Sodium is said to be important for athletes, and different arguments are used to explain why it is so essential. We will dive into the evidence a little more but the infographic below will already provide a short summary of the analysis.
Sodium plays several essential roles during exercise. Firstly, sodium plays a crucial role in water balance. It does this due to its effect on osmolality of the extracellular fluid (fluid in the bloodstream and surrounding the outside of cells). Osmolality is the number of dissolvable particles within the liquid that it’s dissolved in. In the body, sodium is the greatest contributor to the number of particles in the extracellular fluid. A drop in the sodium concentration in the blood (hyponatremia) means a drop in osmolality, and this can have serious consequences for fluid balance. If the concentration of sodium in the blood drops, fluids will move into tissues to even out the osmolality between the inside and outside of cells. One of those tissues is the brain and this can result in brain swelling and even death, but this requires both reduced osmolality to direct water into the cells, as well as excessive total water to cause the cells to expand to dangerous levels. So whilst this is thankfully rare, clearly it is important to make sure that sodium concentration in the blood is well maintained.
Does sodium increase water absorption?
Sodium is often part of drinks that are designed for consumption during exercise (carbohydrate-electrolyte drinks or sports drinks). There are a number of reasons why carbohydrate and sodium are added to these drinks. Some glucose can increase the absorption of sodium, and water will follow the sodium and glucose as a result of osmotic drag. It is sometimes suggested or claimed that sodium helps water absorption, but this effect is only very small. To illustrate this I want to show you the data of one study we performed. In this study different amounts of sodium were provided in a 6% glucose solution.
The Na group ingested the following beverages:
Na0: 6% Glucose
Na20: 6% Glucose + 20 mmol/L sodium = 460mg/L (roughly 0.5 g/L)
Na40: 6% Glucose + 40 mmol/L sodium = 920 mg/L (roughly 1.0 g/L)
Na60: 6% Glucose + 60 mmol/L sodium = 1380 mg/L (roughly 1.5 g/L)
You can see the results in the figure below. If there are differences with moderate to very large amounts of sodium in a drink, the effects are minimal.