1.1 Background to the Study
Hypertension, defined as a systolic blood pressure in excess of 140mmHg or a diastolic pressure higher than 90mmHg, is also a globally increasing public health concern. Roughly 1 billion individuals worldwide are estimated to exhibit clinically significant elevated blood pressure with about 50million of those residing in the United States. Hypertension, in turn is associated with increased risk for CVD, Stroke, renal disease and all cause mortality. The JNC VII report defines stage 1 hypertension as blood pressure levels between. 140 and 159mmHg systolic and 90 and 99 diastolic. Additionally, the report establishes a category of prehypertension (Systolic blood pressure between 120 and 140mmHg or diastolic between 80 and 89mmHg). These two blood pressure classification are deemed to be appropriate primary targets for lifestyle modification interventions, including weight loss. Higher levels of blood pressure or stage 1 hypertension that is maintained over a long period, should be addressed primarily with medications or other physician directed treatments.
There is a positive relationship between overweight or obesity and blood pressure and risk for hypertension. As early as the 1920s, a significant association between body weight and blood pressure was noted in men (Symonds, 1923; Dublin, 1925). In the intervening years, epidemiological studies have routinely confirmed this association. The Framingham study found that hypertension is about twice as prevalent in the obese as the non obese of both sexes (Hubert et al., 1983). Stammer and colleagues (1978) noted an odds ratio for hypertension of obese relative ti non obese (BMI of less than 25) of 2.43 for younger adults and 1/54 for older ones. The Nurses Health study (Manson et al., 1995) compared women with BMIs of less than 22 with those above 29 and found a 2- to 6- fold greater prevalence of hypertension among the obese.
More recent data from the Framingham study further support this relationship. Divided into BMI quintiles. Framingham participants of both sexes demonstrated increasing blood pressure with increased over weight. In this instance those in the highest BMI quintile exhibited 16mmHg higher systolic a.d 9mmHg higher diastolic blood pressures than those in the lowest quintile. For systolic blood pressure this translated into an increase of 4mmHg for each 4.5kg of increased weight (Higgins et al., 1998) noted a 5-fold greater incidence of hypertension in individuals with BMIs of more than 30 relative to those less than 20 for both sexes.