It’s easy to say that coffee has a good or bad effect on people’s health, but it’s not easy to know, suggests a study about studies about whether coffee has a good or bad effect on people’s health.
The study is: “Dietary research on coffee: Improving adjustment for confounding,” David R. Thomas, Ian D. Hodges, Current Developments in Nutrition, 2019, nzz142. The authors, at the University of Auckland, and at HealthSearch Limited, New Zealand, explain:
this narrative review critically evaluated the methods and analyses of cohort studies investigating coffee and mortality. A specific focus was adjustment for confounding related to smoking, healthy and unhealthy foods and alcohol. Assessment of 36 cohort samples showed many did not adequately adjust for smoking. Consuming 1–5 cups of coffee per day was related to lower mortality among never smokers, in studies which adjusted for pack-years of smoking, and studies adjusting for healthy and unhealthy foods. Possible reduced health benefits for coffee with added sugar have not been adequately investigated. Research on coffee and health should report separate analyses for never smokers, adjust for consumption of healthy and unhealthy foods, and for sugar added to coffee.