Myriad components factor into this gradual drop-off in verve, including changes in hormones, sleep, nutrition, heart and lungs. Some of these changes are nonnegotiable. But there's much that guys can do to restore some of their lost vigor.
* Hormones: As a man rounds the bend into middle age, testosterone production begins to decrease by about 1.5% a year. (The amount of testosterone a man has at age 45 to 50 could easily be 20% less than what it was in his 20s and 30s.) Growth hormone also begins to drop. But while both can affect stamina, a more direct contributor to lethargy could be thyroid hormone, which regulates how fast cells burn food to produce energy.
Up to 5% of 45-year-old men may have sluggish thyroid glands, a condition treatable with hormone supplements. "It often gets mistaken for normal aging," says Dr. S. Mitchell Harman, director and president of the Kronos Longevity Research Institute in Phoenix.
* The heart: Middle age brings about a 1%-per-year decline in the ability of the heart to pump blood as well as the amount of blood it can pump. Just as skeletal muscles do, the heart muscle weakens as levels of testosterone fall, says Dr. Daniel Vigil, director of sports medicine at Kaiser Permanente Los Angeles Medical Center, and an American Heart Assn. spokesman.
A slower heart rate means the body finds it more difficult to move and function. Carrying around excess body fat and having arteries clogged with plaque tax the heart even further, contributing to less vim and vigor.
Cardiovascular or aerobic exercise, such as running, cycling or swimming, can help the heart stay fit. Studies have shown that the heart responds best to repeated stimulation, so Vigil and others recommend engaging in aerobic activity for at least 20 to 30 minutes three to five times a week.
* The lungs: The lungs lose about 30 milliliters to 35 milliliters of function a year, says Dr. Jeffrey Kupperman, a Santa Barbara-based pulmonary specialist and spokesman for the American Lung Assn. of California. That means that at age 50, a man could have about 80% of the lung function he had at age 25. He'll feel less energetic because less oxygen is getting to his body.
This decline is due to a gradual loss of lung elasticity, caused partly by a decrease in a protein, elastin. "The lungs don't get in the air in and out as well as they do you when you're younger," Kupperman says.
Lung elasticity can't be improved, but the muscles around them can, compensating by helping to expand and contract the lungs, like a bellows. Exercising the diaphragm is most key, but also important are shoulder and neck muscles, as well as ones surrounding the rib cage, Kupperman says.
Drills include pushing the diaphragm muscle out while inhaling, and breathing through pursed lips. (Consult with a physical therapist specializing in pulmonary rehab or a knowledgeable personal trainer to learn proper form before performing them.)
* Sleep: Not getting enough sleep is a badge of honor among some men who believe ultra busyness translates into success. And, as men age, getting enough sleep becomes harder.
Around the age of 40, sleep patterns begin to change, even though the standard eight hours is still needed, says Mark Rosekind, president of Alertness Solutions, a Cupertino-based sleep consulting firm, and a former sleep researcher with NASA and Stanford University. People begin to have less deep sleep, where rest and rejuvenation take place. Studies suggest that a natural drop in growth hormone may be a cause. They also suggest that sleep deprivation renders blood sugar metabolism less efficient -- another source of tiredness.
Exercise may help, because it triggers the body's need to repair and rejuvenate, bringing on deeper sleep. But don't work out three hours or less before bedtime or you'll be too revved up for shut-eye. Fostering routines -- sleeping and waking at the same time -- can help too.
And if you snore or have excessive daytime sleepiness, consider a test for obstructive sleep apnea. About 4% of men age 35 and older have it.
* Nutrition: Starting at about age 50, "there are some nutrients that as we get old we're not able to absorb as efficiently" and that can affect energy levels, says Jennifer Sacheck, assistant professor of nutrition at Tufts University's Friedman School of Nutrition.
One is vitamin B12, important in red blood cell formation (red meat, fish and poultry are good sources). Another is vitamin B6, also important in metabolism (found in meats, whole grains, bananas and spinach). Zinc is a third (the best sources are meat and oysters).
Two chemicals found naturally in the body -- acetyl-L-carnitine and alpha-lipoic acid -- have been shown, in rats, to help boost energy. (The animals were more active and did better on memory tests.) But these supplements can cause gastrointestinal problems, and human studies aren't yet complete.
As men lose muscle, they also lose some ability to regulate blood sugar, which can cause tiredness, Sacheck says. Muscles have receptors that take up blood sugar to replace what's been used during exercise. With less muscle, sugar regulation can become less efficient.
So spurn doughnuts and cookies with their refined sugar and starches that cause crashes and peaks in blood glucose. Embrace complex carbohydrates (found in fruits, vegetables and whole grains) that will release glucose more steadily into the blood. And eat more small meals instead of a few large ones: That'll also help stabilize blood sugar.