People who eat diets with small amounts of meat — or no meat at all — have lower risks for developing certain cancers, a new study found.
For some types of cancer, including colorectal cancer, diet may be a major lifestyle factor contributing to the risk for the disease. But for other cancers, like breast cancer and prostate cancer, it's not clear how direct the link really is, experts said.
The study included nearly half a million participants
The study, published this week in BMC Medicine, analyzed data for more than 470,000 people that was collected as part of the UK Biobank medical database project. When they were recruited, none of the participants had cancer.
A little more than half of the participants reported that they were regular meat-eaters, meaning they ate processed meat, red meat, or poultry more than five times per week. Another 205,000 of the participants were categorized as low meat-eaters, meaning they ate meat fives times per week or less. There were also about 11,000 pescatarians and 8,700 vegetarians in the study. (The 446 vegans in the study were included in the vegetarian group.)
Their results showed that, after more than a decade of follow-up, regular meat-eaters had a higher risk for all cancers compared to the other dietary groups. Those who followed a low-meat diet — especially men — had a reduced risk for colorectal cancer, vegetarian postmenopausal women had a lower risk for breast cancer and being vegetarian or pescatarian was associated with a reduced risk for prostate cancer in men.
“The take-home message would definitely be that there could be a lower risk of cancer amongst vegetarians and pescatarians,” Watling said, but he added that more research is needed, particularly with larger groups of vegetarians, Cody Watling, lead author on the study and a doctoral student in the cancer epidemiology unit at the University of Oxford, told TODAY.
Watling noted that although the results are interesting, he cautioned against making real-world recommendations based on the findings. “I don’t know if we’re at that stage yet,” he said.
This study builds nicely on previous work from another major research project that looked at cancer risk in 10 European countries, Stephen Hursting, professor in the department of nutrition and the Lineberger Comprehensive Cancer Center at the University of North Carolina at Chapel Hill, told TODAY. "It clearly suggested vegetarians had a lower cancer risk, " he explained, "but it wasn't cancer site-specific."
The new study adds to those findings by identifying that a diet involving less meat could be protective against specific cancer types, Hursting said, particularly colorectal cancer, breast cancer in postmenopausal women and prostate cancer. And because the researchers are drawing from UK Biobank data, "this is going to get better and better" as time goes on and the authors are able to draw in more from that data set, Hursting said. "It's sort of like we're getting the first taste of an early wine; as it ages it's going to get more and more powerful."
Diet is important for cancer risk, but other factors are too
The study's findings fit generally with the "hundreds" of other papers looking at diet and cancer risk, Dr. Steven K. Clinton, professor in the divisions of medical oncology and urology at The Ohio State University, told TODAY. "This is one out of many papers that kind of fits into the conclusion that you should focus on a healthy dietary pattern (to help prevent cancer)."
But there were some limitations. Clinton said the study's measure of meat-eating was "kind of a crude way of looking at things." The one-time measurement, based on a self-reported questionnaire, lumped all types of meat together rather than focusing on just processed meat products and red meat, which have consistently been linked to cancer and other health issues more frequently than leaner poultry, for instance.
And the study didn't account for the possibility that participants may have changed their eating habits over the years. Hursting also noted that the study wasn’t able to identify how the participants prepared the meat they ate.
"That's definitely a limitation of this study," Watling said. "So, definitely, individuals could have switched diets, and that may be influencing the results that we observed."
For some of the associations, other variables may have been at play. "Maybe some people, particularly on the far end of the spectrum that eat a lot of red or processed meat, may also have other health behaviors that are impacting this outcome," Clinton said. For instance, rates of smoking and alcohol intake were both higher among the meat-eaters than other groups.
While diet can be an important factor in cancer risk, "there's multiple factors that go into whether or not someone may develop cancer throughout their lifetime," Kristin Kirkpatrick, registered dietitian and founder and president of KAK Consulting, LLC, told TODAY. That might include some of these other behaviors (smoking, drinking alcohol), as well as genetics and environmental factors, she said.
In the case of postmenopausal breast cancer, participants’ BMI seemed to explain much of the association between diet and cancer risk. “That’s an established risk factor, but I don’t know that we’ve had a lot of data on whether the plant-based diet is protective independent of weight,” Hursting said. “Their data says maybe not; it’s really switching to that more vegetarian diet that helps the women control their weight better and that’s what’s driving the reduced risk.”
The reduction in prostate cancer the researchers saw among vegetarians was also interesting — and puzzling. "you don't see these huge reductions typically," Hursting said. But Clinton noted that vegetarians in this study were also much less likely to get screening than meat-eaters, though Watling said it's not clear why that's the case.
"Well, right there's a critical factor: If you're not doing screening, you're not going to find the cancer," Clinton said. "So I really can't put much credence in that."
Focus on your overall dietary patterns, not just one food
It's important to remember that a diet that contains a lot of meat is likely missing many other nutrient-rich foods, like vegetables and whole grains, Clinton said. But the current study only looked at the amount of meat its participants ate.
Our focus should really be on the overall balance of the foods we eat rather than trying to eat more or less of one specific food, Clinton said. "The whole dietary pattern is probably more important than any one food," he said. "And a person can choose a healthy dietary pattern that still has some red meat in it." Even among those who've had colorectal cancer in the past, eating red meat doesn't necessarily affect their chances for recurrence, other new research suggests.
Current guidelines from the World Cancer Research Fund and the American Institute for Cancer Research already recommend limiting processed and red meats to help prevent cancer, Clinton noted. Among other lifestyle goals, the guidelines also suggest limiting alcohol, reducing fast foods, staying physically active as well as eating higher amounts of vegetables, whole grains, beans and fruit.
"I tell my patients to look at red meat as you would dessert," Kirkpatrick said. "I always say, 'Listen, if you love a cheeseburger every once in a while, I don't see any reason why you have to cut that out. But let's define what that looks like.'" She might suggest that someone have their cheeseburger just twice a month, or to think about tweaking the recipe by swapping in a whole wheat bun or adding other nutrient-dense foods on top, like mushrooms or onions.
In fact, the field is heading more and more in the direction of precision nutrition, Hursting said, adding that the National Institutes of Health just provided major funding for centers across the U.S. to investigate these issues further. "Ultimately, in the food and nutrition space, you want to go from the general public health guidelines — which we know are good — into a more personal, prescribed recommendation," Clinton said.