Sodium is an essential electrolyte mineral, meaning that it must be obtained through the diet. One of its key functions is to regulate the body’s fluid balance and therefore blood pressure. The general population are advised to restrict their consumption of sodium to 2.4g per day, which equates to 6g of salt (which is composed of 40% sodium and 60% chloride, another electrolyte) to help prevent the development of high blood pressure or hypertension. But the sodium requirements for those participating in endurance sports are generally higher. Endurance exercise is associated with significant sodium loss as a result of sweating, and this additional sodium loss needs to be replaced through the diet. As regular physical activity also reduces the risk of hypertension, sodium recommendations for endurance athletes need to be different to those for the general population. However, older athletes and those with a family history of hypertension, stroke or heart disease should regularly monitor their blood pressure.
The sodium losses in sweat are affected by two factors: concentration and volume. The average sodium content of human sweat was found in one study to be 900mg per litre. 1 But the range in healthy athletes varies considerably, from as little as 200mg/L to as much as 1700mg/L. Variation of the other electrolytes – calcium, magnesium and potassium is far less pronounced. Some sodium is reabsorbed by the sweat gland; an individual’s rate of reabsorption differs and affects the concentration of sodium in sweat. Becoming acclimatised to exercising in the heat, for example, is one way to increase sodium reabsorption.2 Sweat volume is affected by the temperature you are exercising in, and by the type, intensity and duration of your exercise session, together with fitness levels and genetic factors. As a result of these various factors, sweat rates and sodium losses in individual athletes vary hugely, and consequently the sodium replacement strategy for endurance athletes needs to vary as well.
In 2007, the American College of Sports Medicine published their latest Position Stand on Exercise and Fluid Replacement. 3 This recommends that athletes weigh themselves before and after exercise to estimate their sweat rate and use this information to customise their own fluid replacement strategy, aiming to replace any fluid electrolyte deficit. According to the statement, athletes can lose up to 5g of sodium in sweat during a single high intensity workout. Ingesting sodium during exercise plays an important role in rebalancing fluid balance (through water retention) and electrolytes but also in minimising dehydration as sodium ingestion promotes thirst, thus encouraging the athlete to drink more. However, athletes should avoid over-drinking, as this can lead to the development of exercise associated hyponatremia (EAH), a medical condition where sodium levels in the blood are diluted to a dangerous level. Symptoms of EAH include nausea, light-headedness, dizziness and fatigue. To avoid this, the ACSM Position Stand recommends ensuring that fluid intake does not greatly exceed fluid loss, and that energy drinks including sodium, usually at a concentration of 0.5-0.7g/L (21-30mmol/L), should be used.
But, as discussed earlier, the sodium needs of athletes vary significantly, so how can you ensure that you are meeting your own requirements? Here are some suggestions:
- If you are training or competing for less than 60 minutes, any sodium losses should easily be covered by your recovery meal or snack. Sodium is found in a wide range of foods, including wholegrains, vegetables, cheese, pulses, nuts, tinned fish, soy sauce and olives.
- For sessions lasting over an hour, use a sports drink containing sodium at a concentration of 0.5-0.7g/L (21-30mmol/L) or sodium-containing energy gels/bars plus water. In meeting your hydration and carbohydrate needs, you should also meet your basic sodium needs.
- If you are a “heavy sweater” (eg you frequently notice a dried white residue on your kit after training), you may need to take on additional sodium. This can be done by adding electrolyte tablets, eg Nuun (http://www.nuun.co.uk/) or High 5 Zero (http://highfive.co.uk/product/hydrate/zero) to your water supply. The sodium content of these tablets varies, so read the label for guidance on how much to use. You may also need to use electrolyte tablets with water in hot conditions when training for less than 60 minutes or in addition to other sports nutrition products for longer sessions in the heat.
- Elete Electrolyte is a liquid alternative to electrolyte tablets, which gives you more flexibility over how much you add to water or use to top up sodium levels in energy drinks which contain lower amounts. See http://eletewater.co.uk/ for more details. It is covered by the Informed Sport scheme, which minimises the risk of contamination with banned substances, making it a good choice if you are subject to drug testing. It also contains no artificial sweeteners.
- Another option worth considering if you want to individualise your sodium needs to the greatest possible extent is to have a sweat test. This measures the precise amount of electrolytes in your sweat, and can now be performed by a qualified practitioner without having to visit a sports science lab. See http://myh2pro.com for details. Varying strength electrolyte tablets, delivering sodium at 250-1500mg/L, are also available to purchase.
- Lastly, practise meeting your sodium needs in training, bearing in mind the conditions you will be competing in, and develop a race plan accordingly. This might involve carrying an additional electrolyte product to use if required. Check which products will be available at aid stations too.
Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sport. Based in London, she also works as a Registered Nutritional Therapist, conducting one–to–one consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit www.nutritionforendurancesports.co.uk
1 Mountain SJ, Cheuvront SN, Lukaski HC. Sweat mineral element responses during 7 hours of exercise-heat stress. Int J Sport Nutr Exerc Metab. 2007. 17(6): 574-82
2 Buono MJ, Ball KD and Folkhorst FW. Sodium ion concentration vs sweat rate relationship in humans. J Appl Physiol. 2007. 103:990-4
3 Sawka MN et al. American College of Sports Medicine position stand: Exercise and fluid replacement. Med Sci Sports Exerc. 2007. 39(2):377-90