Carotenoids and Breast Cancer Prevention
AICR/WCRF’s expert report and its continuous updates show that foods high in carotenoids protect against cancers of the mouth, pharynx, larynx and lung. Two reviews of the research that looked at blood levels now point to the possibility that sweet potatoes, tomatoes and the many other colorful fruits and vegetables high in carotenoids may also reduce women’s risk of breast cancer.
Carotenoids are a large group of phytochemicals (see sidebar) well recognized by their red, orange and yellow hues; many dark green vegetables, such as kale and spinach, are also rich sources. Earlier population studies on dietary carotenoids and breast cancer have had mixed findings.
It’s possible measurement errors by dietary questionnaires may be obscuring the link between carotenoids and reduced breast cancer risk, says Dagfinn Aune, an epidemiologist at Imperial College London and lead author of an analysis that assessed carotenoid intake by both dietary recall and blood concentrations.
Diet and Blood
Aune and his colleagues reviewed 24 publications on breast cancer risk and six carotenoids. Some of the studies used dietary recall to estimate carotenoid intake and others measured carotenoid blood levels. The carotenoids studied include beta-carotene, alpha carotene, lycopene and lutein. For the dietary studies, no link was found between five of the dietary carotenoids and breast cancer risk. Estimates of high beta-carotene intake showed a slightly reduced risk. Published in The American Journal of Clinical Nutrition, the study was funded by the World Cancer Research Fund as part of AICR/WCRF's Continuous Update Project.
Yet the studies measuring blood concentration showed a strong link between carotenoids and reduced breast cancer risk. The reduced risk was seen for total carotenoids and the individual phytochemicals.
“In our study, we saw about a 20 to 30 percent decreased risk of breast cancer when comparing the highest blood concentration of carotenoids to the lowest, but not for diet,” said Aune. The analysis suggests that dietary studies add blood measures when possible to better determine carotenoid intake.
The second analysis, published in the Journal of the National Cancer Institute, also focused on blood levels of carotenoids in order to overcome potential problems with dietary recall. The study was a pooled analysis of eight population studies. Each of the studies collected blood samples from women who were initially healthy and then tracked their health over time. At the time their blood was drawn, most of the women were postmenopausal.
Together, the studies made up over 80 percent of all the published literature on the topic, note the authors, including approximately 3,000 women who were diagnosed with breast cancer and 4,000 women who were not.
When comparing the women who measured the highest versus the lowest blood levels, those with the highest levels of total carotenoids linked to almost 20 percent lower breast cancer risk. Lower risk was also associated with higher levels of many individual carotenoids, including lycopene, alpha-carotene and beta-carotene.
The Whole Package
Carotenoids may influence carcinogenesis directly or through metabolism to retinol, a form of vitamin A. Lab studies show that many carotenoids have antioxidant properties and improve immune function.
There are several alternative explanations for the observed risk reduction, researchers add. For example, women with higher blood concentrations of carotenoids are also more likely to have healthy lifestyle habits, such as being more physically active and a healthy weight, and it’s possible these factors played a role, although many studies adjusted for these.
Carotenoid-packed plant foods are also rich in a variety of phytochemicals that could interact with the carotenoids. “Most of the published studies are small, and we need bigger studies on the subject with biomarkers,” said Aune. “We know that carotenoids are biomarkers of fruits and vegetables… it could be specific benefits of carotenoids, but it could also be the whole package of antioxidants and other beneficial things you find in fruits and vegetables.”
This article is excerpted from AICR's ScienceNow.