As millions of Americans flock to the gym armed with New Year's resolutions to get in shape, medical experts are offering an additional reason to exercise: Regular workouts may help fight off colds and flu, reduce the risk of certain cancers and chronic diseases and slow the process of aging.

    Physical activity has long been known to bestow such benefits as helping to maintain a healthy weight and reduce stress, not to mention tightening those abs. Now, a growing body of research is showing that regular exercise -- as simple as a brisk 30- to 45-minute walk five times a week -- can boost the body's immune system, increasing the circulation of natural killer cells that fight off viruses and bacteria. And exercise has been shown to improve the body's response to the influenza vaccine, making it more effective at keeping the virus at bay.

    'No pill or nutritional supplement has the power of near-daily moderate activity in lowering the number of sick days people take,' says David Nieman, director of Appalachian State University's Human Performance Lab in Kannapolis, N.C. Dr. Nieman has conducted several randomized controlled studies showing that people who walked briskly for 45 minutes, five days a week over 12 to 15 weeks had fewer and less severe upper respiratory tract infections, such as colds and flu. These subjects reduced their number of sick days 25% to 50% compared with sedentary control subjects, he says.

    Medical experts say inactivity poses as great a health risk as smoking, contributing to heart disease, diabetes, hypertension, cancer, depression, arthritis and osteoporosis. The Centers for Disease Control and Prevention says 36% of U.S. adults didn't engage in any leisure-time physical activity in 2008.

    Even lean men and women who are inactive are at higher risk of death and disease. So while reducing obesity is an important goal, 'the better message would be to get everyone to walk 30 minutes a day' says Robert Sallis, co-director of sports medicine at Fontana Medical Center, a Southern California facility owned by managed-care giant Kaiser Permanente. 'We need to refocus the national message on physical activity, which can have a bigger impact on health than losing weight.'

    Regular exercise has been shown to combat the ongoing damage done to cells, tissues and organs that underlies many chronic conditions. Indeed, studies have found that exercise can lower blood pressure, reduce bad cholesterol, and cut the incidence of Type 2 diabetes.

    Building on that earlier research, scientific studies are now suggesting that exercise-induced changes in the body's immune system may protect against some forms of cancer.

    Researchers are also investigating whether exercise can influence aging in the body. In particular, they are looking at whether exercise lengthens telomeres, the strands of DNA at the tips of chromosomes. When telomeres get too short, cells no longer can divide and they become inactive, a process associated with aging, cancer and a higher risk of death.

    In a study published in November in Circulation, the medical journal of the American Heart Association, German researchers compared two groups of professional athletes （32 of whom were in their early 20s, and 25 who were middle-aged） with two groups （26 young and 21 middle-aged） who were healthy nonsmokers, but not regular exercisers. The athletes had significantly less erosion in telomeres than their more sedentary counterparts. The study concluded that physical activity has an anti-aging effect at the cellular level, suggesting exercise could prevent aging of the cardiovascular system.

    The guidelines, developed by the Department of Health and Human Services and available online at health.gov/paguidelines, recommend that adults get at least two hours and 30 minutes weekly of moderate-intensity aerobic physical activity, or one hour and 15 minutes a week of vigorous-intensity aerobic exercise, or an equivalent combination of both. The guidelines also say that additional health benefits can be had from as much as doubling the minimum recommendation for aerobic exercise. Also recommended: muscle-strengthening activities two or more days per week, which protects against a decline in bone mass, especially that experienced by post-menopausal women.

    在數百萬美國人懷著健美身材的新年決心涌入健身房之際，醫學專家找到了運動的又一個理由：定期健身可以幫助你抵抗感冒和流感、降低患某些癌癥和慢性病的風險、并延緩衰老。

    很早以前，人們就知道體育活動有各種好處，比如幫助保持健康的體重、緩解壓力，更不用說可以讓你擁有結實的腹肌。如今，越來越多的研究表明，定期運動──簡單如每天快走30到45分鐘，每周堅持五天──可以增強人體免疫系統，促進抗擊病毒和細菌的自然殺傷細胞的循環。此外，運動還可以增強人體對流感疫苗的反應，提高疫苗抗病毒的有效性。

    北卡羅來納州阿巴拉契亞州立大學（Appalachian State University）人類表現實驗室主任尼曼（David Nieman）說，在減少患病天數方面，沒有哪一種藥物或營養補充品比得上幾乎每日堅持的適度運動。尼曼進行了數項隨機對照研究，結果發現每天快走45分鐘、每周堅持五天的人，在12至15周后，他們患感冒和流感等上呼吸道感染的幾率要小，嚴重程度要低。他說，與活動較少的對照組相比，這些人患病天數少25%-50%.

    醫學專家說，不活動帶來的健康風險與吸煙類似，會導致心臟病、糖尿病、高血壓、癌癥、抑郁、關節炎和骨質疏松。美國疾病預防控制中心（Centers for Disease Control and Prevention）說，2008年，有36%的成年美國人在業余時間不參加任何體育活動。

    甚至是體型瘦削的人，如果不運動，死亡和患病的風險也會增大。有鑒于此，南加州芳塔娜醫學中心（Fontana Medical Center）運動醫學聯席主任薩利斯（Robert Sallis）說，在將減少肥胖作為重要目標的同時，更有益的信息或許是讓人人每天都走上30分鐘。該中心隸屬于管理式醫療護理巨頭凱薩醫療機構（Kaiser Permanente）.他說，我們需要將全國的注意力轉移到體育運動上來，相比減肥，體育運動對健康的影響要更大。

    研究發現，有規律的運動可以抗擊細胞、組織和器官正在遭受的損傷，而這些是很多慢性病的根源。實際上，研究發現，運動可以降低血壓、減少壞膽固醇、減少II型糖尿病的發病。

    在上述前期研究的基礎上，現在的科學研究顯示，運動給人體免疫系統帶來的改變可以保護人們遠離某些種類的癌癥。

    研究人員也在研究，運動是否可以影響人體的衰老過程。尤其值得一提的是，他們正在研究運動是否可以延長端粒──即染色體末端DNA片斷──的長度。當端粒長度過短時，細胞就不再分裂、失去活性，這個過程會造成衰老、癌癥和高死亡率。

    11月份《循環》（Circulation）雜志上發表的一項研究中，德國研究人員將兩組職業運動員（一組是32名20多歲的女運動員，一組是25名中年女運動員）和兩組不吸煙、但也沒有定期運動的健康女性（一組是26名年輕人，一組是21名中年人）進行了對比。運動員組的端粒受損情況明顯好于運動較少的對照組。研究得出的結論是，體育運動在細胞水平上具有抗衰老的效果，顯示出運動可以防止心血管系統的衰老。《循環》是美國心臟協會（American Heart Association）出版的醫學期刊。

    美國衛生與公共服務部（Department of Health and Human Services）提出的指導（可登錄health.gov/paguidelines參看全文）建議，成人每周至少進行兩個半小時的中度有氧運動，或每周1小時15分鐘的高強度有氧運動，或二者結合起來達到相應的運動量。指導性文件還說，將建議的有氧運動量增加一倍，還可以獲得更多的好處。它還建議：每周進行兩次及以上的肌肉增強運動，可以預防骨密度的降低，對更年期之后的女性來說尤其如此。