June 12, 2011
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.
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.
***********************************************
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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Baked Fish with Portobello Mushrooms
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The Good Food Book
http://www.drmirkin.com/goodfood/index.html
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