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Why You Need Salt During Prolonged Exercise in Hot Weather

The only mineral that you need to make extra effort to take during prolonged exercise is sodium. The amount of salt people need varies greatly from person to person. If you exercise regularly for more than an hour, particularly in hot weather, you probably need extra salt.

You do not need extra potassium, magnesium or calcium:
Healthy athletes and exercisers do not need to take potassium, magnesium, calcium or any other minerals (1). Athletes do lose minerals through increased sweating, but compared to blood, sweat is very dilute in minerals, so they can get all the minerals they need from food. A deficiency of potassium, magnesium, or calcium has not been reported in healthy athletes who eat a normal diet.

Sodium:
The definitive studies on minerals and exercise were done during World War II. Dr. James Gamble of Harvard Medical School paid Harvard medical students to lie on a raft or exercise in his swimming pool, take various amounts of fluids and salt, and have blood drawn to measure salt and mineral levels. He showed that salt requirements increase significantly when you exercise for several hours in hot weather. I was fortunate enough to be among the doctors training at Harvard Medical School who heard Dr. Gamble give his lectures on minerals and exercise. Now, more than sixty years later, nobody has improved on his research.

You need extra salt in your food when you exercise in hot weather:
If you don't take salt and fluids during extended exercise in hot weather, you will tire earlier and increase your risk for heat stroke, dehydration and cramps.

Not taking in salt when you exercise for more than two 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. You will not feel thirsty until the concentration of salt in the blood rises high enough to trip off thirst osmoreceptors in your brain, and it takes a loss of two to four pints of fluid to do that (2).

You need salt to retain the fluid you drink while exercising:
In one study, female competitive distance runners took in drinks with different concentrations of salt during a four-hour run (3). 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, hyponatremia, the low salt syndrome that can kill athletes, is usually caused by taking in far too much fluid, rather than from not taking in enough salt.

How salt can improve performance in competition:
Taking extra salt just prior to competition can help you exercise longer and harder (4). Fatigue during hot-weather exercise is caused by lack of water, salt, sugar or calories. Of the four, exercisers are most ignorant of their sodium needs.

Salt after exercising in the heat:
You should always replace fluids, salt, sugar, and protein after you exercise in hot weather. (5). Just salting your food to taste should replace the salt you lose through heavy sweating (6). If your kidneys are normal, you should be able to rid yourself of any excess salt that you may take in.

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Can you harm your health by causing salt deficiency?
If you are not replacing salt that you lose from sweat, you will suffer fatigue, muscle cramps and injuries that can prevent you from continuing to exercise. Severe salt deficiency can cause high blood pressure and metabolic syndrome. While moderate salt restriction can lower high blood pressure, severe salt restriction can raise blood pressure. When you don't get enough salt, your adrenal glands put out large amounts of aldosterone that constricts arteries and raises blood pressure, and your kidneys put out extra renin that also constricts arteries and raises blood pressure (7).

Severe salt restriction can raise blood sugar and insulin levels:
A study from Columbia University Medical School showed that salt restriction raises blood sugar and insulin levels, while adding salt lowers them (8).

Salt your food, not your drink:
Salty drinks taste awful, 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, potato chips, or anything else that tastes salty.

How can you tell that you need more salt?
Salt deficiency causes tiredness, lethargy and cramps. It also weakens muscles, causing you to slow down and lose strength. If you suffer any of these symptoms, you can get a blood test for sodium and chloride on the day after a hard workout. Low blood levels of sodium are most likely to occur on the morning after you have replaced fluid lost from heavy exercise. If your blood sodium level is below 130, you are deficient and need to add more salt to your food.

Who is most likely to suffer from salt deficiency?
Vegetarians and people who limit meat are at increased risk for salt deficiency because plants are naturally low in salt. Meat, fish and chicken naturally contain far more salt. Most processed foods are high in salt because manufacturers know that salt makes food taste good and is also a preservative.

Why don't casual exercisers suffer from salt deficiency?
The North American diet typically contains up to 10 times the minimal daily salt requirement. If you doubled or tripled your salt loses through sweating, you may still not be deficient because you probably take in far more salt than you need.

