SALT-SENSITIVITY--CLASSIFICATION AND DIETARY SODIUM AND CALCIUM

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It has been recommended that ingestion of sodium be reduced in the general population. However, there is evidence indicating that not all individuals will benefit from such a dietary change and that, in some, it may lead to a rise in blood pressure and serum lipids. Individuals whose blood pressure rises or declines in response to elevations or reductions of sodium intake have been termed salt-sensitive (SS) and salt-insensitive, respectively. Procedures designed to identify SS individuals, i.e., those likely to benefit from a reduction of salt intake, have been proposed but, none is suitable for population-based screening. One aim of this project will be to explore differences between SS and SI individuals on selected physiological characteristics that may be more suitable for measurement as population-based screening indices. Specifically, assessments will be made of salivary flow and sodium composition, taste intensity and hedonic judgements of NaCI, glucose and citric acid, the initial natriuretic response to saline ingestion, and glucose tolerance and plasma insulin levels. A primary purpose for identifying SS individuals is to increase the efficiency and effectiveness of dietary interventions but, little is known about the factors influencing long-term adherence to a reduced sodium diet among the SS. A second aim of this project will be is to monitor SS and SI individuals on such a diet and to document noted obstacles to compliance. One hundred and fifty normotensive adults will participate in this approximately 6 month protocol. During the first five days, baseline values will be obtained for blood pressure, body composition (by anthropometry and bioelectrical impedance) and sodium intake (by urinary exertion). In addition, salt taste responsiveness and preference, salivary function (flow and Na concentration) as well as plasma glucose and insulin levels before and after an oral glucose tolerance test will be measured to permit analysis of their predictive power for SS classification. During the next 11 days blood pressure will be monitored every other day while participants adhere to a low (4 days) and high (7 days) salt diet. At least a 5mmHg increase of mean arterial pressure between the last day of each diet period will provide the basis for SS classification. During week 4, initial diuretic response to oral exposure to isotonic saline will be determined as another potential predictive index. Following this, a reduced Na diet will be prescribed for 4 months. Half of the participants will also receive a calcium supplement while the others will be given placebos. Sensory tests and inquires about obstacles to dietary compliance will be conducted monthly during this period. Dietary compliance will be determined by diet records, hormone (parathyroid, calcitonin) levels and urinary excretion of Na, K, nitrogen and Ca.