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.

    公共健康與社會福利部（HHS）于年末發布了新的“美國人體育鍛煉指南”，呼吁年齡18至64歲的成年人每周至少參加適度運動（如漫跑或水上有氧健身操）2小時30分鐘或劇烈運動（跑步、游泳或以每小時10英里以上的速度騎車）1小時15分鐘。

    遵照這一經十余年科學研究得出的建議，你運動的時間越長、強度越大、越頻繁，你的健康受益就越大，包括減少患諸如癌癥和糖尿病等疾病的風險。

    據HHS咨詢委員會主席、斯坦福大學醫學教授William Haskell的研究顯示，按照聯邦政府所推薦的運動量進行鍛煉的人比那些成天呆在家里的人平均多活3至7年。但運動又是如何做到這一點的呢？還有，那些反對者所聲稱的運動不僅對健康無益反而有害的說法又是怎么回事呢?他們（所說的）有可信之處嗎?

    有益于心臟和血管

    在過去數十年中,涉及數千參與者的各種研究顯示，戶外運動能夠降低患心臟病的風險。“運動的確對心血管疾病的所有危險因素有著有益效果”，位于加利福尼亞帕洛阿而托退伍軍人健康事務系統的健康研究科學家Jonathan Meyers如是說。他說，其原因是：當一個人運動時，心肌強有力且頻繁的收縮，增加了動脈內的血流量。這使得控制血管收縮和舒張的自主神經系統發生了微妙的變化。他說，這一微調促成了一個較低的靜息心率（向全身泵血的次數較少），較低的血壓和變量更大的心率，所有這些因素都降低了誘發心血管病的風險。

    Meyers說運動還能抑制心臟病相關的炎癥，例如可以導致心臟病發作的動脈硬化。近期的許多研究均專注于C-反應蛋白，一種炎癥的標志。Meyers說，研究表明整天坐著工作的人們參加3到6個月的運動，其C-反應蛋白的平均水平為30％—大概和用過斯他汀（一種降膽固醇和消炎的藥物）的人的水平相當。換句話說，對于很多人來講，運動或許和降低炎癥這一心血管病關鍵誘因的治療方案同樣見效。

    Haskell強調，運動還能通過降低血漿中甘油三酸酯—動脈中形成的與血小板相關的多脂分子來改善心血管健康。除此之外，他補充道，體育鍛煉還有助于減小血液中低密脂蛋白（LDL）或所謂的壞膽固醇的顆粒大小，并提高能夠減少動脈阻塞的高密脂蛋白（HDL）亦稱作好膽固醇的數量。

    但運動并非對每個人的心血管系統都能起到同樣的作用，Arthur Leon，位于明尼阿波利斯的明尼蘇達州心臟病防治大學高級心臟病專家這樣評論。“大體上是見效的，但差異性較大，并且家庭內亦存在差異，”他說。就拿HDL膽固醇來說吧，廣泛的研究表明，體育鍛煉最多能夠使HDL的水平提高5％，但進一步檢查顯示，該比率從0％到25％不等，這取決于研究項目（的不同），他說，似乎只有大約一半人是因為運動而令HDL增長的。

    癌癥減少

    多年來涉及數千科目的多項研究（包括正在進行的聯邦國民健康與營養問卷調查）顯示，定期運動能夠降低某些癌癥的患病風險，特別是乳腺癌和結腸癌，Demetrius Albanes，馬里蘭州貝什斯達國家癌癥協會研究員這樣表示。科學家們也不得不描述其機理，但許多說辭似乎并非特別另人信服。

    “體育鍛煉有益于（控制）體重，”Albanes說道，瘦人的胰島素循環水平較低，（胰島素是）一種由胰腺分泌、幫助細胞吸收其原始能量源—葡萄糖的荷爾蒙。肥胖和超重者更容易產生胰島素抵抗現象，在這一條件下細胞對荷爾蒙沒有反應而無法吸收葡萄糖。這時，胰腺就會分泌大量的（胰島素）來補償，使血流中的胰島素大大超標；血液中胰島素的水平偏高（已證實）與［某些］癌癥相關。“胰島素本質上是一種生長激素，”Albanes說，“胰島素能夠通過速度遞增的細胞分裂而形成腫瘤，或者只是讓小腫瘤長大。

    Albanes說，運動或許還能抑制癌癥和其它疾病（的發作），因為它能增強人體的免疫系統。運動還能降低血液中雌激素和黃體酮的水平，也就從根本上降低了與這些激素水平過高相關的乳腺癌和子宮癌的患病風險。

    盡管體育鍛煉與較低的癌癥（患病）幾率有著明顯的關系，Albanes承認，這其中一定有其它因素在起作用。“［因為］大多數研究是不可控的，應該有一些生活方式的因素［這幫助解釋了較低的癌癥患病風險］，”他說，另外，運動的人可能飲食更健康一些。