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Dangers of Prolonged Sitting

A study from The University of Toronto reviewed 47 studies and found that sitting for more than eight hours a day is associated with increased risk for death from heart attacks, diabetes, and cancers of the breast, colon, uterus, and ovary, even if a person exercises (Annals of Internal Medicine, 2015;162(2):123-132). The authors believe that people are harmed by spending more than four to five hours a day sitting, driving a car, using the computer or watching television. One study cited in the review showed that people who sit less than eight hours a day have a 14 percent lower risk of being hospitalized. Other studies also associate prolonged time spent sitting with increased risk for weight gain, disease and premature death (Br J Sports Med, 2015 Jan;49(2):95-9 and J Am Coll Cardiol, 2013 Jun 11;61(23):2346-54).

The study found that even people who exercised were harmed by prolonged sitting time. The authors stated that "Exercising one hour per day should not give us the peace of mind to remain seated for the remaining 23." However the authors did find that exercise is still better than no exercise. Those who exercised regularly but still spent a large amount of time sitting and lying down, were 30% less likely to die of any cause than those who get little to no exercise.

Principal author David Alter recommends:
• While working at a desk, get up frequently
• While watching TV, stand up or exercise during the advertisements
• Reduce sitting
• Try to exercise every day.

How Sitting Can Harm Casual Exercisers and Non-Exercisers
A high rise in blood sugar causes sugar to stick to the outside surface of every cell in your body and cause significant damage (American J. of Clinical Nutrition, April, 2010 and Archives of Internal Medicine, May 2010). Resting muscles pull virtually no sugar from your bloodstream, and insulin is required for the little amounts of sugar the muscles use. Contracting muscles can draw large amounts of sugar from the bloodstream and don’t even need insulin to do so. People who do not move their muscles have much higher blood sugar levels than those who exercise vigorously, so they are at increased risk for weight gain, diabetes, heart attacks, strokes, cancers and premature death.

Vigorous Exercisers Were Not Studied
Virtually none of the people in this study were doing enough intense exercise to be at a high level of fitness, let alone to be able to compete in sports. The authors defined high levels of physical activity as "at least 20 minutes a day of moderately vigorous exercise or at least seven hours a week of moderately vigorous exercise." This meager amount of exercise is far less than any serious exerciser would do, and is certainly too little to compete successfully in any sport.

Why Prolonged Rest Does Not Harm Vigorous Exercisers
Exercise helps muscles to control blood sugar levels by drawing large amounts of sugar from the bloodstream. The more vigorously you exercise,
• the more effectively your muscles remove sugar from the bloodstream without needing insulin, and
• the longer your muscles continue to draw sugar from your bloodstream without needing insulin after you finish exercising (Am J Clin Nutr, 2008(July);88(1):51-57).
After a vigorous exercise session, your muscles can continue drawing sugar without needing insulin for up to 17 hours (J Appl Physiol, 2005;99: 338-343 & 2005;8750-7587). Preventing a high rise in blood sugar helps to prevent diabetes heart attacks, strokes, impotence, dementia and premature death.

Why Athletes Need Rest
People who exercise intensely need to spend a lot of time sitting or lying down because muscles are damaged by intense exercise. If vigorous exercisers and athletes do not rest, their muscles do not heal from their previous intense workouts, they will be unable to train intensely and they are at high risk for injuries.

All successful athletes train by stressing and recovering. They take a workout that is intense enough to damage their muscles, and then they need to rest their muscles to allow them to recover for their next workout. Without the muscle damage caused by intense exercise, muscles do not grow and become stronger. If they don’t spend enough time resting their muscles after an intense workout, their muscles will take longer to heal, which will delay recovery. It will take longer for them to be able to take their next workout, or worse, they will suffer an injury.

Most serious athletes train twice a day or even more. They take an intense workout that damages muscles and within a few hours they develop DOMS (delayed onset muscle soreness). Then they should do easier workouts until the muscle heal and the soreness disappears. Most athletes allow 48 hours between intense workouts. Those who train once a day follow each intense workout with one easy workout. Those who train twice a day follow an intense workout with three easy ones.

How Bicycle Racers Recover during the Tour de France
In multi-day races such as the Tour de France, bicycle racers race flat out over more than 100 miles most days for three weeks. After they finish each day's race, they immediately eat large amounts of food, drink a lot of fluid and lie down until their next ride. They try to stay still instead of moving and sleep as much as possible.

Athletes and Vigorous Exercisers Can Ignore This Study
Vigorous exercisers and competitive athletes in sports that require prolonged, intense exercise should rest their muscles after intense workouts to help them recover for their next workout or competition. For them, prolonged sitting or lying down is not likely to increase risk for diabetes, heart attacks or any other disease. I think that the authors should have recommended that all sedentary people and casual exercisers should exercise more intensely as well as more frequently.

March 29th, 2015
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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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