One of the open questions is whether there’s some abnormal response that determines who gets sick from overdrinking. In most cases, we’re pretty good at peeing out extra water. Do the people who end up with hyponatremia have some malfunction of that fluid regulation process, or is it purely a question of how much you drink? It would have been nice to see more discussion of this question.
Another controversy is whether drinking sports drinks, which contain sodium, lowers your risk of hyponatremia compared to drinking water. This has been the topic of vigorous debate between critics like Tim Noakes and Gatorade-funded scientists. Logic would suggest that if you’re drinking any given amount of fluid, getting at least some sodium would lower your risk of sodium dilution. On the other hand, blood has a far higher concentration of sodium (~140 mmol/L) than sports drinks (10-38 mmol/L), so you’re diluting the sodium content of your blood no matter what you drink.
So how do you balance these factors? The overall recommendation from the consensus group is that you should simply drink to thirst, which will avoid the overdrinking problem entirely. That’s certainly the message that CrossFit seized on its press release: “‘Drink Only to Thirst,’ Scientists Say: Consensus Statement Addresses Preventable Condition Responsible for at Least 17 Deaths since 1993.”
I generally agree with the suggestion to drink when you’re thirsty, though I think the press release makes the message sound simpler than it is. The consensus statement includes a few more caveats, noting for example that “Fluid intake recommendations suggesting that athletes begin to drink fluids before the onset of the sensation of thirst were targeting those exercising in situations where high sweat rates were present and dehydration could evolve rapidly with known medical and performance outcomes.”
It’s worth noting that this describes many common situations, like running a marathon on a warm day. That’s the challenge in formulating general recommendations: one size doesn’t fit all. Telling people to drink early and often backfires for casual exercisers who aren’t sweating that much anyway; but telling people only to drink when they’re thirsty could also backfire in situations where sweat rates are high and opportunities to drink are constrained. You can’t drink whenever you’re thirsty if you only pass water stops every 5K.
Last point: as I mentioned at the top, the strength of evidence behind each recommendation in the consensus statement is assessed with a grade. The drink-to-thirst recommendation gets a grade of 1C. The “1” means the researchers view it as a strong recommendation where the benefits clearly outweigh the risks. The “C” means it’s based on “low-quality or very low quality evidence.” I agree with the overall message of these new hyponatremia guidelines, and I think it’s good to raise awareness, but I think we should keep in mind that the evidence is still pretty thin, and avoid turning this into a politicised PR battle.