Cognitive function improves with aerobic exercise, but not for individuals who had been exposed to high levels of mercury in the womb, according to new research at the Harvard T.H. Chan School of Public Health.
The study is one of the first to look at how methylmercury exposure before birth may affect cognitive function in adults.
“We know that aerobic exercise is an important part of a healthy lifestyle, but these findings suggest that early-life exposure to pollutants may reduce the potential benefits,” said Gwen Collman, Ph.D., director of the NIEHS Division of Extramural Research and Training.
“We need to pay special attention to the environment we create for pregnant moms and babies.”
Mercury comes from industrial pollution in the air that falls into the water, where it turns into methylmercury and accumulates in fish. The researchers suggest that prenatal exposure to methylmercury — known to have toxic effects on the developing brain and nervous system — may limit the ability of nervous system tissues to grow and develop in response to increased aerobic fitness.
“We know that neurodevelopment is a delicate process that is especially sensitive to methylmercury and other environmental toxins, but we are still discovering the lifelong ripple effects of these exposures,” said Collman. “This research points to adult cognitive function as a new area of concern.”
The study involved 197 participants from the Faroe Islands, 200 miles north of England, where fish is a major component of the diet. Researchers have been following the subjects since they were in the womb in the late 1980s.
At age 22, the participants went to a follow-up exam that included estimating the participants’ VO2 max, or the rate at which they can use oxygen, which increases with aerobic fitness. A variety of cognitive tests were performed related to short-term memory, verbal comprehension and knowledge, psychomotor speed, visual processing, long-term storage and retrieval, and cognitive processing speed.
The findings show that adults with high prenatal exposure to methylmercury, which mainly comes from maternal consumption of fish with high mercury levels, did not experience the faster cognitive processing and better short term memory benefits of exercise that were seen in individuals with low prenatal methylmercury exposures.
Overall, higher VO2 max values were tied to better neurocognitive function, as expected based on previous research. Cognitive efficiency, which included cognitive processing speed and short term memory, benefitted the most from increased VO2 max.
When the participants were divided into two groups based on the methylmercury levels in their mothers while they were pregnant, they found that these benefits were confined to the group with the lowest exposure.
Participants with prenatal methylmercury levels in the bottom 67 percent — levels of less than 35 micrograms per liter in umbilical cord blood — still exhibited better cognitive efficiency with higher VO2 max. However, for participants with higher methylmercury levels, cognitive function did not improve as VO2 max increased.
The U.S. Food and Drug Administration recommends that children and women of childbearing age eat two to three weekly servings of fish low in mercury as part of a healthy diet. Low mercury fish include salmon, shrimp, pollock, canned light tuna, tilapia, catfish, and cod. Fish to be avoided due to high mercury levels include tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel.
The findings are published in the journal Environmental Health Perspectives.