Exercise Can Help Control Weight
Obesity results from energy imbalance: too many calories in, too few calories burned. A number of factors influence how many calories (or how much “energy”) people burn each day, among them, age, body size, and genes. But the most variable factor—and the most easily modified—is the amount of activity people get each day.
Keeping active can help people stay at a healthy weight or lose weight. It can also lower the risk of heart disease, diabetes, stroke, high blood pressure, osteoporosis, and certain cancers, as well as reduce stress and boost mood. Inactive (sedentary) lifestyles do just the opposite.
Despite all the health benefits of physical activity, people worldwide are doing less of it—at work, at home, and as they travel from place to place. Globally, about one in three people gets little, if any, physical activity. (1) Physical activity levels are declining not only in wealthy countries, such as the U.S., but also in low- and middle-income countries, such as China. And it’s clear that this decline in physical activity is a key contributor to the global obesity epidemic, and in turn, to rising rates of chronic disease everywhere.
The World Health Organization, the U.S. Dept. of Health and Human Services, and other authorities recommend that for good health, adults should get the equivalent of two and a half hours of moderate-to-vigorous physical activity each week. (2-4) Children should get even more, at least one hour a day. There’s been some debate among researchers, however, about just how much activity people need each day to maintain a healthy weight or to help with weight loss, and the most recent studies suggest that a total of two and a half hours a week is simply not enough.
This article defines physical activity and explains how it is measured, reviews physical activity trends, and discusses the role of physical activity in weight control.
Though people often use “physical activity” and “exercise” interchangeably, the terms have different definitions. “Physical activity” refers to any body movement that burns calories, whether it’s for work or play, daily chores, or the daily commute. “Exercise,” a subcategory of physical activity, refers to “planned, structured, and repetitive” activities aimed at improving physical fitness and health. (5) Researchers sometimes use the terms “leisure-time physical activity” or “recreational physical activity” as synonyms for exercise.
Experts measure the intensity of physical activity in metabolic equivalents or METs. One MET is defined as the calories burned while an individual sits quietly for one minute. For the average adult, this is about one calorie per every 2.2 pounds of body weight per hour; someone who weighs 160 pounds would burn approximately 70 calories an hour while sitting or sleeping. Moderate-intensity physical activity is defined as activities that are strenuous enough to burn three to six times as much energy per minute as an individual would burn when sitting quietly, or 3 to 6 METs. Vigorous-intensity activities burn more than 6 METs.
It is challenging for researchers to accurately measure people’s usual physical activity, since most studies rely on participants’ reports of their own activity in a survey or daily log. This method is not entirely reliable: Studies that measure physical activity more objectively, using special motion sensors (called accelerometers), suggest that people tend to overestimate their own levels of activity. (6)
Worldwide, people are less active today than they were decades ago. While studies find that sports and leisure activity levels have remained stable or increased slightly, (7-10) these leisure activities represent only a small part of daily physical activity. Physical activity associated with work, home, and transportation has declined due to economic growth, technological advancements, and social changes. (7,8,10,11) Some examples from different countries:
- United States. In 1950, 30 percent of Americans worked in high-activity occupations; by 2000, that proportion had dropped to only 22 percent. Conversely, the percentage of people working in low-activity occupations rose from about 23 percent to 41 percent. (8) Driving cars increased from 67 percent of all trips to work in 1960 to 88 percent in 2000, while walking and taking public transit to work decreased. (8) About 40 percent of U.S. schoolchildren walked or rode their bikes to school in 1969; by 2001, only 13 percent did so. (12)
- United Kingdom. Over the past few decades, it’s become more common for U.K. households to own second cars and labor-saving appliances. (13) Work outside the home has also become less active. In 2004, about 39 percent of men worked in active jobs, down from 43 percent in 1991–1992. (11)
- China. Between 1991 and 2006, work-related physical activity in China dropped by about 35 percent in men and 46 percent in women; women also cut back on physical activity around the house—washing clothes, cooking, cleaning—by 66 percent. (10) Transportation-related physical activity has also dropped—no surprise, perhaps, given that car ownership is on the rise: Sales of new cars in China have gone up by about 30 percent per year in recent years. (14)
The flip side of this decrease in physical activity is an increase in sedentary activities—watching television, playing video games, and using the computer. Add it up, and it’s clear that globally, the “energy out” side of the energy balance equation is tilting toward weight gain.
