November 3, 2004.
A study from Harvard school of public health in Boston questions whether eating lots of fruit and vegetables reduces the risk of developing cancer.
Although a diet high in fruits and vegetables (the currently recommended 5 or more a day) protects against cardiovascular disease, the study says, such a diet has no protective effect against cancer.
This study's conclusions clash with advice from organizations such as American Cancer Society, which advises eating 5 or more servings of fruits and vegetables daily.
Analyzing diet and health reports from more than 100,000 doctors and nurses over 14 years, the Harvard researchers conclude that diets high in fruits and vegetables led only to a "modest" overall reduction in chronic disease, largely because of the impact on preventing heart disease and stroke
Increased fruit and vegetable consumption, the authors say, is associated with a decreased risk of cardiovascular disease but not cancer.
In an editorial published online Arthur Schatzkin and Victor Kipnis of the National Cancer Institute look at every angle that possibly might have distorted the Harvard researchers' results. While they present no solid evidence for rebuttal, they do note several possible flaws.
How this study collected information
The Harvard study, managed by Walter C. Willett, MD, Department of Nutrition at the university's School of Public Health, set out to look at benefits of fruit and vegetable consumption in relation to overall health. Previous studies, they claim, are "limited."
Willett and team evaluated the relationship between fruit and vegetable intake and the incidence of cardiovascular disease and cancer and of deaths from other causes in two large groups of people -- 71, 910 female participants in the Nurses' Health study and 37, 725 male participants in the Health Professionals' Follow-up Study.
All the participants were healthy ("free of major chronic disease") when they joined the study in 1984 and 1986, respectively, and filled out out baseline food-frequency questionnaires. The participants updated their dietary information a couple of times over about a decade -- in 1986, 1990, and 1994 for women and in 1990 and 1994 for men.
Participants were followed up for incidence of cardiovascular disease, cancer, or death for 12 -14 years (through May 1998 for women and January 1998 for men). Then their data was analyzed.
Willett's team measured 9329 "events" (1964 cardiovascular diseases, 6584 cancers and 781 other deaths) in women and 4957 events (1670 cardiovascular diseases, 2500 cancers, and 787 other deaths) in men during follow-up.
Men and women in the top 20 per cent for total fruit and vegetable intake had a 0.95 relative risk for major chronic diseases compared to people in the bottom 20 per cent. People who ate five servings daily had 0.88 risk for cardiovascular disease and a normal (1.00) risk for cancer.
Of the food groups analyzed, green leafy vegetable intake showed the strongest protective value for major chronic disease and cardiovascular disease. For an increment of one serving per day of green leafy vegetables, relative risks were 0.95 for major chronic disease and 0.89 for cardiovascular disease.
Increased fruit and vegetable consumption, the Harvard researchers conclude, have "a modest although not statistically significant reduction in the development of major chronic disease. The benefits appeared to be primarily for cardiovascular disease and not for cancer."
Questions raised immediately
In an editorial accompanying the article, Drs. Schatzkin and Kipnis from
the nutritional and genetic branches of the National Cancer Institute raise the possibility of"
systematic error" in this research.
Firstly, they say, errors may have occurred in measuring diet by asking people to recall what they ate during the previous year. Along with other confounding factors in the studythis basic method may have distorted true associations between fruit and vegetable intake and cancer.
In their editorial, Schatzkin and Kipnis conclude: "the evidence is simply inadequate at this time to determine whether fruit and vegetable intake confers modest protection against cancer."
The Harvard team do recognize possible benefits against specific cancers, such as the cabbage family against bladder cancer. Green leafy vegetables lowered the doctors' and nurses' risk of cardiovascular disease although, overall, fruits seemed more important than vegetables.
"Participants in our study eating at least five servings of fruits and vegetables daily had a 28% lower risk of vascular disease than participants eating fewer than 1.5 portions a day, probably due to higher intake of multiple nutrients," they said.
"Consumption of five or more servings of fruits and vegetables per day has been recommended in the National 5 A Day for Better Health Program for cancer prevention, but the protective effect of fruit and vegetable intake may have been overstated," the authors write. However, they add, "our findings for cardiovascular disease still support the recommendations of the American Heart Association of consuming at least five servings of fruits and vegetables per day."
References:
Journal of the National Cancer Institute (JNCI), Vol. 96, No. 21, 1577-1584, November 3, 2004
Fruit and Vegetable Intake and Risk of Major Chronic Disease Hsin-Chia Hung , Kaumudi J. Joshipura , Rui Jiang , Frank B. Hu , David Hunter , Stephanie A. Smith-Warner , Graham A. Colditz , Bernard Rosner , Donna Spiegelman , Walter C. Willett
Could Exposure Assessment Problems Give Us Wrong Answers to Nutrition and Cancer Questions? Arthur Schatzkin , Victor Kipnis
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