Editor's note: Fluids and Electrolytes - The Forgotten Nutrients, Part 1 is available here.
Electrolytes and Nutrition
You will notice earlier in this article a references to electrolytes. Athletes need to be concerned with electrolyte balances if their goal is to constantly perform at an optimum level. One of the reasons why an athlete needs to know about electrolytes is than an excessive amount of electrolytes can be lost through sweat. Blood, one of the main body fluids, also contains water and electrolytes. Our diet provides electrolytes and as such, attention to the diet is also important, as is refueling the body after exercise.
Electrolytes generate electricity in the body and help in a number of important bodily processes. These processes include heart and nerve function, muscle coordination and control, balancing bodily fluid levels and preventing dehydration, carrying glucose and other nutrients throughout the body, excreting waste products and excess water out of the body, and also the regulation of blood pressure.
The balance of electrolytes in the body is controlled by a variety of hormones, and also through the thirst mechanism that stimulates thirst when the body becomes dehydrated; it’s commonly known that if you feel thirsty, then you have already been dehydrated for a while.
The most common electrolytes in the body are; calcium, chloride, potassium, magnesium, and sodium. Hormones in the body help to regulate the intake of electrolytes, with the kidneys filtering any excessive levels.
Calcium is necessary for contraction of muscles, for nerve function, blood clotting, division of body cells and for healthy teeth and bones. Found in dairy products, green leafy vegetables and legumes. Recommended intake 1-2 grams per day.
Chloride is necessary for maintenance of fluid balance in the body. Found in a variety of foods but mostly comes from table salt. Recommended intake 1 gram per day.
Potassium regulates contraction of the heart and helps to maintain fluid balance. Found in fruits, vegetables, legumes, meat, poultry, potatoes, and dairy products. Recommended intake 2-4 grams per day.
Magnesium is necessary for muscle contraction, nerve functions, heart rhythm, bone strength, generation of energy and building protein. Found in nuts, legumes, whole grain breads and cereals, and green leafy vegetables. Recommended intake 0.4g per day.
Sodium maintains fluid balance and necessary for muscle contractions and nerve functions. Found in small amounts in all foods and mostly comes from table salt, and processed foods. Recommended intake 1-2 grams per day.
As mentioned above electrolytes are lost through sweating. Sodium content in sweat averages approximately 800 mg per litre of sweat and the loss may be greater when training in a hotter and more humid environment where the sodium concentration can be as high as 1000-1100 mg per litre. Also lost in sweat is potassium (195 mg/litre), magnesium (25 mg/litre), calcium, (50 mg/litre).
For an average person obtaining adequate electrolytes can easily be met by having a balanced diet that includes the following foods, whole grain breads and cereals, dark green leafy vegetables, low fat dairy products, a variety of fruit, nuts, beans and legumes, and occasionally using table salt on foods and drinking plenty of water.
Consuming too many or not enough electrolytes can cause electrolyte imbalances. In athletes the most common imbalances involve sodium and potassium. In the case of sodium, excessive sodium can cause hypernatraemia, and low sodium levels can cause hyponatraemia. In the case of potassium, an excessive amount of potassium causes hyperkalaemia, and low levels hypokalaemia. Natraemia means sodium in the blood, and kalaemia means potassium in the blood, both are causes of serious medical conditions if left untreated.
Fluid Intake Before, during and after exercise
Below is a discussion of recommended fluid intakes before, during and after exercise. Remember that along with fluid intake, before, during and after exercise, food intakes are also an important part of each of the phases. The term before exercise normally refers to the period of one to four hours prior to the start of exercise, and the main goal during this time is to consume foods and fluids that will provide glucose, prevent dehydration, delay fatigue and minimize gastrointestinal distress. With the pre-exercise meal being high in carbohydrates, low in fat and provide adequate fluid and energy. A period one hour before exercise would focus only on carbohydrate and fluid intake. The period during exercise, athletes focus mainly on carbohydrates and or fluids to prevent dehydration, hypoglycaemia, and gastrointestinal distress with the intensity and duration being the governing factor. The period after training is the time for glycogen and amino acid resynthesis and the replenishment of fluids and electrolytes lost during training.
Fluid Intake Before exercise
The period before exercise is a time of between one to four hours before exercise begins. The pre exercise meal normally contains high carbohydrates, moderate protein, be low in fat and must be accompanied by fluid. Athletes should consume 400 – 600 ml of fluid two to three hours before exercising. This amount of fluid will help to ensure the athlete is not dehydrated and will allow enough time for absorption of the fluid and eliminate urine before training or competition commences. Athletes who exercise in the heat should consume a further 250 – 500 ml within two hours of the start of exercise. Athletes who train or perform for a time longer than one hour may choose to take a carbohydrate beverage instead of, or as part of their pre-exercise fluid intake. They often take in 400-600 ml of a carbohydrate beverage two hours before exercising. The concentration of the carbohydrate beverage should not be more than 8% concentration, although some athletes may be able to tolerate concentrations at a higher percentage of carbohydrates. Fifteen to thirty minutes before exercise a further carbohydrate beverage is consumed containing 300-500 ml.
Fluid and Electrolyte Intake during Exercise
Athletes who perform continuous exercise for more than an hour such as marathon runners, or athletes who perform high intensity intermittent exercise for one to four hours, such as soccer or basketball players, are at risk of dehydration, fatigue and low blood sugars, these factors are known to decrease an athlete’s performance. By using fluid and carbohydrates we can delay the above factors, but we also have to take into account the possibility of causing gastrointestinal upset when we are deciding on fluid and carbohydrate intake when performing high intensity training. Protein and fats are not taken during exercise as they cannot prevent the above conditions. Taking foods that contain fats can cause gastrointestinal distress so should be avoided totally. Both carbohydrates and fluids are needed.
