Should we eat meat?

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Homer talks about the vegetarians in ancient Greece. Leonardo da Vinci reportedly abstained from meat, as did 19th-century poet Percy Bysshe Shelley. Throughout history there have been small groups of people arguing against eating meat, largely for ethical reasons.

Today’s equivalents are nutrition experts — and they increasingly have data on their side.

Although researchers disagree about exactly how much meat is OK to eat, most agree that less is better. Harvard nutrition guru Dr. Walter Willett says he eats red meat only once or twice a year. New York Times food writer Mark Bittman doesn’t eat any meat products for breakfast or lunch, and only sparingly later in the day. Dr. Neal Barnard believes all animal products, including fish, are bad for both the heart and the brain, so he doesn’t eat any at all.

Research consistently shows that regularly dining on red meat, pork, or cured meats is bad for the heart and increases the risk of colon cancer. The studies aren’t clear, however, on how much meat causes problems, or how liberally we can include chicken or fish in our diets.

“I think there’s strong evidence that cutting back on red meat is beneficial,” said Willett, chairman of the department of nutrition at the Harvard School of Public Health, who says he eats chicken and fish in moderation.

Gary Fraser, who runs a long-range study of 35,000 vegetarians at Loma Linda University in California, said vegetarians fare better than moderate meat-eaters on measures of longevity, heart disease, diabetes, high blood pressure, obesity, and a few cancers. Giving up all animal products, including fish, dairy, and eggs is even better in measures of weight, diabetes, and high blood pressure, his research suggests.

Among non-red meat eaters and vegetarians, there are degrees of improvements: chicken and fish eaters are healthier than those who also eat red meat; people who eat only fish are better off than those who also consume chicken; those who eat no meat but eat eggs and cheese — lacto-ovo vegetarians — are better still; and vegans have the lowest levels of obesity, diabetes, and high blood pressure, his research suggests.

But, “we’re not quite at the point yet of being able to publish definitive evidence” that a vegan diet is best, said Fraser, who eats eggs, a little dairy, and an occasional piece of fish.

It’s possible, Fraser said, that future research will find little difference between those who avoid meat altogether and those who indulge less than once a week. “For many people, it might be easier to do that.”

Barnard, an adjunct associate professor of medicine at George Washington University, is more dogmatic. He recently ran an international conference on nutrition and the brain, and he believes all meat — including fish — is bad for the heart, and by extension, the brain.

“There is no [nutritional] requirement for meat. Zero,” said Barnard, adding that his own vegan diet is tasty, easy, and varied. “This is not punishment.”

So in the face of such studies, why haven’t we stopped eating meat?

More research might help turn the tide. People could be more willing to limit their meat consumption to once or twice a week if there was definitive evidence showing benefits of cutting back.

But it’s tough to be definitive about dietary advice, because scientists can’t tell one group of people to eat one way for decades and a different group to eat another. Most research, therefore, is in mice, or looking at large groups of people and asking them to remember how they ate in the past or to keep food diaries — which are notoriously inaccurate. Measures of longevity and obesity that test people after-the-fact don’t meet the gold standard of scientific research.

In decades of offering nutrition advice, Dr. Dean Ornish said he’s learned one thing: Don’t tell people what not to eat.

“Fear is not a sustainable motivator,” he said. “Even more than being healthy, most people want to feel in control.”

For the last 36 years, Ornish has studied a comprehensive diet and lifestyle program, including a whole foods, plant-based diet, stress management, moderate exercise, and social support, including friendship and intimacy.

This kind of lifestyle, which he described in his book “The Spectrum,” can reverse heart disease, may stop or reverse early stage prostate cancer, and perhaps even slow aging, his research has shown.

“The more you change the more you improve at any age,” said Ornish, a clinical professor at the University of California, San Francisco, and founder and president of the Preventive

Medicine Research Institute in Sausalito, Calif.

Ornish said he was a strict vegetarian for 30 years, but now can’t resist an occasional piece of sushi.

Anecdotally, there’s more interest in vegetarianism in recent years, with food writers and scientists regularly extolling its virtues.

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Bittman, who’s been eating vegan before 6 p.m., or “VB6” (the title of his new book), for more than six years, said he doesn’t think the details matter as much as the general trend. “I think it’s important that people move toward eating more food and less crap,” he said.

Bittman said the evidence of growing interest in vegetarianism is obvious: “When Chipotle is doing a vegan burrito with tofu, when TCBY is doing almond frozen yogurt — this is becoming mainstream.”

The trend isn’t reflected in national polling data, though.

Gallup Inc. has found little change between 1999 and 2012, with only 5 to 6 percent of Americans considering themselves vegetarians and 2 percent vegans.

Primary care doctors used to respond with horror when patients announced they had given

up eating meat. Doctors might cluck their tongues and worry aloud about the nutrients their charges were missing.

Now, most doctors and medical organizations have come around at least to a neutral stance on vegetarianism.

The American Heart Association, for instance, touts the apparent heart-healthy benefits of vegetarian diets on its website, though it warns of possible vitamin deficiencies: Vegetarians who subsist on pizza, chips, and cake are unlikely to be any healthier than most meat-eaters, researchers agreed.

Chef Mollie Katzen, whose cookbooks have educated people about vegetarian eating for decades, said she’s seen a sea change in attitudes in recent years.

The explosion of farmers’ markets has made great produce easier to get, said Katzen, whose newest book “The Heart of the Plate: Vegetarian Recipes for a New Generation” comes out in September. Cooking is now considered a worthwhile endeavor, instead of a chore, she said, and people who have to watch their budgets realize that it’s far cheaper to cook than eat out.

Katzen said she has nothing against meat. “I’m just profoundly pro-vegetable.”