As part of learning how to read primary research papers, students wrote the following adapted paper. The original paper is Sagiv et. al (2012) Prenatal Exposure to Mercury and Fish Consumption During Pregnancy and Attention-Deficit/Hyperactivity Disorder–Related Behavior in Children (PubMed – paywall).
Note: the pieces of the paper are written by different groups with minimal edits so there may be different styles throughout. The sections were written by each blog group and a link is provided for you to check out their other science writing. If you have suggestions for improvement, please leave a comment and I’ll share it with the students.
Written by Science Avengers
Childhood behaviour disorders have long reaching effects. ADHD is one of the most common childhood disorders, affecting 8 to 12% of children across the world. Studies are suggesting heavy metals can cause ADHD.
It is an accepted fact that mercury causes neurological damage. Mercury has been shown in rodents to cause changes in dopamine levels, providing an explanation for ADHD behaviours in humans. The effects on the body with lower level of mercury are inconclusive.
Mercury intake comes mostly from fish and government organizations have advised pregnant women to avoid eating less than 12 ounces of fish weekly. But fish also has great benefits to brain development making it difficult to identify the exact damage caused. The association of the mercury levels in a pregnant woman’s hair and the fish intake with the ADHD-related behaviors of a child at age 8 is investigated.
Two groups covered this section: one decided to do point form (Through the Looking Flask) and the other did paragraph summaries (Radioactive Lab Rats). Due to the stepwise nature of results, the two different styles are placed together.
- 604 singleton births who had an 8-year examination
- 421 out of 604 or 69.7% had prenatal mercury data
- 515 out of 604 or 85.3% had fish consumption data
- Summary statistics for mercury exposure, fish consumption, and outcome measures
Of the 604 singleton births surveyed, 421 had information on prenatal mercury levels, and 515 had information on fish consumption.
- Child’s age at end of examination was between 7-10 years
- Higher mercury levels in mothers who were older, married, higher household income, did not smoke during pregnancy or use drugs in the year before birth
- Higher mercury levels if either parent did not graduate high school or went to college
The participants were very varied in terms of sociodemographic levels. Mothers who were older, were married, had a higher household income, did not smoke during pregnancy or use illicit drugs the year before birth, consumed more fish during pregnancy, and had children of white race tended to have higher mercury levels. The lowest and highest educated also tended to have the higher mercury levels.
- Mercury levels were positively correlated with HOME score (measure of a child’s learning environment)
- Mercury levels were positively correlated with prenatal ?-3 (omega 3)
- Mercury levels were positively correlated with fish servings per week
- Mercury levels were positively correlated with cord serum PCB levels
- Mercury levels were inversely correlated with maternal depression symptoms (the mother’s mood disorder caused by childbirth)
Higher mercury levels also resulted in a higher Home Observation for Measurement of the Environment score, prenatal -3 and fish servings per week, and cord serum polychlorinated biphenyls levels and resulted in lower maternal depression symptoms
- adjusted and unadjusted associations between prenatal mercury levels and ADHD-like behaviours in 8-year-old children from New Bedford, 1993-98
- apparent associations when mercury levels are greater than or equal to 1?g/g – higher risk of being inattentive or impulsive/hyperactive
- adjusting total fish consumption did not consistently change associations
Table 4 shows the adjusted and unadjusted relationship between hair mercury levels and behaviors associated with ADHD. An association between mercury levels and ADHD behaviors was found, along with an increasing risk of these behaviors accompanying increasing mercury levels no matter how high. Higher mercury levels were associated with a higher risk of commision and errors and lower scores on the WISC-III Subscales, mainly in processing speed. After adjusting for total fish consumption, PCB levels, two year lead levels and excluding children treated with ADHD medication, it was found that the mercury-ADHD associations weren’t modified.
- Both sexes had similar associations between mercury level and CRS-T score
- However, girls had a noticeably faster mean reaction time and lower reaction time variability (at mercury levels less than 1?g/g)
- Boys had a slower processing speed as well
Protective associations between hair mercury levels and ADHD behaviors were stronger in boys than in girls. Associations between mercury at levels less than 1 ?g/g and CPT reaction time were found in girls. However there is no significant difference between fish consumption and ADHD levels between the sexes.
- Prenatal fish consumption was found to be strongly related to CRS-T results (more fish equals to a lower score), especially for the DSM-IV impulsivity/hyperactivity
- After adjustments were made, fish consumption was also linked to higher scores for the WISC-III scores (in particular the processing speed)
- Again, differences between the results of the two sexes didn’t remain consistent throughout
The average maternal hair mercury level was 0.45 ?g/g
MATERIALS AND METHODS
Two groups covered this section: one decided to do point form (Mighty Wizards) and the other did paragraph summaries (Dear World, Sincerely Science). Because the students wrote the information in a different order, both sets of Materials and Methods are written out separately.
From 1993 to 1998, 788 newborns in the New Bedford area (where PCBs-industrial chemicals- had been released into the harbour in the past) were recruited for the study regarding the effect of these chemicals on brain development. Despite New Bedford’s relatively low PCB levels (in umbilical cord fluids,) these chemicals were still associated with ADHD-like symptoms in the area’s children.