The normal method of taking carbohydrates during training is via a carbohydrate drink, containing both carbohydrates and water. Studies have shown that consuming 30 – 60 grams of carbohydrates per hour during prolonged exercise will improve performance and delay fatigue by sparing both liver and muscle glycogen. It will also prevent a fall in blood glucose levels as low blood glucose levels causes light headedness, lack of concentration and irritability. As it is well documented that taking in a drink containing both water and carbohydrates during exercise is beneficial, we have to take into account the amount and timing of the carbohydrate drink. As mentioned previously in this article the rate of gastric emptying is known to decrease when the exercise intensity is greater than 70% of VO2max. Athletes who perform at this intensity will find that smaller amounts of carbohydrates are necessary. The rate of intestinal absorption must also be considered, as taking a carbohydrate drink is not slowed when exercise intensity is less than 75% of VO2max. When performing at a high intensity greater than 75% of VO2max it should be remembered that the rate of absorption and gastrointestinal upset may appear. Concentrated carbohydrate solutions (greater than 8%) are known to slow gastric emptying but can provide more carbohydrates, but smaller concentrations are emptied faster although the amount of carbohydrates provided is small. Most athletes consume sports drinks that contain 6 to 8% carbohydrate, and at this concentration, each litre (1000 ml) provides 60 to 80 grams of carbohydrate. An important fact is the maximum absorption of carbohydrate from the intestinal tract is approximately one gram of glucose per minute, meaning a maximum of 60 grams per hour. Remember also that concentrations that are greater then 8% may cause gastrointestinal upset.
The main reason why fluid is needed during training is to prevent dehydration. Athletes who take part in events lasting less than one hour may not have any opportunity to ingest fluid during the event. Athletes who take part in events that last longer than an hour their fluid intake is important especially if training in the heat. If competition lasts less than 60 minutes it is recommended that 180 to 240 ml of cold water is consumed every 10 to 15 minutes to prevent dehydration. Drinking cold water is an excellent choice for the reason that cold water leaves the stomach much quicker than fluid which is at room temperature. If the athlete has paid attention to their intake of carbohydrates prior to exercise then their glycogen stores would be sufficiently ample for exercise performance so only water would be needed to be consumed during exercise. Sometimes these same athletes may decide to take a carbohydrate drink because they may feel they require the extra carbohydrates or the fact that they don’t like unflavoured water. Athletes who perform exercise that lasts for more than one hour or high intensity intermittent exercise for one to four hours are recommended to consume 150 – 300 ml of a carbohydrate/electrolyte drink every 15 to 20 minutes during exercise. The reason for this is that the water in the drink will help prevent dehydration, while the carbohydrates will provide glucose to prevent muscle glycogen depletion. The electrolyte sodium which is included in most sports drinks (along with other electrolytes) helps the body to retain fluid and stimulates the desire to drink more fluid. Dehydration and glycogen depletion both hasten the onset of tiredness and feelings of fatigue.
Some loss of electrolytes occurs during exercise due to sodium and chloride being lost in sweat. Athletes who compete in ultra endurance events or events such as Iron Man events have to carefully monitor sodium and fluid intakes. The reason for this is that these athletes are at risk of a condition called hyponatremia a condition that is caused by low levels of sodium in the plasma, causing blood and sodium levels falling too low. These athletes are recommended to take sports beverages containing sodium to prevent the possibility of hyponatremia. Athletes who compete in endurance events sometimes become dehydrated despite their best efforts not to. Sweating can lose approximately 50 ml of fluid per minute but gastrointestinal absorption of fluid is only approximately 30 ml per minute. The recommendation for these athletes is to take in a minimum of 600 ml of a mixture of carbohydrates and electrolytes every hour.
Fluid Intake and Rehydration after Exercise
Athletes are normally dehydrated following exercise or competition and it is therefore vitally important that they follow a rehydration strategy quickly after exercise so that they don’t embark on the next session or event in a compromised state. Using sodium post training is of benefit because it causes the body to retain fluid and to provide a desire to drink. There is sodium in sports drinks but normally the amount of sodium is relatively small. It is therefore important that athletes slightly salt food after exercise.
A trained athlete has a maximum sweat rate of approximately 2-3 litres per hour so it wouldn’t take long for this athlete to lose 2-3 percent of their body weight. Two litres of sweat equates to a 2 kg loss of body weight. Losing this much fluid during exercise will have an effect on the athletes energy levels and can cause premature fatigue. In order to know how much fluid has been lost during exercise the athlete should weigh themselves pre training and comparing that weight to the post training bodyweight. For every pound of body weight lost during exercise the athlete should take in a minimum of 1 pint per pound of body weight lost. A better guideline to follow is to take in a pint and a half of fluid per pound of body weight lost through exercise. In order that body weight lost through exercise is correctly calculated by the scales the athlete must be fully hydrated prior to exercise. Another method athlete’s use is monitoring the colour of their urine. Going to the toilet frequently and passing large amounts of light coloured urine would mean adequate hydration, but dark coloured concentrated urine may mean that the athlete is dehydrated.
- Sports Nutrition for Health and Performance: M Manore. J Thompson
- Nutrition, Concepts and Controversies: F Sizer. E Whitney
- Nutrition for Health, Fitness and Sport: M.H.Williams