Does extra salt cause high blood pressure and heart attacks?
A review of the world's literature shows that salt restriction does not lower high blood pressure for most people with high blood pressure (9). Eating salty foods and drinks when you exercise for more than two hours is unlikely to raise blood pressure. I found only six long-term follow-up studies of salt intake and heart attacks. Three studies suggest that very low salt intake may cause heart attacks.

Exercise can prevent a rise in blood pressure with extra salt intake:
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 (10). This study shows that many people who develop high blood pressure from a high-salt diet when they are sedentary, will not develop high blood pressure on the same diet when they exercise.

People with metabolic syndrome are the ones most likely to develop high blood pressure from excess salt intake:
A high-salt diet causes high blood pressure most commonly in people who suffer from metabolic syndrome and are pre-diabetic or diabetic (11). Metabolic syndrome occurs when cells lose their ability to respond adequately to insulin and blood levels of sugar rise too high. It is caused by eating too much sugar and other refined carbohydrates, being overweight, not exercising, and lacking vitamin D. Metabolic syndrome is characterized by storing fat primarily in the belly, having a thick neck, high blood triglycerides, low blood good HDL cholesterol, high blood sugar, and eventually liver damage and all the side effects of diabetes. People with metabolic syndrome have a greater rise in blood pressure with increased salt intake and a drop in blood pressure with salt restriction.

Buy a blood pressure cuff:
If you are concerned about your blood pressure, you can buy an inexpensive wrist cuff and check your systolic blood pressure at bedtime. If it is below 120, you probably do not need to worry about salt. If it is above 120, and particularly if you store fat primarily in your belly rather than your hips, your good HDL cholesterollis below 40, your triglycerides are above 175, or you have a blood sugar above 100 two hours after a meal or an HBA1C above 5.9, you probably should restrict salt and definitely should work to correct the causes of metabolic syndrome:
• lack of exercise,
• overweight,
• eating too much red meat,
• taking sugared drinks and foods,
• lack of vitamin D, and
• not eating enough fruits and vegetables.

Summary:
If you do not exercise, you do not sweat very much and you do not need very much salt. Too much salt can increase blood volume which raises systolic blood pressure. Being fat is the primary cause of elevated diastolic blood pressure. On the other hand, if you exercise vigorously, you sweat tremendously and lose a lot of salt. Without the extra salt that you need, you will not recover from your hard bouts of exercise and you will be more likely to be injured and tired all the time.

References:
1. Medicine and Science in Sport and Exercise 1999(October);31(10):1406-13
2. American Journal of Emergency Medicine, 1999;17(6):532-539
3. British Journal of Sports Medicine, August 2003
4. Medicine and Science in Sports and Exercise, January, 2007; and Clinical Journal of Sport Medicine, January 2007
5. J Sports Sci 2007:15:297-303
6. Eur J Appl Physiol 1996:73:317-325
7. Clinical Autonomic Research, 2002;12(5):353-357
8. American Journal of Hypertension, 2001;14(7, Part 1:653-659
9. Journal of Hypertension. May 2011;29(5):821-828
10. Journal of Human Hypertension, May 2006
11. Lancet, published online March 2, 2009

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Dear Dr. Mirkin: Does extreme endurance exercise prolong life?

Probably. Data on fifty percent of Tour de France participants show they died at an average age of 81.5 years, compared to 73.5 in the general populations of France, Italy and Belgium (International Journal of Sports Medicine, May 2011). Certainly exercising repeatedly to severe exhaustion and training for up to eight hours a day, seven days a week, and suffering severe pain during daily racing for 21 days did not harm these men.

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Update on vitamin D deficiency and heart attacks:

Ninety-six percent of 239 people suffering acute heart attacks in 20 hospitals had very low levels of vitamin D. They were also markedly overweight and had high levels of parathyroid hormone (American Journal of Cardiology, June, 2011). Normal vitamin D3 level is above 30 ng/ml (75 nmol/L).

The single highest risk factor for a heart attack is diabetes. The major function of vitamin D is to increase calcium absorption from the intestines. When blood levels of vitamin D are low, ionized calcium drops, causing the parathyroid glands to put out large amounts of parathyroid hormone. This blocks insulin receptors to cause diabetes (Journal of Nephrology, 06/06/2011).

If your vitamin D3 level is below 30 ng/ml, your doctor will probably recommend more sunlight or vitamin D pills.

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Recipe of the Week:

Baked Fish with Portobello Mushrooms

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June 12th, 2011
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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