To test the feasibility and the efficacy of nutritional interventions in the primary prevention of hypertension in individuals predisposed to the development of hypertension; specifically, to test the hypothesis that reduction of weight and/or decreased sodium intake in obese individuals, or decreased sodium intake with or without increased potassium intake (in men and women, regardless of weight) would prevent the elevation of blood pressure and the incidence of hypertension.
BACKGROUND:
The nature of essential hypertension as a mass public health problem throughout the United States, and internationally as well, is recognized generally. Prevalence estimates exceed l5 percent in most adult populations. Further, the results of the Hypertension Detection and Follow-up Program (HDFP) demonstrated the efficacy of antihypertensive therapy even at the 90-l05 mmHg level, which comprises 72 percent of the total hypertensive population. The Joint National Committee on the Detection, Evaluation and Treatment of High Blood Pressure recommended treatment of mild hypertension by pharmacologic or non-pharmacologic therapy to the lowest diastolic pressure consistent with safety and tolerance. The recommendation was consonant with the goal blood pressure guidelines for those with baseline pressures from 90-99 mmHg. Hence, there appeared to be a growing consensus that reduction of blood pressure to levels as low as 80 mmHg might be desirable for optimal cardiovascular health. The Joint National Committee further recommended that, in young patients with uncomplicated mild hypertension, the benefits from pharmacologic therapy must be weighed against the cost, inconvenience, and possible side effects. There was, therefore, a growing concern over the need for pharmacologic management of hypertension and the public health implications of such a course of action. These recommendations toward intervention in mild hypertension set a firm basis for the primary prevention of hypertension.
The association between obesity and elevated blood pressure was widely recognized. Among hypertensive individuals, numerous studies had shown that short term weight reduction by caloric restriction resulted in a reduction of blood pressure. Results of several reports of long-term follow-up of weight reduction efforts indicated that long-term control of weight at levels compatible with the objectives of the HPT was feasible. A number of investigators had achieved these levels one or more years after treatment despite minimal intervention efforts during the follow-up period.
There was strong circumstantial evidence relating sodium intake to hypertension. Most impressive evidence came from the study of the 'low-salt tribes', subsisting on a very low sodium intake. They failed to develop hypertension, or even gradual rise of blood pressure with age. Many studies published on the effect of lowering sodium intake on blood pressure of hypertensive patients had been positive. In one investigation, blood pressure fell approximately 9 (systolic)/6 (diastolic) mmHg when sodium intake was reduced from approximately l80 to l00 mEq/day. Reduction of sodium intake to a level of 70-l00 mEq/day appeared within the capability of most people and maintenance up to a period of two years had been indicated with minimal follow-up procedures.
The role of potassium intake in the development and treatment of hypertension was less clear. As noted above, primitive cultures studied epidemiologically had low sodium intakes associated with low prevalence and incidence of hypertension but these cultures also consumed higher levels of potassium. It has been suggested that the ratio of sodium to potassium may be the most important determinant of blood pressure. Hygienic intervention to increase potassium intake in free-living populations was the least studied of the procedures used in the HPT. Potassium intakes > l00 mEq could be achieved without the use of dietary supplements if individuals consumed large quantities of fruits and vegetables. This natural intake could also be increased by use of potassium-based baking powder and seasonings.
The study was carried out by four clinical and two resource centers (a Coordinating Center and a Nutrition and Educational Resource Center). It was to be Phase l of a long-term intervention trial to determine whether development of hypertension could be prevented in a population at special risk. The initial phase of the study was directed towards testing the feasibility of the intervention, alternative modes of recruitment, quantification of sodium and potassium intake, trends in blood pressure, and the pre-testing of various study procedures.
Each of the four clinical centers recruited 200 or more eligible 25 to 49 year old individuals (total 841), either identified through any of a number of multiple community sources, or through an index of hypertensive cases. The clinical investigators aided by statistical and data processing support from the Data Coordinating Center and intervention support through a special Nutrition and Education Resources Center, formed the collaborative group to test the working hypothesis. The study protocol was completed in the fall of 1982. Recruitment ended September 30, 1983. The three-year follow-up period ended August 1986. Analysis of the data ended in December 1988.
DESIGN NARRATIVE:
A randomized, non-blind design with three treatment groups (non-obese) or five groups (obese), including a no-treatment group in each weight stratum. Intervention methods were dietary only, and included sodium restriction, sodium restriction plus potassium supplementation (obese and non-obese), weight reduction, and weight reduction plus sodium restriction (obese only). This was a pilot study with measures of feasibility including recruitment success, dietary adherence, and blood pressure differences among the treatment groups.
