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FAITH LAPIDUS: This is the VOA Special English program SCIENCE IN THE NEWS. I'm Faith Lapidus.
BOB DOUGHTY: And I'm Bob Doughty. Today, we will tell about physical exercise. We will tell why exercise is important, and some of the popular ways to get in shape.
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FAITH LAPIDUS: Summer officially returns to the United States in less than two weeks. For many Americans, summer is a time to put on swim wear and spend time at the sea or a lake. But before going anywhere, they may want to lose any extra weight gained during the winter.
So, where does one get started? Diet is surely important, but diet alone will not do much good without an exercise plan. Health experts have long noted the importance of physical activity.
Exercise not only improves your appearance. It can also improve your health. Exercise helps to reduce the risk of some diseases. They include heart disease, stroke, type-two diabetes, osteoporosis and even some kinds of cancer.
BOB DOUGHTY: The Centers for Disease Control and Prevention says heart disease is the leading cause of death in the United States. In 2006, heart disease killed more than 630,000 Americans. High blood pressure and high cholesterol levels in blood can increase your risk of heart disease. Medical experts say both can be reduced through normal exercise.
Physical activity is also known to increase the release of endorphins. These chemicals reduce feelings of pain. They also help people feel more happy and peaceful. There is some debate about exactly what causes the brain to release endorphins. Some experts believe it is the act of exercising itself. Others say it is the feeling one gets from having met an exercise goal. Either way, the two things work together when it comes to improving one's emotional health.
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FAITH LAPIDUS: Surprisingly, exercise improves your energy levels by increasing the flow of blood to the heart and blood vessels. One of the main reasons people exercise is to control or reduce their weight. Physical activity burns calories – the energy stored in food. The more calories you burn, the easier it is to control or reduce your weight.
So exactly how much exercise do you need to do to gain all of these great health effects? Experts say it is easier than you think. Two years ago, the Centers for Disease Control released its first ever Physical Activity Guidelines for Americans. The report included suggestions for young people, adults, disabled persons and those with long-term health problems. One of the major ideas noted in the report is that some activity is better than none. So if you are not doing anything, now is the time to get started.
BOB DOUGHTY: The C.D.C. defines physical activity as anything that gets your body moving. And, it says there are two separate, but equally important kinds of physical activity. Aerobic or cardio exercise gets your heart rate going faster and increases your breathing. Some examples are activities like walking at an increased speed, dancing, swimming or riding a bicycle.
Muscle-strengthening activities help build and strengthen muscle groups in the body. This kind of exercise includes things like lifting weights, or doing sit-ups and push-ups.
FAITH LAPIDUS: To get the most from your exercise plan, experts say adults should get at least two and a half hours of aerobic exercise each week. More intense activities reduce the suggested amount of time to one hour and 15 minutes. Examples are playing basketball, swimming and distance running.
Earlier advice from the C.D.C. said people need to exercise 30 minutes each day for at least five days to get the health benefits of exercise. More recent research suggested that those gains are the same whether you exercise for short periods over five days or longer sessions over two or three days.
In addition, the newer suggestions say any exercise plan should include at least two days of muscle training. Each exercise period should be at least ten minutes long. The total amount of activity should be spread over at least three days throughout the week. Most importantly, experts say people should choose physical activities that they find fun. This helps to guarantee that they stay with the program.
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BOB DOUGHTY: So, what are some of the most popular physical activities in the United States? Walking tops the list. A 2006 report from the C.D.C. found that more than 79 million Americans walk to stay physically fit. For many people it is considered the easiest way to get exercise. It does not require a health club membership. Walking is safe. And, it is said be to as valuable for one's health as more intense forms of exercise like jogging. Walking is also reported to be less damaging to the knees and feet. This makes it a better choice of exercise for older adults.
FAITH LAPIDUS: Another popular form of exercise is jogging, or running at a slow to medium speed. USA Track and Field Hall of Famer, Bill Bowerman, is credited with bringing jogging to the United States in the 1970s. He did so after witnessing the popularity of the activity himself during a trip to New Zealand in the 1960s. He started the first running club in America and wrote a book about jogging for fitness. Bill Bowerman also helped establish Nike, the tennis shoe company.
Jogging provides great physical conditioning for the heart and lungs. And, it increases the flow of blood and oxygen in the body. All of these things combined help to improve heart activity, lower blood pressure and cholesterol levels, and reduce bone and muscle loss. Running is also a good way to lose weight. People burn an average of 160 calories a kilometer while running.
BOB DOUGHTY: The Census Bureau says swimming was the third most popular sports activity in the United States in 2007. The top two were walking and exercising with equipment. Swimming is said to be one of the best ways to exercise. Nearly all of the major muscle groups are put to work.
Swimming also presents less risk of muscle and joint injury because of the body's weightlessness in water. This makes it a great choice of exercise for people with special needs, like pregnant women, older adults, and persons who are overweight.
Water aerobics is another popular form of exercise. This can be anything from walking or running against the resistance of water, to doing jumping jacks in the water. There is a debate about whether or not swimming burns as many calories as other forms of exercise. But one thing is sure: the effects on your health are just as great.
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FAITH LAPIDUS: Whatever kind of exercise you choose, experts agree that you should start small and work your way up. Start by exercising ten minutes a day two times a week. After a few weeks, increase your time to 15 or 20 minutes, and increase the number of days.
Next, try to increase the intensity of your workout. If you have been walking, try walking faster, or take turns between walking and jogging. And try not to forget those muscle strengthening exercises. The more time you spend exercising, the more health benefits you get.
Health experts advise people who have been physically inactive to have a complete physical exam before beginning a new exercise program.
BOB DOUGHTY: If one of your goals is to lose weight, you will also need to change how and what you eat. To lose weight, you must use or burn off more calories than you take in.
A pound of fat is equal to about 453 grams or 3,500 calories. To lose that fat in a week, you have to burn off at least that amount in calories or eat that much less. The best thing to do is to combine both ideas. Eat fewer calories and increase your activity so that you burn off more.
The National Institutes of Health suggests that women limit their calories to no less than 1,200 calories a day without medical supervision. For men, the number is no less than 2,500. The American government also says a healthy diet is one that is high in fruits, vegetables and whole grains.
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FAITH LAPIDUS: This SCIENCE IN THE NEWS was written by June Simms. Our producer was Brianna Blake. I'm Faith Lapidus.
BOB DOUGHTY: And I'm Bob Doughty. Join us again next week for more news about science in Special English on the Voice of America.
osteoporosis: a condition in which the bones become weak and are easily broken 骨质疏松；骨质疏松症
endorphin: a hormone produced in the brain that reduces the feeling of pain 内啡肽（内分泌激素，有镇痛作用）
cardio: connected with the heart 心脏的
push-up: exercises to strengthen your arms and chest muscles. They are done by lying with your face towards the floor and pushing with your hands to raise your body until your arms are straight 俯卧撑
water aerobics: 水中有氧运动
jumping jack: 开合跳
（来源：VOA 编辑：陈丹妮）
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