Avoiding Hyponatremia:
Specific recommendations for Ironman athletes

Alan Couzens, M.S. (Sports Science)

Aug 12th, 2015

Endurance planet posted a very interesting podcast recently on the (literally) hot topic of hyponatremia in endurance events. Dr. Tim Noakes & Dr. Phil Maffetone – 2 guys who I respect immensely, weighed in on the topic and suggested that the problem is a simple one of athletes not listening to their body and drinking to a fixed schedule instead of “drinking to thirst”.

As somebody who loves a good debate, this sort of advice frustrates the “you know what” out of me. Not so much because I disagree with it but simply because it is so general that it’s hard to argue against (like when Madame Zara foretells 'some trouble in your future') &, more significantly, for the athletes we’re advising, lacks a whole lot of practical applicability. This is exemplified in a quote from the podcast…

Tawnee asks “So what if an athlete does ‘drink to thirst’ and still gains weight over the course of the race?”

Dr. Noakes replies something to the effect of “well, in that case the athlete has got ‘drinking to thirst’ wrong”(!)

I don’t know about you, but during a race, my mind is too full of other matters to be 100% engaged in the philosophical question of “am I really thirsty?”, i.e. Is what I’m feeling thirst or are my airways just dry from the altitude or nature of the unfamiliar location? Am I thirsty enough to pound a bottle right now? And more practically, will I be thirsty enough between here and the next aid station to ditch the half bottle I have on my frame, slow down and get a fresh bottle?

While, in principle, I agree with being introspective during a race and running constant diagnostics, in practice, I’m also aware that under race conditions our ability to do this is (as a species) not very good! Just as importantly, if I'm honestly 'cool, calm and collected' enough to run these high level diagnostics, my risk of developing hyponatremia from overdrinking is significantly diminished (see point 1 below).

When it comes to the true, 'nitty gritty' reality of Ironman racing, we could look at our hydration plan and our race pacing plan in a similar light. While it would be great for athletes to develop the ability to throw the gadgets and pacing plans away during a race and pace intuitively, when we look at the reality of the way that athletes actually race in the absence of this feedback, it speaks to the practical value of going in with a (flexible) plan!

The above withstanding, I get Doctor Noakes' hesitancy to be more concrete. The fear of giving specific guidelines on the subject is a legitimate one. Hyponatremia is a serious, potentially life threatening condition! If the numbers are off by too much, it can be fatal! For this reason, its prevention is worthy of some serious time and thought.

What is Hyponatremia?

Hyponatremia is the dilution of sodium levels within the body. This causes an osmotic shift where fluid begins to move from its ‘proper place’ in the extracellular space into the cells causing swelling. When this occurs in the brain cells it is very bad news!

So, what causes hyponatremia?

This is the million dollar question and there are likely a combination of factors at play in different proportions in each case. The prevailing theory (& one that I agree with) is that, in most cases, rather than electrolytes ‘running low’ due to sweating, water is ‘running high’ due to over-drinking.

This brings up an interesting follow up qu… Shouldn’t our body have protective mechanisms to stop this? And more practically, how come after a ‘night on the town’ where we're drinking a whole lot & certainly not dropping Nuun’s into our beer, we’re not in the hospital with hyponatremia? The answer is a fairly simple (& annoying) one – because in this situation, we pee…a lot.

So, why don’t we just pee out the excess if we over-do it on the bike? 3 letters answer this qu. A.D.H.

Anti-Diuretic Hormone (Vasopressin) is a hormone whose prime function is, unsurprisingly enough, anti-diuresis, i.e. ‘holding onto’ fluid. It is secreted when osmolality of the body is elevated, i.e. sodium levels become too high, but it also serves a secondary role as a ‘messenger’ for the release of cortisol in response to..you guessed it…STRESS!

Point 1 – Control stress.

Going back to our question of why you’re not a hyponatremic mess after a night on the town, a large part comes down to the fact that (unless you’re an introvert like me :-) your stress levels in the context of having a few drinks with friends at the bar vs. putting your game face on during a race are miles apart. So, in terms of mitigating the risk of hyponatremia in a race situation (& having your best race) your number 1 piece of advice is this – relax, control your emotions and keep perspective. For most of us, the term ‘race’ is used a little too liberally – it’s actually a survival test – the longest ‘session’ that you will do all year. Keep that perspective when you’re expending psychological (or physical) energy at a high rate at the start of the ‘race’!

