We all get thirsty at times, especially during intense physical exertion. This is a normal reaction of the body when it is getting low on water. Dehydration occurs when the body loses more water than it is takes in, usually through sweating and open mouth breathing. Thirst is an annoyance; dehydration is a serious problem that may lead to leg cramps, upset stomach, severe headaches, and death. Dehydration is not a single event; it is a combination of events over time. Therefore, proper intake of fluids is an integral part of the training process. Training without replacing fluid and electrolyte losses does not condition, make you tougher, or improve your endurance, but it may kill you.
During even light exercise, serum (the liquid part of blood) volume is slightly reduced. During intense exercise, serum volume is reduced even more. During normal conditions, blood flow to muscles is relatively small. However, exercise dramatically increases blood flow to the muscles, which decreases volume flow to the rest of the body. The body compensates by reducing flow to the organs least necessary during exercise, e.g. the indigestion system. As the level or exercise increases, blood flow through the heart is modified since it depends on the return of blood from the body to supply what is needed for the next beat. As the exercise level increases, we sweat, which further reduces serum volume, which results in blood that is more viscous. The thicker blood reduces the efficiency of the heart.
The body has now reassigned as much of its blood supply as possible. If you suddenly do something that further reduces the volume of blood flow to the heart, then the heart cannot maintain enough blood pressure to get blood to its primary user, the brain, so you pass out.
Dehydration is not shock. Shock is the global failure of cellular oxidative metabolism. One form of shock is hypovolemic shock in which the circulating blood volume is low. Dehydration may cause shock but is not synonymous with shock. Other forms of shock may have normal blood volume but the volume is mal distributed, e.g. anaphylactic shock. In some cases of shock, blood volume may in fact be increased but the heart may be unable to circulate the blood and oxygen to the tissues, e.g. cardiogenic shock.
Dehydration is the loss of total body water. Hypovolemia is a low circulating blood volume. Total body water amounts to about 0.6 liters per Kg body weight. For the typical 70 kg person, this means a total body water of 42 l. About 2/3 of this (about 28 l) is in the cells (intracellular water) and about 1/3 (14 l) is outside of the cells (extracellular water). About 2/3 of the extracellular water (9 l) is in the space between the cells (interstitial fluid). About 1/3 (4 l) is in the vascular space (intravascular water). Usually when we lose fluid, we lose it from the intravascular space. For example, sweat glands take fluid from the blood and put it on the skin or the kidneys take fluid from the blood and produce urine. Thus, most of our fluid losses are from the intravascular space.
If fluid loss is slow, the body is able to move fluid, first from the interstitial space and then more slowly from the intercellular space, into the intravascular space to maintain the intravascular volume. If we lose fluid suddenly, then there is no time for the body to so this. The body can move a maximum of 1-2 l of fluid per day from the interstitial and intercellular spaces into the intravascular space. If you dehydrate slowly, you will not display symptoms until you have lost about 5-10% of your total body water (2-4 l). At that point, if you suddenly stand up, your heart rate will increase but your blood pressure will remain normal. If you lose 10-15% (4-6 l) and suddenly stand up, your blood pressure will drop and you may feel lightheaded. If you lose 15-20% (6-8 l), your blood pressure will be low even lying down. At this point, you would probably meet diagnostic criteria for hypovolemic shock. If you suddenly lose fluid from your intravascular space without time to move fluid from other areas back into the space, you will have symptoms at lower levels of fluid loss. This is why some people feel lightheaded when they stand up after donating only a half-liter of blood.
Most of the fluid we lose each day is in the form of urine. Healthy people make at least 0.5 ml of urine per kg body weight per hour. We also lose some fluid each day in stool, sweat, and in our breath. This amounts to about half-liter per day. In hot conditions, when we have a fever, or when we exercise, this increases.
When we exercise, need to replace fluid loss and the sodium, potassium, and other minerals that we lose in sweat. When we become hypovolemic several things happen. First, the heart speeds up; it pumps less volume per beat but it beats more often to maintain cardiac output. Second, the blood levels of catecholamines (epinephrine, norepinephrine, and dopamine) increase, which causes blood vessels in the critical organs, such as heart, brain, liver, and kidneys, to dilate, and the vessels in the less critical areas, such as gut and muscles, to constrict. Changes in the output from our sympathetic nervous system act to reinforce this. The net effect is reduced blood flow to the muscles and gut and more to the brain etc. Thirdly, receptors that measure our blood pressure, sodium level, etc. tell the brain to increase our fluid volume, which triggers the thirst mechanism. Fourthly, the atria of our heart, since they are smaller in diameter, reduce their production of ANP, a peptide hormone that stimulates the kidneys to make urine. Thus, the kidneys reduce their secretion of sodium and urine.
