Karen Collins, MS, RD, CDN
American Institute for Cancer Research
Studies keep linking diets high in red meat with increased cancer risk. Could part of the risk from large amounts of meat reflect an unhealthful balance between vegetables and meat? The Mediterranean and Asian eating patterns have a well-established link with lower cancer risk. These diets both limit meat and strongly emphasize an abundance of vegetables.
One reason the vegetable-to-meat balance might be so important are compounds in vegetables that act on potentially harmful substances produced by meat. When meat is processed, smoked, grilled, fried or cooked well done, it can lead to the formation of carcinogens that can damage DNA and begin the process of cancer development. Several types of compounds in vegetables (and fruits) stimulate enzymes that convert such carcinogens to inactive forms, which are then excreted from the body.
Cruciferous vegetables such as broccoli, cauliflower and cabbage are especially known for compounds called isothiocyanates. These compounds take aim at meat-formed carcinogens. Studies suggest that meat is one of the major causes of colon cancer. Research shows that people who eat about four, half-cup servings of cruciferous vegetables weekly may reduce their risk of colon cancer by 20 to 50 percent.
Garlic and onions contain sulfide compounds that can also inactivate meat-formed carcinogens. In a large analysis of several studies, people who ate four to five cloves of garlic a week were 31 percent less likely to develop colon cancer compared to people who ate from zero to one clove per week.
Vegetables also provide dietary fiber, which can bind to heterocyclic amines (HCAs), one class of carcinogens produced when meat is cooked at high temperatures. When fiber binds to HCAs, it may prevent HCAs from being absorbed out of the digestive tract. The HCAs are then excreted before they can travel through the body and damage cells.
Diets high in meat and low in vegetables may promote inflammation throughout the body. Inflammation can cause an increase in free radicals, which can damage DNA. It can also lead to cells reproducing too quickly, leaving less time to repair possible DNA damage that can lead to cancer. Vegetables provide antioxidant vitamins and protective compounds that prevent and possibly repair DNA damage from free radicals. People who followed a Mediterranean-style diet for two years had blood levels that showed a 40 percent lower level of inflammation than that of a control group. The control group was asked to follow a low-fat diet without the emphasis on such healthful foods as fruits, vegetables and olive oil.
Researchers now suggest that one reason red meat is linked to so much more cancer risk than poultry and fish may involve its higher content of a particular form of iron called heme iron. Studies indicate that heme iron may damage the lining of the colon. Some preliminary laboratory studies show that heme iron may interact with estrogen in promoting hormone-related cancers, such as breast and prostate cancers.
A diet high in vegetables may help to balance the risks of heme iron. A recent study found that rats given food with added heme iron showed a 50 percent increase in colon cell growth. The amounts fed to the rats are equivalent to humans eating two-and-a-half cups of cooked spinach and five ounces of red meat daily.
Follow-up studies showed that chlorophyll the substance in vegetables that makes them green kept heme iron from damaging colon cells and beginning the cancer process. If this comparison to humans is accurate, green vegetables have a reasonable chance to reduce risk from red meat kept to the recommended limit of three ounces or less a day.