Sport Drinks and Diving

Last month we looked at the many reasons why divers might be dehydrated both before and after diving. This month we will continue the discussion of this very important issue.

In any discussion of dehydration, the topic of electrolytes must come up. So what are electrolytes? In Stedman's medical dictionary, an electrolyte is defined as "any compound that, in solution, conducts electricity and is decomposed by it; an ionizable substance in solution". Ions are atoms or molecules that have either gained or lost electrons and therefore have a charge. In the body the main electrolytes are sodium (Na+), potassium (K+) and chlorine (Cl-). Sodium and chlorine are the main electrolytes in the extracellular fluid while potassium is mainly inside the cells. Table salt is sodium chloride and the source of most of the Na+ and Cl- in the body while potassium comes from various foods. These ions have many functions in the body including control of the amount of water in the various compartments and control of the electrical gradients maintained across most cell membranes. It is far too complex to get into here but the electrical gradient across cell membranes is the way that nerves conduct a signal, the way that muscles are given the signal to contract (including the heart) and the way that the glands work. Suffice it to say that the maintenance of the proper levels of these electrolytes in the body is necessary for life to continue.

The body can tolerate a wide variation in the intake of these substances and still maintain their concentration in the body constant. For example, we actually need about 500 mg of sodium per day. However, the typical North American diet contains about 4500 mg of sodium per day! Under these circumstances, the hormone aldosterone tells the kidney to dump the extra sodium into the urine and get rid of it. When the intake of sodium is low, the kidneys retain virtually all of the sodium and very little is lost in the urine. The system is not perfect however and about 1/3 of the people suffering from hypertension have high blood pressure because they consume too much salt in their diets. Even if we avoid all salty foods and do not add salt to our food, either in cooking or at the table, we will still get far more salt in our diets than we need. All of us would be much better off if we consumed a lot less salt.

What is the significance of electrolytes in dehydration? One of the main causes of dehydration is sweating and sweat contains significant amounts of sodium, plus some potassium and chloride. In fact, one litre of sweat contains about 1.5 grams (1500 mgs) of sodium and under extreme conditions, a person can sweat up to three litres an hour! You would assume that this would quickly result in a deficit of sodium in the body and the manufacturers of "sport drinks" with added electrolytes would like us to think so. Unfortunately for them, it is usually not so. In one study, runners competed in a 20 day road race in Hawaii. Even though they did not consume any electrolyte supplements, they maintained normal electrolyte levels. The combination of a normal diet and the body's remarkable ability to conserve electrolytes when required, completely offset the high electrolyte losses the runners where incurring through sweating. As a point of interest, when a person is first exposed to a hot environment, they have limited ability to sweat and their sweat contains relatively high concentrations of electrolytes. After a couple of weeks acclimatization, they can sweat much larger quantities and the sweat contains much lower concentrations of electrolytes.

You might ask "why does sweat contain any electrolytes"? The water in sweat comes mainly from the blood. If sweat contained no electrolytes, the concentration of electrolytes in the blood would quickly rise and this could result in death. Sweat actually contains electrolytes in slightly lower concentration than blood and over time the blood does become a bit "thicker". Therefore, you need to drink water when sweating. Under extreme conditions, if you are sweating a great deal, not eating, and drinking only water, you can deplete your reserves of electrolytes and become low in sodium (hyponatremic). This condition can be fatal, but this will virtually never be a concern under recreational diving conditions.

Is thirst a good indicator of dehydration? There are two conditions that cause you to feel thirsty. The first is a reduction in total body water (dehydration). If you drink plain water you will dilute the electrolytes in your body and the body responds to this reduction in electrolyte concentration by peeing out the extra water. Not only that, the reduction in electrolyte concentration will turn off your sensation of thirst and cause you to stop drinking. The second and more important stimulus to thirst is the concentration of electrolytes. We are all familiar with this and so are establishments that serve alcohol and other drinks. They almost all provide free snack foods that are high in salt. The salt will increase the sodium concentration in the body and that will stimulate your thirst, even though you are already drinking! Therefore, if you are drinking a solution that contains electrolytes, you will drink more of it than if you are drinking plain water and your body will retain more of the water you take in. The first commercial drink developed to take advantage of this fact was Gatorade (University of Florida). The electrolytes in these drinks are similar to or less than the concentration of electrolytes in sweat and therefore you do not have to dilute them and they will not cause dehydration.

Conversely, if you drink sea water you will become dehydrated. The sodium concentration in sea water is several times higher than the concentration in blood. The body has to excrete the extra salt in the urine and more water is required to get rid of the salt than was in the sea water in the first place. Therefore, you will literally "dry up" drinking sea water. Some sea birds, like penguins, sea gulls and albatross, can drink sea water but they have special glands in their heads to excrete the excess salt.

