The findings suggest the link is strongest for ER negative (ER-) breast cancers, which tend to be more aggressive, have a poorer prognosis and fewer prevention and treatment options. ER negative means the cancer is not driven by estrogen.
Lead author A. Heather Eliassen, of the Channing Laboratory in the Department of Medicine at Brigham & Women's Hospital and Harvard Medical School, and colleagues, write about their findings in a paper published online on 6 December in the Journal of the National Cancer Institute.
Previous studies have linked carotenoids with reduced risk for breast cancer, but with mixed results depending on which specific carotenoid compound they looked at.
The authors analyzed data pooled from studies covering a total of 7,000 women (3,055 with breast cancer and 3,956 matched controls), and looked for links between breast cancer and total levels of circulating carotenoids, as well as individual ones.
The data came from 8 cohort studies that between them covered 80% of the data published around the world from prospective studies of blood carotenoids and breast cancer.
The researchers found a statistically significant inverse association between breast cancer risk and circulating levels of total and individual carotenoids.
"This comprehensive prospective analysis suggests women with higher circulating levels of alpha-carotene, beta-carotene, lutein+zeaxanthin, lycopene, and total carotenoids may be at reduced risk of breast cancer."
They found no statistically significant link for beta-cryptoxanthin.
The authors note they found the strongest links were for ER negative breast cancers, and there was also some evidence that carotenoids may slow the growth of ER positive cancers too, but warn it's possible the effect is obscured by hormone-related associations that drown out the other risk factors.
Nevertheless, they conclude:
"A diet high in carotenoid-rich fruits and vegetables offers many health benefits, including a possible reduced risk of breast cancer."
Carotenoids in the DietCarotenoids are yellow, orange, and red pigments found in plants. The types the researchers looked at in this study are the ones most common in North American diets, namely alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene.
Alpha-carotene and beta-carotene are provitamin A carotenoids, meaning they can be converted in the body to vitamin A.
Orange and yellow vegetables like carrots and winter squash, are rich sources of alpha- and beta-carotene. Spinach is also a rich source of beta-carotene, although the chlorophyll in the leaves hides the yellow-orange pigment. Sweet potato and kale (a leaf vegetable from the cabbage family) also have high amounts of beta-carotene.
Dark green leafy vegetables like spinach, kale and turnip greens have the highest amounts of lutein and zeaxanthin, which although are separate compounds, studies often group them together.
Tomatoes are a rich source of lycopene, the carotenoid that also gives pink grapefruit, watermelon, and guava their red color.
Some Fat Required to Digest CarotenoidsThere is evidence that for the body to absorb carotenoids from fruits and vegetables, they should be eaten with some fat.
A study by food scientists published mid-2012 in the journal Molecular Nutrition & Food Research suggests you won't get much benefit from a salad without the right type and amount of salad dressing.
They concluded that the bioavailabilty of carotenoids is affected by the type and amount of fat consumed at the same time.
While not advocating a high-fat diet, lead author Mario Ferruzzi notes:
"Overall, pairing with fat matters. You can absorb significant amounts of carotenoids with saturated or polyunsaturated fats at low levels, but you would see more carotenoid absorption as you increase the amounts of those fats on a salad."