Question
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 common genotype (Genotype 1) of the OBCT gene that regulates the metabolism of sugar in the blood, whereas the other common genotype of this gene (Genotype 2) does not cause obesity following consumption of SSBs.
a. You conducted a prospective cohort study in which children, non-obese at recruitment, 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. The following table shows the results you obtained at the end of the 10 years:
Obese | Not Obese | Total | |
SSB Consumer | 250 | 250 | 500 |
Non-SSB Consumer | 500 | 1000 | 1500 |
Total | 750 | 1250 | 2000 |
a. Calculate the risk ratio and risk difference for the crude association between SSB consumption and obesity status.
b. Blood was drawn from the children at enrollment and all were genotyped. Calculate the risk ratio and risk difference for the association between SSB consumption and obesity status within each genotype stratum shown below.
Genotype 1:
Obese | Not obese | Total | |
SSB consumer | 90 | 190 | 280 |
Non-SSB consumer | 217 | 483 | 700 |
Total | 307 | 673 | 980 |
Genotype 2:
Obese | Not obese | Total | |
SSB consumer | 170 | 50 | 220 |
Non-SSB consumer | 300 | 500 | 800 |
Total | 470 | 550 | 1020 |
c. What do you conclude about the relationship of the genotype to the association between SSB consumption and obesity status? Is there evidence of effect modification on the additive and/or multiplicative scales?
d. Using the stratified tables above, fill in the table below.
Genotype | SSB Consumption | Observed cumulative incidence | Expected cumulative incidence(show calculation) |
1 | No | --- | |
2 | No | --- | |
1 | Yes | --- | |
2 | Yes |
e. Considering only the data shown in the table, is there evidence of effect modification between genotype and SSB consumption status? Explain.
f. When you write up the study results, should you report the crude (unadjusted) associations for the association between SSB consumption and obesity, the stratum-specific associations, or calculate and report an adjusted association? Why?
Source: PH W250 2016 Problem Set 5
