- Substances that naturally occur in plants are continually being studied as possible ways to prevent cancer, because some studies have suggested that people who eat more fruits and vegetables are less likely to develop cancer.
- Researchers have studied connections between soy and breast cancer and lycopene and cruciferous vegetables, like broccoli and cabbage, and the prevention of several cancers.
- Although many studies show mixed results, there is enough evidence to suggest that adding more fruits and vegetables may be protective against cancer and provide other health benefits.
About plant-based foods
Naturally occurring substances in plants, animals, fungus (such as mushrooms), and bacteria (such as probiotics and fermented foods) that lower the risk of disease are called biologically active food constituents or bioactives. Research suggests that eating foods with bioactives may reduce the risk of cancer. Most of the research about bioactives has been related to phytochemicals or phytonutrients, which are naturally occurring chemicals in plants.
Phytochemicals/phytonutrients are in nearly all fruits and vegetables and many times can be identified with the color of the food. They include:
- Carotenoids, the chemicals that give fruits and vegetables an orange or yellow color, such as carrots and sweet potatoes
- Phenolics, a group of substances found in many herbs, spices, vegetables, fruits, green tea, and berries
- Organosulfur compounds, the substances found in cruciferous vegetables and allium vegetables, such as garlic, leeks, and onions.
Other common food sources of bioactives that may help prevent some types of cancer include tomatoes, spinach, red wine, citrus fruits, soy, and turmeric, a spice found in Indian curry.
The chemicals found in plants protect against cancer in several 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, which is 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 caused by substances called oxidants, such as tobacco or ozone. Read more about antioxidants and vitamins and minerals .
Soy and breast cancer. Some laboratory studies show that soybean products may help protect against breast, prostate, colon, and lung cancers; however, clinical studies in people have been conducted to more clearly determine the role in cancer prevention.
The relationship between soy, which contains phytochemicals, and breast cancer risk is especially complex, and research study results are conflicting. In addition, some studies suggest that soy may act like the hormone estrogen, which may be a 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 like soy milk and tofu is unlikely to increase the risk of breast cancer growing and spreading. However, taking a type of estrogen from plants found in soybeans, called concentrated isoflavone, or soy supplements is not recommended, because these products could act like estrogen in the body.
Lycopene. Lycopene is a carotenoid found in tomato products, such as tomato sauce. Other important sources of lycopene include pink grapefruit, watermelon, and apricots. Lycopene studies show that it may protect against several cancers, including lung, stomach, prostate, and cancers of the digestive tract, such as colon, oral and esophageal. However, a direct link between eating lycopene and lowering the risk of cancer has not been found yet. Additional clinical trials are needed.
Cruciferous vegetables. These vegetables, which include broccoli, cauliflower, cabbage, Brussels sprouts, bok choy, and kale, are likely to protect against some types of cancers. A protective effect has been shown for cancers of the mouth, a part of throat called the pharynx, the voice box or larynx, esophagus, and stomach. Several laboratory studies have suggested that cruciferous vegetables help regulate the body's complex system of enzymes that defend against cancer and that components of the vegetables can stop the growth of cancer cells.
Adding plant-based foods to your diet
Although association studies show mixed results in terms of the connection between plant-based foods to cancer prevention, there is enough evidence to suggest that adding more fruits and even more importantly, vegetables, to your diet may be protective. In addition, eating fruits and vegetables has been shown to provide additional health benefits, such as lowering the risk for heart disease and stroke.
Here are some suggestions to help increase the amount of bioactives in your diet:
- Make half your plate a combination of vegetables and to a slightly lesser extent, fruit at every meal. How many fruits and vegetables you need each day depends on your age, sex, and how much physical activity you do. Visit www.choosemyplate.gov  for more information.
- 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.
- In addition to buying fresh fruits and vegetables, keep frozen, canned, and packaged vegetables on hand.
- Shred fresh vegetables or fruit into items such as stews, soups, and casseroles.
National Cancer Institute: Cruciferous Vegetables and Cancer Prevention