Mothers that participated were of legal age, spent their pregnancy near the contaminated harbour and spoke English or Portuguese. Infants had to be healthy enough to be studied at birth. About eight years later, 78% of the children who were studied at birth, were examined for ADHD/hyperactive behaviors. 421/607 children had evidence of mercury in their hair and 515 had fish in their diet.
Ten days after the births, mothers gave hair samples which were tested for Mercury. The samples were thoroughly cleaned to avoid contamination and dried at 60?C for a day. We analysed the hair within three centimetres of the scalp, when possible.
We weighed the samples, decomposed them, and recombined them. We then rapidly heated the sample, releasing the mercury from it. Using a technique known as atomic absorption spectroscopy, we measured the concentration this mercury, which was now hot and liquid.
Soon after the births, mothers took questionnaires on how frequently they ate different types of seafood (dark fish, tuna, shellfish, eel etc.,) which allowed us to estimate how much fish the mothers ate per week.
Two different tests, and a rating scale, were used to assess inattentive and impulsive behaviours. The CRS-T was used to assess inattentive, hyperactive-impulse, and both behaviours, in different ages and genders, based on the book ‘DSM-IV’. The second test showed animal silhouettes where the children were instructed to press a button whenever a cat appeared. Long, incorrect, and no responses were used to show attention disorders. The third test evaluated their intellectual abilities, based on a scale where children with attention disorders score lower.
We wanted to know the relationship between prenatal mercury and fish consumption and behavioral outcomes. First, we used “multivariable regression” (a bunch of statistics) and found that children in above the 86th percent mark had ADHD. Then, scientists experimented the average reaction time of these kids in the eighty sixth percentile. These results could be modeled linearly. Afterwards, the scientists separated the kids in the eighty sixth percentile into those who ate fish and those who didn’t. The kids who had fish usually had about 2 servings of fish per week. That is where their mercury comes from.
The test subjects also had to the some tests at the age of 2 weeks and the age of 8. These tests included the KBIT test (basic intelligence test), the BDI test (test for depression symptoms) and the HOME test (test about their environment they live in). They also studied the variable cases for the mothers of these children. They looked at their ages at their child’s birth, prenatal smoking status, alcohol consumption, illicit drug use in the year of their child’s birth, and at the 8 year examination, education level, marital status, IQ and depression. They also studied the child’s sex, race and the household income paternal educational level, and another HOME score.
Scientists estimated the total fish consumption and intake of polyunsaturated fatty acids that the mother ate from food frequency questionnaires. Scientists also accessed the sensitivity of the results to ADHD medication use, 2-year blood lead levels, and cord serum PCB.
Materials and Methods 2
- They analyzed data for children examined at the age of eight years old with peripartum maternal hair mercury measures or maternal report of fish consumption during pregnancy.
- Inattentive, impulsive and/or hyperactive behaviours were assessed using a teacher rating scale and neuropsychological testing (CRS-T)^32 on approximately 607 children.
- 788 infants’ births between 1993 and 1998 at St. Luke’s Hospital of South Coast Hospital Groups were tested for exposure to PCBs.
- They collected data on prenatal fish consumption using a food frequency questionnaire administered shortly after birth.
- Food frequency questionnaire included consumption of dark fish (e.g. salmon, mackerel), tuna, shellfish, eel, etc…
- They estimated associations of prenatal mercury exposure and fish consumption with behaviour outcomes using multi-variable regression.
- They dichotomized outcomes at the 86th percentile .(t>/-61)
- Identifies children with mild-markedly atypical scores and used log risk models to estimate risk ratios (RRs).
- They assessed sex differences.
- 8 year examination assessment of maternal intelligence and depression symptoms using KBIT, BDI, and HOME
- Characteristics of mothers (Status and alcohol consumption) were also recorded.
Written by Function of a Rubber Duck
By examining the mercury levels in maternal hair, researchers found that prenatal mercury exposure was consistently associated with ADHD in children. The mother’s hair can accurately reflect the amount of mercury present in the fetus’ brain and blood.
Since fish consumption is the most common way for humans to contact mercury, mercury levels and ADHD can also be linked to the mother’s fish consumption during pregnancy. However, fish is also known to have many nutrients, such as omega 3 fatty acids. While mercury negatively impacts brain development, omega 3 has a positive effect on neurodevelopment. Therefore, the presence of both substances can make it hard for scientists to measure the effect of mercury alone on neurological development.
The results showed that 1 microgram/gram of mercury seems to be a borderline: When mercury in the body is below this level the amount of nutrient one gains from eating seafood outweighs the potential neurodevelopmental harm from the mercury. When the the bodies mercury level is above 1 microgram/gram, the toxicity of it overshadows the nutritional benefits one may gain from eating that amount of seafood.
Another interesting observation that was made was that boys were approximately 1.7 times more affected by this link between fish consumption during their mothers’ pregnancies, mercury levels, and ADHD.
Since consuming fish can have both positive and negative effects, pregnant women should be well informed of the pros and cons of it.