In Western countries people are consuming far more salt than they should in their daily diet, which is recognized to be a bad habit, potentially leading to augment the risk of cardiovascular diseases. The marked increase in the incidence of autoimmune diseases observed in the past half-century has been also attributed to high salt intake as an environmental risk factor, since the epidemiological changes cannot be related to genetic factors.

Indeed, two independent studies in murine models of autoimmune diseases have recently shown a modulation in T cell immune response towards a pro-inflammatory subset, i.e. T helper 17 lymphocytes, following an excess salt intake. Based on the findings, the research group headed by Prof. Guido Valesini, rheumatologist at Sapienza University of Rome and Head of the Division of Rheumatology, Policlinico Umberto I, Rome, Italy, has conducted a study aimed at investigating whether the dietary salt intake may cause a pro-inflammatory phenotype in the cells of immune system from patients with rheumatoid arthritis and systemic lupus erythematosus, two of the most common autoimmune diseases.

The study, published recently in the PLOS ONE journal, has examined for the first time the biological effect of salt in patients with autoimmune diseases, analyzing both T helper 17 lymphocytes and regulatory T cells, which are functional antagonists of T helper 17 lymphocytes. Given the harmful effects of excess sodium consumption, patients underwent a dietary regimen starting with a restricted daily sodium intake followed by a normal-sodium daily intake, thus avoiding the exposure to a surplus in sodium dietary intake. The study was planned to cover a five-week period: in the first 3 weeks patients were asked to maintain a low-sodium dietary regimen following the indications in the leaflet given to each of them. After that, patients entered the last 2 weeks of the study, where they followed a normal-sodium dietary regimen, meaning that they were supposed to meet the WHO recommendations requiring not to exceed 5 g salt/day.

Therefore, the aim of the study was to recognize possible changes in T helper 17 and regulatory lymphocyte number in relation to the 2 different dietary conditions. “Adherence to the dietary regimen was deemed as an unavoidable requirement in order to provide believable research findings. Therefore, we used the most reliable method available to day for measuring sodium intake, that is the 24-hour urinary sodium excretion. Because 1 gram of salt contains 17 mEq of sodium, the 24-hour urinary sodium excretion should be less than 85 mEq in a normal-sodium dietary regimen", Prof. Valesini says. Fourteen patients with rheumatoid arthritis and 15 with systemic lupus erythematosus complying with the dietary regimen were identified. The group was under observation at the outset of the study, and then after 3 and 5 weeks to analyze T helper 17 and regulatory lymphocytes.

The findings of the study confirmed the initial intuition of the research group: all the patients showed a reduction in the frequency of T helper 17 cells after the first 3 weeks of low-sodium dietary regimen, and an increase after the 2 further weeks characterized by an augmented salt intake, albeit within the limits of WHO recommendations. Conversely, regulatory T lymphocytes, which are typically anti-inflammatory cells, exhibited the opposite trend: an increase was observed in the first 3 weeks, followed by a reduction over last 2 weeks. Notably, at the end of the study, a decrease in the levels of some molecules capable of amplifying the immune response in patients with autoimmune diseases was described.

“Our results support the idea that a restricted sodium dietary intake may reduce the inflammatory response in patients with autoimmune diseases”, Prof. Valesini claims. Therefore, excess in sodium intake seems to be a modifiable risk factor in the management of autoimmune diseases. “It is worrying that the levels of 24-hour urinary sodium excretion at baseline far exceeded the recommended limit of 85 mEq”, Prof. Valesini adds. This consideration inserts the caveat that lifestyle measures based on healthier dietary regimens should be broadly adopted.

References:

The role of dietary sodium intake on the modulation of T helper 17 cells and regulatory T cells in patients with rheumatoid arthritis and systemic lupus erythematosus. Scrivo R, Massaro L, Barbati C, Vomero M, Ceccarelli F, et al. (2017) PLOS ONE 12(9):e0184449. https://doi.org/10.1371/journal.pone.0184449