There's a good bit of medical lore that says that caffeine will increase your blood pressure. It's true in the sense that there are short-term clinical studies that show that caffeine intake can raise blood levels of stress hormones associated with hypertension, but these studies have all been only up to a week or so in length. Recently a team of researchers at Harvard published the first study of the effects of long-term caffeine intake among women (JAMA. 2005;294:2330-2335 ).

The study followed two groups of female registered nurses:

1. 53,175 women, 32-55 years of age, between 1990 and 2002
2. 87,369 women, 25-42 years of age, between 1991 and 2003

Any woman who was diagnosed with hypertension was excluded from the study.

Each woman filled out a questionnaire every four years, answering questions about diet, exercise, and caffeine intake. The relevant beverages on the questionnaire were:

• low-calorie cola (e.g., Diet Coke or Diet Pepsi with caffeine),
• regular cola (e.g., Coke, Pepsi, or other cola beverages with sugar),
• tea with caffeine,
• tea without caffeine,
• coffee with caffeine, and
• decaffeinated coffee.

The researchers adjusted their analyses for other risk factors, like family history of hypertension, BMI level, alcohol use, amount of exercise, and use of oral contraceptives. Their results are very interesting: those women whose caffeine intake was in the midrange of the study (between 144mg and 297 mg per day) had a 14-15% HIGHER risk of hypertension than those whose caffeine intake was less than 45mg per day. But here's the interesting part: those women whose caffeine intake was at the high end of the study (over 417mg/day) had almost the same risk of hypertension as those women in the low end of the study. These results were so surprising that the researchers went on to analyze the results according to what kind of caffeine the women in the study were drinking: coffee, tea, or soda?

Although there was some slight difference between the two groups of women, those women who drank four or more cans (or glasses) of caffeinated soda (regular or diet) had between 37% and 60% greater risk of hypertension than those who drank one or less.

By comparison, those who drank coffee had an increased risk of hypertension of only 6% for those who only drank one cup per day (Those who drank more had no increase in risk or their risk actually decreased.) Those who drank tea had increased risk with four or more cups per day