Sodium and Fluid Balance: Reading Your Result

Sodium, a fundamental element, is a critical component for human health due to its multifaceted roles within the body. Sodium primarily exists in the extracellular space in the form of sodium ions (Na+), where it maintains fluid balance and regulates blood pressure. Sodium collaborates with another ion, potassium (K+), to ensure that bodily fluids remain at appropriate levels, which is crucial for cardiovascular health and muscle function. Moreover, sodium is integral to nerve signaling and muscle contractions, facilitating the transmission of electrical impulses within the nervous system and enabling movement. Sodium is also involved in the transport of nutrients and other substances across cell membranes (Shrimanker and Bhattarai 2023; Strazzullo and Leclercq 2014).

As sodium plays multiple critical biological roles, it is necessary for blood sodium concentrations to be maintained within a relatively tight range. When sodium concentrations deviate significantly and/or suddenly from this range, there is a risk of severe neurologic damage and, in some cases, death. While mild abnormalities in sodium concentration are not acutely life-threatening, chronically high or low sodium levels are associated with poor health outcomes. For example, low sodium levels (“hyponatremia”) are associated with increased risk of cardiovascular events and mortality (Sajadieh et al. 2009; Wannamethee et al. 2016), while high sodium levels (“hypernatremia”) are associated with increased biological aging and higher mortality (Dmitrieva et al. 2023; Wannamethee et al. 2016).

The finding of a mildly low or high sodium level can provide information about hydration status, and in some cases, could indicate an underlying health condition. A low sodium level indicates dilute blood (i.e. relatively more water than sodium), which can happen when we drink excess water without also taking in electrolytes–during intense or prolonged exercise, for example. Hyponatremia can also be caused by diuretic medications, antidepressants, pain medications, seizure medications, kidney problems, heart failure, adrenal insufficiency, hypothyroidism, and a condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH), in which the body retains too much water (Adrogué, Tucker, and Madias 2022).

A high sodium level indicates concentrated blood (i.e. relatively more sodium than water), which can happen when we do not drink enough water. In addition to dehydration from not drinking enough fluid, hypernatremia can be caused by excess fluid loss (sweating, diarrhea, vomiting), kidney problems, overactive adrenal glands (hypercortisolism), or a condition called diabetes insipidus which impairs the kidney’s ability to concentrate urine (Patel et al. 2023).

While there are some data suggesting that serum sodium levels between 138-142 mmol/L are associated with the best health outcomes (Dmitrieva et al. 2023), this is based on observational data and therefore does not necessarily reflect a causal relationship. Without evidence that modulating serum sodium levels within the normal range can improve outcomes, use of a distinct optimal range is not recommended, and may contribute to harm via overcorrection.

For NiaHealth, the following reference categories, provided by our lab testing partner, are used: serum sodium levels below 136 mmol/L are considered low (at risk), levels between 136–146 mmol/L are considered normal (optimal), and levels above 146 mmol/L are considered high (at risk).