A journal article, “Blood pressure response to chronic intake of coffee and caffeine: a meta-analysis of randomized controlled trials” (Journal of Hypertension: May 2005, Volume 23 (Issue 5)) examined the consumption of coffee: Coffee is a widely consumed beverage and small health effects of substances in coffee may have large public health consequences. It has been suggested that caffeine in coffee increases the risk of hypertension. We performed an analysis of randomized controlled trials of coffee or caffeine and blood pressure (BP).

Study selection: A total of 16 studies with a randomized, controlled design and at least 7 days of intervention was selected, comprising of 1010 subjects. Both coffee and caffeine effects were examined.

Results: A significant rise of 2.04 mmHg in systolic BP and 0.73 mmHg in diastolic BP was found. When coffee trials (median intake: 725 ml/day) and caffeine trials (median dose: 410 mg/day) were analysed separately, BP elevations appeared to be larger for caffeine than for coffee. Effects on heart rate were negligible.

Conclusions: Regular caffeine intake increases BP. When ingested through coffee, however, the blood pressure effect of caffeine is small.

(a) What are the treatments of this study?

(b) What conclusion can be drawn from this study? Why can we make this conclusion?

(c) Could there be a lurking variable in the above study? If so, identify one plausible lurking variable and fully explain why it is a lurking variable.