Diving Medicine

By Dr. David Sawatzky

In any discussion on 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 chloride (Cl-).  Sodium and chloride are the main electrolytes in the extracellular fluid while potassium is mainly inside cells.

Table salt is sodium chloride and the source of most of the sodium in the body while potassium comes from various foods (bananas are an excellent source of potassium).  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 cell membranes.  This 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 maintenance of appropriate 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 4,500 mg of sodium!  Under these circumstances the hormone aldosterone tells the kidney to eliminate the extra sodium in the urine.  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 a third of the people suffering from hypertension have high blood pressure because they consume too much salt in their diets.  This has recently become a popular topic in North American health discussions and education programs to try and get people to eat less salt.  What tastes good to a person is basically what they have been eating for the previous three years.  Therefore, if you reduce the amount of salt in your diet you can reprogram your taste so that less salt tastes good.  As a general guideline, salt should never be added at the table and is seldom required in cooking.  It is also an excellent idea to eat less salty food.

Don’t Sweat It

One of the main causes of dehydration is sweating.  One litre of sweat can contain up to 1.5 grams (1,500 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.  Fortunately this is not the case.  In one study runners competed in a 20-day road race in Hawaii.  Even though they did not consume any electrolyte supplements, they maintained their electrolytes at normal 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.  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 of 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 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.  However, this will never be a concern under recreational diving conditions.

Thirsty Signals

Is thirst a good indicator of dehydration?  There are two conditions that cause you to feel thirsty, a reduction in total body water (dehydration) and more importantly, a reduction in the concentration of electrolytes in the body.  If you drink a lot of plain water you’ll dilute the electrolytes in your body.  The body responds to this reduction in electrolyte concentration by peeing out the extra water, increasing the concentration of electrolytes and eliminating the sensation of thirst.  Establishments that serve alcohol are well aware of how this system works.  They often provide free snack foods that are high in salt.  The salt will increase the sodium concentration in the body and stimulate your thirst, causing you to drink more.  The increased profit from drink sales more than offsets the cost of the snacks!

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 concentrations of electrolytes in sports drinks are similar to or less than the concentration of electrolytes in sweat and they will not cause dehydration.

Conversely, if you drink seawater you will become dehydrated.  The sodium concentration in seawater 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 seawater in the first place.  Therefore, you will literally dry up drinking seawater.  Some sea birds, like penguins, sea gulls, and albatross, can drink seawater but they have special glands in their heads to excrete the excess salt.

Sport Drinks

What purpose do carbohydrate drinks serve?  Many sport drinks contain carbohydrates (sugar) and claim to enhance athletic performance.  Muscles use glucose (sugar) to produce almost all of the energy that they need to contract.  Muscles use glucose delivered to them in the blood and they contain a store of glucose in the form of glycogen.  At a low level of activity, muscles get most of their energy from glucose delivered to them by the blood.  At a high level of activity, blood will not deliver glucose fast enough and 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, 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 (32 km) 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.  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.  For example, Gatorade contains 6.0 percent carbohydrate, Exceed contains 7.0 percent and Bodyfuel 450 contains 4.5 percent carbohydrates.

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 two hours is useful), the glycogen stores will be replaced faster.  High carbohydrate drinks have been developed to take advantage of this window.  Any food high in carbohydrates will work but carbohydrate drinks are fast and easy.  I have been told chocolate milk works well.

We can summarize the effects of sport drinks as follows.  They taste better than plain water (sometimes) so we will drink more.  The electrolytes they contain help retain water in the body.  The electrolytes stimulate our thirst and encourage us to drink more.  They provide carbohydrates and delay fatigue during long periods of exercise or when meals are missed.  Finally, they help replace glycogen stores after strenuous exercise.

Caffeine is a methylxanthine and methylxanthines are found naturally in coffee beans, tea leaves, chocolate, cocoa beans and cola nuts.  Caffeine is often added to soft drinks and nonprescription medications (e.g., decongestants).  Caffeine has many complex effects on the body.  Some good effects are that it enhances endurance performance (probably through enhanced fat metabolism thus sparing glycogen), reduces the subjective effort of exercise, increases the force of a muscle contraction at submaximal levels and enhances tolerance of cold.  Some bad effects are that caffeine causes headache, insomnia, irritability and irregular heartbeats.  Caffeine causes an increase in the production of urine and thereby dehydration.  Caffeine should usually be avoided for several hours before diving although before a short cold dive it might be useful to help keep warm.

Dehydration is a significant factor for all divers and all divers are usually suffering some degree of dehydration after diving.  Dehydration increases the risk of developing DCS, reduces athletic performance and reduces tolerance to heat.  Special care should be taken to avoid dehydration before a dive and to treat it after diving.  Thirst is a poor indicator of dehydration and therefore, you should drink before you are thirsty and drink more than you think you need.  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.  One of the primary advantages of sport drinks is that the electrolytes will reduce urine production during the dive and thereby reduce the need to pee during the dive.  So remember, drink lots (not alcohol and usually not caffeine), and have fun!