The clock is ticking for each of us, and with each tick we age. For those who manage to avoid major medical problems, the aging is slow and gradual, but the specific changes that occur do add up. Aging begins as early as the third decade of life. For example, after age 25, the average man's maximum heart rate declines by approximately one beat per minute per year, and his heart's peak capacity to pump blood drops by 5 to 10 percent per decade. That's a reason why a healthy 25-year-old can pump 2 ½ quarts of blood a minute, but a 65-year-old heart can't get above 1 ½ quarts, and an 80-year-old heart can pump only about one quart. In lay terms, this diminished aerobic capacity means fatigue and breathlessness with even modest daily activities.

As we reach the middle age years, our blood vessels begin to lose elasticity and blood pressure creeps up. The blood itself also changes, becoming more viscous (thicker and stickier) and harder to pump through the body.

Our body composition changes as we age. By middle age, we've put on more than a few pounds (the average American adds 3-4 pounds a year). We also lose muscle mass in our 40s and that process continues each decade. The extra body fat contributes to a rise in LDL (bad) cholesterol and fall in HDL (good) cholesterol. It also helps explain why blood sugar levels rise by about 6 points per decade, making type 2 diabetes a possibility for seniors.

With the loss of muscle mass (in some as much as 50 percent) comes weakness and disability along with stiffer, tighter muscles. Both men and women lose bone calcium as they age, although the risk of osteoporosis is higher in women. Men also lose testosterone at the rate of about 1 percent per year after age 40. This often results in a gradual decline in libido and sexual vigor.

The nervous system changes as we age. Reflexes are slower, coordination is reduced, and memory lapses become more frequent. The average person gets less restful sleep in maturity than in youth. Not surprisingly then, spirits often sag as the body slows down.

If all of this information comes as grim news, then don't despair. Even though we can't stop the clock, each of us can slow its tick. Research shows that many of the changes attributed to aging are actually caused by disuse. In other words, while exercise may not be the fountain of youth, it's a good long drink of vitality. And this is not new information. Famed 18th-century Scottish physician wrote "Of all the causes which conspire to render the life of man short and miserable, none have greater influence than the want of proper exercise." And about the same time, the British poet John gay scribed, "Exercise thy lasting youth defends."

Let's fast forward to the present. A new study from Texas shows just how valuable exercise can be. In 1966, five healthy men volunteered to spend 3 weeks of their summer vacation resting in bed. At the end of the trial, they were tested and found to have higher resting heart rates, higher blood pressures, reduced heart pumping capacity, rises in body fat and declines in muscle strength. In short, the young men had developed the physical profiles of men twice their age.

The researchers then put the men on an 8-week exercise program, reversing all of the physical deterioration brought on by bed rest.

Thirty years later the subjects were revisited at the age of 50. All five were relatively healthy and none required long-term medication. However, the 30-year interval had not been kind. The men had gained an average of 50 pounds, doubled their body fat from 14 percent to 28 percent. Their cardiac function had been reduced, including rises in resting heart rate and blood pressure and a decline in maximum pumping capacity. Interestingly, their overall physical condition was better at age 50 than it was 30 years earlier after three weeks of bed rest.

As they did earlier, the researchers put the subjects on an exercise regimen, this time a gradual 6-month program of walking, jogging and cycling. At the end of the program, the men had lost 10 pounds and restored much of their resting heart rates, blood pressures and maximum pumping capacity to baseline levels. In short, slow but steady endurance training had carried the day.

Clearly, inactivity is a huge threat to our longevity and vitality. Exercise is the best way to protect the body's metabolism from the effects of age.

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