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?
How does exercise effect our blood sodium levels?
If we look at effect of endurance exercise on blood sodium levels we find something very interesting. Tim Noakes in his book Waterlogged has a table (pg 129-130) listing 30 studies over a 45 year period that measured blood sodium levels pre and post a number of different endurance events. In only one study were there any subjects that had a lower blood sodium level then when they started. In every other study EVERY participant had a higher plasma sodium level post race compared to pre race.
Think about that for a second – over a period of 45 years researchers consistently found that post exercise blood sodium levels ROSE regardless of what they drank (in most of the studies the participants only drank water) . The only logical conclusion to this is that blood sodium concentrations always rise during exercise.
How can blood sodium levels rise when we know we lose sodium in sweat and urine?
Remember it is the concentration of sodium in blood that is tightly regulated not the actual amount of sodium. So if sodium is lost and blood volume is reduced (through dehydration) in an equal amount then concentration stays the same . So as we sweat we lose both fluid and sodium but since our sweat is less concentrated than blood our blood sodium concentration increases.
What about salty sweaters – surely they need more sodium ?
In fact the opposite has been shown
A study on Indian soldiers in 1960 (1) measured sodium loses in urine and sweat for soldiers exercising 2 hours a day in 35 degree heat. The soldiers were split into different groups, each group on a strictly controlled sodium diet . Researchers found the more sodium in their diet the more sodium in their sweat and urine.
Another study of U.S Soldiers in the desert by Adolph 1947 (2) found that those that ate a higher salt diet had more salt in their sweat and urine
How much sodium can we lose before we begin to suffer any effects?
In a study by Armstrong, Hubbard et al 1983 (3) subjects on a 4g a day sodium diet ( average Australian intake is around 6g per day) exercised for 30 minutes each hour for 8 hours per day in 41 degree heat . (Not a study many of you would be keen on signing up for thats for sure!) After 17 days of the experiment there were no cases of heat exhaustion, heat cramps, heat stroke and blood sodium levels remained in the normal range for the duration. Consider the subjects lost 60 litres of sweat over the period and drank nothing but water. It seems obvious that we can survive a long time with very little sodium, well beyond the length of any endurance event. Even multi day events don’t pose a problem since at the end of each day you consume a normal meal containing salt.
What about people who found that they can now complete races without cramping through the use of sodium supplementation?
Since we know that blood sodium levels aren’t low in crampers one possible explanation is that sodium triggers a reflex in the back of the throat which then triggers a neural reflex that relaxes the cramp. I’ve discussed this before here
So if you cramp often then first of all look at training and tapering and if you feel a cramp coming use a salty solution as a mouthwash – no need to ingest it.
Is there a danger of taking in too much sodium ?
According to Dr Marty Hoffman ( Director of Research at the Western States Endurance Race) in the 2014 Practise Guidelines for Wilderness Emergency Care “Adverse effects associated with abnormal water retention from excessive sodium intake have been reported” and he has also shown data from the 2011 Western States Endurance Run where some runners finished the race hyper hydrated ( i.e. carrying more weight then when they started) and had sodium levels that were above the normal range. These runners consumed between 25 and 60 S! Caps each. Now that may seem like a lot by for 24 hour runners thats only 1-2 per hour which is well within recommendations given by the manufacturers.
This conclusion by Earhart et all (4) sums it up
“High-dose sodium supplementation does not appear to impact thermoregulation, cardiovascular drift, or physical performance in trained, endurance athletes. However, in light of the possibility that high sodium intakes might have other adverse effects, such as hypertension, it is our recommendation that athletes interpret professional recommendations for sodium needs during exercise with caution.”
What about the arguments for sodium supplementation
Despite the above there are some people who still claim to have evidence to support sodium supplementation. But if low sodium levels are truly the cause of cramp or poor performance then we must see a correlation between low blood sodium levels and cramp or poor performance and thats not the case at all in numerous studies. They all confirm there is no difference in incidences of cramp or a correlation to performance if you take sodium or not.
There are claims that people who cramp have higher sweat sodium losses which makes sense when you think about it. The more acclimatised you are the lower your sodium loss in sweat . If you take two people with the same fitness but one is more acclimatised than the other then the more acclimatised athlete will be able to perform at a higher level of intensity in warmer conditions compared to the unacclimatised person. We all know what happens if you go out too hard in hot conditions – cramp. The central governor is telling the unacclimatised person that they are going out too hard and one way to slow them down is cramp, nothing to do with low sodium levels.
The pro salt side may tell you that there is a greater risk of hyponatremia when consuming water alone rather than a liquid containing sodium. Given that the sodium concentration we are able to digest is far lower than the concentration of sodium levels in the blood, whether you take water or a sports drink containing sodium you are still diluting your blood sodium levels. If you drink too much water it doesn’t matter if it contains electrolytes or not you’ll end up with hyponatremia.
If not consuming salt was a risk factor in hyponatremia then we would expect to see athletes that only drank water during endurance exercise in hot conditions to have a greater chance of dangerously low sodium levels. As we discussed earlier Noakes listed over 30 studies showing that blood sodium levels remained in normal ranges during endurance events when drinking only water .
The biggest risk factor for hyponatremia is a body weight gain during exercise which comes from drinking too much water. Whether that water contains sodium or not makes no difference. This was confirmed in study in 1991 by Barr et al that found that blood sodium levels dropped in subjects that drank 1.15 litres of water per hour by equal amounts whether they drank a saline solution or plain water. The volume consumed was calculated to be the volume needed to prevent any weight loss during exercise.
If you only take one point from this article let it be this one from the study by Barr:
In subjects that drank sufficient fluid to suffer no weight loss at all during exercise, blood sodium levels dropped whether they consumed plain water or sodium.
Another excellent argument NOT to weigh yourself before and after exercise and use that as your basis for how much fluid you should consume during exercise. Drink to thirst and dont bother taking sodium – it makes no difference.
So assuming you consume salt in your diet there is absolutely no need to take it during training or racing. If you often cramp then look at your training, tapering and pacing in races and take a small solution of pickle juice to hold the cramps at bay if they occur.
For those that argue some of the studies quoted are old and therefore not valid here is the conclusion from The Clinical Journal of the American Society of Nephrology in 2007 on Hyponatremia and consuming sodium.
“Currently, there is insufficient evidence to support the suggestion that ingestion of sodium prevents or decreases the risk for EAH; neither is there any evidence that consumption of sports drinks (electrolyte-containing hypotonic fluids) can prevent the development of EAH (1,35–38,42,100,101).”
You can read the full article here The studies quoted in the summary are all in the last 20 years.
1. Malhoutra MS Salt and water requirement of acclimatised people working outdoors in severe heatInd J Med Res 1960
2. Adolph EF Urinary excretion of water and solutes. Physiology of man in the desert 1947
3. Armstrong LE Hubbard RW, Askew EW et al – Responses to moderate and low sodium diets during exercise-heat acclimation. Int J Sport Nut 1993
4. Elizabeth L. Earhart, Edward P. Weiss, Rabia Rahman, Patrick V. Kelly
Effects of Oral Sodium Supplementation on Indices of Thermoregulation in Trained, Endurance Athletes. J Sports Science Medicine (2015) 14,
5. Barr SI, Costill DL, Fink WJ, Fluid replacement effects of water, saline or no fluid. Med. Sci. Sports Exerc. 1991