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Most Exercisers Do Not Raise Metabolism

Many people believe that exercise controls weight by increasing your metabolism so you burn extra calories all day long. A review of the world's literature from the University of South Australia in Adelaide shows that you have to be in very good shape to exercise vigorously enough to increase your metabolism (Journal of Sports Science, December 2006). This means that most exercisers are not able to exercise hard enough to burn extra calories for a significant time after they finish exercising, so increased post-exercise metabolism does not cause most exercisers to lose weight.

Researchers monitor changes in metabolism by measuring how much oxygen your body uses over a period of time. The maximum amount of oxygen that you can use during exercise in a given time is called VO2max. To increase the amount of oxygen that your body uses after exercising, you must exercise at an intensity of at least 50 percent of your VO2max, which is too much for casual exercisers. You have to exercise very vigorously to increase your oxygen consumption and body temperature for more than a few minutes.

This study shows that if a person wants to increase his metabolism for from 3 to 24 hours, he must exercise for more than 50 minutes at 70 percent of his VO2max, or more than 6 minutes at 100 percent of his VO2max. You need to be very fit to be able to exercise at these levels. For most people, weight control depends on more on how long you exercise, and far less on the extra calories that you burn after you finish exercising.


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Dear Dr. Mirkin: How can I know if I need to eat while I am exercising?

Fatigue during a workout or sporting event is usually caused by lack of water, salt or sugar. Most athletes in sports that last more than a couple of hours know that they should drink and take in some salt, but they also need a source of sugar.

When you exercise, you get your energy from sugar and fat stored in your muscles and sugar and fat in your bloodstream, and, to a lesser extent, from protein. At first you get more than 80 percent of your energy from fat and sugar stored in muscles. Usually at the start of exercise, almost 45 percent of the energy comes from stored muscle sugar. As you continue to exercise, you use up fat and sugar stored in muscles and get far less from these stores. After two hours of exercise, you have used up most of your stored muscle sugar (glycogen) and get less than 15 percent of your energy from that source. At four hours, your muscles have almost no stored sugar at all.

When your muscles are depleted of their stored sugar, they become difficult to coordinate and feel heavy and hurt. Your muscles can get some sugar from gluconeogenesis, a process in which your liver makes sugar from protein (branched chain amino acids), but that is not enough for all-out exercise. During intense exercise, you need a source of sugar. It can come from sugared drinks or any carbohydrate-rich foods. You can use special sugared exercise drinks, sugar gels, carbonated soft drinks, exercise bars or any food that contains sugar or flour.


Dear Dr. Mirkin: How can fat cause diabetes?

A fascinating review from Tufts University in Boston shows how excess fat stored in muscles causes diabetes (American Journal of Clinical Nutrition, March 2007). Before insulin can do its job of driving sugar into cells, it must first attach on special hooks on the surface of cell membranes called insulin receptors. When excess fat is stored in muscles, the insulin receptors internalize so that insulin cannot attach on the hooks. This markedly increases the amount of insulin that is necessary to drive sugar into cells, and eventually huge amounts of sugar accumulate in the bloodstream to cause diabetes and damage every cell in the body.

Excess deposition of fat into muscles is caused by eating too many calories, not getting enough exercise and eating too much saturated and partially hydrogenated fats. Most cases of Type II diabetes are caused by a faulty lifestyle, not genetics, and are curable with proper exercise and diet.


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Ratatouille with Baby Potatoes
New Potato Salad

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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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