How do you know when you are dehydrated? Unfortunately, thirst may come too late. Therefore, when you sweat a lot or the humidity is low, you must drink a lot, even if you are not thirsty. When dehydrated, the urine has strong color and smell, since the kidneys retain more water and urine is more concentrated. Stronger symptoms are headache, fogginess, weakness, and finally, unconsciousness. Dehydration may also lead to renal failure and death. When you exercise vigorously, some muscle destruction always occurs. The muscle cells are lysed (broken up) and their contents (electrolytes, myoglobin, and enzymes) circulate in the bloodstream. The more dehydrated you are, the higher the concentration of these items. When a person has been exercising, it is normal to find a few red cells (blood) in urine. It is not normal to find”smoke" colored urine, which indicates either the breakdown of blood (hemoglobin) or muscle (myoglobin). The enzymes that are released with the breakdown of the muscle cells are the most destructive part and can damage the kidney to the point of failure. This is more likely to occur when the person is dehydrated. The kidney failure is usually temporary (1-7 days), some people die from it.
Avoiding dehydration is simple: drink frequently so the water can be gradually absorbed and used. Drink plenty of water before you exercise, and continue to drink during exercise (3-5 cups of water/hour for an average sized person). Electrolyte mixes (Gatorade, PowerAde, etc.) only have a marginal advantage over the best drink on the planet: water. As a rough gauge, water weighs about 8 pounds per gallon and there are 8 pints in a gallon, so, during and after working out, drink a pint of water for every pound you lose.
A recent report from the Institute of Medicine found that to be adequately hydrated, men need about 125 ounces, about 16 glasses, of water a day. Women need about 125 ounces, about 16 glasses. The water may come from many sources, such as colas, coffee, juice, milk, tea, fruit, vegetable, and other foods and beverages, except for alcohol, which increase the body’s water needs. Caffeine, although it is a diuretic and increases the excretion of urine, the effect is small and fleeting. Most people get adequate amount of water from normal drinking behavior and by letting their thirst guide them. You need more water if you exercise, perspire a lot, your skin shrivels, you get muscle cramps, or if you are chronically constipated. It is difficult to drink too much water but athletes who drink a lot of water should also replace the salt lost in sweat to prevent abnormal heart rhythms. Water may also be useful for weight control by drinking a glass before and during meals to fill you up and aid digestion. Recent studies have shown that merely drinking more water speeds metabolism.
Americans consume too much sodium chloride, or salt. The average man takes in 7.8 to 11.8 grams of salt each day. The average woman takes in 5.8 to 7.8 grams. These numbers are estimates since they only include prepared foods and do not include salt that people add at the table. The average person only requires about 3.8 grams of salt a day (about two-thirds of teaspoon, or 1500 milligrams of sodium) to replace that lost in sweat. The upper limit should only be 5.8 grams of salt a day (about a teaspoon).
Consuming too much salt increases the risk of developing high blood pressure, which leads to strokes, heart attacks, and kidney disease. Some people are sensitive to the harmful effects of salt, including the elderly, African-Americans, and people with chronic diseases such as hypertension, diabetes, and kidney disease.
Salt is an acquired taste. Three-fourths of the salt consumed comes from prepared and processed convenience foods. Enough sodium is present in natural foods to meet the body’s needs without adding more.
Along with too much salt, Americans also consume too little potassium, which counters the harmful effects of too much salt. The average man and woman requires 4.7 grams of potassium a day, but consume only about 50% of this amount. They consume less than 4.7 grams of potassium as day. Among the best dietary sources of potassium are : spinach, cantaloupe, almonds, Brussels sprouts, mushrooms, bananas, oranges, orange juice, grapefruits, potatoes, dried fruits, peanut butter, bran, meats, dried beans, peas, coffee, tea, and cocoa.
It has been commonly accepted that a body-weight loss of water of more than 2% during exercise impairs athletic performance. However, a French study of marathoners found that the fastest finishers were the MOST DEHYDRATED, having lost 3.1% of their body weigh, while the slowest finishers lost only 1.8%. It appears the body has hidden reserves that can generate several pints of water during exercise. Drinking water when thirsty appears to be the best course of action.
Joseph F. Kras, M.D., D.D.S.; Robert S. Joseph, RPh, MD, FCCP; David Jackson, Ph.D.