Everyone knows some people who can eat ice cream, cake, and whatever else they want and still not gain weight. At the other extreme are people who seem to gain weight no matter how little they eat. What are the causes of obesity? What allows one person to remain thin without effort but demands that another struggle to avoid gaining weight or regaining the pounds he or she has lost previously?

On a very simple level, your weight depends on the number of calories you consume, how many of those calories you store, and how many you burn up. But each of these factors is influenced by a combination of genes and environment.

Both can affect your physiology such as how fast you burn calories as well as your behavior the types of foods you choose to eat, for instance. The interplay between all these factors begins at the moment of your conception and continues throughout your life.

The balance of calories stored and burned depends on your genetic makeup, your level of physical activity, and your resting energy expenditure the number of calories your body burns while at rest.

If you consistently burn all of the calories that you consume in the course of a day, you will maintain your weight. If you consume more energy calories than you expend, you will gain weight.

Excess calories are stored throughout your body as fat. Your body stores this fat within specialized fat cells adipose tissue — either by enlarging fat cells, which are always present in the body, or by creating more of them. If you decrease your food intake and consume fewer calories than you burn up, or if you exercise more and burn up more calories, your body will reduce some of your fat stores.

When this happens, fat cells shrink, along with your waistline. To date, more than different genes have been implicated in the causes of overweight or obesity, although only a handful appear to be major players.

Genes contribute to the causes of obesity in many ways, by affecting appetite, satiety the sense of fullness , metabolism, food cravings, body-fat distribution, and the tendency to use eating as a way to cope with stress. The strength of the genetic influence on weight disorders varies quite a bit from person to person.

Having a rough idea of how large a role genes play in your weight may be helpful in terms of treating your weight problems. Genes are probably a significant contributor to your obesity if you have most or all of the following characteristics:. Genes are probably a lower contributor for you if you have most or all of the following characteristics:.

These circumstances suggest that you have a genetic predisposition to be heavy, but it's not so great that you can't overcome it with some effort. At the other end of the spectrum, you can assume that your genetic predisposition to obesity is modest if your weight is normal and doesn't increase even when you regularly indulge in high-calorie foods and rarely exercise.

People with only a moderate genetic predisposition to be overweight have a good chance of losing weight on their own by eating fewer calories and getting more vigorous exercise more often. These people are more likely to be able to maintain this lower weight. When the prey escaped or the crops failed, how did our ancestors survive?

Those who could store body fat to live off during the lean times lived, and those who couldn't, perished. Today, of course, these thrifty genes are a curse rather than a blessing.

Not only is food readily available to us nearly around the clock, we don't even have to hunt or harvest it! In contrast, people with a strong genetic predisposition to obesity may not be able to lose weight with the usual forms of diet and exercise therapy.

Even if they lose weight, they are less likely to maintain the weight loss. For people with a very strong genetic predisposition, sheer willpower is ineffective in counteracting their tendency to be overweight.

Typically, these people can maintain weight loss only under a doctor's guidance. They are also the most likely to require weight-loss drugs or surgery. The prevalence of obesity among adults in the United States has been rising since the s. Genes alone cannot possibly explain such a rapid rise.

Although the genetic predisposition to be overweight varies widely from person to person, the rise in body mass index appears to be nearly universal, cutting across all demographic groups.

These findings underscore the importance of changes in our environment that contribute to the epidemic of overweight and obesity. Genetic factors are the forces inside you that help you gain weight and stay overweight; environmental factors are the outside forces that contribute to these problems.

They encompass anything in our environment that makes us more likely to eat too much or exercise too little. Taken together, experts think that environmental factors are the driving force for the causes of obesity and its dramatic rise. Environmental influences come into play very early, even before you're born.

Researchers sometimes call these in-utero exposures "fetal programming. The same is true for babies born to mothers who had diabetes.

Researchers believe these conditions may somehow alter the growing baby's metabolism in ways that show up later in life. After birth, babies who are breast-fed for more than three months are less likely to have obesity as adolescents compared with infants who are breast-fed for less than three months.

Childhood habits often stick with people for the rest of their lives. Kids who drink sugary sodas and eat high-calorie, processed foods develop a taste for these products and continue eating them as adults, which tends to promote weight gain.

