For whatever reasons, super-size portions, more time at the computer, less time exercising, people are gaining weight. Today as many as 55% of adult Americans are obese.

Obesity is an excess amount of body fat, and occurs when a person's calorie intake exceeds the amount of energy he or she burns. Everyone needs a certain amount of body fat for stored energy, heat insulation, shock absorption. But it is generally agreed upon that men with more than 25 percent body fat and women with more than 30 percent body fat are obese.

Precisely measuring a person's body fat is not easy. The most accurate method is to weigh a person underwater - a very inconvenient procedure and thus not widely used. Body mass index, or BMI is the measurement that is easiest for most people.. BMI uses a mathematical formula that takes into account both a person's height and weight.

Being overweight is defined as BMI of 25 to 29.9 and obesity is a BMI of 30 and above. BMI does not provide information on a person's percentage of body fat. So someone who is very muscular may have a high BMI, and not be overweight.

It is important to not only determine how much fat a person has, but where the fat is on the body.

Women typically collect fat in their hips and buttocks, giving their figures a "pear" shape. Men, on the other hand, usually build up fat around their bellies, giving them more of an "apple" shape. This is not a hard and fast rule, though. Some men are pear-shaped and some women become apple-shaped, especially after menopause. People whose fat is concentrated mostly in the abdomen are more likely to develop many of the health problems associated with obesity.

The waist-to-hip ratio is a simple way to measure whether you are an apple or a pear.

To find your waist-to-hip ratio, measure the waist at its narrowest point, then measure the hips at the widest point. Divide the waist measurement by the hip measurement. A woman with a 35-inch waist and 46-inch hips would do the following calculation:

35 ÷ 46 = 0.76

Women with waist-to-hip ratios of more than 0.8 or men with waist-to-hip ratios of more than 1.0 are "apples".

Obesity often has more than one cause: genetic, environmental, psychological, and other factors all may play a part.

Obesity tends to run in families, suggesting that it may have a genetic cause. However, family members share not only genes but also diet and lifestyle habits that may contribute to obesity. Americans tend to have high-fat diets, often putting taste and convenience ahead of nutritional content when choosing meals. Most Americans also don't get enough exercise.

Separating these lifestyle factors from genetic ones is often difficult. Nevertheless, people who feel that their genes have doomed them to a lifetime of obesity should take heart. Many people genetically predisposed to obesity do not become obese or manage to lose weight and keep it off, by changing what they eat and becoming more active.

One of the most painful aspects of obesity may be the emotional suffering it causes. Americans place great emphasis on physical appearance, often equating "attractiveness" with "slimness", especially in women. The messages, intended or not, make overweight people feel unattractive. Many people assume that obese people are gluttonous, lazy, or both. However, more and more evidence contradicts this assumption. Obese people often face prejudice or discrimination at work, at school, while looking for a job, and in social situations.

Obesity is more then a cosmetic problem. It's a health hazard, the more obese a person is, the more likely he or she is to have health problems. Someone who is 40 percent overweight is twice as likely to die prematurely as a person who has a normal weight. As BMI levels rise over 25, blood pressure and total cholesterol levels increase and average HDL or good cholesterol levels decrease. Obesity has been linked to diabetes, heart disease, high blood pressure, and stroke. It is also associated with higher rates of certain types of cancer. Obese men are more likely than non-obese men to die from cancer of the colon, rectum, and prostate. Obese women are more likely than non-obese women to die from cancer of the gallbladder, breast, uterus, cervix, and ovaries. The risk of death increases as the body mass index reaches and surpasses 30.

If you are overweight, losing as little as 5 to 10 percent of your body weight may improve many of the problems linked to being overweight, such as high blood pressure and diabetes. For example, if you weigh 200 pounds and are considered overweight, you would need to lose 10 to 20 pounds. Even a small weight loss can improve your health.

Slow and steady weight loss of no more than 1 pound per week is the safest way to lose weight. Very rapid weight loss can cause you to lose muscle rather than fat. It also increases your chances of developing other problems, such as gallstones, gout, and nutrient deficiencies. Making long-term changes in your eating and physical activity habits is the best way to lose weight and keep it off over time.