Fruits & Veggies May Protect Against Some Cancers

June 1, 2007

There's nothing new about the value of eating lots of fruits and vegetables, but the effect of this diet on cancer risk has been difficult to show.

LOS ANGELES—There's nothing new about the value of eating lots of fruits and vegetables, but the effect of this diet on cancer risk has been difficult to show. At the 2007 American Association of Cancer Research annual meeting, however, several large-scale studies linked consumption of these healthful foods with significant reductions in several types of cancer.

Pancreatic Cancer

A diet high in flavonols may help reduce pancreatic cancer risk, especially in smokers, according to the Multiethnic Cohort Study of 183,518 residents of California and Hawaii (abstract 856). Flavonols are ubiquitous in plant-based foods, but concentrations are highest in onions, apples, berries, broccoli, and kale.

Subjects were recruited to the prospective study, conducted at the Cancer Research Center of Hawaii, Honolulu, between 1993 and 1996 and answered dietary frequency questionnaires. After 8 years of follow-up, 529 participants developed pancreatic cancer. The intake of total flavonols was compared for these subjects vs those without pancreatic cancer, calculated by quintiles according to a multivariate Cox regression model.

Adults who consumed the largest amounts of flavonols had a 23% reduction in the risk of developing pancreatic cancer, compared with those who ate the least (P = .046). Smokers gained the most benefit, with those in the highest quintile reducing their risk by 59%, "perhaps because they are at increased pancreatic cancer risk already," said principal investigator Ute Nothlings, DrPH, who is now at the German Institute of Human Nutrition Potsdam-Rehbruecke. Smoking, the only established risk factor for pancreatic cancer, doubles the risk of the disease, compared to never smoking.

Among three individual flavonols that were evaluated, kaempferol (found in spinach and some cabbages) was associated with the greatest protection, a 73% reduction of risk in smokers and a 22% reduced risk overall.

Across quintiles of intake, total flavonols, and the individual compounds quercetin (found in onions and apples), kaempferol, and myricetin (found in red onions and berries), were all associated with a significant trend toward reduced pancreatic cancer risk in current smokers but not in never smokers or former smokers. The interaction with smoking status was significant for total flavonols, quercetin, and kaempferol, she said.

Interestingly, smokers fell mainly into the lowest quintile of flavonol intake, yet had the most benefit. The highest quintile of intake contained the lowest percentage of smokers, Dr. Nothlings said.

"While our study has large statistical power because of the number of pancreatic cases that were observed, more evidence is still needed. It certainly validates current recommendations for diets high in vegetables," she said.

Head and Neck Cancer

A diet rich in fruits and vegetables was protective against head and neck cancer, in the NIH-AARP Diet and Health Study, a prospective 5-year study of nearly 500,000 AARP members (abstract 849). Subjects eating 6.0 servings of fruit and vegetables per day per 1,000 calories had a 29% reduced risk, compared with those who ate 1.5 servings/d/1,000 calories. Considering that the average adult consumes about 2,000 calories/d, an abundance of fruits and vegetables are required for the greatest protection, but adding just one fruit or vegetable serving per day, per 1,000 calories, was associated with a 6% reduction in risk, said Neal Freedman, PhD, a cancer prevention fellow at NCI.

The associations were similar in men and women. In models that were mutually adjusted for fruit and vegetable intake, a significant protective association was observed for vegetables, but not fruits.

In an analysis of anatomic subsets of head and neck cancer, the association between vegetable consumption (per serving/d/1,000 calories) and cancer was significant for the oral cavity (16% reduced risk); suggestive inverse associations were also seen for cancers of the orohypopharynx (10% reduction) and larynx (9% reduction).

By classifying foods into 13 separate botanical groups, the greatest reduction in risk was afforded by dried leguminosae (beans, peas); rosaceae (apples, peaches, nectarines, plums, pears, strawberries); and carrots, Dr. Freedman reported.

Colorectal Cancer

Also from the NIH-AARP Diet and Health Study, other investigators correlated dietary pattern with colorectal cancer occurrence among the nearly 500,000 subjects (abstract 371). From the dietary recall analysis, they identified three prominent dietary patterns—fruits and vegetables, meat and potatoes, and fat-reduced/diet foods—ranked each subject in each pattern, and put each into quintiles of intake. Multivariate adjusted models controlled for age, ethnicity, tobacco use, physical activity, body mass index, education, and use of hormone replacement therapy.

The fat-reduced/diet-foods pattern was associated with decreased risk of colorectal cancer in men (19%, compared with the lowest quintile) and was suggestive of reduced risk in women (13%). The fruit-and-vegetable pattern was associated with a reduced risk of colorectal cancer in men (19%), but not in women; and the meat-and-potatoes pattern was associated with an increased risk, especially in women (48%), said Andrew Flood, PhD, of the University of Minnesota. This study supports previous meta-analyses showing a modest but significant increase in risk for a meat-and-potatoes type diet, especially red meat, Dr. Flood noted.