Overweight and obesity are defined as abnormal or excessive fat accumulation that may impair health.
Body mass index (BMI) is a simple index of weight-for-height that is commonly used to classify overweight and obesity in adults. It is defined as a person’s weight in kilograms divided by the square of his height in meters (kg/m2).

Adults
For adults, WHO defines overweight and obesity as follows:

overweight is a BMI greater than or equal to 25; and
obesity is a BMI greater than or equal to 30.
BMI provides the most useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults. However, it should be considered a rough guide because it may not correspond to the same degree of fatness in different individuals.

For children, age needs to be considered when defining overweight and obesity.

Children under 5 years of age
For children under 5 years of age:

overweight is weight-for-height greater than 2 standard deviations above WHO Child Growth Standards median; and
obesity is weight-for-height greater than 3 standard deviations above the WHO Child Growth Standards median.

Charts and tables: WHO child growth standards for children aged under 5 years
Children aged between 5–19 years
Overweight and obesity are defined as follows for children aged between 5–19 years:

overweight is BMI-for-age greater than 1 standard deviation above the WHO Growth Reference median; and
obesity is greater than 2 standard deviations above the WHO Growth Reference median.
Charts and tables: WHO growth reference for children aged between 5–19 years
Facts about overweight and obesity
Some recent WHO global estimates follow.

In 2016, more than 1.9 billion adults aged 18 years and older were overweight. Of these over 650 million adults were obese.
In 2016, 39% of adults aged 18 years and over (39% of men and 40% of women) were overweight.
Overall, about 13% of the world’s adult population (11% of men and 15% of women) were obese in 2016.
The worldwide prevalence of obesity nearly tripled between 1975 and 2016.
In 2019, an estimated 38.2 million children under the age of 5 years were overweight or obese. Once considered a high-income country problem, overweight and obesity are now on the rise in low- and middle-income countries, particularly in urban settings. In Africa, the number of overweight children under 5 has increased by nearly 24% percent since 2000. Almost half of the children under 5 who were overweight or obese in 2019 lived in Asia.

Over 340 million children and adolescents aged 5-19 were overweight or obese in 2016.

The prevalence of overweight and obesity among children and adolescents aged 5-19 has risen dramatically from just 4% in 1975 to just over 18% in 2016. The rise has occurred similarly among both boys and girls: in 2016 18% of girls and 19% of boys were overweight.

While just under 1% of children and adolescents aged 5-19 were obese in 1975, more 124 million children and adolescents (6% of girls and 8% of boys) were obese in 2016.

Overweight and obesity are linked to more deaths worldwide than underweight. Globally there are more people who are obese than underweight – this occurs in every region except parts of sub-Saharan Africa and Asia.

What causes obesity and overweight?
The fundamental cause of obesity and overweight is an energy imbalance between calories consumed and calories expended. Globally, there has been:

an increased intake of energy-dense foods that are high in fat and sugars; and
an increase in physical inactivity due to the increasingly sedentary nature of many forms of work, changing modes of transportation, and increasing urbanization.
Changes in dietary and physical activity patterns are often the result of environmental and societal changes associated with development and lack of supportive policies in sectors such as health, agriculture, transport, urban planning, environment, food processing, distribution, marketing, and education.

What are common health consequences of overweight and obesity?
Raised BMI is a major risk factor for noncommunicable diseases such as:

cardiovascular diseases (mainly heart disease and stroke), which were the leading cause of death in 2012;
diabetes;
musculoskeletal disorders (especially osteoarthritis – a highly disabling degenerative disease of the joints);
some cancers (including endometrial, breast, ovarian, prostate, liver, gallbladder, kidney, and colon).
The risk for these noncommunicable diseases increases, with increases in BMI.

Childhood obesity is associated with a higher chance of obesity, premature death and disability in adulthood. But in addition to increased future risks, obese children experience breathing difficulties, increased risk of fractures, hypertension, early markers of cardiovascular disease, insulin resistance and psychological.

Obesity Treatment:
There are countless weight-loss strategies available but many are ineffective and short-term, particularly for those who are morbidly obese. Among the morbidly obese, less than 5 percent succeed in losing a significant amount of weight and maintaining the weight loss with non-surgical programs — usually a combination of dieting, behavior modification therapy and exercise.

People do lose weight without surgery, however, particularly when they work with a certified health care professional to develop an effective and safe weight-loss program. Most health insurance companies don’t cover weight-loss surgery unless you first make a serious effort to lose weight using non-surgical approaches.

Many people participate in a combination of the following therapies.

Dietary Modification
Many of us have tried a variety of diets and have been caught in a cycle of weight gain and loss — “yo-yo” dieting — that can cause serious health risks by stressing the heart, kidneys and other organs.

