Obesity is a leading cause of preventable illness and death in North America. In recent years, the number of overweight people in industrialized countries has increased significantly, so much so that the World Health Organization (WHO) has called obesity an epidemic. In the United States, 69% of the adult population are overweight or obese. In Canada, the self-reported data shows that 40% of men and 27% of women are overweight, and 20% of men and 17% of women are obese.

People who are obese are at a much higher risk for serious medical conditions such as high blood pressure, heart attack, stroke, diabetes, gallbladder disease, and different cancers than people who have a healthy weight.

Obesity occurs when your body consumes more calories than it burns. In the past, many people thought that obesity was simply caused by overeating and under-exercising, resulting from a lack of will power and self-control. Although these are significant contributing factors, doctors recognize that obesity is a complex medical problem that involves genetic, environmental, behavioural, and social factors. All these factors play a role in determining a person's weight.

Recent research shows that in some cases, certain genetic factors may cause the changes in appetite and fat metabolism that lead to obesity. For a person who is genetically prone to weight gain (e.g., has a lower metabolism) and who leads an inactive and unhealthy lifestyle, the risk of becoming obese is high.

Although a person's genetic makeup may contribute to obesity, it's not the primary cause. Environmental and behavioural factors have a greater influence – consuming excess calories from high-fat foods and doing little or no daily physical activity over the long run will lead to weight gain. Psychological factors may also foster obesity. Low self-esteem, guilt, emotional stress, or trauma can lead to overeating as a means to cope with the problem.

Certain medical conditions such as binge eating disorder (BED), Cushing’s disease, and polycystic ovary syndrome can also lead to weight gain and obesity. BED is an eating disorder where a person has recurrent episodes of binge eating. During these episodes, the individual eats a large amount of food quickly and feels a lack of control over this eating.

The health risks associated with obesity include:

• breathing disorders (e.g., sleep apnea, chronic obstructive pulmonary disease)
• certain types of cancers (e.g., prostate and bowel cancer in men, breast and uterine cancer in women)
• coronary artery (heart) disease
• depression
• diabetes
• gallbladder or liver disease
• gastroesophageal reflux disease (GERD)
• high blood pressure
• high cholesterol
• joint disease (e.g., osteoarthritis)
• stroke

People who are obese may have the symptoms of the medical conditions mentioned above. High blood pressure, high cholesterol levels, breathing problems, and joint pain (in the knees or lower back) are common. The more obese a person is, the more likely they are to have medical problems related to obesity.

Aside from the medical complications, obesity is also linked to psychosocial problems such as low self-esteem, discrimination, difficulty finding employment, and reduced quality of life.

The diagnosis of obesity is usually based on a physical examination and a patient history (i.e., eating and exercise habits).

A measurement called the body mass index (BMI) does not directly measure body fat, but it is a useful tool to assess the health risk associated with being overweight or obese. A BMI of 18.5 to 24.9 is considered within the healthy range. The BMI is calculated using kilograms (kg) and metres (m) instead of pounds (lb) and inches/feet. Keep in mind that 1 lb equals 0.45 kg and 1 inch equals 0.0254 m.

BMI is calculated as follows:

BMI = body weight (kg) ÷ height² (m)

Example: if you weigh 150 lbs (68 kg) and are 5'8" (1.73 m) tall, divide 68 by (1.73 × 1.73), or 2.99. The result is 22.74, which is right in the middle of the healthy range.

Health Canada classifies BMI according to the associated risk of developing health problems:

*The above classification does not apply to people younger than 18 years of age, pregnant women, or breast-feeding women. For people 65 years and older, the "normal" range is higher, beginning slightly above 18.5 and extending into the "overweight" range.

Doctors may also use other measurements, such as waist size, to evaluate health risks associated with excess abdominal fat. When BMI and waist size indicate a high risk for health problems, additional tests may also be performed.

The Canadian Guidelines for Body Weight Classification in Adults are available on the Internet.