What purpose do carbohydrate drinks serve? Many sport drinks contain carbohydrates (sugar) and purport to enhance athletic performance. The muscles use glucose (sugar) to produce almost all of the energy that they need to contract. As I explained in my column in April 1993 (leg cramps), the muscles use the glucose delivered to them in the blood and have a store of glucose in the form of glycogen within them. At a very low level of activity, the muscles get most of their energy from glucose delivered to them by the blood. At a high level of activity, the blood will not deliver glucose fast enough and the muscles will start to use the glucose they have stored as glycogen. If a high level of activity is maintained for a long enough period of time, the muscles will exhaust their glycogen reserves and their ability to contract will be severely reduced. Almost all long distance runners are familiar with this phenomenon and call it "hitting the wall". It typically happens around the 20 mile mark. This is one reason that runners drink carbohydrate drinks during exercise. They are trying to deliver as much glucose to the working muscles as possible so that the glycogen stores will last longer, hopefully until the end of the race. As can be seen from the above discussion, recreational scuba diving will never be intense enough for a long enough period of time to exhaust our glycogen reserves. However, carbohydrate drinks are a good source of energy, especially if circumstances do not allow you to eat regularly or if your stomach is a bit queasy. Gatorade contains 6.0% carbohydrate, Exceed is 7.0% and Bodyfuel 450 is 4.5%.

A second group of carbohydrate drinks are called "carbohydrate replacers". These drinks contain higher concentrations of carbohydrates. When a person has significantly depleted their glycogen reserves, it will often take two to three days to replace them. However, if the person consumes carbohydrates within 30 minutes of stopping exercise (up to the first two hours is useful), the glycogen stores will be replaced much faster. The high carbohydrate drinks have been developed to take advantage of this window. Any food high in carbohydrates will work as well but the drinks are fast and easy. We can summarize the effect of sport drinks as follows. They taste better than plain water so we will drink more. They both replace the needed water and they help the body retain the water. They stimulate our thirst and thereby encourage us to drink more. They provide carbohydrates and thereby delay fatigue during long periods of exercise or when meals are missed. Finally, they help replace glycogen stores after strenuous exercise. For a more complete discussion of this topic, I recommend a book called "Diving Physiology in Plain English" by Jolie Bookspan.

Alcohol has many physiological characteristics that make it completely incompatible with diving, not the least of which is that it can cause dehydration! Beer does not contain enough electrolytes to be useful as a replacement drink and alcohol actually causes a reduction in ADH, thereby enhancing fluid loss by the kidneys. Alcohol can actually cause the body to lose more water in the urine that is contained in the drink and some people believe that a "hangover" is due to dehydration of the brain! Other negative effects of alcohol include the fact that it reduces the output of glucose by the liver, is not a ready source of energy, interferes with mental and motor control, enhances the effect of narcosis, etc. As can easily be seen, alcohol should not be consumed at any time near a dive.

Drinks containing caffeine must also be mentioned. Caffeine is a "methylxanthine" and methylxanthines are found naturally in coffee beans, tea leaves, chocolate, cocoa beans and cola nuts. Caffeine is also often added to soft drinks and nonprescription medications (decongestants). Caffeine has many complex effects on the body, some good and some bad. Some of the good effects are that it enhances endurance performance (probably through enhanced fat metabolism thus sparing glycogen), reduces the subjective effort of exercise, it increases the force of a muscle contraction at submaximal levels and it enhances your tolerance of the cold. Some of the bad effects are that caffeine causes headache, insomnia, irritability and irregular heart beats. The most significant effect of caffeine in this discussion is that it causes an increase in the production of urine and thereby dehydration. In the final analysis, caffeine should usually be avoided for several hours before a dive although before a short cold dive it might be useful.

In the last two columns we have had a fairly detailed look at dehydration and fluid replacement drinks. Dehydration is a significant factor for all divers and all divers are usually suffering some degree of dehydration by the end of a dive. Dehydration increases your risk of developing DCS, reduces your athletic performance and reduces your tolerance to heat. Special care should be taken to avoid dehydration before a dive and to treat it after diving. Remember that thirst is a poor indicator of dehydration and therefore you should drink before you are thirsty and drink more than you think you need. In this column we primarily focused on sport drinks. Although plain water works well and you do not "need" the electrolytes and carbohydrates in sport drinks, they do have some advantages. The electrolytes help the body retain the water in the drink and the carbohydrates are a good source of energy, especially if you are not able to eat regularly during the dive day. One of the primary advantages of sport drinks, especially before diving, is that the electrolytes will reduce the urine production during the dive and thereby reduce the need to "go" during the dive. Alcohol has no place around diving and caffeine should really be avoided.