Prepared Foods October 31, 2005 e-newsletter

Consuming fish at least once a week was associated with a 10% per year slower rate of cognitive decline in elderly people, according to a new study posted online from Archives of Neurology. The study will be published in the December print edition of the journal.

Fish is a direct source of omega-3 fatty acids, which have been shown to be essential for neurocognitive development and normal brain functioning, according to background information in the article. Fish consumption has been associated with lower risk of dementia and stroke, and recent studies have suggested that consumption of one omega-3 fatty acid in particular, docosahexaenoic acid (DHA), is important for memory performance in aged animals.

Martha Clare Morris, ScD, of Rush University Medical Center, Chicago, and colleagues analyzed six years of data from an ongoing study of Chicago residents, 65 years and older, first interviewed between 1993 and 1997 and every three years in two follow-up interviews. Interviews included four standardized cognitive tests and dietary questions on the frequency of consumption of 139 different foods, as well as questions of daily activities, exercise levels, alcohol consumption and medical history.

"Dietary intake of fish was inversely associated with cognitive decline over six years in this older, biracial community study," the researchers report. "The rate of decline was reduced by 10% to 13% per year among persons who consumed one or more fish meals per week, compared with those with less-than-weekly consumption. The rate reduction is the equivalent of being three to four years younger in age." The researchers examined whether overall dietary consumption patterns accounted for the association of cognitive decline and fish consumption, but the rate differences did not change after adjusting for consumption of fruit and vegetables.

"Cognitive decline is common among older people and is very much associated with advancing age," the authors write. "Our data offer no insight as to whether this cognitive decline is pathological or the result of a normal aging process. Nonetheless, data from the U.S. and other countries indicate that it is a widespread and increasing public health problem."

"This study suggests that eating one or more fish meals per week may protect against cognitive decline associated with older age," the authors conclude. "More precise studies of the different dietary constituents of fish should help to understand the nature of the association."

In other fishy news, recent recommendations by the FDA advising pregnant women to limit mercury-containing fish in their diets may have the unintended consequence of depriving fetuses of essential nutrients, according to a study published recently in the October issue of the peer-reviewed journal Environmental Health Perspectives (EHP).

Although excessive mercury intake during pregnancy can harm the neurological development of fetuses, a recent study found that nutrients in fish, such as n-3 polyunsaturated fatty acids, may play a critical role in an infant's neurocognitive development.

Researchers tested six-month-old infants' cognitive ability and compared it to both the amount of fish consumed by the mother during pregnancy and the amount of mercury found in the mother's hair. As had been found in previous studies, elevated maternal mercury levels were associated with a deficit in infant cognition. However, higher fish intake was associated with higher infant cognition, especially after adjusting for mercury levels.

While these results may seem contradictory, researchers found that the infants who scored highest on cognitive tests were those whose mothers ate more fish and had lower levels of hair mercury.

"The most likely explanation is that the benefit is conferred by consuming fish types with the combination of relatively little mercury and high amounts of beneficial nutrients," wrote the authors of the study. Fish that tend to be higher in n-3 fatty acids but lower in mercury include salmon, canned light tuna, and sardines.

The study, conducted by researchers at Harvard Medical School, examined data from 135 woman-infant pairs who participated in Project Viva, a prospective pregnancy and child health cohort study.

The women completed a food frequency questionnaire that recorded how often during the second trimester of pregnancy they ate four different fish types (canned tuna, shellfish, dark meat fish such as salmon, and all other fish). Maternal hair samples collected at delivery provided a separate measure of mercury intake during the second trimester.

The final stage of research called for infant cognitive testing conducted at approximately six months of age. Infants took the visual recognition memory test, which analyzes the child's ability to recognize an initial stimulus and record into memory a novel stimulus. This test has been shown to correlate to IQ later in life.

The researchers sought to better understand the sometimes opposing opinions regarding the consumption of fish by pregnant women. "The net effect of the beneficial nutrients and harmful contaminants contained within fish has not been well studied and remains unclear," they wrote. The study authors suggest that future research include more detailed dietary information to help pregnant women make informed decisions about which fish species may be better or worse for their child's cognition.