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Exercising in Hot Weather

With each increase in environmental temperature from 62 to 89 degrees Fahrenheit, cyclists have higher body temperatures, lose more power and ride slower (International Journal of Sports Physiology and Performance, June, 2011).

YOU LOSE POWER BEFORE YOUR TEMPERATURE RISES SIGNIFICANTLY: The loss of power precedes a marked rise in body temperature (Journal of Sports Science, January 2010). Muscles are made up of millions of individual nerve fibers. You contract only a few, never all, muscle fibers at the same time. The greater the percentage of fibers you contract at the same time, the greater the force generated and the faster you can go. When you exercise in hot weather, you lose power, speed and endurance before your body temperature rises because you contract fewer fibers at the same time (Pflugers Archiv, July 2004).

MECHANISM: The limiting factor in how fast and long you can exercise is the time it takes to bring oxygen into muscles. When you can't keep up with your needs for oxygen, you slow down. More than 70 percent of the energy supplied to power your muscles is lost as heat. The faster you go, the more heat your muscles produce. A rise in muscle temperature makes your muscles require even more oxygen, so it slows you down.

Your heart pumps oxygen-rich blood to your muscles. As your temperature rises, your heart has to work harder to pump the hot blood from your muscles to your skin to dissipate the heat. This extra blood pumped to your skin reduces the amount of oxygen-rich blood available to be pumped to your muscles, so you have to slow down.

TAKING CARBOHYDRATES HELPS YOU RIDE FASTER IN THE HEAT: Cyclists who take sugar during races do not slow down as early as those who take placebos (Journal of Applied Physiology, April, 2008), and they are able to exercise longer (International Journal of Sport Nutrition and Exercise Metabolism, April 2004). Taking sugared drinks during races helps you ride faster at any environmental temperature, but in hot weather, it helps you maintain your power and speed even more. Since sugar requires less oxygen to power your muscles than fat and protein do, taking extra sugar reduces oxygen needs and allows you to ride faster.

COLD DRINKS ARE BETTER THAN WARM ONES: Cyclists who took cold drinks (4 degrees C) every ten minutes had slightly lower body temperatures, and rode faster and longer than those who took their drinks at warmer temperatures (Journal of Sports Science, September 2010). A review of ten studies on the subject showed that cold drinks lower body temperature and can improve performance by as much as 10 percent (International Journal of Sport Nutrition and Exercise Metabolism, April 2010).

ACCLIMATIZATION: If you want to race at your best in hot weather, you have to train in the heat and live in the heat. You can't just go from a cold climate to a hot one and expect to be able to tolerate the heat or race effectively. Acclimatization to hot weather exercise can be accomplished only by exercising intensely in the heat, not just by living in hot weather. It takes at least a couple of weeks of exercising intensely in the heat to acclimatize. The more fit you are, the better you can tolerate the heat.

CAUTION: Exercising in the heat increases your risk for heat stroke and heat exhaustion. See

FOR THE COMPETITIVE ATHLETE: Pre-cooling helps you to ride faster in the heat. If you lower your temperature and drink cooled fluids before you start to race, you can ride faster at the start and during that race. The most effective way to do this is to: *take a sugared and iced drink before the race (J Sports Sci, 2007) and *immerse the racer up to his neck in cold water (Med Sci Sports Exerc, 1997;29(7). The fastest time trials were accomplished by cyclists who preceded their time trials by taking large volumes of a sugar-ice drink (up to one liter ) and by plunging their bodies into cold water for ten minutes (Med Sci Sports Exerc, Jan, 2011). However I can find no studies on the safety of pre-cooling. I know that people with blocked arteries leading to their hearts could suffer irregular heartbeats and even heart attacks.


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Dear Dr. Mirkin: Do your genes determine how long you live?

Genes appear to be more important in determining who will live beyond 100, but in people who live into their nineties, how sick you become and how long you live are determined more by what you do than by your genes.

WHO IS LESS LIKELY TO DIE? Middle-aged people are 63 percent less likely to die within 18 years if they avoid smoking, eat a low-fat and low-sugar diet, exercise regularly, and drink alcohol moderately (American Journal of Public Health, published online August 18, 2011). Compared to those who followed none of these healthful living habits, those who followed all four were protected for:
• 11.1 years from all-causes of death,
• 14.4 years from malignant cancers,
• 9.9 years from heart attacks, and
• 10.6 years from other causes of death.
Smoking was the strongest risk factor for premature death.

WHO IS LESS LIKELY TO SUFFER POOR HEALTH? Middle-aged people who never smoked, were not overweight, exercised at least 3.5 hours per week, and ate a healthful diet had an 80 percent decreased risk of chronic diseases. After being followed for almost eight years, they had:
• 93 percent lower risk of diabetes,
• 81 percent lower risk of heart attacks,
• 50 percent lower risk of strokes, and
• 36 percent lower risk of cancers
If you added in taking no more than two drinks a day, the benefits would have been even greater. Being overweight was the strongest risk factor for serious disease. Next came smoking, followed by lack of exercise, and then by a high fat and sugar diet (Archives of Internal Medicine, August 10/24, 2011).


Dear Dr. Mirkin: Can taking calcium supplements cause heart attacks?

Possibly. DO NOT TAKE MORE THAN 500 MG/DAY IN CALCIUM PILLS. Many studies show that taking more than 500 mg/day of calcium pills is associated with an increased risk for heart attacks (Endocrine Practice, published online August 19, 2011). Therefore most medical organizations recommend that you never take more than 1200 mg/day of calcium from pills plus food (Journal of Bone and Mineral Research, April 2011).

DO NOT RAISE BLOOD LEVELS OF VITAMIN D ABOVE 125 NMOL/L. Since vitamin D markedly increases the absorption of calcium, you probably should never take vitamin D pills that raise blood levels of vitamin D3 beyond 50 ng/mL (125 nmol/L). A review of the world's literature shows that the upper safe level of intake for vitamin D is between 4,000 and 10,000 IU/day and for calcium, from food and pills, between 2,000 - 3,000 mg/day.


Recipe of the Week:

Sweet Potato Bisque

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September 4th, 2011
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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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