By Marie Spano, M.S., R.D., Contributing Editor
Conventional wisdom says five servings of fruits and vegetables each day helps prevent cancer. But does it?
The European Prospective Investigation into Cancer and Nutrition (EPIC), a study conducted in 23 centers in 10 European countries, assessed relationships between intake of total fruits, total vegetables, and total fruits and vegetables combined and cancer risk over an eight-year period in 142,605 men and 335,873 women. They found a very small inverse association between total fruit and vegetable intake and cancer risk, with a stronger association tied to an increased intake of vegetables versus fruit. Additionally, a high intake of fruits and vegetables in current smokers was associated with a decreased risk of lung cancer (Journal of the National Cancer Institute, 2010; 102:529-537; European Journal of Cancer, 2010; 46:2,555-2,562). Though these results may seem to debunk the belief that fruits and vegetables are preventive weapons in the war against cancer, the incidence of cancer in the population studied at the time of initial publication wasn’t large enough to detect an association with fruit and vegetable intake and specific cancers (Journal of the National Cancer Institute, 2010; 102:529-537).
A systematic, comprehensive analysis of all relevant, methodologically sound research on food, nutrition, physical activity and cancer consisting of 7,000 papers and 17 systematic literature reviews, published by the World Cancer Research Fund and American Institute for Cancer Research, concluded “non-starchy vegetables and fruits probably protect against some cancers," and, because they are low in energy density, they likely protect against weight gain, an independent risk factor for some cancers. In particular, they found a convincing association between non-starchy vegetable intake and decreased risk of the following cancers: mouth, pharynx, larynx, esophagus and stomach. Additonally, non-starchy vegetable intake was associated with a probable decreased risk of nasopharynx, lung, colorectum, ovary and endometrium cancers. Fruit intake was inversely associated with a convincing decrease in risk of mouth, pharynx, larynx, esophagus, lung and stomach cancers, and a probable decreased risk of liver, colorectum, nasopharynx and pancreas cancers.
A strong association between fruit and vegetable intake and a reduced risk of renal cell cancer was found in a pooled analysis of 13 prospective studies, including 1,478 cases of renal-cell cancer among 530,469 women and 244,483 men followed for 7 to 20 years. And, a statistically significant inverse association was found for intakes of root vegetables, broccoli and carrots and renal-cell cancer risk (Cancer Epidemiology, Biomarkers and Prevention, 2009; 18:1,730).
Action behind the activity
Fruits and vegetables may affect cancer cells by modulating detoxification enzymes, stimulating the immune response to cancer, modulating hormones and inhibiting cancer-cell growth (Cancer Causes Control, 1991; 2:427-442).
Though some studies suggest antioxidants play a role, science has not elucidated many of the potential compounds within fruits and vegetables that could be responsible (American Institute of Cancer Research, 2007). However, the EPIC study found a high intake of dietary fiber was associated with decreased colorectal-cancer risk, and high plasma levels of vitamin C, some carotenoids, retinol and alpha-tocopherol were inversely associated with gastric-cancer risk (European Journal of Cancer, 2010; 46:2,555-2,562).
Recommended amounts and types
The World Cancer Research Fund and American Institute of Cancer Research emphasize eating mostly foods of plant origin, including non-starchy vegetables, fruits and legumes, all of which contain substantial amounts of dietary fiber, micronutrients and are low in energy density, making them an important part of weight management. Five servings (14 oz. or 400 grams) of a variety of different colors of non-starchy vegetables and fruits, including red, green, yellow, white, purple and orange, as well as tomato-based products and allium vegetables, such as garlic, are recommended daily.
The average American adult consumes half of the recommended intake of fruits and vegetables, about 2.47 and 2.16 cups of vegetables daily (including fried potatoes), for men and women, respectively, and 0.61 cups of fruit daily for both men and women. And, Americans are falling short on variety—a very small amount of vegetables consumed consists of dark green and orange vegetables. In addition, fruit juice accounts for approximately one-fifth of total fruit intake (Medscape Journal of Medicine, 2009; 11(1)).
Though past research has, at times, been conflicting, more recent research is lending support to the association between fruit and vegetable intake and some types of cancer. A systematic review and meta-analysis of prospective studies examining fruit and vegetable intake and breast-cancer risk found a high intake of fruits, and fruits and vegetables combined, but not vegetables, was associated with a weak reduction in risk of breast cancer (Breast Cancer Research & Treatment, 2012; 134:479-493). And, a meta- analysis of 19 prospective studies found a weak but statistically significant nonlinear inverse association between fruit and vegetable intake and colorectal-cancer risk (Gastroenterology, 2011; 141:106-118). Additionally, a meta-analysis of case control studies found fruit and vegetable consumption reduced risk of pancreatic cancer (Cancer Epidemiology, 2012; 36:60-67). And finally, recent analysis of data from the EPIC study found an inverse association between total intake of vegetables, onion and garlic and risk of intestinal gastric cancer, and an inverse association between citrus fruit and risk of cardia gastric cancer (International Journal of Cancer, doi: 10.1002/ijc.27565). The latest data from EPIC also found fiber from fruit and vegetables was inversely associated with colon-cancer risk (PLoS ONE, 2012; 7(6):e39361). And, a small meta- analysis of seven cohort and six population-based case-control studies found fruit and vegetable intake was tied to a significantly decreased risk of prostate cancer (International Journal of Urology, 2012; 19:134-141).
Taken together, the current body of scientific data suggests increased intake of non-starchy vegetables and fruits can decrease risk of some cancers and possibly play a role in weight management; adult weight gain, overweight and obesity are independent risk factors for some types of cancer.
Marie Spano, M.S., R.D., CSCS, is a nutrition communications expert whose work has appeared in popular press magazines, e-zines and nutrition-industry trade publications. She has been an expert guest on NBC, ABC and CBS affiliates on the East Coast. For more information, visit mariespano.com.