Several studies over the past five years have presented evidence that cognitive decline can be a precursor to Alzheimer's, in which diet is thought to be a contributing factor.
When women experience the menopause, their natural estrogen levels decline gradually over two to ten years. The presence of estrogen receptors in the central nervous system indicates its role in cognitive function, but the side effects of estrogen replacement therapy (ERT) may include increased risk of breast cancer and cardiovascular disease, and vaginal bleeding.
Given these drawbacks, interest has been piqued in the effects of phytoestrogens obtained from the diet, which may provide the same benefits as ERT, without the risks.
But according to the Dutch researchers, whose study is published in the Journal of Nutrition (135, 5:1190-1195, 2005), data on the relationship between phytoestrogens and cognitive function are still sparse.
They set out to examine how dietary intake of two phytoestrogens, lignans and isoflavones, in the typical Western diet may affect cognitive function in postmenopausal women.
The main dietary sources of lignans are oil-seeds, linseeds, broccoli and berries. Isoflavones occur in soy products, beans, peas, nuts, tea and coffee.
The study involved 394 healthy postmenopausal women who had an intact uterus, at least one intact ovary and had not used hormonal replacement therapy since their last menstrual period. Of these women, 196 experienced a natural menopause between 1969 and 1979 and 207 between 1987 and 1989.
Their intake of phytoestrogens during the year prior to enrollment in the study was estimated by a validated food frequency questionnaire. Cognitive function was assessed by a mini-mental state examination (MMSE), which involved questions and tasks associated with orientation, registration, attention, calculation, recall and language.
Since diagnosis of dementia was not the aim, the cut-off score for intact cognitive function was relatively high - 26 or more out of a possible 30. After adjustment for confounders, women who consumed higher levels of lignans performed better in the MMSE, and the results were more pronounced amongst the women who experienced menopause between 1969 and 1979.
Establishing a reason for this difference was outside the scope of this study but possible explanations were put forward by the researchers, such as some other age-related mechanism. The 1969 to 1979 group had a mean age of 69.2 years, compared with 63.5 years for the 1987 to 1989 group.
"An alternative explanation is that the protective effect of endogenous estrogens on the nervous system us dependent on cumulative time of exposure rather than on actual levels of intake," they wrote.
However the researchers could establish no relationship between isoflavone intake and cognitive function - an apparent contradiction to the findings of a study published in Psycopharmacology in 2001, which concluded that isoflavones from soy may indeed positively effect cognition.
Other research underway in The Netherlands is investigating lignans' potential to alleviate the effects of age-related conditions in men.
Earlier this month Acatris released the results of an animal study suggesting that flax lignans may help treat benign prostate hyperplasia, a condition said to affect more than half of all men over the age of 50. A human clinical study to test the theory further is expected to take place later this year, and will also look at the effect of lignans on hair loss.