A July 2012 Gallup poll puts the percentage of American adults who say they consider themselves vegetarian at 5%, and those who consider themselves vegans—who eat no meat or dairy products—at 2%.

Far more Americans in a 2006 Gallup poll said they eat red meat and dairy regularly: 60% and 71%, respectively.

But of course, that isn’t necessarily confirmation of the benefits of meat and dairy: Good health, like good sense, does not always reside with the majority.

No one is arguing that Americans should be required to eat meat or dairy products—or broccoli, for that matter. For many people, the decision comes down to convenience, habit and taste.

But whatever you currently like to eat, digging into some of the issues that define this debate could be good for your health. Indeed, there’s obviously more at stake here than pleasing our taste buds.

What does science say on the subject? Here, two scientists offer their thoughts.

T. Colin Campbell, who argues that a vegan diet is healthier than diets that include meat and dairy products, is professor emeritus of nutritional sciences at Cornell University and co-author of “The China Study.” Nancy Rodriguez, who says it’s healthy to eat meat and dairy products as part of a balanced diet that includes each of the major food groups, is a professor of nutritional sciences at the University of Connecticut, in Storrs.

Yes: Cut Animal-Based Protein

By T. Colin Campbell

I was raised on a dairy farm. I milked cows until starting my doctoral research over 50 years ago at Cornell University in the animal-science department. Meat and dairy foods were my daily fare, and I loved them.

When I began my experimental research program on the effects of nutrition on cancer and other diseases, I assumed it was healthy to eat plenty of meat, milk and eggs. But eventually, our evidence raised questions about some of my most-cherished beliefs and practices.

Our findings, published in top peer-reviewed journals, pointed away from meat and milk as the building blocks of a healthy diet, and toward whole, plant-based foods with little or no added oil, sugar or salt.

My dietary practices changed based on these findings, and so did those of my family. So, what is this evidence that has had such an impact on my life?

In human population studies, prevalence rates of heart disease and certain cancers strongly associate with animal-protein-based diets, usually reported as total fat consumption. Animal-based protein isn’t the only cause of these diseases, but it is a marker of the simultaneous effects of multiple nutrients found in diets that are high in meat and dairy products and low in plant-based foods.

Trojan Horse

Historically, the primary health value of meat and dairy has been attributed to their generous supply of protein. But therein lay a Trojan horse.

More than 70 years ago, for example, casein (the main protein of cow’s milk) was shown in experimental animal studies to substantially increase cholesterol and early heart disease. Later human studies concurred. Casein, whose properties, it’s important to note, are associated with other animal proteins in general, also was shown during the 1940s and 1950s to enhance cancer growth in experimental animal studies.

Casein, in fact, is the most “relevant” chemical carcinogen ever identified; its cancer-producing effects occur in animals at consumption levels close to normal—strikingly unlike cancer-causing environmental chemicals that are fed to lab animals at a few hundred or even a few thousand times their normal levels of consumption. In my lab, from the 1960s to the 1990s, we conducted a series of studies and published dozens of peer-reviewed papers demonstrating casein’s remarkable ability to promote cancer growth in test animals when consumed in excess of protein needs, which is about 10% of total calories, as recommended by the National Research Council of the National Academy of Sciences more than 70 years ago.

One of the biggest fallacies my opponent presents is that a diet including meat and dairy products is the most efficient way of giving the body the nutrients it needs with a healthy level of calories. Plant-based foods have plenty of protein and calcium along with far greater amounts of countless other essential nutrients (such as antioxidants and complex carbohydrates) than meat and dairy.

Higher-protein diets achieved by consuming animal-based foods increase the risks of cancer, cardiovascular diseases and many similar ailments, caused by excess protein and other unbalanced nutrients as well.

It’s also worth noting that the government recommendations for certain population groups to increase their protein and iron consumption come from the U.S. Department of Agriculture, an agency long known to be subservient to the meat and dairy industries.

The dairy industry has long promoted the myth that milk and milk products promote increased bone health—but the opposite is true. The evidence is now abundantly convincing that higher consumption of dairy is associated with higher rates of bone fracture and osteoporosis, according to Yale and Harvard University research groups.

Pain Relief

Some of the most compelling evidence of the effects of meat and dairy foods arises when we stop eating them. Increasing numbers of individuals resolve their pain (arthritic, migraine, cardiac) when they avoid dairy food. And switching to a whole-food, plant-based diet with little or no added salt, sugar and fat, produces astounding health benefits. This dietary lifestyle can prevent and even reverse 70% to 80% of existing, symptomatic disease, with an equivalent savings in health-care costs for those who comply.

