Nocturnal dipping behaviour in normotensive white children and young adults in response to changes in salt intake.

BACKGROUND

Nocturnal nondipping is a feature of salt-sensitive, hypertensive individuals. In normotensive children and adults, the impact of salt intake on circadian blood pressure (BP) rhythm is not well defined.

OBJECTIVE

To test whether a high-salt diet abolishes nocturnal dipping in salt-sensitive, normotensive individuals.

METHODS

In normotensive, healthy individuals dichotomized for age (children: n = 28, age 11.9 +/- 0.8 years, 43% girls; adults: n = 41, age 25.7 +/- 0.9 years, 46% women), 24-h ambulatory BP monitoring was performed and 24-h urine collections were obtained during the steady-state phase of a low and a high-salt diet. Salt-sensitivity was defined as at least 3-mmHg increase in 24-h mean arterial pressure during the high-salt diet.

RESULTS

Salt-sensitive children and young adults (n = 11 in each group) and salt-resistant individuals (n = 17 children and n = 30 adults) were recruited. Circadian BP rhythm was maintained irrespective of age, salt intake and salt sensitivity. In contrast to the pronounced pressure response to high salt, a low-salt diet lowered the BP of salt-sensitive individuals as compared with salt-resistant individuals at daytime (SBP 107.6 +/- 1.2 vs. 114.8 +/- 1.6 mmHg, P = 0.002 in adults and SBP/DBP 103.1 +/- 1.6/68.6 +/- 1.5 vs. 111.2 +/- 1.3/74.5 +/- 1.1 mmHg, P = 0.005 in children), yet left night-time BP unchanged. Nonlinear mixed effects modelling indicated a steeper downward slope of BP from daytime to night-time in salt-sensitive as compared with salt-resistant children and all adults (P < 0.0015). Without exception, daytime mean arterial pressure disclosed salt-sensitive individuals upon salt loading.

CONCLUSION

Normotensive children and young adults maintain normal nocturnal BP dipping irrespective of salt intake and of individual salt sensitivity. Thus, daytime BP assessment is sufficient to characterize salt responsiveness in normotensive individuals.