What is obesity?

Obesity is defined as having a body weight that is greater than that considered healthy for your height. The main contributor to an increase in body weight is excess fat, which can have a range of serious consequences for your health. Levels of obesity have been increasing in the US community over the past 20 years. Obesity levels in 2013-2014 were:


  • Percent of adults aged 20 and over with obesity: 37.9%
  • Percent of adults aged 20 and over with overweight, including obesity: 70.7%

Children and adolescents

  • Percent of adolescents aged 12-19 years with obesity: 20.6%
  • Percent of children aged 6-11 years with obesity: 17.4%
  • Percent of children aged 2-5 years with obesity: 9.4%

Source: Centers for Disease Control and Prevention FastStats - Overweight Prevalence

Measurement of overweight and obesity

As carrying excess weight can affect many aspects of your health, your doctor may ask to weigh and measure you and regularly discuss your weight.

There are a number or measures that can be applied to determine whether a person's weight may cause health problems. Two of the most commonly used are:

  • Body mass index (BMI), and;
  • Waist circumference.

Body mass index

BMI is a tool that is used to classify the weight of adults into healthy, overweight and obese. BMI is also used to classify the weight of boys and girls aged 2-18, but it is interpreted differently to adults.

It is recommended by the World Health Organization and used as the main measurement in many guidelines for obesity around the world.

Your BMI is calculated by dividing your weight (in pounds or kilograms) by your height (in inches or meters) squared described in more detail below.

Pounds and inches: Formula: weight (lb) / [height (in)]2 x 703
Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.
Example: Weight = 150 lbs, Height = 5'5″ (65″)
Calculation: [150 ÷ (65)]2 x 703 = 24.96
Formula: weight (kg) / [height (m)]2

Kilograms and meters (or centimeters): With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Because height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters.
Example: Weight = 68 kg, Height = 165 cm (1.65 m)
Calculation: 68 ÷ (1.65)2 = 24.98

BMI score

Weight range




Healthy weight





It is important to remember that the BMI is an approximate guide and is not accurate for everyone. One reason for this is that people with the same weight can carry different amounts of fat compared to their lean body mass (bone and muscle).

Also, BMI does not take into account where body fat is accumulated on the body. Not all body fat contributes equally to health problems. Fat underneath the skin, particularly on the hips and thighs, does not tend to cause as many problems as fat around your abdomen. In particular, fat that builds up around the organs inside the abdominal cavity can cause inflammation within the body and contributes to many of the health problems associated with carrying extra weight.

At any given weight, different people will carry varying amounts of fat around their abdomen. The greater the abdominal fat, the greater the risk of health problems. People from some ethnic groups can tend to carry more fat around their abdomen.

For this reason, waist circumference, which can be a good indicator of how much abdominal fat a person is carrying, is also often used to assess how much of a risk a person's extra weight may pose to their health.

Waist circumference

To measure waist circumference with a tape measure:

  • Stand straight with feet fairly close together, about 15cm apart, and breathe normally;
  • Place a tape directly on the skin by removing any bulky clothes before measuring;
  • Place the tape horizontally around the waist at around the level of the bellybutton, and;
  • Breathe out and take the measurement.

Man measuring his waist circumference.Measuring waist circumference. 

Waist measurements and health risk


Waist measurement in inches and centimeters

Health risk


> 37.01 inches (94 centimeters)


> 40.15 inches (102 centimeters)



> 31.5 inches (80 centimeters)


> 34.64 inches (88 centimeters)


Just as with BMI, waist circumference does not apply equally to everyone:

  • For people of South Asian, Japanese and Chinese heritage, increased risk occurs for men at 35.43 inches (90 centimeters) and women at 31.5 inches (80 centimeters), and;
  • Health risk may be slightly overestimated for people of some ethnic groups, such as Pacific Islanders and people of African-American descent.

Also, for people who have greater-than-average waist circumferences for medical reasons, such as women during pregnancy and people with health conditions that cause the belly to swell or distend, these measurements are not a good health indicator.


The part of the body that lies between the chest and the pelvis.


In some respects, the cause of obesity is quite simple; it occurs because you take in more energy through food and drink than your body uses, so the extra energy gets converted into fat.

Energy in

Energy in food is measured in kilojoules or calories (4.2 kilojoules = 1 calorie). Foods vary in the amount of energy they provide. For example, vegetables tend to be low in energy and high in dietary fiber and nutrients that the body requires. Foods that contain a lot of carbohydrates and fats tend to be high in energy.

Beverages also contribute to energy intake. While water contains no energy, soft drinks, fruit juices and alcoholic beverages are all high in energy.

The portion size of the foods you eat is also important. Even foods with moderate amounts of energy can contribute to weight gain if they are eaten to excess.

Energy out

Metabolism describes the chemical processes that occur inside the cells in your body in order for the body to function. These processes require energy and this is called your basal metabolism.

Physical activity also uses up energy. The more vigorous the exercise (such as running), the more energy the body will use, although all activities, such as housework, walking to the shops and even standing still, contribute to the amount of energy the body uses every day.

With high-energy foods, you can take in large amounts of energy very quickly, while it takes much more time and effort to work off that energy.

Control of body weight

Taking in small amounts of unneeded extra energy on a daily basis can eventually add up to carrying quite a lot of extra weight if you do it over a long period of time. However, the way the body regulates weight is more complex. It involves a complex system of hormones, including insulin, leptin and ghrelin, that:

  • Increase hunger;
  • Decrease appetite and food intake, and;
  • Increase metabolism and the amount of energy the body uses.

