An analysis of data from three large prospective cohort studies in the USA has demonstrated a link between the risk of type 2 diabetes and changes in coffee intake. Increasing coffee consumption over a four-year period was associated with a lowering of risk and vice versa.

The data for analysis came from the Nurses’ Health Study (NHS), the Nurses’ Health Study II (NHS II), and the Health Professionals Follow Up Study (HPFS). Participants received validated questionnaires at baseline, and every two years thereafter to give a record of medical history, lifestyle, potential risk factors and new disease diagnosis.

Diet was assessed every four years in all studies with a 126-item semi-quantitative food frequency questionnaire. The specific questions on tea and coffee related to how often on average during the previous year they had consumed caffeinated and decaffeinated coffee (one cup) or tea (one cup or glass). The answers were chosen from never to ≥ 6 daily.

The final study population consisted of 48,464 women in the NHS study, 4,7510 women in the NHS II study and 27,759 men in the HPFS. The primary endpoint was incident type 2 diabetes. Those who reported new diabetes were sent a further questionnaire to validate the diagnosis.

In 1,663,319 person-years of follow-up, 7,269 cases of incident type 2 diabetes were confirmed. Participants who increased their coffee intake by more than one cup a day (median change 1.69 cups/day) over a four-year period had an 11% (95% CI:3-18%) lower risk of type 2 diabetes.

Conversely, reducing intake of coffee by more than one cup a day (median change two cups/day) was associated with a 17% (95% CI:8-26%) higher risk of type 2 diabetes.

Changes in tea consumption were not found to affect the four-year risk of type 2 diabetes.

Tea and coffee have both been shown in previous meta-analyses to reduce the risk of new onset type 2 diabetes.

The measurement of risk of diabetes against changes in intake of coffee and tea, in this study, was considered a better estimate of any true effect on diabetes, and the timescale needed for it to appear, than baseline reported consumption.

The long follow-up period and large cohort sizes add power to this analysis, although without a randomised controlled trial it is not possible to be certain that there is not a reverse causality, i.e. that those with medical conditions that predispose to diabetes reduce their coffee intake after diagnosis.

However, a balanced diet, normal BMI, active lifestyle and avoiding smoking and excessive alcohol intake remain the cornerstone of diabetes prevention and the promotion of good health.

Dr Matthew Lockyer

• Bhupathiraju SN, Pan A, Manson JE et al. (2014). Changes in coffee intake and subsequent risk of type 2 diabetes: three large cohorts of US men and women. Diabetologia doi:10.1007/s00125-014-3235-7