Likewise, kids who watch television and play video games instead of being active may be programming themselves for a sedentary future. Many features of modern life promote weight gain. In short, today's "obesogenic" environment encourages us to eat more and exercise less.

And there's growing evidence that broader aspects of the way we live — such as how much we sleep, our stress levels, and other psychological factors — can affect weight as well. According to the Centers for Disease Control and Prevention CDC , Americans are eating more calories on average than they did in the s.

Between and , the average man added calories to his daily fare, while the average woman added calories a day. What's driving this trend? Experts say it's a combination of increased availability, bigger portions, and more high-calorie foods.

Practically everywhere we go — shopping centers, sports stadiums, movie theaters — food is readily available. You can buy snacks or meals at roadside rest stops, hour convenience stores, even gyms and health clubs.

In the s, fast-food restaurants offered one portion size. Today, portion sizes have ballooned, a trend that has spilled over into many other foods, from cookies and popcorn to sandwiches and steaks.

A typical serving of French fries from McDonald's contains three times more calories than when the franchise began. A single "super-sized" meal may contain 1,—2, calories — all the calories that most people need for an entire day.

And research shows that people will often eat what's in front of them, even if they're already full. Not surprisingly, we're also eating more high-calorie foods especially salty snacks, soft drinks, and pizza , which are much more readily available than lower-calorie choices like salads and whole fruits.

Fat isn't necessarily the problem; in fact, research shows that the fat content of our diet has actually gone down since the early s. But many low-fat foods are very high in calories because they contain large amounts of sugar to improve their taste and palatability.

In fact, many low-fat foods are actually higher in calories than foods that are not low fat. The government's current recommendations for exercise call for an hour of moderate to vigorous exercise a day.

Our daily lives don't offer many opportunities for activity. Children don't exercise as much in school, often because of cutbacks in physical education classes. Many people drive to work and spend much of the day sitting at a computer terminal.

Because we work long hours, we have trouble finding the time to go to the gym, play a sport, or exercise in other ways. Instead of walking to local shops and toting shopping bags, we drive to one-stop megastores, where we park close to the entrance, wheel our purchases in a shopping cart, and drive home.

The widespread use of vacuum cleaners, dishwashers, leaf blowers, and a host of other appliances takes nearly all the physical effort out of daily chores and can contribute as one of the causes of obesity.

The average American watches about four hours of television per day, a habit that's been linked to overweight or obesity in a number of studies. Data from the National Health and Nutrition Examination Survey, a long-term study monitoring the health of American adults, revealed that people with overweight and obesity spend more time watching television and playing video games than people of normal weight.

Watching television more than two hours a day also raises the risk of overweight in children, even in those as young as three years old. Part of the problem may be that people are watching television instead of exercising or doing other activities that burn more calories watching TV burns only slightly more calories than sleeping, and less than other sedentary pursuits such as sewing or reading.

But food advertisements also may play a significant role. The average hour-long TV show features about 11 food and beverage commercials, which encourage people to eat. And studies show that eating food in front of the TV stimulates people to eat more calories, and particularly more calories from fat.

In fact, a study that limited the amount of TV kids watched demonstrated that this practice helped them lose weight — but not because they became more active when they weren't watching TV.

It's important to note that some people use weight scales to measure their body composition. One study even compared the results from three different commercially available scales to a DXA scan and found the weight scales were inaccurate. A variety of factors can affect your body composition.

Unlike the BMI, your body composition takes the following factors into consideration:. Your healthcare provider can help you better understand important details about the percentage of body fat and lean mass in your body and offer advice on how to move forward.

If you want to change your body composition either by increasing lean mass or reducing body fat , here are some safe, effective ways to do so:. Body composition is the breakdown of body fat and lean mass that you carry. Unlike the BMI which only considers your height and weight, factors like your age, sex, hormone levels, lifestyle habits, and genetics all play a role in your body composition.

If you want a holistic assessment of the state of your health, knowing your body composition is more useful than just knowing your body weight or BMI. By looking at the breakdown of lean mass compared to fat in your body, you and your healthcare can develop a plan to make certain lifestyle changes that can help you reach your health goals and lower the risk of developing certain health conditions.

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