This treatment effect is broad in scope, exceptionally rapid in response (days to weeks) and often, lifesaving. It cannot be duplicated by animal-based foods, processed foods or drug therapies.

By contrast, any evidence that low-fat or fat-free-dairy foods reduce blood pressure is trivial compared with the lower blood pressure obtained and sustained by a whole-foods, plant-based diet.

Based on the scientific evidence, and on the way I feel, I know beyond any doubt that I am better off for having changed my diet to whole and plant-based foods.

Dr. Campbell is professor emeritus of nutritional sciences at Cornell University and co-author of “The China Study.” He can be reached at reports@wsj.com.

No: It’s a Question of Balance

By Nancy Rodriguez

For years a wealth of scientific research has supported the idea that healthy nutrition begins with a balanced diet consisting of the basic food groups: fruits, vegetables, grains and protein and dairy.

Each group offers nutrients that are essential to our health. Experts agree that the most important thing to remember when considering a vegetarian or vegan lifestyle is that essential nutrients removed from the diet with the elimination of meat or dairy need to be obtained from other foods.

Individuals who stop eating meat and dairy products are at risk of not getting enough calcium, vitamin D, protein, vitamin B12, zinc and iron in their diets—all nutrients that come mostly from food products derived from animals.

What happens then? Insufficient calcium and vitamin D can compromise bone structure. Lack of zinc can hinder growth in children. B12 and iron assist production of red blood cells, which deliver oxygen throughout the body. Proteins are essential for building and maintaining muscle and keeping our brains healthy. And animal proteins provide all the essential amino acids, nutrients our bodies cannot make on its own.

Calorie Efficiency

Including dairy and meat in a balanced diet can be an important way to get essential nutrients without excess calories—a key consideration given concerns about our overweight and undernourished nation. Our average daily consumption of dairy products, for example, provides more than half of the recommended daily amount of calcium and vitamin D in our diets, for only one-tenth of the calories. A three-ounce serving of beef has less than 10% of the calories in a typical 2,000-calorie-a-day diet while supplying more than 10% of the daily value for 10 essential nutrients.

Contrary to popular belief, Americans aren’t eating too much protein. According to Economic Research Service data from the U.S. Department of Agriculture, the daily caloric contribution of flour and cereal products increased by about 200 calories per person from 1970 to 2008, compared with only a 19-calorie increase from meat, eggs and nuts.

The Dietary Guidelines (the U.S. government’s science-based nutritional recommendations, compiled and issued every five years) have noted that some Americans need more protein, and that adequate consumption of iron and B12 (both found in lean meat) is a concern for specific population groups. The Dietary Guidelines are founded on evidence-based, peer-reviewed scientific literature, and take into account the entire body of research, not just a single study.

Proponents of a vegan diet paint a grim picture of the effects of animal protein on human health. But the effects of powdered, isolated casein on rats tells us very little about what traditionally consumed forms of milk will do to humans. And it tells us nothing that can be generalized to all “animal nutrients.” Casein is one of many proteins found in milk and is recognized around the world for its nutritional quality.

It is simply untrue to suggest that animal protein causes cancer. The American Cancer Society, along with other leading health organizations, emphasizes that the effects of foods and nutrients need to be considered in the context of the total diet. Research from many sources shows that other factors, such as not smoking, responsible alcohol consumption, maintaining a healthy weight and regular physical activity, are much more important to reducing cancer risk than eating or avoiding any individual food.

There is scientific evidence that low-fat or fat-free dairy and lean meat, as part of a balanced diet, produce specific health benefits such as reducing blood pressure. Fat-free, low-fat and reduced-fat options are widely available, as are lactose-free milk and milk products. Many of the most popular beef cuts are lean, including top sirloin, tenderloin, T-bone steak and 95% lean ground beef.

Calcium Question

Finally, contrary to my opponent’s assertions, dairy’s role in strengthening bones has long been established by the nutrition and science community. Don’t take just the Dietary Guidelines’ word. Dozens of randomized, controlled, clinical trials—the gold standard in research—have demonstrated that calcium and dairy products contribute to stronger bones. These trials far outweigh any observational studies which, by their very design, cannot show a causal relationship between eliminating meat and dairy foods and a subsequent improvement in health.

Government and public health organizations around the globe encourage daily consumption of dairy foods to promote good health and help prevent disease. We all have emotional and cultural connections to various foods; many of us have opinions on what to eat, how much and why. But appreciating the science behind nutrition helps us make smart choices about the best way to feed ourselves and the world.

Dr. Rodriguez is a professor of nutritional sciences at the University of Connecticut, Storrs. She can be reached at reports@wsj.com.