Relationship of salt usage behaviours and urinary sodium excretion in normotensive South African adults

Background: Dietary salt intake in the South African population exceeds the physiological need. Excessive salt intake is associated with elevated blood pressure levels which may lead to hypertension and cardiovascular accidents. A lifestyle modification such as dietary salt restriction is an inexpensive, effective disease prevention option. Objective: The overall main objectives of this investigation was to: 1) compare salt intake, estimated from a short salt frequency intake questionnaire, with the 24-hour urinary salt excretion and blood pressure of young normotensive healthy white and black South Africans; and 2) compare 24-hour salt excretion and 24-hour blood pressure profiles of normotensive white and black individuals in terms of their knowledge, attitude and behaviour towards dietary salt intake. Study design: The study design was cross-sectional and nested in the baseline phase of the African Prospective Study on the Early Detection and Identification of Cardiovascular Disease and Hypertension in South Africa (African-PREDICT) study. Methods: Multiple methods of data collection were used including anthropometry, biochemical analyses, dietary intakes and cardiovascular measurements. Participants in the study completed the short salt frequency intake questionnaire, describing and quantifying habitual salt intake, and a questionnaire describing knowledge, attitude and behaviour regarding salt intake. Responses to the questionnaires were compared with actual salt intakes estimated from a single 24-hour urine sample and with the 24-hour blood pressure measurements. Results: There was no significant correlation between salt intake based on the questionnaire and 24-h urinary excretion in the white (r=0.07; p=0.40) and black (r=-0.53; p=0.56) participants before and after adjustment for covariates. Estimated salt intake from the questionnaire significantly correlated with systolic blood pressure in white participants (r=0.22, p=0.005) before adjustment for covariates and was no longer significant after adjustment. None of the correlations (unadjusted or adjusted) were significant for the black participants (all p>0.05). The Bland-Altman plots for salt intake showed that the mean difference between the methods used to determine salt intake for the white group is 0.5 g/day, and for the black group is -1.9 g/day. The urinary salt excretion may estimate salt intake to be 9.6 g/day above or 11.1 g/day below the questionnaire’s estimation in the white, and 10.8 g/day above and 18.4 g/day below in the black groups. The level of agreement (Cohen’s Kappa analyses) between the salt frequency questionnaire and the 24-hour urinary salt excretion were determined by categorising the participants in groups who meet the target of <5 grams salt per day or do not. The value of Kappa for the white participants was 0.17 (slight agreement) and for the black participant it was i -0.06 (no agreement). In the white participants were a significant increase in both SBP and DBP with increasing tertiles of salt intake according to the questionnaire (p<0.006 and p<0.02 respectively). In the black participants there were no significant difference in BP levels (all p>0.05). The five foods/food groups that contributed most to dietary salt intake in both ethnic groups were discretionary salt, bread, gravy made with stock or gravy powder, soup and biltong. There were no differences in the BP levels between those who answered questions about their knowledge and attitude towards salt intake in both ethnic groups (all p>0.05). Also, there were no differences in their urinary salt excretion (all p>0.05). Only certain behaviours mentioned in the questionnaire were reflected in the salt intake levels and blood pressure. Conclusions: The short salt frequency intake questionnaire can be used to identify food items that contribute to total salt intake. However, the questionnaire considerably underestimates the dietary salt intake. The application of this questionnaire may be helpful in epidemiological studies that evaluate foods which contribute to the total salt intake in order to monitor the average salt intake of a population and to assess the proportion of the population that does not meet the target of less than 5 grams of salt intake per day. It cannot, however, be used to assess the salt intake of an individual. The knowledge, attitude and behaviour of women and men of both ethnic groups are poorly reflected in their actual salt intake and blood pressure, especially among the black participants. The majority of the participants in both ethnic groups consume dietary salt in much higher quantities than the recommended less than 5 grams per day. The current public awareness campaign to decrease salt intake to the target level of less than 5 grams per day by the South African National Department of Health and the Heart and Stroke Foundation is commendable. Key terms: Salt, Sodium, Urinary sodium excretion, Knowledge, Attitude, Behaviour, Salt frequency intake, Questionnaire