Implications for practice and research

• Salt restriction (<6 g/day) improves blood pressure (BP).

• The effect in hypertensives is more significant than normotensives.

• Small increase in renin-angiotensin-aldosterone (RAAS) markers.

• Long-term data regarding the effects of sodium restriction is limited.

Context

Hypertension is a primary cause of death and disability.1 Despite great strides in achieving BP targets, the majority of patients remain undertreated. Current recommendations include the dietary approaches to stop hypertension (DASH) diet and sodium reduction (6 g/day salt).1

DASH-sodium is the sequel to the DASH study, as salt was not a factor originally.2 Reductions in the BP by decreasing salt, while not as impressive as with DASH, were still substantial. A reduction of 4 g/day of salt for 30 days had similar effects to single drug therapy. Two recent meta-analyses have shown dissimilar long-term sodium reduction effects.3

Findings

The meta-analysis included 34 studies (n=3230). The salt reduction intake by 4.4 g/day resulted in a -4.18 mm Hg change in systolic BP (SBP) and -2.06 mm Hg diastolic BP. Changes in 24 h urinary sodium, age, BP and ethnicity were all significantly associated with SBP changes. Hypertensives (vs normotensives) and black patients (vs white patients) had greater BP reduction. A 1- year increase in age was associated with a 0.06 mm Hg greater decrease in SBP with salt reduction.

There was a statistically significant increase in plasma renin activity, aldosterone and norepinephrine. There was no change in cholesterol, however, <25% of trials measured low-density lipoprotein, high-density lipoprotein, triglycerides and total cholesterol

Commentary

The role of dietary sodium in health and disease has been a topic of debate for many decades.4 The meta-analysis shows that a modest reduction in salt intake (5–9.4 g/day) reduces BP in hypertensives and normotensives. Small individual BP changes can have an important clinical and public health impact on reducing cardiovascular complications. This dietary plan is attuned to the current recommendations of 5–6 g/day of salt.1

Salt is an essential nutrient required to maintain extracellular fluid volume and serum osmolality. Under specific circumstances, such as heat stress or vigorous physical activity, the body requires a higher intake to maintain homeostasis. Salt is ubiquitous in food, important for food preservation and taste enhancement and consumption in large amounts is consequently widespread. The salt intake in most industrialised societies is close to 9 g/day, a figure far in excess of the minimal daily requirements.

Salt reduction is one of the most cost-effective measures to improve public health worldwide. Reductions in salt intake are imperative to control hypertension. He and colleagues suggest that recommendations should be up to 3 g/day, given the results of only two of the studies, instead of the current recommendations of 5–6 g/day.1 Yet, questions remain regarding this very low-salt diet. First, is it feasible and achievable as a long-term plan? Second, what are the long-term cardiovascular consequences? Graudal et al3 suggest that salt restriction adversely affect lipid profile and they have shown that there was an increase in RAAS markers. Third, will individuals use other taste enhancers, such as sugar or fat, to compensate for salt reduction? Lastly, should this be a global recommendation or should the focus be on individuals that are more salt sensitive? Rigorous outcome trials for the long-term effects of salt restriction are urgently needed.