The Department of Health and Human Services (HHS) late this year released its new Physical Activity Guidelines for Americans, calling for adults between the ages of 18 and 64 to exercise moderately (such as brisk walking or water aerobics) for at least two hours and 30 minutes or vigorously (running, swimming, or cycling 10 mph or faster) for at least an hour and 15 minutes weekly.

    The longer, harder and more often you exercise, the greater the health benefits, including reducing the risk of diseases such as cancer and diabetes, according to the recommendations, which were based on a decade of scientific research.

    Studies have shown that people who engage in the amount of exercise recommended by the feds live an average of three to seven years longer than couch potatoes, according to William Haskell, a medical professor at Stanford University who chaired the HHS advisory committee. But how exactly does exercise accomplish this? And what about claims by naysayers that exercise not only isn't healthy but may actually be bad for you? Is there any truth to them?

    Good for the heart and blood vessels
    In the past decade or so, various studies involving thousands of participants have shown that workouts lower the risk of heart disease. "Exercise has a favorable effect on virtually all risk factors of cardiovascular disease," says Jonathan Meyers, a health research scientist at the Palo Alto Veterans Affairs Health System in California. The reason, he says: when a person exercises, the heart muscle contracts forcefully and frequently, increasing blood flow through the arteries. This leads to subtle changes in the autonomic nervous system, which controls the contraction and relaxation of these vessels. This fine-tuning leads to a lower resting heart rate (fewer beats to pump blood through the body), lower blood pressure and a more variable heart rate, all factors that lower the risk of developing cardiovascular disease, he says.

    Meyers says that exercise also limits inflammation associated with heart trouble, such as arteriosclerosis or hardening of the arteries around the heart, which may lead to heart attacks. Many recent studies have focused on C-reactive protein, a marker of inflammation. Meyers says that research showed that sedentary folks who embarked on three- to six-month exercise programs, on average, experienced a 30 percent dip in their C-reactive protein levels – about the same drop as someone given a statin (a cholesterol and inflammation-lowering drug). In other words, in many people, exercise might be as effective as an Rx in tamping down inflammation, one of the key risk factors for cardiovascular disease.

    Exercise also boosts cardiovascular health by decreasing the amount of plasma triglycerides—fatty molecules in the blood that are associated with plaque build-up in the arteries— notes Haskell. What's more, he adds, physical activity helps reduce the particle size of low-density lipoprotein (LDL) or so-called bad cholesterol in the blood, and increase amounts of high-density lipoprotein (HDL), aka good cholesterol, which translates to less artery clogging.

    But exercise may not have the same effect on every person's cardiovascular system, notes Arthur Leon, chief cardiologist at the University of Minnesota's Heart Disease Prevention Clinic in Minneapolis. "On average, there is a response but there is great variability, and that variability runs in families," he says. Take, for example, HDL cholesterol. Most broad studies show physical exercise leads to up to a 5 percent increase in HDL levels, but a closer examination shows that the percentages vary from zero to 25 percent, depending on the study subject, he says, noting that only about half of the population seem to experience HDL increases as a result of exercise.

    Less cancer 
    Several studies (including the ongoing federal National Health and Nutrition Examination Survey) following thousands subjects for several years, show that regular exercise lowers the risk for certain cancers, particularly breast and colon cancer, says Demetrius Albanes, a researcher at the National Cancer Institute in Bethesda, Md. Scientists have yet to pinpoint the mechanisms involved but have come up with several plausible explanations.

    "Physical activity beneficially affects body weight," says Albanes, noting that leaner people have lower circulating levels of insulin, a hormone produced by the pancreas that helps cells absorb glucose, their primary energy source. Obese and overweight people, are more likely to develop insulin resistance, a condition in which the cells no longer respond to the hormone and absorb glucose. When this happens, the pancreas produces greater amounts to compensate, flooding the bloodstream with insulin; high levels of insulin in the blood have been linked to [some types of] cancer. "Insulin is essentially a growth hormone," Albanes says. "Insulin could create new tumors by increasing rates of cell division, or it could just make small tumors grow."

    Albanes says that exercise may also ward off cancer and other diseases because it appears to beef up the body's immune system. Exercise may also help reduce levels of the female hormones estrogen and progesterone in the blood, potentially also lowering the risk of developing breast and uterine cancers linked to high levels of those hormones.

    Despite the apparent link between physical exercise and lower odds of cancer, Albanes acknowledges that there could be other factors at work. "[Because] most of these studies are not controlled trials, it could be some other lifestyle factor [that helps explain the lower cancer risk], " he says, noting that people who exercise may also eat healthier diets.

