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Rules for Eating and Drinking During Exercise

Muscle tiredness and weakness during and after prolonged exercise are caused by lack of water, sugar, salt and calories. You do not need extra potassium, calcium or any other mineral. You don't need to take protein during exercise lasting less than five hours to improve performance or to hasten recovery, but you will recover faster if you take sugar and protein within an hour after finishing exhausting exercise.

The rules for eating and drinking during exercise are quite simple. For exercise lasting:

Less than an hour -- fit athletes do not need to take any food or drink, except they may need water on the hottest days.

More than an hour -- Take sugared drinks just before you begin and frequently during intense exercise.

More than three hours -- Take sugared drinks before you begin and frequently during intense exercise. Eat the food of your choice (fruit or sugar- added foods such as whole grain bars, etc.), plus a source of salt. We use potato chips or peanuts. You cannot get enough salt in a drink because it would taste awful.

More than five hours -- You need water, sugar, salt, and protein and lots of calories. Up to ten percent of the energy to power your muscles during exercise is supplied by protein stored in muscles. However, you do not lose enough muscle to harm performance during exercise until you have been exercising for more than five or six hours. Protein sources include whole grain bars, nuts, meat, fish, chicken, or dairy products.

Why You Need Sugar for Sports Lasting More than Two Hours

Muscles use mostly carbohydrates and fats, and small amounts of proteins, to supply energy for exercise. You have an almost infinite amount of fat stored in your body, but only a very limited amount of sugar stored in your blood, liver and muscles. Recent research shows that you also store a very limited amount of sugar in astrocyte cells in your brain, but the amount is so small that this will not protect you from passing out when your blood sugar levels drop during exercise; see "bonking" below.

You Go Faster When Muscles Burn More Sugar

When you exercise, your muscles use mostly sugars and fat for energy. The faster and more intensely you exercise, the greater the percentage of sugar your muscles use. The greater percentage of sugar that your muscles use, the faster you can go. The limiting factor to how fast you can run, swim or pedal is the time it takes for oxygen to get into muscles. Since sugar requires less oxygen to be turned into energy for your muscles, you have to slow down when your muscles run out of sugar.

"Hitting the Wall"

When your muscles run out of their stored sugar called glycogen, you "hit the wall" and have to slow down. Your muscles hurt and you have to work harder and you move more slowly. All athletes learn, sooner or later, that they have greater endurance as long as they have sugar stored in their muscles. Hitting the wall is common in marathon runners after about 18 miles or three hours. To prevent this from happening to you, take sugar during races or competitions lasting more than an hour.

Running out of Brain Sugar ("Bonking")

Your brain gets almost all of its fuel from sugar in your bloodstream. When your blood sugar level drops, your brain cannot get enough fuel to function properly, you feel tired and confused and can pass out.

During exercise, your muscles draw sugar rapidly from your bloodstream and your blood sugar is quickly used up. There is only enough sugar in your bloodstream to last three minutes. To prevent your blood sugar level from dropping too low, your liver releases sugar constantly from its cells into your bloodstream. But there is only enough sugar in your liver to last up to 12 hours at rest, and less than an hour during vigorous exercise. Your liver can also manufacture sugar from protein.

Bonking is common in bicycle races when a rider does not eat frequently, but is rare in long distance running races. When you run, your leg muscles are damaged from the constant pounding on the roads and you must slow down. However, you pedal in a smooth rotary motion which does not damage your muscles, so you can continue to pedal at a rapid cadence for many hours as long as you take in adequate amounts of sugar.

Hot Weather Increases Your Need for Sugar

When you move intensely for more than an hour, you run low on your muscle and liver stores of sugar. In hot weather you run out of sugar earlier than in cold weather. Almost 80 percent of the energy used to power your muscles is lost as heat. In hot weather, your heart has to work much harder, using more calories to pump this extra heat in your blood from your muscles to your skin where the heat can be dissipated.

Try to Avoid or Restrict Sugar When You Are Not Exercising

Sugars can cause very high rises in blood sugar, particularly if you do not exercise or are overweight or diabetic. Every cell in your body is like a balloon full of fluid. When blood sugar levels rise too high, sugar sticks to the outer membranes of cells. Once there, sugar can never get off. Eventually sugar is converted by a series of chemical reactions to sorbitol, which destroys the cells. This cell destruction causes all of the side effects of diabetes: blindness, deafness, heart attacks, strokes, kidney failure and so forth.

Resting muscles are inactive. They need insulin to remove sugar from your bloodstream to protect you from cell damage. However, contracting muscles actively remove sugar from the blood stream without even needing insulin. So during vigorous exercise, contracting muscles protect you from a high rise in blood sugar. The effect is maximal during vigorous exercise and for up to an hour after you finish. This benefit usually disappears completely in less than 17 hours, so you should try to restrict sugar when you are not exercising.


Reports from

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Carnitine in Meat and Energy Drinks

This week a study from the Mayo Clinic shows that giving carnitine to people who have just had a heart attack is associated with fewer deaths or irregular heartbeats, and less chest pain (Mayo Clinic Proceedings, published online April 12, 2013). Carnitine is found in meat and many energy and sports supplements and drinks.

Last week, a study from the Cleveland Clinic showed that carnitine can be converted in the intestines to TMAO, a chemical that causes heart attacks by punching holes in arteries to cause plaques to form in them (Nature Medicine, published online April, 2013; see my report in last week's eZine).

No Conflict

No conflict exists between the two studies. One shows that taking carnitine causes a specific type of bacteria in the intestines to convert carnitine to TMAO, which may increase risk for a future heart attack. The other study shows that carnitine can help to repair heart muscle damaged by a heart attack.

Athletes have known for many years that carnitine helps to clear breakdown products of metabolism from accumulating in skeletal muscles to delay recovery, so they take carnitine to help their skeletal muscles heal faster from hard exercise. In a like manner, carnitine may help heart muscle to heal faster when it is damaged by a heart attack.

This Week's Study

The authors reviewed 13 controlled trials of carnitine given to 3,629 heart attack patients. They had 250 deaths, 220 cases of new heart failure, and 38 recurrent heart attacks. Carnitine was associated with:

• 27 percent reduction in sudden death,

• 65 percent reduction in irregular heartbeats (ventricular arrythmia),

• 40 percent reduction in the development of chest pain (angina),

• reduction in size of the part of the heart muscle that is damaged (infarct size).

How Could Carnitine Treat a Heart Attack?

When heart muscle is deprived of oxygen during a heart attack, toxic fatty acids accumulate in the dying muscle tissue to cause further damage, and carnitine levels drop precipitously. Carnitine helps to move these fatty acids into the mitochondria where they can be burned for energy. This helps to increase the circulation of blood, which increases oxygen supply to the oxygen-starved, damaged part of the heart muscle.

My Recommendations

Since research data show that eating red meat is associated with increased risk for heart attacks, I believe that you should restrict your intake of red meat. I also feel that you are probably better off not taking supplements or energy drinks that contain carnitine. We have data that carnitine can be converted to TMAO by bacteria in your intestines, and TMAO can damage arteries to increase risk for a heart attack. If you have the misfortune to suffer a heart attack, your treatment may include taking carnitine to help repair the damage.

This week's medical history:
Johnny Miles Was My Hero

For a complete list of my medical history biographies go to Histories and Mysteries


Recipe of the Week:

Incredible Creamy Clam Chowder

You'll find lots of recipes and helpful tips in The Good Food Book
- it's FREE


April 21st, 2013
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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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