The ability to measure our sodium losses in sweat during exercise has led some to thinking that we need to replace at least some of what we lose in sweat to ensure our blood sodium levels don’t fall to levels that effect performance or health.
The thinking goes that somewhere between 230-1700mg of sodium can be lost per hour during exercise in hot conditions and we have a typical daily intake of 4g. As a consequence it can take only 2-3 hours before we deplete our sodium stores levels that effect performance. But measuring sweat sodium levels is only part of the picture – our body doesn’t particularly care what our sweat sodium concentration is (and in fact it reduces sweat sodium concentration as it acclimatises to exercising in the heat). What the body very tightly controls is our blood sodium levels and has several mechanisms to keep it within 135-145 mmol/litre range that is required for normal human function.
One thing to understand is its not the actual amount of sodium in our body that is the critical factor, its the concentration of sodium in our blood. If there is less blood then we need less sodium to keep the concentration in normal range.
Deciding we need sodium supplementation based solely on what we sweat out is like basing fat consumption based purely on how much fat we burn during exercise or basing our hydration strategy purely on how much weight we lose or our carb intake on how much energy we burn when we run. What we burn or sweat out doesn’t matter – its whats left in the body that we need to be concerned about. So when we look at fat burnt during exercise we know that we have ample supplies of fat so there is no need to take on additional fat, we know that our carb supplies will eventually run out so we need to take in additional carbs in a race but we know we don’t have to replace the whole amount we burn, we also know that the body can handle a certain level of dehydration with no adverse effects provided we drink to thirst. As far as sodium goes what we should be looking at is what happens to our blood sodium levels during exercise NOT how much sodium we lose in sweat.
Do we need to replace all of our sodium sweat losses, some of them or none of them?