A new Swedish study has reported that dietary folate intake is associated with reduced risk of breast cancer recurrence, especially among women with estrogen receptor negative (ER-) tumors. The study was designed to investigate the association between dietary folate intake and survival after a diagnosis of breast cancer. Folate is thought to influence breast cancer development and progression. However, population studies on the relation between folate and breast cancer survival are limited. The study included 3,116 women in the population-based Swedish Mammography Cohort who were diagnosed with breast cancer. The women filled out a 67-item food frequency questionnaire in 1987 and were followed through 2008. A total of 852 deaths, including 381 from breast cancer, occurred during follow up.
Dietary folate intake was found to be inversely associated with death from any cause and breast cancer-specific death. Women in the highest fourth of folate intake were found to have a 21% lower risk of death from any cause compared to those in the lowest quartile. These women also had a 22% lower risk of death from breast cancer, however this result did not reach statistical significance. However, a stronger protective association between dietary folate intake and breast cancer death was found among those with ER- tumors, who had a 58% lower risk of death, again comparing those with the highest to the lowest quartile of dietary folate intake. The authors conclude that dietary folate intake before breast cancer diagnosis may improve breast cancer and overall survival. While these findings need to be confirmed in future studies, they do offer assurance that dietary folate intake at the levels observed in the Swedish population does not unfavorably affect survival after breast cancer, according to the authors.
Food sources of folate (folic acid)
The study results are not a reason for breast cancer survivors to take folic acid supplements. Folic acid is a synthetic form of folate found in supplements and fortified foods. Unlike in Sweden, the Canadian and U.S. governments require food manufacturers to add folic acid to white flour, enriched pasta, and cornmeal to ensure that pregnant women receive enough of this B vitamin. There is enough evidence of an association between folic acid supplementation and increased risk of breast cancer to warrant caution.
Folate deficiency is thought to contribute to increased risk of cancer through increased DNA stand breaks, impaired DNA repair, and increased mutagenesis. Some studies have linked adequate folic acid to a decrease in neuroblastomas, leukemia, some brain tumors, and colon cancer. Approximately 10% of the U.S. population overall (and a higher percentage of the poor) is estimated to have a level of folate deficiency sufficient to cause chromosome breaks. It is unclear how folic acid supplementation influences breast cancer risk in populations with low levels of folate deficiency. However, based on laboratory evidence, folic acid supplementation and food fortification may prevent the initiation of breast cancer in normal tissues.
On the other hand, folic acid supplementation might promote the progression of existing tumors in some women. Population studies have reported inconsistent results. These contradictory results suggest that different subgroups may experience varying outcomes from folic acid supplementation, and, given folate's role in DNA synthesis and repair, that these differences may have a genetic basis. In fact, there is some evidence of this. A Swedish study which examined plasma folate concentration in relation to genotypes of a folate-metabolizing enzyme found a genetic variant that was associated with high circulating folate levels. This variant was also associated with increased breast cancer risk. In women with other genotypes, no significant associations between breast cancer and folate levels were observed. A Singapore Chinese study which examined the relationship between green tea (which reduces folate absorption) and risk of breast cancer found that the effect of folate consumption varied based on whether the women possessed certain genotypes.
In addition to the impact of individual genetic differences, the effects of folate in the diet may be different from those of folic acid supplements and there appears to an interaction between folic acid, vitamin B-12, and cancer risk that is not well understood. Therefore, we suggest that breast cancer survivors include foods in their diets that incorporate folate. The following foods are good sources of folate, as well as being on our recommended food list for breast cancer in general:
- Beans, dry
- Bell peppers
- Broccoli
- Cabbage
- Cauliflower
- Celery
- Collard greens
- Lettuce, romaine
- Liver, chicken or turkey
- Mustard greens
- Parsley
- Spinach
- Squash