Increased Salt Intake Decreases Diet-Induced Thermogenesis in Healthy Volunteers: A Randomized Placebo-Controlled Study

Abstract

High salt intake ranks among the most important risk factors for noncommunicable diseases. Western diets, which are typically high in salt, are associated with a high prevalence of obesity. High salt is thought to be a potential risk factor for obesity independent of energy intake, although the underlying mechanisms are insufficiently understood. A high salt diet could influence energy expenditure (EE), specifically diet-induced thermogenesis (DIT), which accounts for about 10% of total EE. We aimed to investigate the influence of high salt on DIT. In a randomized, double-blind, placebo-controlled, parallel-group study, 40 healthy subjects received either 6 g/d salt (NaCl) or placebo in capsules over 2 weeks. Before and after the intervention, resting EE, DIT, body composition, food intake, 24 h urine analysis, and blood pressure were obtained. EE was measured by indirect calorimetry after a 12 h overnight fast and a standardized 440 kcal meal. Thirty-eight subjects completed the study. Salt intake from foods was 6 g/d in both groups, resulting in a total salt intake of 12 g/d in the salt group and 6 g/d in the placebo group. Urine sodium increased by 2.29 g/d (p < 0.0001) in the salt group, indicating overall compliance. The change in DIT differed significantly between groups (placebo vs. salt, p = 0.023). DIT decreased by 1.3% in the salt group (p = 0.048), but increased by 0.6% in the placebo group (NS). Substrate oxidation indicated by respiratory exchange ratio, body composition, resting blood pressure, fluid intake, hydration, and urine volume did not change significantly in either group. A moderate short-term increase in salt intake decreased DIT after a standardized meal. This effect could at least partially contribute to the observed weight gain in populations consuming a Western diet high in salt.

Keywords: 24 h urine analysis; blood pressure; body composition; food intake; indirect calorimetry; salt challenge.

Figure 1

Consolidated Standards of Reporting Trials (CONSORT) flow diagram.

Figure 2

Primary outcome. Diet-induced thermogenesis (DIT) within 4 h after a 441 kcal breakfast before and after 2 weeks of placebo (n = 19, (a) or salt (n = 19, (b). Comparison of DIT changes between groups (n = 38, (c)). P value by Wilcoxon signed rank test (a,b) or Mann–Whitney U test (c).

Figure 3

Indirect calorimetry. Energy expenditure (EE) and respiratory exchange ratio (RER) following a 441 kcal breakfast, before (open circles) and after (closed circles) 2 weeks of placebo (n = 19, (a,c)) and salt (n = 19, (b,d)). Data as mean and SEM.