Coffee May Actually Be Good For You!

Summarized by Robert W. Griffith, MD
February 27, 2004

Introduction

Type 2 diabetes is becoming increasingly common, and this can largely be related to poor lifestyle factors - overweight and obesity, low physical activity, smoking, and a diet high in trans-fatty acids and having a high glycemic load.¹ Where does coffee drinking stand? In small, short studies, it seems to decrease the body's sensitivity to insulin and impair glucose tolerance, suggesting it might be a risk factor for diabetes. But a recent analysis of two large long-term studies of caffeine intake has produced a different result; this has been published in the Annals of Internal Medicine, and is summarized here.

What was done

The two studies analyzed were the Health Professional Follow-up Study (HPFS) and the Nurses' Health Study (NHS). The first, which started in 1986, was all men, and the second, started in 1976, all women. The scientists followed 41,934 men for 12 years, and 84,276 women for 18 years.

At baseline, the participants didn't have diabetes, cardiovascular disease of any kind, or cancer. They were sent a questionnaire that requested detailed information on their medical history, lifestyle, and other risk factors. Fresh questionnaires were sent out every two years, and included questions about newly diagnosed diseases.

Participants were asked how often on average during the previous year they had drunk coffee or tea. Decaffeinated coffee and caffeinated soft drinks were assessed once in each study. Total caffeine intake was calculated, based on the intake of coffee as: 'never', 'less than 1 cup/day', '1-3 cups/day', '4-5 cups/day', or '6 or more cups/day'.

People reporting they had been diagnosed with diabetes were asked for diagnostic details. They were counted as having diabetes if they had: classical symptoms and a fasting blood glucose over 140 mg/dL (7.8 mmol/L); or repeated raised blood glucose levels over 200 mg/dL (11.1 mmol/L).

Statistical analyses were done to find any link between coffee drinking and the risk for developing diabetes.

What was found

The average age of the men at baseline was 53, and the women 46. There were 1,333 new cases of diabetes in the two studies.

Adjustments to the risk of developing diabetes were made, to exclude any influence of age, overweight, physical activity level, smoking, and a family history of diabetes. The likelihood of developing diabetes was calculated for the different groups of coffee drinkers, setting 'no coffee' as 1.0. The results are shown in the table:

Number of Cups of Coffee per Day	Likelihood of Developing Diabetes
	Men	Women
0	1.0	1.0
Fewer than 1	0.98	1.16
1 to 3	0.93	0.99
4 to 5	0.71	0.70
6 or more	0.46	0.71

Clearly, the more coffee you drink, the less likely you are to develop diabetes, according to these results. Thus men drinking 6 or more cups of coffee a day halved their risk of getting type 2 diabetes.

The scientists also analyzed the effect of drinking decaffeinated coffee. A similar but smaller decrease in risk was found. Men who drank 4 cups or more had a 0.74 likelihood and women a 0.85 likelihood of developing diabetes, compared with those who drank no coffee at all.

Tea consumption had no apparent effect on the risk of developing diabetes. However, total caffeine from all sources (i.e. including caffeine-containing soda drinks) was clearly protective.

Comment

This study shows that long-term coffee drinking is linked to a decreased risk of developing type 2 diabetes. This might come about in two ways: a) caffeine increases the metabolic rate, and could therefore contribute to the removal of carbohydrates and fats; and b) the potassium, magnesium and antioxidants in coffee could actually improve glucose metabolism.

Before we all start drinking numerous cups of coffee to take advantage of this apparent effect, we should remember that the results were obtained in what is called an "observational study". They don't prove a cause-effect relationship. However, the findings are what scientists call "robust", and should allow you to continue drinking coffee without feeling that you are doing something bad for your health.

Source

• Coffee consumption and the risk for type 2 diabetes. E. Salazar-Martinez, WC. Willett, A. Ascherio,  et al., Ann Intern Med, 2004, vol. 140, pp. 1--8

Footnotes
1. The glycemic load (GL) gives a fuller picture than the glycemic index (GI) does. A GI value tells you how rapidly a carbohydrate turns into sugar, but it doesn't tell you how much of that carbohydrate is in a serving of the food. That's where glycemic load comes in. The carbohydrate in watermelon, for example, has a high GI. But there isn't a lot of it, so watermelon's glycemic load is relatively low, and therefore more healthy.

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