Ninety percent of people participating in all diet programs regain the weight they’ve lost within two years. For people who have weight-loss surgery, dieting is an instrumental part of maintaining weight loss after surgery.

If you decide to go on a diet, we recommend that you work with a health professional who can customize a diet to meet your needs. A diet should greatly restrict your calorie intake, but maintain your nutrition. Calorie-restrictive diets fall into two basic categories.

Low calorie diets (LCDs)
are individually planned to include 500 to 1,000 calories a day less than you burn.
Very low calorie diets (VLCDs)
typically limit intake to only 400 to 800 calories a day and feature high-protein, low-fat liquids.

Behavior Modification
The goal of behavior modification therapy is to change your eating and exercise habits to promote weight loss. Examples include:

√Setting realistic weight loss goals — short term and long term.
√Recording your diet and exercise patterns in a diary.
√Identifying high-risk situations and avoiding them.
√Rewarding specific actions, such as exercising for a longer time or eating less of a certain type of food.
√Adopting realistic beliefs about weight loss and body image.
√Developing a support network, including family, friends and co-workers, or joining a support group that can help you focus on your goal.
Although some people experience success with behavior modification, most patients achieve only short-term weight loss for the first year. If you plan on having weight-loss surgery, behavior therapy and dieting will be instrumental in helping you maintain your weight loss after surgery.

√Surgery is a tool to get your body to start losing weight. Diet and behavior modification will determine your ultimate success.

Exercise:
Exercise greatly increases your chance of long-term weight loss. It is a key component for any long-term weight management program, particularly weight-loss surgery.

Research shows that when you reduce the number of calories you consume, your body reacts by slowing your metabolism to burn fewer calories, rather than promote weight loss. Daily physical activity can help speed up your metabolism, effectively reducing the “set point” — a sort of thermostat in the brain that makes you resistant to either weight gain or loss — to a lower natural weight.

Starting an exercise program can be intimidating if you’re morbidly obese. Your health condition may make any level of physical exertion extremely difficult. But you can learn strategies to help you start a realistic exercise routine. The following strategies can help you start exercising and can be incorporated into your daily routine.

•Park your car at the far end of parking lots and walk through them.Walking is considered one of the most effective forms of exercise. You can start slowly and build up over time.

•Reduce the time you spend watching television.
•Ride an exercise bike.
•Swim or participate in low-impact water aerobics.
•Take the stairs instead of the elevator.
•Walk briskly for five minutes in the morning and five minutes in the afternoon.

Medications
A variety of over-the-counter and prescription weight loss drugs are available. Some people find these drugs help curb their appetites. Studies show that patients on drug therapy lose around 10 percent of their excess weight, and that the weight loss plateaus after six to eight months. As patients stop taking the medication, weight gain usually occurs.

Weight loss drugs, approved by the U.S. Food and Drug Administration (FDA) for treating obesity, include:

Beta-methyl-phenylethylamine (Fastin)
This is a stimulant that increases fat metabolism

•Orlistat (Xenical)
This drug works by blocking about 30 percent of dietary fat from being absorbed. Alli is a lower-dose, over-the-counter formula of the same medication.

•Phentermine
Phentermine, an appetite suppressant, has been available for many years. It is half of the “fen-phen” combination that remains available for use. The use of phentermine alone has not been associated with the adverse health effects of the fenfluramine-phentermine combination.

•Sibutramine (Meridia)
This is an appetite suppressant approved for long-term use.
Medications are an important part of the morbid obesity treatment process but weight-loss drugs can have serious side effects. We recommend that you visit a certified health care professional who can prescribe appropriate medications. Before insurance companies will reimburse you for weight-loss surgery, you must follow a well-documented treatment plan that typically includes medications.

•Surgery
Many people ,who are morbidly obese and who have been unsuccessful in losing and keeping off the weight, opt for bariatric or weight-loss surgery.

Bariatric surgery, which involves sealing off most of the stomach to reduce the quantity of food you can consume, can be an effective means for morbidly obese people to lose weight and maintain that weight loss.

To be considered for weight-loss surgery, you must meet at least one of the following qualifications:

Be more than 100 pounds over your ideal, recommended body weight.
Have a body mass index (BMI) of 40 or higher (20 to 25 is considered a normal). BMI is a number based on both your height and weight. Surgery may be considered with a BMI as low as 35 if your doctor determines that there’s a medical need for weight reduction and surgery appears to be the only way to accomplish the targeted weight loss.
To qualify for surgery, you must complete a medical and psychological pre-evaluation process, and show how that you are committed to long-term, follow-up care after surgery. Most surgeons require that you demonstrate serious motivation and a clear understanding of the extensive dietary, exercise and medical guidelines that must be followed for the remainder of your life.