The Blood Pressure Villain is Cola, not Caffeine

Summarized by Robert W. Griffith, MD
February 27, 2006

Introduction

Like overweight and other side-effects of western-style living, high blood pressure is on the increase. It's a major risk factor for coronary heart disease, stroke, and heart failure, so anything one can do to lower its occurrence is worth noting. Scientists have known for years that injection of caffeine into lab animals raises their blood pressure, and short-term studies in humans find a small effect of this nature, but it fades with continued intake of the 'drug'. Until now, no one has conclusively demonstrated a link between prolonged caffeine intake and blood pressure increases. A report in the Journal of the American Medical Association remedies this.

What was done

Women enrolled in the US Nurses' Health Studies I and II between 1990 and 1991 formed the subjects of this study. Every 2 years they provided data on diet, smoking, alcohol use, height, weight, physical activity, medication use, and any family history of high blood pressure. The food frequency questionnaires allowed beverage selections to specify low-calorie colas (e.g. Diet Coke or Diet Pepsi with caffeine), regular colas (Coke, Pepsi, or others), tea with caffeine, tea without caffeine, coffee with caffeine, and decaffeinated coffee.

Daily caffeine intake was calculated, using the following standards: cup of coffee 137 mg, cup of tea 47 mg, can or bottle of cola drink 46 mg, and chocolate candy 7 mg. The participants were asked about physician-diagnosed high blood pressure, and the year of diagnosis. In a small group of subjects, these reports were checked against medical records, and it was found that self-reporting was accurate.

The subjects from each study who had normal blood pressure at enrollment were divided into five groups, or quintiles, according to their average daily caffeine intake. This was to allow analysis of the possible effects of caffeine intake on the development of high blood pressure.

What was found

There were 53,000-plus women enrolled in the NHS-I part of this study, and 87,000 in NHS-2. The average age of NHS-I subjects at enrollment was 55, and that in NHS-II was 35 years. Over 33,000 women were diagnosed with high blood pressure during the study period.

The quintiles for daily caffeine intake ranged from 0-45 mg for the lowest quintiles, and from 410-1900 mg for the highest quintiles (there were separate quintiles for each part of the NHS). Using the frequency of high blood pressure in the lowest quintile as 1.0, the following percentage increases were obtained:

Only in case of the 3rd quintiles (133-297 mg caffeine daily) was there a statistically significant increase in the development of high blood pressure. The authors interpret these results as showing that there is no association between caffeine consumption and an increased risk of high blood pressure. Further analysis showed there was no link between coffee drinking and high blood pressure, either.

Analyzing the results for cola beverages, with or without sugar, showed them to have a much closer link with the occurrence of high blood pressure (less than 1 can a day = 1.0):

These changes are statistically significant - i.e. they cannot be considered to have occurred by chance alone.

What these results mean

First, the results refute the idea that coffee consumption is linked to an increased risk of high blood pressure in women. Second, and more importantly, in both studies sugared and diet colas were clearly linked to an increase in hypertension. If, indeed, this is a cause-and-effect phenomenon, it has considerable public health implications. Therefore it should be investigated further, with some urgency. And the non-caffeine, non-sugar constituents of cola drinks need to be examined particularly carefully (although it seems that the sugared colas were a little more active in raising blood pressure - see the table above).

It might be wise for cola drinkers, if there is a family history of raised blood pressure, to try to find a different non-sugared drink - or switch to water.

Source

• Habitual caffeine intake and the risk of hypertension in women. WC. Winkelmayer, MJ. Sampfer, GC. Curhan, JAMA, 2005, vol. 294, pp. 2330--2335

Related Links
Study Shows Coffee Not Hazardous to Heart Health
Coffee May Actually Be Good For You!
Diet and the Metabolic Syndrome