    公共健康与社会福利部（HHS）于年末发布了新的“美国人体育锻炼指南”，呼吁年龄18至64岁的成年人每周至少参加适度运动（如漫跑或水上有氧健身操）2小时30分钟或剧烈运动（跑步、游泳或以每小时10英里以上的速度骑车）1小时15分钟。

    遵照这一经十余年科学研究得出的建议，你运动的时间越长、强度越大、越频繁，你的健康受益就越大，包括减少患诸如癌症和糖尿病等疾病的风险。

    据HHS咨询委员会主席、斯坦福大学医学教授William Haskell的研究显示，按照联邦政府所推荐的运动量进行锻炼的人比那些成天呆在家里的人平均多活3至7年。但运动又是如何做到这一点的呢？还有，那些反对者所声称的运动不仅对健康无益反而有害的说法又是怎么回事呢?他们（所说的）有可信之处吗?

    有益于心脏和血管

    在过去数十年中,涉及数千参与者的各种研究显示，户外运动能够降低患心脏病的风险。“运动的确对心血管疾病的所有危险因素有着有益效果”，位于加利福尼亚帕洛阿而托退伍军人健康事务系统的健康研究科学家Jonathan Meyers如是说。他说，其原因是：当一个人运动时，心肌强有力且频繁的收缩，增加了动脉内的血流量。这使得控制血管收缩和舒张的自主神经系统发生了微妙的变化。他说，这一微调促成了一个较低的静息心率（向全身泵血的次数较少），较低的血压和变量更大的心率，所有这些因素都降低了诱发心血管病的风险。

    Meyers说运动还能抑制心脏病相关的炎症，例如可以导致心脏病发作的动脉硬化。近期的许多研究均专注于C-反应蛋白，一种炎症的标志。Meyers说，研究表明整天坐着工作的人们参加3到6个月的运动，其C-反应蛋白的平均水平为30％—大概和用过斯他汀（一种降胆固醇和消炎的药物）的人的水平相当。换句话说，对于很多人来讲，运动或许和降低炎症这一心血管病关键诱因的治疗方案同样见效。

    Haskell强调，运动还能通过降低血浆中甘油三酸酯—动脉中形成的与血小板相关的多脂分子来改善心血管健康。除此之外，他补充道，体育锻炼还有助于减小血液中低密脂蛋白（LDL）或所谓的坏胆固醇的颗粒大小，并提高能够减少动脉阻塞的高密脂蛋白（HDL）亦称作好胆固醇的数量。

    但运动并非对每个人的心血管系统都能起到同样的作用，Arthur Leon，位于明尼阿波利斯的明尼苏达州心脏病防治大学高级心脏病专家这样评论。“大体上是见效的，但差异性较大，并且家庭内亦存在差异，”他说。就拿HDL胆固醇来说吧，广泛的研究表明，体育锻炼最多能够使HDL的水平提高5％，但进一步检查显示，该比率从0％到25％不等，这取决于研究项目（的不同），他说，似乎只有大约一半人是因为运动而令HDL增长的。

    癌症减少

    多年来涉及数千科目的多项研究（包括正在进行的联邦国民健康与营养问卷调查）显示，定期运动能够降低某些癌症的患病风险，特别是乳腺癌和结肠癌，Demetrius Albanes，马里兰州贝什斯达国家癌症协会研究员这样表示。科学家们也不得不描述其机理，但许多说辞似乎并非特别另人信服。

    “体育锻炼有益于（控制）体重，”Albanes说道，瘦人的胰岛素循环水平较低，（胰岛素是）一种由胰腺分泌、帮助细胞吸收其原始能量源—葡萄糖的荷尔蒙。肥胖和超重者更容易产生胰岛素抵抗现象，在这一条件下细胞对荷尔蒙没有反应而无法吸收葡萄糖。这时，胰腺就会分泌大量的（胰岛素）来补偿，使血流中的胰岛素大大超标；血液中胰岛素的水平偏高（已证实）与［某些］癌症相关。“胰岛素本质上是一种生长激素，”Albanes说，“胰岛素能够通过速度递增的细胞分裂而形成肿瘤，或者只是让小肿瘤长大。

    Albanes说，运动或许还能抑制癌症和其它疾病（的发作），因为它能增强人体的免疫系统。运动还能降低血液中雌激素和黄体酮的水平，也就从根本上降低了与这些激素水平过高相关的乳腺癌和子宫癌的患病风险。

    尽管体育锻炼与较低的癌症（患病）几率有着明显的关系，Albanes承认，这其中一定有其它因素在起作用。“［因为］大多数研究是不可控的，应该有一些生活方式的因素［这帮助解释了较低的癌症患病风险］，”他说，另外，运动的人可能饮食更健康一些。