Salt for Warm Weather Exercise

This month, two major reports in medical journals claim that getting people to reduce their intake of salt will save lives, prevent heart attacks and markedly reduce health costs (Annals of Internal Medicine, April 2010; NEJM, April 22, 2010). That may be true for many people who do not exercise, but for heavy exercisers and athletes, particularly those who are also vegetarians, it can cause cramps, fatigue, injuries and even death.

During World War II, Dr. James Gamble of Harvard Medical School showed that salt (sodium) is the only mineral that people need to be concerned about when they work or exercise for long periods in hot weather. After Gamble published his studies, people who work in the heat were given salt tablets, which is such a concentrated form of salt that they can cause nausea and stomach irritation. In the 1960s, doctors became concerned that too much salt can cause high blood pressure, so they started recommending low-salt diets.

Excessive intake of salt causes high blood pressure in some, but not all, people. High blood pressure increases risk for heart attacks, strokes, and kidney damage. Many middle-aged people who start an exercise program lose their tendency to develop high blood pressure when they take in extra salt (Journal of Human Hypertension, May 2006).

Not taking in salt when you exercise for more than two to four hours can prevent you from retaining the water that you drink. It can also block thirst, so you may not know that you are dehydrated. Thirst is a late sign of dehydration. You lose water during exercise primarily through sweating, and sweat contains a far lower concentration of salt than blood. So during exercise, you lose far more water than salt, causing the concentration of salt in the blood to rise. A person will not feel thirsty until the concentration of salt in the blood rises high enough to trip off thirst osmoreceptors in the brain, and it takes a loss of two to four pints of fluid to do that.

You need salt to retain the fluid you drink while exercising. In one study, female competitive distance runners took in drinks with different concentration of salt during a four- hour run (British Journal of Sports Medicine, August 2003). Ninety-two percent of those who took in plain water with no additional salt developed low blood levels of salt. Taking in fluid without also taking in adequate amounts of salt dilutes the bloodstream, so that the concentration of salt in the blood is lower than that in brain cells. This causes fluid to move from the low-salt blood into the high-salt brain, causing the brain to swell which can cause seizures and death. However, the low salt syndrome, hyponatremia, that can kill athletes is usually caused by taking in far too much fluid, rather than from not taking in enough salt. Taking extra salt just prior to competition can help you exercise longer and harder (Medicine and Science in Sports and Exercise, January 2007; Clinical Journal of Sport Medicine, January 2007). Athletes who took the extra salt had larger blood volumes and greater endurance. Salt makes you thirsty earlier so you drink more, and salt in your body holds water so you have more water available to meet your needs. Salty drinks taste bad, so it is easier to meet your needs with salted foods. If you plan to exercise for more than a couple hours in hot weather, drink one or two cups of the liquid of your choice each hour and eat a salty food such as salted peanuts.

Taking extra salt during prolonged exercise increases thirst so you drink more fluids, and prevents blood salt levels from dropping so low that you can become tired, develop muscle cramps, and even die. You can keep yourself fresh during extended exercise by eating foods with salt and drinking frequently, before you feel hungry or thirsty. Once you feel hungry or thirsty, you will find it very difficult to regain your strength. Commercial sports drinks help increase endurance by their caffeine, sugar, salt, and to a lesser degree, protein content. It is unlikely that any other component improves performance (The Physician and Sportsmedicine, April 2010).

What exercisers and athletes should do: I recommend that you buy a blood pressure cuff and take your blood pressure a few times a week before you go to bed. If your blood pressure remains above 120 systolic, you have high blood pressure and should check with your doctor. The amount of salt people need varies greatly from person to person. If you exercise regularly for more than a couple hours, particularly in hot weather, you need extra salt. You also need more sugar in hot weather to increase endurance. When you run out of sugar stored in muscles (glycogen), your oxygen requirements rise significantly and you have to slow down. We drink Pepsi and eat salted peanuts on long rides in the summer. You may prefer pretzels or any other salty snack.