Naturally occurring substances in plants, animals, fungus (such as mushrooms), and bacteria (such as probiotics) that lower the risk of disease are called biologically active food constituents (BAFCs). Recent research suggests that eating foods with BAFCs may reduce the risk of cancer. Most of the research about BAFCs has been with phytochemicals, which are naturally occurring chemicals in plants.

Phytochemicals are in nearly all fruits and vegetables. They include:

• Carotenoids, the chemicals that give fruits and vegetables an orange or yellow color, such as carrots and sweet potatoes; beta-carotene is an example
• 
  
  
• Phenolics, a group of substances present in many herbs, spices, vegetables, fruits, and green tea
• 
  
  
• Organosulfur compounds, the substances found in cruciferous vegetables (such as broccoli and cabbage) and allium vegetables (such as garlic, leeks, and onions)

Other common food sources of BAFCs that may help prevent certain types of cancer include tomatoes, spinach, red wine, citrus fruits, turmeric (a spice found in Indian curry), berries, soy, and some meat and dairy foods.

The chemicals found in plants protect against cancer in dozens of different ways, and it is likely that the various phytochemicals work together to lower cancer risk. Some help regulate hormones, such as estrogen. Others work against cancer cell growth or block inflammation (a process in the body that helps fight infection and may contribute to some diseases). Many are antioxidants and lower the possibility of oxidative damage (damage to cells by oxidants, such as tobacco or ozone). (For more information about antioxidants, read Cancer.Net's Vitamins and Minerals section.)

Soy and breast cancer

The relationship between soy and breast cancer risk is complex and research study results are conflicting. Some laboratory studies have shown that soy products can protect against cancer; however, clinical studies in people have not shown a protective link. In addition, some studies suggest that soy may act like the hormone estrogen, which causes concern for women with a type of breast cancer that is fueled by estrogen (called estrogen receptor-positive breast cancer). Current evidence suggests that eating normal amounts (such as three servings each day) of soy foods, such as soy milk and tofu, is unlikely to increase risk for breast cancer progression. However, taking concentrated isoflavone (a type of estrogen from plants found in soybeans) or soy supplements is not recommended for postmenopausal women, since these products could act like estrogen in the body.

Lycopene and prostate cancer

Lycopene is a carotenoid found in tomato products, such as tomato sauce, pink grapefruit, and watermelon. In a few studies of men with prostate cancer, results suggested that lycopene might reduce cancer risk. However, a direct relationship between eating lycopene and lowering the risk of prostate cancer has not yet been established or consistently observed. Again, additional clinical trials are needed to determine if lycopene reduces the risk of prostate cancer.

Cruciferous vegetables

Eating cruciferous vegetables and risk of cancer has been studied for breast, colorectal, and ovarian cancers. Preliminary studies indicate that cruciferous vegetables (especially broccoli) may lower the risk of these cancers. An interesting observation in some studies is that there are genetic differences in how people metabolize (break down) the chemical in these vegetables. These differences seem to affect cancer risk. It is likely that genetics and foods influence each other with respect to cancer risk.

Adding plant-based foods to your diet

Although few large clinical trials link plant-based foods to cancer prevention, there is enough evidence to suggest that adding more fruits and vegetables to your diet can't hurt. In addition, eating fruits and vegetables has been shown to provide additional health benefits, such as lowering the risk for heart disease and stroke.

The following suggestions can help increase the amount of BAFCs in the diet:

• Aim for five to nine servings of fruits and vegetables each day. This is about two cups of fruit and 2.5 cups of vegetables. Add fruits and vegetables to the foods you normally eat, and choose fruits and vegetables for snacks.
• 
  
  
• Try new fruits and vegetables and choose vegetables with a variety of colors so that you get as many different phytochemicals in your diet as possible.
• 
  
  
• Keep a log of the fruits and vegetables you eat.
• 
  
  
• Ask your doctor or a dietitian about how to include more fruits and vegetables in your diet.

More Information

Cancer Prevention: Vitamins and Minerals

Cancer Prevention: General Nutrition Recommendations