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Why Do People Gain Weight?

The Causes of Obesity


Updated July 02, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Why Do People Gain Weight?

Why do we gain weight?

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Obesity is being described as an epidemic, with more than two-thirds of Americans classified as overweight or obese. The problem continues to rise: the US adult obesity rate has more than doubled since the mid-1970s when 15 per cent of adults were obese, to 35.7 per cent in 2010, according to the US Centers for Disease Control and Prevention (CDC).

Carrying too much weight on your body can lead to a variety of problems which may threaten your health and longevity, like heart disease, diabetes and fatty liver disease. So what really causes obesity?

Fat, carbs, or calories? Despite an explosion of diet books blaming individual nutritional components, like fat and carbohydrates, health agencies around the world warn that people are gaining weight simply because they're taking in more calories every day than they're burning off.

How much are we eating? In a 2008 report compiled by the US Department of Agriculture USDA), economists Hodan Wells and Jean Buzby write that Americans are eating more of everything — including healthy fruits and vegetables — compared with their consumption decades ago. Wells tells me that in 1970, for example, the average daily caloric intake for an American adult was an estimated 2,076 calories. (Intake is estimated by the USDA by factoring in national food production, imports, exports, and loss due to spoilage). By 2010, that number had jumped to 2,534, a difference of 458 calories each day. While that may not sound like much, if those calories aren't being compensated for through greater daily activity, they can translate to an extra 46 pounds per year!

Fortunately, average rates of weight gain are not as high as that, but incremental increases in daily consumption can certainly account for extra pounds over months, or years. How much weight you may gain, and how quickly, will depend on how many calories you eat, how active you are, and to a certain extent on your personal body composition (the ratio of muscle tissue in your body compared with fat tissue). More muscle mass usually means you'll burn more calories over the course of a day, even when your body is at rest.

How important are genetics in obesity? Obesity tends to run in families, though this may have more to do with lifestyle patterns than genetics. Family members often share dietary habits and activity levels, which will each play a large role in an individual's weight. Take heart that maintaining a healthy weight is still possible, even if there's a tendency towards obesity in your family tree.

Why diets work: Any diet regimen will help you lose weight in the short term if it restricts calories to keep your consumption level below the number of calories you expend. While proponents of certain diets may insist that calories "don't count", the common element shared by successful regimens is this calorie deficit. Over the long-term, balancing calories in with calories burned will help you avoid weight "creep", or incremental weight gain over time.

Well-fed but not well-nourished: In their 2008 report, Wells and Buzby warn that even though Americans are eating more, they're still not achieving optimum nutrition. For example, most people consume twice the recommended amount of refined grains each day, and only one-third the recommended 3oz (85g) of whole grains daily.

In order to meet the minimum dietary guidelines laid out by the US government in 2005, they write, people would need to "substantially lower their intake of added fats, refined grains, and added sugars and sweeteners and increase their consumption of fruits, vegetables, whole grains, and lower-fat milk and milk products."


Cynthia L. Ogden; Margaret D. Carroll; Brian K. Kit; and Katherine M. Flegal. "Prevalence of Obesity in the United States, 2009–2010." US National Center for Health Statistics. Data Brief No. 82. January 2012. Accessed June 24, 2013.

Understanding Adult Weight and Obesity. US National Institute of Diabetes and Digestive and Kidney Diseases/Weight Control Network Public Information Sheet. Accessed June 24, 2013.

Wells, Hodan F., and Jean C. Buzby. Dietary Assessment of Major Trends in U.S. Food Consumption, 1970-2005, Economic Information Bulletin No. 33. Economic Research Service, U.S. Dept. of Agriculture. March 2008. Interview with Hodan Wells conducted by phone June 24, 2013.

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