“A bear, however hard he tries, grows tubby without exercise.” – Pooh’s Little Instruction Book (inspired by A. A. Milne)

Here’s a flash that will knock most “health gurus” right off their treadmills: Long-duration exercise is a waste of your time.

When you exercise for long periods of time, you can’t build stamina. Your body responds by making your heart, lungs, and muscles smaller and more efficient — able to go longer by expending less energy. But while smaller may be more efficient, it also reduces an organ’s reserve capacity.

Reserve capacity is the ability of an organ to adapt to the sudden onset of any high demand it may encounter. For your heart, in particular, that’s extremely important. Inadvertently reducing the reserve capacity of your heart by making it smaller through regular, prolonged aerobic activity could be very dangerous.

Remember Jim Fixx? He was a marathon runner who became a famous health guru. He preached long-duration cardiovascular endurance training as a method of achieving peak performance. He insisted this would protect against heart disease … up until the moment he suddenly, in the prime of his life, dropped dead of a heart attack.

If the image of pounding away on a treadmill, sweating and gasping for an hour at a time, only to die of a heart attack anyway, doesn’t leave you thrilled about exercise, I have good news for you. In spite of the government’s recent step in the wrong direction, evidence is mounting for a different approach: Shorter is better.

No matter how much they exercise, most people can’t seem to keep the fat off. In fact, they often actually signal their bodies to make more fat. Fat is the only source of energy that can fuel activity for more than 15 minutes. So the more often you engage in long aerobic sessions, the more adept your body becomes at preserving fat — even if it has to sacrifice energy-expensive tissues like your muscles to fuel your heart and lungs. In addition to that, some of the most important changes from exercise occur after, not during, the session. After any exercise, your body seeks to better prepare for the next time you call upon it to repeat that activity. So, by utilizing fat during long-duration exercise, you are, in effect, telling your body that it needed that fat — and that signals it to make more fat at the next opportunity.

In a recent study, Colorado State University researchers measured how long fat continues to be burned after brief periods of exercise. Participants exercised for two minutes and then rested for one minute. They continued that cycle for 20 minutes. The researchers found that participants continued to burn fat at a high rate 16 hours after the exercise. At rest, their fat oxidation was up by 62%.

This is because during exercise you “key in program changes” that affect your metabolism over the next several days. And that explains my observation that long-distance runners are thin but not very lean. In fact, endurance athletes typically have body-fat percentages ranging from 10% to 20%, which is higher than the fat percentages of most other athletes. Body builders, for instance, who do very short bursts of intense exercise, are quite lean — with 4% to 8 % body-fat.

I was surprised when I first observed these measurements in my clinic, but it has now come to make perfect sense.

If you want to lose fat and maintain the capacity of your heart, lungs, and muscles, exercise in short bursts. When you do that, your body mainly uses energy from carbohydrates stored in muscle rather than fat. Carbs are capable of burning energy at a much higher rate. As a result, you will then burn much more fat for energy during the recovery period as you replenish the carbs. By doing this repeatedly, you teach your body that, for fast access, it needs to store more energy in muscle. At the same time, you teach it that storing energy as fat is inefficient because you never exercise long enough to make good use of it.

(Ed. Note: Dr. Al Sears is the editor of Health Confidential for Men, a publication devoted to men’s health.)

Al Sears has written six books and more than 500 articles in the fields of alternative medicine, anti-aging and nutritional supplementation. He enjoys a worldwide readership of millions spread over 123 countries and has appeared on more than 50 national radio programs, ABC News, CNN and ESPN. In The Doctor’s Heart Cure, Dr. Sears exposes the real causes of the modern epidemic of heart disease with practical how-to advice for building real heart strength and resistance to disease without drugs. It is available in nine languages and remains a bestseller three years after its publication. His 12 Secrets to Virility sheds light on the huge environmental and nutritional problems with virility in our modern world and gives men a step-by step guide for maintaining health, strength and masculinity as they age. It became a bestseller during its first month of release. His latest book Your Best Health Under the Sun addresses the myths of the sun’s dangers and gives readers an action plan for restoring this important natural resource for better mood, strength, energy and nutritional health.Dr. Sears is board certified as a Clinical Nutrition Specialist and was appointed to the international panel of experts at Health Sciences Institute, (HSI) a worldwide information service for alternative nutritional therapies. A master gardener and herbalist, Dr. Sears maintains an herbal apothecary of more than 250 organic herbs used for research, education and treatments. Dr. Sears is the founder and director of The Wellness Research Foundation, conducting original research evaluating natural alternatives to pharmaceutical therapies.Dr. Sears is a member of the American Academy of Anti-Aging Medicine and is board certified in Anti-Aging Medicine. As a pioneer in this new field of medicine, he is an avid researcher and sought-after lecturer to thousands of doctors and health enthusiasts. He is a member of the American College of Sports Medicine and the National Youth Sports Coaches Association. As well as being a sports and fitness coach and a lifelong advocate of exercise programs, Dr. Sears is an ACE-certified fitness trainer. He maintains his integrative clinic and research center in Florida, where he has developed novel exercise and nutritional systems transforming the lives of more than 20,000 patients.