New
research published in Diabetologia (the journal of the European Association for the
Study of Diabetes) shows that increasing coffee consumption by on average one
and half cups per day (approx 360ml) over a four-year period reduces the risk
of type 2 diabetes by 11%. The research is led by Dr Frank Hu and Dr Shilpa
Bhupathiraju, Department of Nutrition, Harvard School of Public Health, Harvard
University, Boston, MA, USA, and colleagues.
Coffee
and tea consumption has been associated with a lower type 2 diabetes risk but
little is known about how changes in coffee and tea consumption influence
subsequent type 2 diabetes risk. The authors examined the associations between
4-year changes in coffee and tea consumption and risk of type 2 diabetes in the
subsequent 4 years.
The
authors used observational data from three large prospective, US-based studies
in their analysis: the Nurses' Health Study (NHS) (female nurses aged 30-55
years, 1986-2006), the NHS II (younger female nurses aged 25-42 years
1991-2007), and the Health Professionals Follow-up Study (HPFS) (male
professionals 40-75 years, 1986-2006). Detailed information on diet, lifestyle,
medical conditions, and other chronic diseases was collected every 2 to 4 years
for over 20 years.
The
availability of these repeated measures and the long-duration of follow-up
allowed the authors to evaluate 4 year changes in coffee and tea intake in
relation to risk of type 2 diabetes in the following 4 years. They also
examined whether the association with diabetes incidence differed between
changes in caffeinated and decaffeinated coffee. Diet was assessed every 4
years using a validated food frequency questionnaire. Self-reported incident
type 2 diabetes cases were validated by supplementary questionnaires. The final
analysis included 48,464 women in NHS, 47,510 women in the NHS II, and 27,759
men in HPFS.
The
authors documented 7,269 incident type 2 diabetes cases, and found that
participants who increased their coffee consumption by more than 1 cup/day
(median change=1.69 cups/day) over a 4-year period had a 11% lower risk of type
2 diabetes in the subsequent 4-years compared to those who made no changes in
consumption. Participants who decreased their coffee intake by 1 cup a day or
more (median change=-2 cups/day) had a 17% higher risk for type 2 diabetes.
Changes in tea consumption were not associated with type 2 diabetes risk.
Those
with highest coffee consumption and who maintained that consumption—referred to
as "high-stable consumers" since they consumed 3 cups or more per
day—had the lowest risk of type 2 diabetes, 37% lower than the "low-stable
consumers" who consumed 1 cup or less per day.
The
authors say that the higher risk of type 2 diabetes associated with decreasing
coffee intake may represent a true change in risk, or may potentially be due to
reverse causation whereby those with medical conditions associated with risk
for type 2 diabetes (such as high blood pressure, elevated cholesterol,
cardiovascular disease, cancer) may reduce their coffee consumption after
diagnosis. However, even when cases of cardiovascular disease or cancer were
excluded during follow-up, the results were very similar.
While
baseline decaffeinated coffee consumption was associated with a lower type 2
diabetes risk, the changes in decaffeinated coffee consumption did not change
this risk. Regarding tea consumption, the authors say: "we found no
evidence of an association between 4-year increases in tea consumption and
subsequent risk of type 2 diabetes. This finding may have potentially been due
to the relatively low number of participants who made significant changes to
their tea consumption over a 4-year period thereby limiting statistical power
to detect true associations. The overall low levels of tea consumption in this
group may also be responsible for these findings."
The
authors say: "In these 3 large prospective cohorts with more than 1.6
million person-years of follow-up, we observed that increasing coffee, but not
tea, intake over a 4-year period was associated with a lower type 2 diabetes
risk in the next 4 years. Decreasing coffee intake was associated with a higher
type 2 diabetes risk. These changes in risk were observed for caffeinated, but
not decaffeinated coffee, and were independent of initial coffee consumption and
4-year changes in other dietary and lifestyle factors."
They
add: "Changes in coffee consumption habits appear to affect diabetes risk
in a relatively short amount of time. Our findings confirm those of prospective
studies that higher coffee consumption is associated with a lower type 2
diabetes risk and provide novel evidence that changes in coffee consumption
habits are related to diabetes risk."
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