Generally speaking, this system works to try to maintain body weight at the same level over time. This level is sometimes called a set-point and when your weight increases or decreases from your set-point, the hormones will try to work together to get your weight back to that level.

When a person gains a lot of weight, the body can change the set-point to a higher level and this is one reason why it can be difficult to lose excess weight once it has been gained.

Medical conditions that can contribute to weight gain

There are some medical conditions that can contribute to weight gain and make it harder to lose weight.

These include:

Medications that can contribute to weight gain

Some medications have been shown to cause weight gain. They include:

  • Medications for bipolar disorder (including lithium, clozapine and olanzapine);
  • Medications for type 2 diabetes (sulphonylureas such as chlorpropamide and thiazolidenes such as pioglitazone);
  • Beta-blocker medications for high blood pressure (hypertension) such as propranolol;
  • Tricyclic antidepressants such as amitriptyline;
  • Steroids, and;
  • Insulin.

Quitting smoking

People who quit smoking tend to gain weight. How much weight is gained varies from person to person and the health benefits of quitting smoking will most likely outweigh any health risks caused by any weight gain.

Dietary fiber

Ingredients in food that are not digestible. They pass through the digestive system relatively unchanged and help to keep the digestive system healthy.


A hormone secreted by the pancreas in order to regulate glucose levels in the body's cells, which is used for energy.

Prader-Willi syndrome

A rare genetic disorder that affects development and growth, typically causing poor muscle tone, short stature, incomplete sexual development, cognitive disabilities, problem behaviors and obsessive eating.


A chemical produced by the fatty tissue that signals to the brain feelings of fullness.


A chemical produced by stomach cells that signals to the brain feelings of hunger.

Cushing's syndrome

A group of disorders characterized by high levels of cortisol, an important chemical in the body with numerous functions including stress responses, maintaining blood sugar levels and helping the immune system.

Risk factors

Beyond the personal choices about what foods you eat and how physically active you are, there are many factors that can affect your weight. These include:


The genes you inherit from your parents strongly affect your weight.


Epigenetics describes how genes can be turned on and off (called gene expression) without the DNA sequence of the genes themselves being changed. It is thought some epigenetic changes may play a significant role in controlling body weight.

Early life

Factors in early life, such as the nutrition of the mother during pregnancy, low or high birth weights and the amount of weight gained during the first few months of life can affect body weight later in life.


Environment affects the way people behave. Factors that contribute to weight gain are sometimes called obesogenic influences. Research into how they contribute to weight gain in the community is ongoing. These factors can include the kinds of foods that are easily available, neighborhood design and infrastructure that makes physical activity difficult or unsafe, sedentary jobs and poor quality or disrupted sleep (which can affect metabolism and hormones that control appetite).

Psychological factors

Stress has been linked to increased eating and a lack of energy, which can reduce the motivation to be physically active. Mental health issues and obesity are strongly linked. For example, people with depression are more likely to become obese and people who are obese are more likely to become depressed.


The genetic material of all living cells and some viruses. The full name is deoxyribonucleic acid.


A unit of inheritance (heredity) of a living organism. A segment of genetic material, typically DNA, that specifies the structure of a protein or related molecules. Genes are passed on to offspring so that traits are inherited, making you who you are and what you look like.

Health consequences of being overweight or obese

Carrying extra weight has a range of consequences on the body. Not only can it increase levels of inflammation in the body and affect the body's metabolic processes, it also places additional stress on the joints, airways and bladder.

Generally speaking, the more weight a person carries, the greater the risk they will also develop a range of other serious health conditions, including:

Carrying very large amounts of extra weight can also make it more difficult for doctors to treat a wide range of other health problems. For example, surgery can be more complicated and it can take longer to recover.

Types of treatment

Losing weight can be challenging and its important to have realistic expectations about how fast you can lose weight and how much weight you plan to lose. Your doctor and other health professionals can provide guidance and support towards achieving a healthy weight.

The good news is that even relatively small reductions in weight (as little as a few kilograms) can be effective in reducing your risk of many of the health conditions associated with overweight and obesity.

Recommended approaches to weight loss include:

  • Reducing energy intake via a healthy eating plan, and;
  • Increasing your physical activity levels to increase your energy output.

A woman stirring a pot on the stove, cooking a healthy meal.Healthy eating is important for maintenance of a healthy weight and prevention of obesity. 

Options such as medication to assist weight loss and bariatric surgery, also known as obesity surgery, may be considered for people who:

  • Have not been able to lose weight or prevent weight gain despite trying lifestyle changes;
  • Are obese and have a lot of weight to lose, and;
  • Have related health problems such as type 2 diabetes.

Your doctor can discuss whether these options are suitable for you.

Bariatric surgery

Also known as obesity surgery. Surgical techniques used to treat obesity and promote weight loss. Techniques include gastric banding and stapling surgery and bowel bypass surgery.


Being overweight or obese increases your risk of a range of serious health conditions and can be linked with reduced life expectancy; however, even losing relatively small amounts of weight can help to reduce these risks.

For example, for people who are overweight or obese, losing 5% of their initial body weight can help to:

With greater weight loss, the health benefits can be largely increased.

Healthy weight – it’s not a diet it’s a lifestyle! Centers for disease control and prevention. Accessed 31 March 2015 from

External link


It is easier to prevent weight gain than to try and lose it. The same lifestyle changes that can help to lose weight can also help to prevent weight gain.

Physical activity and healthy eating will prevent obesity.Exercise will enable weight loss and prevention of obesity. 

Healthy weight – it’s not a diet it’s a lifestyle! Centers for disease control and prevention. Accessed 31 March 2015 from

External link

FAQ Frequently asked questions