Point 2 – Don’t over-drink (i.e. don't drink so much that you gain weight)

At its most obtuse, our advice might be ‘drink to thirst’ but, given the importance of getting this right, I think we have an obligation to do better than that!

The key, easily observable, metric which is consistently tied to hyponatremia is a gain in weight over the course of the event (Urso et al., 2014). While (unfortunately) we don’t have ready access to scales during the event, us OCD types certainly do during our preparation for the event! Your estimated sweat rate under conditions similar to your event is a number that you should know! During your key sessions weigh yourself before and after to get an idea of that relationship between how much you drink and how much weight you lose. In health terms, some level of weight loss (i.e. some level of dehydration) over the course of your event is a very positive thing. The key is to pace this over the full duration of your event. If your event is 12 hours long and you’ve already lost 6kg 3 hrs in, in things aren’t looking so good! The above is why I’m not a big fan of the current advice that athletes “shouldn’t be concerned with a bit of dehydration”. When we’re dealing with athletes performing over 8,9,10,12 hours, we need to do better in quantifying this ‘bit’.

We’re all familiar with the 2% drop in bodyweight will affect performance folk lore. Like all folk lore, this is an over-simplification that doesn’t take into account individual and event differences. Over the course of an Ironman, some level of dehydration (and cardiac drift) is unavoidable. The question is, how much can we allow to minimize the risk of hyponatremia while also keeping heatstroke at bay and preventing Ironman performance from plummeting?

Studies have shown that, when exercising in the heat, for every 2% of bodyweight loss, core temperature will rise by ~1C (Wyndham & Strydon, 1969) and HR will rise ~3% (Gonzalez-Alonzon et al., 1999). There is a level of consistency in the ‘meltdown’ point - a core temperature of 40-41C, with full-blown heatstroke typically occurring at ~42C. Therefore, athletes can ‘afford’ to lose ~5-6% bodyweight over the course of the event before significant performance decrement is expected. Incidentally, this will tend to correspond with HR ‘decoupling’ of 10-12% over the course of the entire race – falling in nicely with Joe Friel’s recommendation to try and limit decoupling to ~5% over the course of the bike (so you have some additional ‘ room to play with’ for the run).

Point 3 – Be extra mindful if you’re a ‘salty sweater’*
(*and get tested to see if you're a 'salty sweater')

As I stated above, for most athletes who experience hyponatremia, fluid gain rather than electrolyte loss is the primary cause. As a species, we have relatively large sodium stores that take a lot of sweating to deplete. Montain et al. (2001) looked at this question in considerable depth and concluded that to decrease serum sodium to hyponatremic levels of ~120mEq/L, a 70kg athlete would need to develop a sodium deficit of ~660mEq. Over an Ironman duration, at average sweat rates, this would amount to sodium sweat concentrations in the ball park of ~60 mEq/L. This is certainly on the high end of what we see when performing electrolyte testing of sweat, but it is not unheard of. For some athletes, with unusually high sodium concentrations in their sweat, it will indeed be possible to become hyponatremic over a long event even if they are not drinking excessively. While unusual, if you are one of the unlucky few, this is certainly information that you might find useful from a sweat test!

Beyond this, if we compare a little from column A and a little from column B, i.e. an athlete with slightly higher than average sodium in their sweat who is drinking at slightly too high a rate of fluid ingestion, the likelihood of developing hyponatremia over a long race goes up significantly. In the same paper, Montain looked at this question and found that a 70kg athlete with only moderate sweat sodium concentrations of ~40mEq/L but a high sweat rate of ~1.5L/hr would only need to accrue a fluid surplus of 1.6L (160ml too much fluid -about a fifth of a bottle- per hour) over a 10 hr race to become hyponatremic.

This sodium imbalance is, of course, mitigated if you’re also taking in an adequate amount of sodium in your hot weather race nutrition. Just how much sodium you should take in to keep things in balance will be a function of your size, the expected duration of the event, your sweat rate, your sweat concentration, & the amount of fluid you are going to take in over the course of your event. All of these variables can be tested and, in the words of a wiser man than I, knowledge is power!


When it comes to maintaining good health, neither heat stroke nor hyponatremia is a viable option. It is our responsibility as athletes (and husbands, wives, fathers, mothers) to put some time into experimenting, testing and learning what it takes for us (as individuals) to stay in that healthy ‘green zone’ under race conditions. While ‘race conditions’ will always be a moving target that demand attentiveness and flexibility on the day, the ‘no plan’ plan rarely ends well.

Race smart,