The news for red meat seems to be getting worse and worse.
In December, a survey of more than 494,000 people by the National Institutes of Health found that men who ate more than 5 ounces of red meat each day and women who ate more than 3 ounces had a 51% greater risk of esophageal cancer, 61% of liver cancer and 24% of colorectal cancer than those who ate less than an ounce of red meat daily.
In October 2007, the World Cancer Research Fund and the American Institute for Cancer Research, two charities that fund research on the effects of diet and activity on cancer risk, declared that the evidence linking red meat consumption and colorectal cancer was "convincing."
And though previous reports for breast cancer have been contradictory overall, findings published in July from a Harvard study of more than 39,000 young nurses suggested that the risk of getting breast cancer before menopause goes up for every extra daily serving of red meat a woman ate as a teenager, a time period that had not been studied before.
Add the numerous studies linking red meat to other cancers, cardiovascular disease, Type 2 diabetes and even Alzheimer's disease, and it sounds like the hamburger you had for lunch might as well have been laced with rat poison.
In fact, there is a place for red meat in a healthful diet, scientists say, but they recommend choosing smaller portions of lean cuts and cooking them well but not at high temperatures.
The question is which meat components are responsible for the observed health risks. Scientists have several theories, though none seems to tell the whole story.
Red meat can contain a lot of saturated fats and cholesterol, known contributors to cardiovascular disease. "We know that dementia is strongly related to vascular disease, so it's likely we'll find a relationship there as well," says Dr. Walter Willett, chair of the department of nutrition at the Harvard School of Public Health.
Meat from commercially raised livestock also contains a high amount of omega-6 fats, which have been associated with poor cardiovascular health, but a low amount of omega-3 fats, which may be protective.
Another potential culprit is the iron in meat. Iron is essential for health, but iron from meat comes in a different form than that from vegetables and legumes, one that is absorbed whether the body needs it or not. "This type of iron can cause oxidative damage to all the components of the cell -- the protein, lipid, DNA, RNA," says Al Tappel, professor emeritus of food science at UC Davis.
Many of the studies that found an association between meat consumption and health risks did not differentiate between unprocessed meat, such as a steak, and processed or cured meats such as salami, bacon, pepperoni, bologna and hot dogs. Chemicals in processed meats may account for some of the cancer risk.
Finally, high-temperature cooking methods, such as grilling over charcoal, can cause the formation of known carcinogens such as heterocyclic amines, or HCAs, and polycyclic aromatic hydrocarbons, or PAHs.
Mary Young, a registered dietitian from the Centennial, Colo.-based National Cattlemen's Beef Assn., says that a study it commissioned on the science of red meat reached a very different conclusion (the study has not yet been published). "Red meat does not cause cancer," she says. "Beef is really one of the most underappreciated nutrient-rich foods out there" -- rich in protein and eight other nutrients, including B vitamins and zinc.
Some scientists, too, think that the risk of red meat has been overplayed. "The proof is not as strong as some people would like to think," says Iowa State University animal science professor Don Beitz. "Cancer is such a multifactorial [problem]. I don't see how one can just pin it on certain pollutants or nutrients."
Rock-hard conclusions require carefully controlled, long-term, well-defined studies of many people. Each one of these requirements can be difficult to meet, so scientists rely heavily on epidemiological studies in which the normal habits of large numbers of people are tracked, often pooling the results of multiple studies.
But unlike lab rats, humans don't live in a perfectly controlled environment, which makes it difficult to determine if it's meat or something else in the diet or environment that leads to an observed cancer risk. Also, some studies ask people to recall what they ate years ago, and many studies don't even define red meat the same way.
Even when a correlation between meat consumption and illness is found, the effect can be significant but small. In the December 2007 study, for example, high meat consumption resulted in only a 50% increased risk of developing esophageal cancer -- by way of comparison, smoking can increase a person's risk of developing lung cancer by 1,000% or more.