Epidemiology

Consumption of sugar-sweetened beverages (SSBs) is hypothesized to increase the risk for childhood obesity.It is further hypothesized that SSBs increase the risk for obesity only among those with a genetic variant that dysregulates the metabolism of sugar in the blood.The gene is not associated with consumption of SSBs.

a.If the hypothesis is correct, what associations do you expect to see in your data after stratifying on genetic variant status?

b.Below are the data you collect, based on a prospective cohort study in which non-obese children were followed over 10 years.Children in the top 25th percentile of self-reported SSB consumption during the first year of the study were categorized as SSB consumers.All other children were categorized as non-SSB consumers.Blood was drawn from the children at enrollment, and all were tested for the presence of the genetic variant.

Calculate the appropriate measures of the crude association between SSB consumption and obesity status, with 95% confidence intervals.

Obese Not Obese Total Total

SSB consumer 125 125 250

Non-SSB consumer 250 500 750

Total 375 625 1000

c.Calculate the association between SSB consumption and obesity status within strata of the genetic variant, with 95% confidence intervals.

Genetic variant present:

Obese Not Obese Total Obese Not obese Total

SSB consumer 85 25 110

Non-SSB consumer 150 250 400

Total 235 275 510

Genetic variant absent:

Obese Not Obese Total Obese Not obese\ Total

SSB consumer 40 100 140

Non-SSB consumer 100 250 350

Total 140 350 490

d.What do you conclude about the relationship of the genetic variant to the association between SSB consumption and obesity status?

e.Will you report the crude association between SSB consumption and obesity or the stratum-specific association? Why?

f.Using the stratified tables above, calculate the:

i.Risk of obesity among those with the genetic variant who consume SSB (R_G+SSB+)

ii.Risk of obesity among those with the genetic variant who do not consume SSB (R_G+SSB-)

iii.Risk of obesity among those without the genetic variant who consume SSB (R_G-SSB+)

iv.Risk of obesity among those without the genetic variant who do not consume SSB (R_G-SSB-)

g.Using the information above and the formula below, determine if there is interaction on the additive scale.

((R_G+SSB+) - (R_G-SSB-)) =?= ((R_G+SSB-) - (R_G-SSB-)) + ((R_G-SSB+) - (R_G-SSB-))

h.Using the information above and the formula below, determine if there is interaction on the multiplicative scale.(Note that these risk ratios are different from the ones you calculated in the above stratified tables: these all have a common denominator).

((R_G+SSB+)/(R_G-SSB-)) =?= ((R_G+SSB-)/(R_G-SSB-))*((R_G-SSB+)/(R_G-SSB-))