Background: Increased heart rate variability (HRV) is associated with a low risk of mortality, as is consuming a low sodium diet. As the survival benefits of a low sodium diet may be mediated partly by an increase in HRV, we have tested the hypothesis that adopting a low sodium diet increases HRV.
Methods: We used a randomised double-blind placebo-controlled trial design. Participants were aged 18-65 years old, had a physician diagnosis of asthma. All adopted a low sodium diet and they were randomised to receive either 80 mmol/day of oral sodium supplements (normal sodium intake - NSI) or matched placebo (low sodium intake-LSI) for 6 weeks. The primary outcome was change in SDNN (standard deviation of the N-N intervals); secondary outcomes were changes in other time domain and frequency domain measures of HRV.
Results: In those allocated to the LSI, mean daily urinary sodium excretion decreased by 22 mmol; and in those allocated to the NSI mean daily urinary sodium excretion increased by 31 mmol. There were no differences between the two groups for either the primary or secondary outcome measures. The mean difference in change in SDNN between those who received the LSI compared to the NSI was -2.7 ms (95% Confidence Intervals CI; -18.0 to +12.6).
Conclusions: Adopting a low sodium diet does not have an impact on SDNN over a 6 weeks period. Future studies should aim to achieve a larger change in dietary sodium intake for a longer duration than 6 weeks.