The two groups that are best to enroll and compare for this purpose in a case-control study are:
1) The group of CHD cases: Individuals who have been diagnosed with Coronary Heart Disease (CHD).
2) A control group of individuals without CHD: These individuals should be selected from the same population as the CHD cases and should be free of CHD at the time of enrollment.
Given that CHD is rare in younger people, the researcher should focus on enrolling older individuals as CHD is more likely to manifest in older age due to cumulative exposure to potential risk factors over time.
Additionally, since the researcher wants to investigate if tea consumption increases the risk of CHD, they should ensure that both the case and control groups are matched for age, sex, and other potential confounding factors related to CHD risk. Moreover, it is essential to assess tea consumption using the biomarker for tea metabolism to ensure accurate and objective measurement.
By comparing the tea consumption patterns (assessed by the biomarker) between the CHD cases and the control group, the researcher can determine if there is an association between tea consumption and the risk of CHD.