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Hyponatremia 2.0

Posted on 27/02/2020

About the Author: Jono (old man) Rumbelow is the familiar and friendly face at Run in Bree street. You may not know, however, that he is an experienced coach and learned athlete himself. He also has some experience singing in a choir, but let’s not get into that. 

The Art of getting our Salt & Water intake horribly wrong 2.0

In March 2017, I was supporting athletes that I was coaching at the time, at the South African Triathlon Championships. Like any other triathlon event, there were a number of athletes cramping but this particular day was fairly challenging – far beyond the normal levels at most other Olympic distance races I have attended. I decided to look into this a little further and after some extensive research have come up with this.

In part 1 (read here if you haven’t already), we spoke about how salt and water is lost in sweat and how they can be replaced unequally (i.e. water replacement is greater than sodium). I also spoke about the interference of kidney function and how this interference can also lead to hyponatremia. Let’s explore this a little more

Changes in Blood Sodium

The hormone that regulates your sodium / water balance is called ADH (anti-diuretic hormone). It is released by a specific group of cells in the posterior pituitary of your brain that monitors blood sodium concentration and blood volume. If the sodium concentration level drops, ADH secretion drops, and the kidneys respond by dumping out more free water (the urine becomes more dilute). Thus the blood sodium concentration increases. When this happens, the secretion of ADH increases and the kidneys hold on to more free water; thus the sodium concentration falls back to normal. 

Hormones, such as antidiuretic hormone (ADH), regulate kidney function. High ADH levels signal the kidneys to decrease water lost in urine. Section Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Changes in Blood Volume

You see your kidneys are not sure what to do and the “Yo Yo” effect is not great for the human body. The pituitary gland cells are less sensitive to changes in blood volume, but the response is of a much greater magnitude than with sodium concentration changes. A fall in blood volume causes a greater release of ADH — to try to get the kidneys to hang on to more water to bring the volume back up.

Conflicting Signals

Now imagine what would happen in your body if the sodium falls and the blood volume falls? The former tells the pituitary to decrease ADH and the latter tells it to increase. The result of these conflicting signals depends upon the magnitude of the changes. Beyond a certain %, the volume pathway predominates and is a much more potent stimulus for ADH release than the sodium pathway. Protecting blood volume is a survival mechanism. Thus, in the case of an athlete who is hyponatremic, the pituitary will continue to release ADH to try to hold on to water, even though this may make the hyponatremia worse.

This means that the kidneys will effectively be told to hold on to more free water and less sodium, thus causing hyponatremia. This is seen in a variety of conditions but under the right conditions, the pain/stress of endurance events may be sufficient to generate hyponatremia and is made worse by drinking water, but corrected by ingesting salt.

Another scenario is, as both sodium and water losses mount during a long race, ADH is released to better protect against dehydration. What may happen as the athlete is slipping towards both dehydration and hyponatremia is that the body must make a tough choice. 

Does it release out a lot of diluted urine, thus clearing free water and raising the sodium concentration in the blood. Unfortunately this would be at the expense of dropping the blood pressure and facing potential circulatory collapse (shock), which we have seen at endurance events in the past. Does the body protect the declining blood volume by holding on to water; keep the blood pressure up to make sure that vital organs get perfused with blood and to heck with the sodium concentration (in the hope that you will stop and get some salt in your body).

The latter is the decision that will best protect survival, but it will cause a lowering of the blood sodium and the consequent hyponatremic symptoms. 

Part 3 of this interesting saga will follow… but it does beg the inevitable question –  have you got your nutritional plan right for your race day yet?

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