Apples, Arsenic, and Risk - Part 13: How much is "19 percent higher levels?"

My last post had me looking at a study Consumer Reports states in their January 2012 report on arsenic in apple juice that:

A study published in 2011 in the International Journal of Environmental Research and Public Health examined the long-term effects of low-level exposure on more than 300 rural Texans whose groundwater was estimated to have arsenic at median levels below the federal drinking-water standard. It found that exposure was related to poor scores in language, memory, and other brain functions.

This study used linear regression to:

Examine the potential association between current and long-term arsenic exposure and detailed neuropsychological functioning in a sample of rural-dwelling adults and elders.

What we know so far is this:

• Linear regression models were created using raw neuropsychological test scores as outcome variables and either current or long-term arsenic exposure estimates as predictor variables.
• Linear regression is when you want to predict values of one variable, given values of another variable.

So what we can see from Table 3:

Source

Is that when the current arsenic level increases by one unit, the  "RBANS Language scores" decrease by 0.458 (controlling for the other independent variables in the equation).  That's the prediction of their linear regression model for RBANS Language Scores [−0.458 (p = 0.008)].

For the 13 tests administered, the study found that current GIS-based groundwater arsenic exposure was significantly related to poorer scores for three of the tests:

• RBANS Language scores B(SE) = −0.458 (0.171), p = 0.008
• Visuospatial skills CLOX2, B(SE) = −0.118 (0.060), p = 0.048
• Executive functioning CLOX1 B(SE) = −0.225 (0.080), p = 0.005

So for the sake of argument (and to give me something to write about) let's say that this model is predictive as the authors claim.  How much "poorer" would the scores be for children who drink apple juice?

According to Consumer Reports:

The resulting [NHANES] analysis of almost 3,000 study participants found that those reporting apple-juice consumption had on average 19 percent greater levels of total urinary arsenic than those subjects who did not.

Interesting....remember this table from NHANES:

Source

The mean and (95% CL) for kids 6-11 years old is 7.08 (5.68-8.84).

With that, we can calculate how much "19% more total urinary arsenic" would be for 6-11 year old kids who drink apple juice.

Do some math...carry the 2....7.08 x 1.19 = 8.43 ug/L

So apple juice drinking 6-11 year old kids have 1.35 ug/L more arsenic.  If the linear regression model in this study is correct, we would predict their scores to decrease as follows.  The "B" represents the slope of the regression line--the amount of change in Y due to a change of 1 unit of X:

• RBANS Language scores: -0.618
• Visuospatial skills CLOX2: -0.159
• Executive functioning CLOX1: -0.304

Is that decrease in score a concern?  Look at the standard deviations for the test results in Table 2.  (CLOX means are not listed by the authors, I am assuming their range is similar to the other tests)

Source

For RBANS Language the standard deviation is 5.46 points.  Our little apple juice drinkers would see a decrease in their scores of 0.618.

"But"...you say..."the results might understate the correlation between juice consumption and urinary arsenic levels because NHANES urinary data exclude children younger than 6, who tend to be big juice drinkers."

Let's up the arsenic to that found in males; 9.50 ug/L.  19% more...carry the 2...9.50 x 1.19 = 11.3 ug/L

So...2.25 ug/L more arsenic...or 2.25 units...2.25 x -0.458 = 1.03 points lower for RBANS Language.  That's assuming their model holds true.   Look at the range of results they got; 8 - 42.  Would a decrease of one point indicate a concern?  Is this what Consumer Reports means when they state:

"Mounting scientific evidence suggests that chronic exposure to arsenic...even at levels below water standards can result in serious health problems."

But all that's for the current arsenic level.  Let's look at what they found when they ran the linear regression with long-term arsenic - ug/L-years.


Next Post: Apples, Arsenic, and Risk - Part 14: A poorer score of -0.001

.