people who eat a more diverse array of plants tend to have a more diverse microbiome,” Brockton said, and there’s increasing evidence that microbiome diversity is linked to survival with certain cancers. “That’s why the advice is to eat a range of fruits and vegetables, and that tends to offer the greatest benefit,” Brockton said.

It’s also true that those who eat a diet heavier in plants tend to exercise more, smoke less, and drink less — all behaviors that reduce cancer risk. Despite that correlation, “the diets seem to be beneficial independently,” Giovannucci said. “For example, we see protective associations in smokers and nonsmokers, in alcohol drinkers and nondrinkers, in overweight/obese and in lean individuals, in active and in sedentary people.” 

Food Quality Matters

Of course, not all plant-based diets are created equally. It would be easy to fill a diet with French fries, potato chips, and doughnuts and call it plant-based, but no one would mistake that diet for helping to prevent cancer.

“It’s important to distinguish between an overall plant-based diet and a healthy plant-based diet,” Giovannucci said. A healthy plant-based diet, for example, positively weights whole grains, fruits, vegetables, vegetable oils, nuts, and legumes, and it negatively weights refined grains, fruit juices, potatoes, sugar-sweetened beverages, and sweets, he explained.

One of the most helpful collations of risk is an evidence matrix developed by the AICR and the World Cancer Research Fund. It uses color coding to indicate the likelihood of increased or decreased risk for specific cancers and the strength of the evidence for that likelihood for over two dozen subgroups of foods, such as red meat, citrus fruits, non-starchy vegetables, whole grains, or vitamin C-rich foods. More evidence suggests a decreased risk with non-starchy vegetables for about a half dozen cancers, for example, than most other subgroups. The matrix also includes 15 dietary patterns or diets rich in particular components, such as beta carotene, retinol or saturated fatty acids.

A different, and more patient-friendly interactive cancer matrix from the same two organizations allows users to mouse over bubbles whose size correlates with effect size on risk. In each bubble, users can read more details about each component of a diet.

Daniel-MacDougall pointed out that food manufacturers frequently grab onto buzzwords that seem trendy so they can use it in their marketing. But just because something is “plant-based” doesn’t mean it’s unprocessed, and some plants can be ultra-processed into a form that’s no longer healthy. “A plant-based diet could be unhealthy if you’re getting all of it off a shelf,” she said.

How to Advise Patients 

With the limited time providers have with patients, delving into diet may seem an impossible task. But Daniel-MacDougall advises clinicians to find out where patients are starting from before offering advice. That doesn’t have to mean requesting a detailed food diary but simply asking what their typical breakfast, lunch, and dinner look like. Providers can then meet patients where they are. If they stop by a convenience every morning for breakfast, help them figure out what they can pick up at that store that’s healthier than their usual.

“Some people have more or less freedom or resources in their lives to make those decisions and changes,” Daniel-MacDougall said. She also suggests that clinicians offer patients more than one form of healthy diet, or more than one strategy for eating more plant-based foods. One way to do that is to focus on one goal at a time, starting with, for example, increasing fiber intake.

“While you’re working on that, you’re going to increase your plant foods by default because you don’t get fiber from animal foods,” she said. “But it’s also going to help you identify plant foods that are higher quality and less refined because those provide more fiber.”

Platz echoed this strategy, suggesting small shifts rather than radical diet makeovers. Incremental steps are particularly helpful for people who are time-poor. One such step could be eating a smaller serving of meat while doubling a vegetable serving, or adding a leafy vegetable or colorful berries.

“These diets should be viewed as aspirational,” Platz said. “People can make changes, moderate, and move in that direction.”

Daniel-MacDougall, Platz, Giovannucci, McTiernan, and Brockton had no disclosures.

Tara Haelle is a Dallas-based science journalist.