|
|
Alex "Iron Doc" McDonald: Water and Salt: Separate but Equal
By Alex Mroszczyk-McDonald
8/4/2008 |
|
|
The issue of hyponatremia (low levels of sodium in the blood stream) in endurance athletes has received a very large amount of publicity in the last several years. At times this issue has been legitimately discussed and has helped many athletes avoid the pit falls and serious medical harm that can occur as a result. However, at other times this issue has been subject to the endurance “rumor mill,” if you will, and there has been a lot of misinformation conveyed. The goal in this article is to provide some concrete well established guidelines from various medical and sport authorities, as well as give a brief explanation behind the balance of salt and water in the body and the pathophysiology behind hyponatremia why it is so dangerous.
The human body is capable of truly amazing achievements, however, it is very fickle and requires numerous factors all remain within a narrow range of “normal” values, sodium included. Blood sodium concentrations are maintained under precise control by various metabolic mechanisms, mainly: stimulation of thirst, secretion of antidiuretic hormone (ADH), aldosterone and handling of water and sodium by the Kidney. An important concept to understand is that sodium levels and free water (plain water with no sodium or other electrolytes) are viewed and treated by the body separate entities, however, are intrinsically linked. At any point the body can have too much free water, or too little free water, while at the same time having too much sodium or too little sodium.
For endurance athletes hypovolemic hyponatremia is of primary concern. That is when both the total body water levels (volemia) as well as total body sodium levels (natremia) are both low. This develops as sodium and free water are lost, primarily via sweat and urine, and replaced by inappropriately hypotonic fluids (low electrolyte concentrations), such as plain water, or an electrolyte drink with too little sodium compared to the body’s sodium losses. When receptors in various parts of the body detect concentrations of sodium and/or water that are above or below the acceptable range the above mentioned and other mechanisms are turned on or off to either retain or excrete sodium and/or free water.
Due to the central role of the Kidney in water and sodium balance, an important caveat to make is avoid Non-Steroidal Anti-inflammatory Drugs (NSAIDs) such as Advil or Motrin/ibuprofen, Aleve/naproxen or aspirin during or after a race. These medications hamper the Kidney’s ability to appropriately handle sodium and free water and can compound electrolyte problems on race day. If you want to take a pain reliever on race day, use Tylenol/acetaminophen as this medication is metabolized by the Liver and does not have a major effect on the Kidney.
Another concept to keep in mind is that of osmosis, remember high school science class? When two solutions with different electrolyte concentrations are separated by a semi-permiable membrane (i.e. blood vessel, cell membrane), water will flow across the membrane from high to low concentration solutions until the electrolyte concentration in each is equal, creating a difference in the amount of water on either side of the membrane.
When the Kidney detects low levels of sodium in the blood, thru various mechanisms, it will reabsorb sodium destine for the urine back into the blood stream. However, when this occurs a concentration gradient is established and water that was destine for the urine will “follow” sodium back into the blood stream as well. As a result when there are low sodium concentrations in the blood stream the body will try to correct the perturbation, however, requires sodium from an outside source to correct the imbalance.
The concept of osmosis is also the cause behind the major health risks of hyponatremia. When blood sodium concentrations fall rapidly (as they would in an endurance event) below that of sodium concentrations inside the cells of the body, waters flows down the concentration gradient from the blood into the cells causing them to swell, which maybe seen as edema (soft tissue swelling) often common after a long race. Most tissues of the body are easily able to tolerate this swelling, however, the brain is not. The neural cells of the brain are only able to swell a finite amount due to the fact that it is confined by the skull. When the brain swells, a condition known as cerebral edema, various symptoms can occur and if the edema is severe enough death can ensue.
Clinically significant hyponatremia is relatively uncommon and presents with nonspecific symptoms including: decreased appetite, nausea and vomiting, difficulty concentrating, confusion, lethargy, agitation, headache and seizures. Often with the early signs an athlete may think they are simply “dehydrated” and drink plain water, however, this will only worsen the problem. At the first sign of nausea, muscle cramps or disorientation, an athlete should drink a sodium containing sports drink, or eat salty foods. If the symptoms are extreme, a medical professional should be sought.
The goal, however, is to never reach this point, and that is where nutrition planning and practice are key, particularly for endurance events lasting 4-5hrs. The American College of Sports Medicine recommends that people who are active for more than one hour consume 500–700mg of sodium for every 32oz(~1L) of water. Make sure you determine your sweat rate in training (see Sweat Rate Caculator , ) and consume sodium based on the amount of fluid you drink, not time you are racing. However, there are some who recommend 500-1000mg of sodium per 32oz of water. Experimental data has demonstrated that sweat rate and sodium loss is highly individual ranging from 460-1840 mg/L of sweat. This can be further influenced by numerous other factors including genetics, fitness, acclimatization, race conditions and others. Determine your sweat rate and estimate your sodium losses in various different training environments and conditions throughout the year, not just before a competition.
Another major reason that athletes cite for sodium supplementation is to avoid muscle cramps. Although there is no definitive cause know for muscle cramps it is most popularly believed to be due to hylovolemia and/or hyponatremia. For shorter events these seem to be less of a factor, however, again for events lasting greater than 4 hours finding and executing an optimal nutrition plan, including hydration and sodium, seems to have some efficacy in avoiding muscle cramps. However, again it is a highly individual and can vary drastically from athlete to athlete.
Lastly, sodium and salt is ubiquitous in a western diet and has received a bad reputation. It is true that those individual with high blood pressure, kidney or cardiac problems as well as some other medical conditions need to avoid sodium. However, as long as a person is healthy their body is able to process large amounts of sodium with minimal immediate health impact. However, there are enormous amounts of sodium in many foods, particularly pre-packaged and restaurant fare. If a person does not pay attention to their daily salt intake they could be potentially putting themselves at risk of health problems later in life. Some sodium is ok and actually required in the daily diet, however, try to keep it to the recommended daily allowance (RDA). A particularly good time for an athlete to consume those salty foods is after a workout when their body maybe slightly hyponatremic and will be craving sodium. Consuming sodium will also help to retain fluid consumed after a workout (remember osmosis) and help to replete total body water volume faster, which may aid in recovery. A second time that maybe beneficial to allow your sodium consumption to climb is in the days prior to a long race, this way you are starting a race fully volume loaded.
The bottom line is that hyponatremia is a complex entity which has multiple variables and factors that are in effect. It is preventable by planning, practice and a little trial and error. It is vital to continually practice your nutrition and sodium plan in training.
Alex M. McDonald, MD is a professional triathlete, coach and a member of the Timex Multisport Team. His interests lie in coaching endurance athletes, sports medicine and exercise physiology. He obtained his MD degree from the University of Vermont. Prior to medical school, Alex graduated from Connecticut College in 2003 with an emphasis on both the biological and social sciences. He currently lives and trains in Durham, NC.
Photo: Rebekah Keat stays cool and hydrated in Kona, 2007 (note salt build up on her shorts!)
|
| Back to Listings |
|
