Sodium is the most talked about electrolyte... it is often talked about in relation to hydration but what exactly is it and what exactly does it do?
Sodium plays a role in fluid balance, absorption of water, regulation of blood pressure and muscle contraction. Sodium, as the principal electrolyte (cation) in the extracellular fluids, serves primarily to maintain normal body water volume, the balance of water between the inside and outside of cells, and blood pressure. Normal body fluid levels of sodium are critical for nerve impulse transmission and muscle contraction. The body has effective hormonal control mechanisms for dealing with wide variations in dietary sodium intake (1).
Sodium and fluid balance
Sodium helps with water absorption from the gut. Drinks that contain carbohydrate and sodium will have increased absorption of water and once it is absorbed the water will also be better retained (reduced urine output). This is the main reason why sodium can be found in so many sports drinks. We will discuss this topic in more detail in one of the future blogs as well. Because sodium is the most important cation in the extracellular fluids it serves primarily to maintain normal body fluid balance and osmotic pressure (the amount of fluids inside and outside the cells as well as overall amount in the body). Changes in sodium concentrations in one compartment will usually result in a movement of water between compartments. For example, if sodium concentration in the blood drops, water will move into tissues (and one of these tissues can be the brain which can have serious consequences).
Sodium and blood pressure
Increased intake of high-sodium foods on a regular basis increases your blood pressure. If blood pressure is chronically high this puts a strain on your cardiovascular system, which can eventually lead to worse cardiovascular disease outcomes over time. For decades it was assumed that sodium did this by causing your body to hold onto more water, however it is now believed that the reason is much more complex and not fully understood (2). That said, individuals with high blood pressure can benefit from sodium restriction and this may reduce both systolic and diastolic blood pressure. But this effect is mainly seen in individuals with a pre-existing condition of high blood pressure. Although there can be small benefits among people with normal blood pressure, these effects are usually not clinically relevant (or possibly non-existent). In other words, in the context of blood pressure, sodium is only a point of concern for some individuals in some situations.
Sodium and muscle cramping
Sodium is often associated with muscle cramping. There is no question about a role for sodium in muscle contraction. However, there is little evidence that sodium is linked to exercise induced muscle cramping. This could partly be due to the fact that muscle cramps are very difficult to study. But existing studies have been far from convincing and it is likely that in the vast majority of cases there are other reasons for exercise induced cramping that are not sodium related. Muscle cramps during exercise are now believed to be a complex phenomenon with many possible causes and contributing factors, which unfortunately are still not well understood. Having said this, it is clear that exercise induced muscle cramping occurs more in hot conditions especially later in exercise when dehydration also occurs. But it is a stretch that the development of cramps is thus related to loss of sodium, as the heat itself, the fatigue of exercise and dehydration could all play a role in its development. Clearly more research needs to be done before we can say with confidence how exercise induced cramps are caused, and what role, if any, nutrition has in their prevention.
Regulation of sodium in the body
Our body regulates the amount of sodium in the body. Once your body takes in enough sodium, the kidneys get rid of the rest in your urine, cleverly adjusting for any that has already been lost from sweating. The sodium concentration in the blood is controlled within a very narrow range between 135 and 145 milliequivalents per liter (mmol/L). Hyponatremia, a potentially life-threatening condition, occurs when the sodium in your blood falls below 135 mmol/L (although symptoms are not always present when this happens). Kidneys play a very important role in controlling the sodium concentration in blood. If your sodium concentration in the blood is too high or too low, it may mean that you have a problem with your kidneys, with hydration (maybe too dehydrated or overhydrated), or another medical condition. Insufficient sodium intake is a highly unlikely cause of low sodium in the blood (this will also be explained in more detail in future blogs).
So now we have discussed what electrolytes are, what salt is and what sodium is, we will discuss the following questions in the next blogs:
1. Jeukendrup and Gleeson. Sport Nutrition. Human Kinetics Champaign IL 2018
2. Titze and Luft Kidney International 91(6): 1324–1335, 2017