Hydration: When Too Much Of A Good Thing Becomes Bad For You
Over the past decade or so, advice on what to drink and when has changed based on both exercise science and sports-drink marketing. Runners were once told to drink a preset number of ounces per mile or per hour, but that apparently didn’t take into account for individual differences in sweat rate.
A better way to measure hydration levels is through what is known as “blood osmolarity,” which is a ratio of how much fluid vs. how many other particles (like sodium) are in your plasma. The hypothalamus in your brain keeps tabs on your osmolality balance. When you sweat enough that your osmolality increases by more than 1 or 2 percent (meaning that you have slightly more particles than fluid) and your hypothalamus triggers your pituitary gland to release a hormone called vasopressin. This tells your kidneys to soak up water that would have otherwise become urine, sending it to your bloodstream to replace what you’ve lost.
If your osmolality increases another 1 to 2 percent or so (the exact amount varies by person), your hypothalamus sends a signal to your conscious brain: It’s time to take a drink. That’s when you become aware of being thirsty.
Below are some other facts about hydration that you might want to keep in mind:
Deadly dangers of drinking too much If you drink more than your body’s telling you, you risk a dangerous drop in blood sodium concentration called hyponatremia. This is when your body starts shuttling it from your blood to your body tissue cells to maintain the right osmolality, which thus becomes a problem for the brain cells packed tightly inside your skull. Brain swelling from hyponatremia can cause confusion and convulsions, and has killed a handful of marathon runners in previous years. Do sports drinks help? Debatable Whether drinking sports drinks instead of water can save you from hyponatremia is also controversial. Of course, they’re marketed as a way to replenish lost sodium. But drinking too much of them still dilutes your blood as their sodium concentrations (about 30mmol/L) are closer to that of water (zero) than of blood (136-145 mmol/L). Dehydration expected Unless taken to extremes, it’s perfectly normal to lose body weight while you exercise and it won’t affect your performance. A meta-analysis of previous research also published earlier this year in the British Journal of Sports Medicine found cyclists who lost up to 4 percent of their body weight didn’t experience any declines in speed, and that those who drank only when they were thirsty performed the best. Your super-simple hydration plan Thirst is literally older than humanity—it’s an inborn process that has been present for millions if not billions of years, and it’s pretty fine-turned. So even if you’re just going out for a half-hour or hour-long run, it’s still best to rely on your body’s cues to know when you need to drink. And by quenching your thirst throughout the day, you’ll feel better starting your run in the perfect state of fluid balance.