Weight gain during adulthood can increase the risk of heart disease, diabetes, and other chronic conditions. Since it’s so hard for people to lose weight and keep it off, it’s better to prevent weight gain in the first place. Encouragingly, there’s strong evidence that staying active can help people slow down or stave off “middle-age spread”: (13) The more active people are, the more likely they are to keep their weight steady; (15,16) the more sedentary, the more likely they are to gain weight over time. (17) But it’s still a matter of debate exactly how much activity people need to avoid gaining weight. The latest evidence suggests that the recommended two and a half hours a week may not be enough.
The Women’s Health Study, for example, followed 34,000 middle-age women for 13 years to see how much physical activity they needed to stay within 5 pounds of their weight at the start of the study. Researchers found that women in the normal weight range at the start needed the equivalent of an hour a day of moderate-to-vigorous physical activity to maintain a steady weight. (18)
Vigorous activities seem to be more effective for weight control than slow walking. (15,19,20) The Nurses’ Health Study II, for example, followed more than 18,000 women for 16 years to study the relationship between changes in physical activity and weight. Although women gained, on average, about 20 pounds over the course of the study, those who increased their physical activity by 30 minutes per day gained less weight than women whose activity levels stayed steady. And the type of activity made a difference: Bicycling and brisk walking helped women avoid weight gain, but slow walking did not.
Exercise can help promote weight loss, but it seems to work best when combined with a lower calorie eating plan. (3) If people don’t curb their calories, however, they likely need to exercise for long periods of time—or at a high intensity—to lose weight. (3,21,22)
In one study, for example, researchers randomly assigned 175 overweight, inactive adults to either a control group that did not receive any exercise instruction or to one of three exercise regimens—low intensity (equivalent to walking 12 miles/week), medium intensity (equivalent to jogging 12 miles/week), or high intensity (equivalent to jogging 20 miles per week). All study volunteers were asked to stick to their usual diets. After six months, those assigned to the high-intensity regimen lost abdominal fat, whereas those assigned to the low- and medium-intensity exercise regimens had no change in abdominal fat. (21)
More recently, researchers conducted a similar trial with 320 post-menopausal women, randomly assigning them to either 45 minutes of moderate-to-vigorous aerobic activity, five days a week, or to a control group. Most of the women were overweight or obese at the start of the study. After one year, the exercisers had significant decreases in body weight, body fat, and abdominal fat, compared to the non-exercisers. (23)
Researchers believe that physical activity prevents obesity in multiple ways: (24)
- Physical activity increases people’s total energy expenditure, which can help them stay in energy balance or even lose weight, as long as they don’t eat more to compensate for the extra calories they burn.
- Physical activity decreases fat around the waist and total body fat, slowing the development of abdominal obesity.
- Weight lifting, push-ups, and other muscle-strengthening activities build muscle mass, increasing the energy that the body burns throughout the day—even when it’s at rest—and making it easier to control weight.
- Physical activity reduces depression and anxiety, (3) and this mood boost may motivate people to stick with their exercise regimens over time.
Being moderately active for at least 30 minutes a day on most days of the week can help lower the risk of chronic disease. But to stay at a healthy weight, or to lose weight, most people will need more physical activity—at least an hour a day—to counteract the effects of increasingly sedentary lifestyles, as well as the strong societal influences that encourage overeating.
Keep in mind that staying active is not purely an individual choice: The so-called “built environment”—buildings, neighborhoods, transportation systems, and other human-made elements of the landscape—influences how active people are. (25) People are more prone to be active, for example, if they live near parks or playgrounds, in neighborhoods with sidewalks or bike paths, or close enough to work, school, or shopping to safely travel by bike or on foot. People are less likely to be active if they live in sprawling suburbs designed for driving or in neighborhoods without recreation opportunities.
Local and state governments wield several policy tools for shaping people’s physical surroundings, such as planning, zoning, and other regulations, as well as setting budget priorities for transportation and infrastructure. (27) Strategies to create safe, active environments include curbing traffic to make walking and cycling safer, building schools and shops within walking distance of neighborhoods, and improving public transportation, to name a few. Such changes are essential to make physical activity an integral and natural part of people’s everyday lives—and ultimately, to turn around the obesity epidemic.
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The aim of the Harvard School of Public Health Obesity Prevention Source Web site is to provide timely information about obesity’s global causes, consequences, prevention, and control, for the public, health and public health practitioners, business and community leaders, and policymakers. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The Web site’s obesity prevention policy recommendations are based primarily on a review of U.S. expert guidance, unless otherwise indicated; in other countries, different policy approaches may be needed to achieve improvements in food and physical activity environments, so that healthy choices are easy choices, for all.