THE EVIDENCE FOR EXERCISE
Does Physical Activity Increase or Decrease the Risk of Bone and Joint Disease?
Most college students don't worry much about fall-related fractures or chronic bone-related illnesses, such as osteoporosis (loss of bone mass) or osteoarthritis (degeneration of the cartilage inside joints). Even so, bone health should be a concern throughout life. This is because girls amass 85% of their adult bone mass by age 18, and boys build the same amount by age 20, but most people begin losing bone mass around age 30. For many, poor diet and lack of exercise accelerate bone loss. According to the National Osteoporosis Foundation, 10 million Americans have osteoporosis. Meantime, 34 million Americans are at risk of the disease because of low bone mass. Overall, osteoporosis is a health threat for about 55% of Americans age 50 and older.
Getting enough nutrients is important for bone health (see Chapter 8), but there is mounting evidence that exercise can also help preserve or improve bone health. For example, several studies have shown an inverse relationship between physical activity and the risk for bone fractures. That is, the more you exercise, the less likely you are to suffer fractures, especially of the upper leg and hip. Research has not determined conclusively how much exercise is required to reduce fracture risk, but people who walk at least four hours per week and devote at least one hour per week to other forms of physical activity appear to reduce that risk. These findings seem to be consistent for women and men, but because some studies disagree on this point, further research is needed.
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One way that exercise helps both men and women is by increasing the mineral density of bones, or at least by decreasing the loss of mineral density over time. Several one-year-long studies found that exercise can increase bone mineral density by 1-2% per year, which is significant especially considering that the same amount of bone mineral density can be lost every one-four years in older persons. Currently, the American College of Sports Medicine recommends that adults perform weight-bearing physical activities (such as walking) three-five days per week and strength training exercises two-three days per week to increase bone mass or avoid loss of mineral density. They also recommend that adults practice neuromotor exercise training exercises, such as yoga or tai chi, to prevent falls and bone fractures. Exercise is particularly important in lactat-ing (breastfeeding) women for preventing bone loss.
The evidence is less conclusive for the effect of exercise on osteoarthritis but still fairly positive. All experts agree that regular, moderate-intensity exercise is necessary for joint health. However, they also warn that vigorous or too-frequent exercise may contribute to joint damage and encourage the onset of osteoarthritis. For this reason, experts try to strike a balance in their exercise recommendations, especially for those with a family history of osteoarthritis.
Research seems to support this cautious approach. Some studies have found that regular physical activity (as recommended for general health) at least does not increase osteoarthritis risk. Other studies show that moderate activity may provide some protection against the disease, but this evidence is limited.
A few studies also reveal that the type of exercise you do may increase your risk. For example, competitive or strenuous sports such as ballet, orienteering, football, basketball, soccer, and tennis have been associated with the disease, whereas sports such as cross-country skiing, running, swimming, biking, and walking have not.
The bottom line is that the earlier in life you become physically active, the greater your protection against bone loss and bone-related diseases. However, if you have a family history of osteoporosis or osteoarthritis, or if you have already developed symptoms of one of these ailments, be sure to talk to your physician before beginning an exercise program. Sources: American College of Sports Medicine. 2013. ACSM's Guidelines for Exercise Testing and Prescription, 9th ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins Health; Kemmler, W., and S. Stengel. 2011. Exercise and osteoporosis-related fractures: Perspectives and recommendations of the sports and exercise scientist. Physician Sportsmedicine 39(1): 142-157; Lovelady, et al. 2009. Effect of exercise training on loss of bone mineral density during lactation. Medicine and Science in Sports and Exercise 41(10): 1902-1907; American College of Sports Medicine. 2004. ACSM position stand: Physical activity and bone health. Medicine and Science in Sports and Exercise 36(11): 1985-1996; National Osteoporosis Foundation. 2011. Bone Basics: