One of the largest studies to investigate the relationship between blood pressure and type 2 diabetes has found that women who have high blood pressure levels are three times more likely to develop diabetes than women with low blood pressure levels. This effect was independent of body mass index and other conditions that are known to predispose people to cardiovascular disease and diabetes.
Writing in the European Heart Journal today , the authors say that clinicians should be aware of the relationships between blood pressure and type 2 diabetes to optimise the management of patients at increased risk for cardiovascular disease.
The researchers from the Brigham and Women's Hospital, Harvard Medical School and the Harvard School of Public Health, USA, followed over 38,000 female health professionals for ten years. At the start of the study in 1993, all the women were free of diabetes and cardiovascular disease. Follow-up continued to the end of March 2004, at which point data were nearly 100% complete (97.2% for morbidity and 99.4% for mortality).
The lead author, Dr David Conen, a cardiologist and research fellow, explained: "Despite several studies finding a close relationship between hypertension and type 2 diabetes, little information exists on the relationship between blood pressure levels and the subsequent development of type 2 diabetes. Data for women are particularly limited. Finding an independent association between blood pressure and new-onset diabetes is important, because it suggests that women with increasing blood pressure levels should have their blood glucose levels monitored. Individuals at high risk for cardiovascular disease may benefit from early intervention."
The researchers divided the women into four groups: those with optimal blood pressure (BP), below 120 mmHg systolic, 75 mmHg diastolic; those with normal BP (120-129 mmHg systolic, 75-84 mmHg diastolic); those with high normal BP (130-139 mmHg systolic, 85-89 diastolic); and those with established hypertension (at least 140 mmHg systolic, 90 mmHg diastolic, and/or self-reported history of hypertension or treatment for the condition).
After 10 years of follow-up 1.4, 2.9, 5.7 and 9.4% of women in the four categories respectively had developed type 2 diabetes. After adjusting for various factors such as age, ethnicity, smoking, alcohol intake, body mass index (BMI), exercise, family history of diabetes etc, the researchers found that women with hypertension had a three-fold risk of developing diabetes compared with women with optimal BP.
Dr Conen said: "We found that obesity was also a strong and independent risk factor for the development of type 2 diabetes. However, statistical analyses showed that the relationship between blood pressure and the onset of type 2 diabetes was similar among women who were normal weight, overweight or obese. There was a three-fold increase in risk from the lowest to the highest BP category within all three weight categories. This analysis showed that the association between blood pressure and diabetes was not explained by weight alone."
Women who had an increase in BP during the study also had an increased risk of developing diabetes. Those whose BP rose but who remained within the range of normal BP had an increased risk of 26% compared to women who had stable or decreasing BP. Women who progressed to hypertension had a 64% increased risk.
Dr Conen said: "Compared with an overall rate of 4.5 events per 1,000 person-years, the incidence rates in the optimal BP category was 1.5 events per 1,000 person-years, showing that these women have a very low risk of developing diabetes. On the other hand, women with high normal BP had a much higher risk compared with women with normal BP, and the risk among those with established hypertension was substantial: after ten years almost 10% of these women had diabetes, a rate of ten events per 1,000 person-years. Taken together, our study demonstrates that BP and BP progression are strong predictors of incident type 2 diabetes, an effect independent of BMI and other components of the metabolic syndrome." 
The authors suggest a possible mechanism for the relation between BP and diabetes may be endothelial dysfunction - a dysfunction of the normal biochemical processes carried out by the layer of cells that line the inner surfaces of blood vessels. "It may be a precursor of both hypertension and diabetes," said Dr Conen. "Thus, the progression of endothelial dysfunction may cause worsening of both BP and blood glucose. This is in line with the fact that both BP and blood glucose occur together as part of the metabolic syndrome."
He concluded: "Our findings provide strong evidence that BP and progression of BP are associated with an increased risk of diabetes. They highlight the fact that cardiovascular risk factors are interrelated and occur in clusters. Thus, an important message for physicians and future guidelines is that none of the cardiovascular risk factors should be looked at individually. The combination of all risk factors should be used to make treatment decisions."
 Blood pressure and risk of developing type 2 diabetes mellitus: The Women's Health Study. European Heart Journal, doi:10.1093/eurheartj/ehm400
 Metabolic syndrome is a combination of medical disorders that increase the risk of cardiovascular disease and diabetes. Components include central obesity, elevated fasting glucose levels, decreased HDL cholesterol, elevated triglycerides and elevated blood pressure.