Metabolic Syndrome Increases Salt Sensitivity

Moreover, the risk of salt sensitivity increased with the number of risk factors for metabolic syndrome, Jing Chen, M.D., of Tulane University and colleagues reported online in The Lancet.

The results suggest that "reduction in sodium intake could be an especially important component in reducing blood pressure in patients with multiple risk factors for metabolic syndrome," the authors concluded.

Several small clinical studies have suggested that insulin resistance plays a role in sodium retention and expansion of extracellular fluid volume. As a result, insulin-resistant individuals might have increased blood pressure sensitivity to dietary salt, the authors said.

Given that insulin resistance is considered the underlying mechanism of metabolic syndrome, people with the syndrome would likely be sensitive to a dietary salt intervention, they theorized.

To test that hypothesis, the investigators studied 1,881 nondiabetic individuals ages 16 and older in rural areas of northern China. Participants completed a seven-day trial of a low-sodium diet (51.3 mmol/day) followed by seven days on a high-sodium diet (307.8 mmol/day).

Blood pressure was measured at baseline and on days two, five, six, and seven of each dietary trial.

Investigators defined metabolic syndrome as the presence of three or more of the following risk factors: abdominal obesity, elevated blood pressure, high triglyceride concentration, low HDL level, and elevated plasma glucose.  By that definition, there were 283 of the study participants who had metabolic syndrome.  Salt sensitivity was defined as >5 mm Hg increase in mean arterial pressure during the high-sodium dietary trial and >5 mm Hg decrease during the low-sodium trial.

Participants with metabolic syndrome exhibited significantly greater changes in blood pressure compared with controls in response to both dietary trials.

During the low-sodium period, mean systolic blood pressure decreased 8.8 mm Hg in participants with metabolic syndrome compared with 5.2 mm Hg in those who did not have the syndrome (P<0.0001).

Diastolic blood pressure declined by 5.3 mm Hg in the metabolic syndrome group and 2.9 mm Hg in the control group (P<0.0001).

A similar pattern emerged during high-sodium intervention.

Mean arterial pressure increased by 6.7 mm Hg systolic and 3.4 mm Hg diastolic in the metabolic syndrome group versus 4.6 mm Hg and 2.0 mm Hg, respectively, in the group without metabolic syndrome (P<0.0001 for both comparisons).

Systolic and diastolic responses in the metabolic syndrome group differed significantly from those of the controls at each blood pressure assessment during the high- and low-sodium dietary trials, the authors reported.

In addition, the between-group differences tended to increase with the duration of each dietary trial.

After adjustment for covariates, the investigators found a graded association between an individual's number of risk factors for metabolic syndrome and blood pressure response during the dietary trials.

During the low-sodium intervention, for example, individuals with no risk factors had a mean blood pressure decline of 4.3/1.86 mm Hg versus a 9.39/6.06 mm Hg decline for people with five risk factors.

Although the underlying mechanisms of salt sensitivity remain unclear, the findings of Chen and colleagues have several implications for health policy, Gonghuan Yang, M.D., of Peking Union Medical College in Beijing, said in an accompanying commentary.

They include:

• The value of lifestyle interventions in people with metabolic syndrome.
• The potential value of lifestyle interventions in people with risk factors for metabolic syndrome.
• Need for comparative research into salt sensitivity, particularly ethnic variations.

He also noted the high salt intake in Chinese diets. "In 2002, the average daily salt intake for a man aged 18 years with a light level of physical activity was 12 g per day, which is roughly twice that recommended by Chinese dietary guidelines. In some rural areas, average salt intake has been as high as 14.7 g per day."

Practice Pearls:
Explain to patients that this study showed that people with risk factors for the metabolic syndrome may have an increased blood pressure response to dietary salt.

Note that the findings came from a study conducted in China, and the results may not apply to other racial/ethnic groups.

Chen J et al. "Salt int in individuals with metabolic syndrome" Lancet 2009; DOI: 10.1016/S0140-6736(09)60145-8.