The Salt-Water Connection

It is a simple fact that something salty can ‘make us thirsty”-that is, the mere taste of salt can sometimes be enough to elicit a sense of thirst.

Yet, the components of salt (sodium and chloride) both play key roles throughout the body and the body is ‘hard-wired’ with taste for salt, as evidence by the specific receptors built into the tongue. 

Blood concentrations of sodium are directly related to the regulation of safe fluid levels in the body. In addition, sodium also leads to the re-absorption of water that would otherwise be excreted in the kidneys.

The Sodium-Potassium Pump

In muscle tissue at the cellular level, a mechanism known as the sodium-potassium pump continually adjusts intra- and extra-cellular fluid pressures. When explaining the sodium-potassium pump to a client, for instance, it is accurate to say that potassium is the electrolyte, or ion, that pulls fluids into the muscles, while sodium pulls water back out. That is, when sodium concentration is higher outside the muscle than is the potassium concentration inside the muscle, water shifts away from the muscle. This is undesirable since, left unchecked, it will lead to cramping and other heat-related heat injuries. Conversely, a desirable situation exists when sodium is less concentrated outside the muscle than is the potassium inside the muscle, which causes water to move into the muscle.

Fortunately, there are several interacting mechanisms, including the productions of the hormones angiotensin and aldosterone, exist to adjust when there is too little salt present in order to protect the body’s nerves and muscles, as well as the action of the sodium-potassium pump. It should be noted that while it is important, the sodium-potassium pump is not the only mechanism at work in terms of transporting water to and from muscle tissue. Water is also carried into the muscles along with carbohydrates (in the form of insulin-borne glucose) at a ratio of 3:1.

Controlling Sodium Intake

While there are various neurotransmitter and hormonal activities that play key roles in this ongoing balancing act, it is sodium intake that is a main means that one can apply conscious effort to control fluid levels–barring the assistance of a physician and/or the use of prescription dose medication.

The sense of thirst is an important bodily mechanism, but it is also an ‘imperfect’ one. That is, it often occurs long after the body needs water and it is not particularly good at indicating how much is really needed.

During exercise, especially in warm conditions, water loss is most often attributed to perspiration, an increase in water dissipation due to heat generated during accelerated chemical reactions, an increased rate of respiration, and water losses that occur inside working muscles from the friction caused by greater contractile component movement.

On the surface, ingesting large amounts of sodium would appear to be a good remedy to a dehydrated condition, regardless of what caused it. This is not the case, however, particularly when it comes to fluid loss induced by exercise. Because electrolytes are necessary for muscle contraction, when the levels of sweating are high and fluid electrolyte replacement is not enough, sodium levels will fall, thereby inhibiting muscle relaxation, and leading to exertional heat cramps. (Replacing electrolytes in the form of sports drinks is a topic for another article.)

Even though several electrolytes are lost in perspiration including sodium, potassium, iron, magnesium, calcium, sodium and potassium account for the majority of the loss. The practice of consuming significant amounts of sodium (3.0 to 5.0 grams) consumed in less than one hour before the performance of intense and/or prolonged activity in hot and humid climates should be avoided. If the vascular system is maintaining a certain overall concentration of sodium, and a large amount of sodium is ingested in a single dose just prior to, or during, exercise, the vascular system will immediately direct fluid to the localized area where the sodium is now too concentrated in order to “dilute” the sodium concentration to normal levels. This is accomplished by drawing upon reserve fluids, which in most cases, come from the intramuscular areas. This temporarily pulls much-needed water away from the working muscles, including in extreme cases, the heart. This can lead to severe dehydration and muscle cramping; when it affects the heart, such temporary fluid loss could result in cardiac failure. For these reasons, salt tablets should not be used.

How Much Sodium is Too Much?

The National Research Council recommends a daily sodium chloride intake of 1 gram per kilogram of water consumed. Too much of sodium intake can cause an increased amount of potassium to be lost through the urine. Further, a prolonged abnormally high amount of sodium in the body will result in fluid retention accompanied by dizziness and swelling of areas such as the legs and face. For example, consumption of 14-28 grams of salt daily is considered excessive. Diets consisting of excessive amounts of salt contribute to the increasing incidents of high blood pressure. Since sodium occurs in so many forms in processed foods, the easiest way to reduce sodium intake is to cut out the use of table salt. 

References

1. Maughan, Ron J. “Fluid and electrolyte loss and replacement in exercise.” Journal of sports sciences 9.S1 (1991): 117-142.

2. Food and Nutrition Board, Institute of Medicine. (2004). Dietary reference intakes for water, potassium, sodium, chloride, and sulfate.

3. Washington D.C.: National Academy Press. Murray, R. and Kenney, W.L. (2008). Sodium balance and exercise. Current Sports Med Reports Suppl, 7, 4, S1-S2.