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Comparing Cycling With Running

Which burns more calories, running or cycling? The standard comparison is that one mile of running equals four miles of cycling, but that's lousy science. Although running requires the same amount of energy per mile at any speed (110 calories per mile), riding is affected by wind resistance so the faster you ride, the more energy you use. So you have to compare running and cycling at different cycling speeds.

Dr. Edward Coyle of The University of Texas in Austin determined average values of oxygen consumption by cyclists to develop a table to estimate the approximate caloric equivalence between running and cycling. He found that if you ride 20 miles at 15 mph, you burn 620 calories (20 miles X 31 calories per mile = 620 calories). Take the 620 calories and divide them by 110 calories per mile for running and you get 5.63 miles to burn the same number of calories. So riding a bicycle 20 miles at 15 miles per hour is equal to running 5.6 miles at any speed.

Dr. Coyle made the calculations easy by providing conversion factors for different riding speeds: 10MPH=4.2, 15MPH=3.5, 20MPH=2.9, 25MPH=2.3, and 30MPH=1.9. Divide the number of miles ridden by the conversion factor for your riding speed to tell you the equivalent miles of running at any speed. Thus, for 20 miles ridden at 10MPH, divide 20 miles by 4.2 which tells you that your ride is equivalent to 4.8 miles of running. This formula is for an average-size adult (approximately 155 pounds). A larger cyclist would divide by a slightly higher number; a smaller cyclist, by a slightly lower one. Wind and hills are not accounted for in the table; nor is drafting (riding behind another cyclist), which can reduce your energy expenditure by up to one-third.

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Dear Dr. Mirkin: Several of my cross-country teammates have stress fractures. Is there any way to prevent them?

Stress fractures, small cracks on the surface of the bones, usually start out as a minor discomfort in the foot or leg that occurs near the end of a long run. Usually the pain goes away as soon as the athlete stops running. On the next day, the pain returns earlier in the run. If she notices that it hurts to touch just one spot on a bone and then stops running for a week, she can return to running quickly, but usually she ignores the pain and develops a full- blown stress fracture that hurts all the time. She now has to avoid the hard pounding of running, but can ride a bike or swim for exercise until the fracture heals in 6 to 12 weeks. The most common sites for stress fractures are the bones in the front of the feet or the long bone of the lower leg, but running can cause stress fractures anywhere, even in the pelvic bones.

Forty-five percent of competitive female runners develop stress fractures. The women most likely to suffer these injuries are those who restrict food and those who have irregular periods. Restricting food can stop a woman from menstruating regularly, which can stop her body from producing the female hormone, estrogen. Lack of estrogen weakens bones. Exercise does not cause irregular periods, but not taking in enough calories can. Women who stop menstruating when they exercise heavily will usually start to menstruate regularly when they eat more food. I often prescribe bone strengthening medications such as Fosamax or Evista to women with stress fractures that do not heal in six months.

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Dear Dr. Mirkin: All my friends are dieting, but I'm too skinny. How can I gain weight?

Weight gain should always be in the form of muscle, not fat. To build muscle, start a weight-bearing exercise program. Go to a gym and learn how to do the weight training circuit. It only takes 15 extra grams of protein a day to build a pound of muscle a week -- so you really won't need to eat a lot more. Muscle is far more dense than fat. Once you are exercising regularly and gaining muscle, your appetite will probably increase and you will eat more without any conscious effort. Most muscular people and heavy exercisers will eat plenty to meet their calorie needs. The training tables for football teams are piled high with every kind of food.

It's never too late to start a weight training program. Underweight older people look and feel frail because they have lost most of their muscles, not because of lack of fat. If you are inactive, you lose muscle mass to the point where you are unable to carry out daily activities -- climbing stairs, getting up out of a chair -- because your muscles are not strong enough to move the weight of your own body. Don't try to add fat to a weak body. Overweight older people often have the double burden of weak muscles AND 20, 40 or more extra pounds to lug around with them every day.

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