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Preserve Muscle Sugar for Speed and Endurance

How fast you can move and how long you can exercise intensely depends on the amount of sugar (glycogen) stored in your muscles. The same rule applies in all sports: when muscles run out of their stored sugar supply, they require more oxygen and you have to slow down.

Fluid is less important than muscle sugar because dehydration will not cause you to slow down until your blood volume is reduced. As you lose fluid from sweating, interstitial fluid stored around cells is released into the blood to maintain blood volume. When you compete is sports at a very high intensity, your muscles run out for stored sugar long before your blood volume is reduced, and you slow down from lack of muscle sugar before you slow down from reduced blood volume (Sports Medicine, April- May 2007).

Fuel for muscles comes from sugar and fat stored in muscles, sugar and fat in the bloodstream and, to a lesser degree, protein. When you start to exercise intensely, more than 50 percent of your energy comes from sugar stored in muscles. Two hours later, most of the sugar stored in muscles is used up and less than 10 percent of energy comes from that source. If you do not supply extra sugar during exercise so that your muscles will use less of their stored sugar, muscles run out of glycogen and your performance will suffer.

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Dear Dr. Mirkin: After a long charity bicycle ride, I found that I had gained four pounds and my whole body was puffy. Should I be concerned?

It is not normal to be puffy after a long ride or any other form of exercise. Your doctor should check for kidney problems, hormone abnormalities or diabetes. If these tests are normal, you drank way too much during your ride and are lucky that you only suffered from excess fluid retention. You could have developed hyponatremia, a serious condition that can kill you.

Normal healthy kidneys have a limit to how rapidly they can rid your body of excess fluid. If you take in more fluid than your kidneys can clear, fluid accumulates in your blood and dilutes the salt level. Since fluid moves from an area of low salt to high salt, and your brain has far more salt than your blood does; the fluid will move into your brain to causes swelling, seizures and even death.

Hyponatremia virtually never occurs in trained athletes competing at a fast pace. It takes so much concentration to run or cycle very fast that it is almost impossible to take in too much fluid. On the other hand, when you slow down, you have the time to overindulge in fluids and hyponatremia can happen to you. A world-classes marathon runner typically takes in about a cup of fluid per hour during a race. On the basis of our present knowledge, it may not be safe for average athletes and casual exercisers to take in more than three to four cups of fluid per hour.

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Dear Dr. Mirkin: When should triglycerides be tested?

For more than 50 years doctors have used fasting blood triglyceride levels to predict a future heart attack, but now two studies in the Journal of the American Medical Association show that non-fasting blood triglyceride levels are far more dependable (July 18, 2007) . When your blood sugar rises too high after eating, your pancreas releases huge amounts of insulin. Insulin converts sugar to triglycerides. Triglycerides are therefore a marker for a high blood sugar levels that damage arteries to cause heart attacks. More than 75 percent of diabetics die of heart disease.

Many people have normal blood sugar and triglyceride levels after an overnight fast, but have their blood sugar levels rise too high after eating and therefore have a high rise in triglycerides only after eating. This means that having normal triglycerides after fasting does not rule out high risk for a heart attack. High triglycerides after eating shows that you have high blood sugar levels and should restrict foods and beverages made with sugar, flour or other refined carbohydrates, the foods that cause the highest rise in blood sugar.

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Recipe of the Week

Quinoa Salad with Melon and Spinach

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June 26th, 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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