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.

    在数百万美国人怀着健美身材的新年决心涌入健身房之际，医学专家找到了运动的又一个理由：定期健身可以帮助你抵抗感冒和流感、降低患某些癌症和慢性病的风险、并延缓衰老。

    很早以前，人们就知道体育活动有各种好处，比如帮助保持健康的体重、缓解压力，更不用说可以让你拥有结实的腹肌。如今，越来越多的研究表明，定期运动──简单如每天快走30到45分钟，每周坚持五天──可以增强人体免疫系统，促进抗击病毒和细菌的自然杀伤细胞的循环。此外，运动还可以增强人体对流感疫苗的反应，提高疫苗抗病毒的有效性。

    北卡罗来纳州阿巴拉契亚州立大学（Appalachian State University）人类表现实验室主任尼曼（David Nieman）说，在减少患病天数方面，没有哪一种药物或营养补充品比得上几乎每日坚持的适度运动。尼曼进行了数项随机对照研究，结果发现每天快走45分钟、每周坚持五天的人，在12至15周后，他们患感冒和流感等上呼吸道感染的几率要小，严重程度要低。他说，与活动较少的对照组相比，这些人患病天数少25%-50%.

    医学专家说，不活动带来的健康风险与吸烟类似，会导致心脏病、糖尿病、高血压、癌症、抑郁、关节炎和骨质疏松。美国疾病预防控制中心（Centers for Disease Control and Prevention）说，2008年，有36%的成年美国人在业余时间不参加任何体育活动。

    甚至是体型瘦削的人，如果不运动，死亡和患病的风险也会增大。有鉴于此，南加州芳塔娜医学中心（Fontana Medical Center）运动医学联席主任萨利斯（Robert Sallis）说，在将减少肥胖作为重要目标的同时，更有益的信息或许是让人人每天都走上30分钟。该中心隶属于管理式医疗护理巨头凯萨医疗机构（Kaiser Permanente）.他说，我们需要将全国的注意力转移到体育运动上来，相比减肥，体育运动对健康的影响要更大。

    研究发现，有规律的运动可以抗击细胞、组织和器官正在遭受的损伤，而这些是很多慢性病的根源。实际上，研究发现，运动可以降低血压、减少坏胆固醇、减少II型糖尿病的发病。

    在上述前期研究的基础上，现在的科学研究显示，运动给人体免疫系统带来的改变可以保护人们远离某些种类的癌症。

    研究人员也在研究，运动是否可以影响人体的衰老过程。尤其值得一提的是，他们正在研究运动是否可以延长端粒──即染色体末端DNA片断──的长度。当端粒长度过短时，细胞就不再分裂、失去活性，这个过程会造成衰老、癌症和高死亡率。

    11月份《循环》（Circulation）杂志上发表的一项研究中，德国研究人员将两组职业运动员（一组是32名20多岁的女运动员，一组是25名中年女运动员）和两组不吸烟、但也没有定期运动的健康女性（一组是26名年轻人，一组是21名中年人）进行了对比。运动员组的端粒受损情况明显好于运动较少的对照组。研究得出的结论是，体育运动在细胞水平上具有抗衰老的效果，显示出运动可以防止心血管系统的衰老。《循环》是美国心脏协会（American Heart Association）出版的医学期刊。

    美国卫生与公共服务部（Department of Health and Human Services）提出的指导（可登录health.gov/paguidelines参看全文）建议，成人每周至少进行两个半小时的中度有氧运动，或每周1小时15分钟的高强度有氧运动，或二者结合起来达到相应的运动量。指导性文件还说，将建议的有氧运动量增加一倍，还可以获得更多的好处。它还建议：每周进行两次及以上的肌肉增强运动，可以预防骨密度的降低，对更年期之后的女性来说尤其如此。