Cadmium exposure in the womb linked to lower IQ scores in 5-year-olds.

Nov 09, 2012

Kippler M, F Tofail, JD Hamadani, RM Gardner, SM Grantham-McGregor, M Bottai and M Vahter 2012. Early-Life cadmium exposure and child development in 5-year-old girls and boys: a Cohort study in rural Bangladesh. Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.1104431.

Synopsis by Glenys Webster

Prenatal exposures to the metal cadmium -- even at low levels common in most countries -- can have long-lasting effects on children's IQ. A study from Bangladesh found that 5-year-olds who were exposed through their mothers to higher levels had IQs that were 2 to 3 points lower than less-exposed children. The new evidence suggests that even low-level exposures before birth may have continued effects on children's brain function.

Children who had higher exposures to cadmium in the womb also had lower IQ scores when they were 5 years old, reports a study from rural Bangladesh. On average, scores dropped by 2 to 3 points in children with the highest exposures when compared to those with the lowest exposures.

Even small drops in IQ may affect a child’s ability to succeed at school and work later in life. Lower IQ's across the population also have large impacts on society.

IQ is a measure of intelligence when compared to others in the same age group. Genetics plays a big role in determining IQ, but nutrition before and after birth is also important, including the mother's diet during pregnancy and breastfeeding.

Cadmium is a soft, bluish-white metal found naturally at low levels in the air, water and soil. Manufacturers use it in a number of applications and processes, including to make nickel cadmium batteries and solar panels, to coat and plate metal and to stabilize plastics. Cadmium is released from natural sources. It is also emitted in car exhaust; from burning industrial waste, coal and oil; during battery and paint manufacturing; and from the hauling and disposing of waste. When spread on fields, fertilizers and sewage sludge contaminated with cadmium can increase levels in cropland soils.

Most people are exposed to cadmium through their diet. Cadmium from the soil can accumulate in some foods, such as spinach and other leafy greens, rice and other cereals, and potatoes. It also can be found in seafood and organ meats. Cadmium also can concentrate in tobacco leaves, exposing smokers and those around them to the toxic metal.

Cadmium is known to affect the development of the brain and nervous system. Childhood exposures have been linked to mental retardation, learning difficulties, dyslexia, poor hand-eye coordination, lower IQ and behavior problems in children.

However, little is known about how pre-birth exposures to cadmium might affect a child's long-term brain development. It is important to understand because the developing fetal brain is extremely sensitive, so even small exposures during pregnancy may have effects.

The researchers measured cadmium levels in urine from 1,305 women living in rural Bangladesh who were approximately eight weeks pregnant. All of the babies were born in 2002 and 2003. The participants were part of a larger study looking at food and micronutrient supplementation during pregnancy.

When the children were 5 years old, the researchers measured cadmium levels in their urine and gave standardized tests that measured verbal, performance and full-scale intelligence quotient or IQ. Child behavior was assessed using a questionnaire. The tests and questionnaire were adapted for use with Bangladeshi children.

Those with higher exposures were compared to those with lower exposures. The researchers took into consideration socioeconomic status and personal information, including the quality of home stimulation, maternal IQ and birth order.

While they found links to exposures before birth and at age 5, the children's IQ was more strongly associated with pre-birth exposures than with childhood exposures. When kids with the top 5 percent of cadmium exposures were compared to the lowest 5 percent, full-scale IQ dropped by 2.7 points when pre-birth levels were compared and 1.7 points when childhood levels were compared.

The effects were slightly stronger in girls than boys and stronger in children from higher-income rather than lower-income families. Children with higher cadmium levels in urine also had poorer behavior compared to less-exposed kids.

Surprisingly, cadmium had a larger effect on child IQ than arsenic, another metal known to affect brain development and commonly found in the well water of the region studied.

The median cadmium level in the Bangladeshi moms’ urine (0.63 micrograms per liter (μg/L), was two to three times higher than levels found in Swedish (0.31 μg/L) and U.S. women (0.21 μg/L).

On average, a doubling of the cadmium levels in the moms’ urine during pregnancy was associated with lower verbal IQ (a drop of 0.84 points), lower performance IQ (lowered by 0.64 points) and lower full-scale IQ (down by 0.80 points) in their children five years later. Similar but weaker patterns were found for verbal and full-scale measures using the child's cadmium levels at age 5.

A rice-based diet is probably the main source of cadmium exposure in this study population. Daily intake in the region – estimated at about 20 - 35 micrograms cadmium – is similar to vegetarian or rice-based diets in other countries, but is lower than the expected exposures in Western countries with typically more mixed diets.

The findings suggest that early life exposures to cadmium, at levels present in many countries, may be harmful for brain development. These results are provocative, but need to be followed up in other study populations.

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