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Low-Carbohydrate Diet Slows Time Trial

A recent study from South Africa shows that eating a low-carbohydrate diet slows extended sprint performance of cyclists (Journal of Applied Physiology, January 2006). Competitive bicycle racers ate a high fat or high-carbohydrate diet for six days followed by a high-carbohydrate diet for one day and completed time trials on their bikes. Then they ate the opposite diet for six days followed by a high carbohydrate diet for one day and repeated their time trial. Diets did not affect their times or power output for 100 kilometers (62 miles), but the high fat diet slowed their sprint performance over one kilometer (0.6 miles).

Muscles get their energy from sugar and fat stored in muscles or from the bloodstream. The limiting factor in how fast an endurance athlete can exercise is the time it takes to transport oxygen from the blood in the lungs to the muscles. Muscles require far more oxygen to burn fat than to burn sugar for energy. So when a muscle runs out of its stored sugar, called glycogen, it becomes less efficient, hurts, is difficult to co-ordinate and slows you down.

Many previous studies show that it doesn’t make any difference what an trained endurance athlete eats on the week before competition because the muscles of trained athletes store the most glycogen when they reduce training for several days, regardless of what they eat. Any sprint that takes less than 50 seconds is not affected by diet, because you can work up to 50 seconds anaerobically, without requiring additional oxygen. This study shows that a high-fat diet before extended sprinting hurts performance. A high fat diet causes muscles to burn a higher percentage of fat. Using fat for energy requires more oxygen than carbohydrates do, and how fast you can sprint 0.6 miles on a bicycle is limited by how rapidly you can deliver oxygen to muscles. Restricting carbohydrates before a sprint taking more than 50 seconds increases oxygen needs which slows you down.


Dear Dr. Mirkin: Should I seek out the special margarines that contain sterols?

Researchers at McGill University in Canada report that eating plant sterols and exercising lowers cholesterol, blood pressure, body fat and weight, but you don’t have to eat special plant sterol margarine. You can get plenty of plant sterols in nuts, seeds, vegetables and beans. In this study, middle-aged men ate margarine containing sterols four times a day and used stair-stepping machines and stationary bicycles three times a week for eight weeks (American Journal of Clinical Nutrition, December 2005). Down went the total cholesterol, bad low-density cholesterol, and triglycerides and up went the good HDL cholesterol. Fifty percent of deaths in North America are caused by heart attacks and strokes. Most people can prevent these catastrophes by exercising, eating lots of plants (fruits, vegetables, whole grains, beans and other seeds), avoiding smoking and smokers, staying monogamous and maintaining a healthful weight.


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Dear Dr. Mirkin: What’s the most efficient way for a beginning weight lifter to become stronger?

A recent study from the University of Sydney in Australia shows that you benefit either from increasing the number of sets of repetitions or from training faster, but not both. Weight lifters were divided into four groups: 1) one set fast 2) three sets fast, 3) one set slow 3) three sets slow. A control group did no lifting. A set was the heaviest weight that they could lift six to eight times in a row. They trained three times a week for six weeks (Medicine & Science in Sports & Exercise, September 2005).

The group that did one slow set increased strength by 25 percent. Three sets produced twice the increase in strength of one set. Fast training resulted in a greater increase in strength than slow training. There was a benefit of training with three sets or fast speeds, but there was no additive benefit of training with both. So unless you are an athlete who needs speed to compete, you can follow a regimen that emphasizes increasing weight, rather than moving faster.

If you want to become strong, check with your doctor to make sure that you do not have a condition that will be aggravated by heavy exercise. Then pick several different exercises, such as a bench press, upright row, and so forth. Start out with a weight that you can lift comfortably six to ten times in a row. Do one set in each exercise, and repeat this workout three times a week. As you become comfortable with this workout, increase to three sets of 6 to 10 repetitions. When you are comfortable with this workout, increase the weight that you lift. More on how muscles get strong


I recommend eating fruits WITH other foods, not alone as snacks, especially if you are diabetic or working to control your weight. Try one of Diana’s new salad recipes that feature fruit . . .

Fruity-Nutty Salad
Mother’s Spinach Salad
Cranberry-Wild Rice Salad

You'll find many more in the Salad section of Diana's Healthful Recipes

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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