Muscle Sugar More Important Than Fluid for 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. More on hyponatremia

Low vitamin D prevents insulin response; sunlight may not be enough

A recent study from the University of Wisconsin Osteoporosis Clinical Research Program in Madison, showed that some people have very low levels of vitamin D in spite of getting a lot of sunlight where they live in sun-drenched Hawaii (Journal of Clinical Endocrinology & Metabolism, June 2007).
The 93 participants in the study averaged 22.4 hours per week outside without sun screen. In spite of abundant sun exposure, 51 percent had low vitamin D levels.

Many people think of vitamin D as the vitamin that helps to prevent rickets, a disease characterized by weak bones that break easily. However, vitamin D does much more than that. It is necessary for your immune system to search out and destroy invading bacteria, viruses and even cancer cells. Recent studies show that lack of vitamin D prevents your body from responding to insulin adequately. The Third National Health and Nutrition Examination Survey shows that having low levels of vitamin D increases risk for high blood pressure, obesity, diabetes and having high blood levels of triglycerides (Archives of Internal Medicine, Volume 167, 2007). Researchers at Tufts University in Massachusetts showed that obesity increases a person's chances of having low vitamin D levels (Journal of Clinical Endocrinology & Metabolism, May 2007). A possible explanation is that fat sops up the available vitamin D so that it is not readily available to be used by the body.

Most people do not get adequate amounts of vitamin D from the food that they eat. They must depend on sunlight. People with dark skin require more sunlight to meet their needs for that vitamin, which could explain the increased risk for diabetes in dark-skinned people who live far from the equator. Calcium blocks the activation of vitamin D so people who take calcium supplements need to get extra sunlight or take vitamin D supplements. The large amount of calcium in milk can lower vitamin D levels even when the milk is fortified with the vitamin.

Vitamin D can be made in the skin when it is exposed to ultraviolet light from the sun. Then the liver converts vitamin D to an active hormone form called 25 hydroxy vitamin D. Then your kidneys convert this hormone to a more active di-hydroxy vitamin D. So you get that vitamin from your skin, liver and kidneys, and lack of sunlight and liver or kidney diseases can cause a deficiency. Now we learn that some people can be deficient even with adequate sunlight exposure and normal kidneys and livers. The only way to find if you are deficient of vitamin D is to get a blood test called 1,25 diyhxdroxy-vitamin D.