The terms overweight and obesity refer to an excess amount body of fat that may be detrimental to health.

The current standard way of classifying overweight or obesity is through calculation of a person’s body mass index (BMI), which is their weight in kilograms divided by the square of their height in meters.

For adults, overweight is defined as a body mass index (BMI) of between 25 and 29.9; obesity as a BMI of 30 to 39.9 and extreme obesity as a BMI of 40 or more. For children, overweight or obesity is classified as a BMI at or higher than the 85th percentile and obesity as a BMI at or above the 95th percentile.

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Recent estimates from the World Health Organization (WHO) show that in 2014, about 39% of adults globally were obese, about 13% were overweight and that there had been a doubling in obesity prevalence, compared with 1980. About 40 million children aged under 5 years classed as overweight or obese and aside from parts of sub-Saharan Africa and Asia, there are more cases of overweight and obesity worldwide than there cases of underweight.

Overweight and obesity used to mainly be considered as a problem in high-income countries, but now, low- and middle-income countries are seeing significant increases of cases, especially in urban areas. Children from such countries are less likely to have been adequately nourished, whether it be prenatally, during infanthood or as young children. These children tend to be exposed to foods of lower nutritional content. The foods are more affordable, but they are also high in fat, sugar and salt. This, in combination with decreased physical activity levels as a result of more sedentary lifestyles and increased transport, has led tosharp rises in childhood obesity, as well as malnourishment problems.

Overweight and obesity are caused by an energy imbalance between calories consumed and calories expended when used as energy by the body. A steady body weight is maintained when the calorie intake is equal to the calories “burned.” When this calorie intake exceeds the amount burned, the energy left over is stored as fat and eventually a person starts to gain weight and is at an increased risk of becoming overweight or obese. Energy intake and expenditure also need to be balanced in children, but the fact that children grow needs to be accounted for. Energy is considered balanced when calories are burned to support growth, but without causing weight gain.

Causes Among AdultsMany factors contribute to weight gain, overweight and obesity including genetics, lifestyle factors such as diet and exercise, medication use, place of habitat, education level and income.

Behaviors: The main aspects of healthy behaviors are a healthy diet and regular physical activity. According to the Dietary Guidelines for Americans, a healthy diet would include fruit, vegetables, whole grains and lean proteins and a limited intake of high-fat foods. According to the Physical Activity Guidelines for Americans, adults should engage in 150 minutes of moderate exercise and/or 75 minutes of intense exercise, as well as strength building exercises. Failing to adhere to these guidelines by eating an unhealthy diet and not exercising enough, increases the risk of overweight, obesity and the associated complications such as heart disease and type 2 diabetes.

Environment and community: People tend to base decisions about their lifestyle on the people and environment they are surrounded by. For example, unsafe routes to work or a shopping outlet may lead people to choose transport instead of walking. People’s work or school environments, healthcare and home life can also influence how a person behaves on a day-to-day basis.

Genetics: Research has suggested that genetic factors do influence how people respond to a high calorie intake or changes in the environment. Gene variants have also been identified that increase hunger and the amount of food people eat.

Disease: Certain diseases can lead to a person becoming overweight or obese, with examples being Cushing’s disease and polycystic ovary syndrome.

Medications: Certain medications such as antidepressants and steroids can lead to weight gain.

Causes Among Children

The causes of overweight and obesity in children are similar to those that cause adult obesity, such as unhealthy day-to-day behaviors and genetic risk factors. Eating a high-calorie diet that is rich in fat and low in nutrients and having a sedentary lifestyle that involves watching lots of TV and playing computer games can increase the risk of a child becoming overweight or obese.

Engaging in regular exercise and eating a healthy diet, on the other hand, can help children to grow without gaining too much weight. Again, environments or communities that do not encourage a healthy lifestyle can make it difficult for children to choose healthy foods and get enough exercise. Other environmental factors include availability and affordability of foods, food advertising and the behaviors of peers.