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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 community over the past 20 years. The most recent Australian Health Survey in 2011-12 reported that 63% of Australians aged 18 years and over were overweight or obese. More men (68%) were found to be overweight or obese than women (56%)  .
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
Body mass index is a tool that is used to classify the weight of adults into healthy, overweight and obese. It is recommended by the World Health Organization (WHO) and used as the main measurement in many guidelines for obesity around the world.
Your BMI is calculated by dividing your weight (in kilograms) by your height (in metres) squared.
18.5 - 24.9
25.0 - 29.9
It is important to remember that the BMI is an approximate guide and is not the best guide 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).
BMI can tend to overestimate in:
- Athletes, particularly those who build a lot of muscle with weight or resistance training; and,
- Some ethnic groups, such as Pacific Islanders, who tend to have higher levels of lean body mass compared to body fat.
BMI can tend to underestimate in:
- Older people, who tend to lose muscle and bone density as they age; and,
- People of South Asian, Japanese and Chinese heritage, who tend to have higher levels of body fat compared to lean body mass.
Also, BMI does not take into account where 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, doesn't tend to cause as many problems as fat around your abdomen. This is because fat which builds up around the organs, inside the abdominal cavity, can cause inflammation within the body and contributes to many health problems associated with obesity.
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, including indigenous Australians, 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.
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.
Waist measurements and health risk
Waist measurement in centimetres
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 90cm and women at 80cm, 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.
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 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 fibre 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.
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.
- Cushing's syndrome;
- Polycystic ovary syndrome (PCOS);
- Inherited conditions, such as Prader-Willi syndrome, and;
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;
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.
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.
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, neighbourhood 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).
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.
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:
- Cardiovascular disease including heart attack and stroke;
- High blood pressure (hypertension);
- Type 2 diabetes;
- A range of cancers including bowel, breast, ovarian, endometrial, oesophageal, gall bladder, pancreatic and kidney cancer;
- Obstructive sleep apnoea;
- Osteoarthritis, spinal problems and lower back pain;
- Kidney disease;
- Stress urinary incontinence and fertility problems in women;
- Gall bladder, liver and pancreatic disease, and;
- Mental health problems such as depression and anxiety.
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.
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.
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:
- Reduce the risk of cardiovascular disease;
- Reduce blood pressure;
- Help to prevent, delay or improve control of type 2 diabetes;
- Improve kidney disease;
- Improve sleep apnoea;
- Reduce heartburn;
- Reduce symptoms of musculoskeletal problems such as osteoarthritis, and;
- Reduce symptoms of urinary incontinence.
With greater weight loss, the health benefits can be largely increased.
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.
- Overweight and obesity. Australian Bureau of Statistics. Accessed 31 March 2015 from link here
- Healthy weight – it’s not a diet it’s a lifestyle! Centers for disease control and prevention. Accessed 31 March 2015 from link here
- Clinical Practice Guidelines for the Management of Overweight and Obesity in Adults Adolescents and Children in Australia (2013) | National Health and Medical Research Council. Accessed July 20 2014. link here
- How Are Overweight and Obesity Diagnosed? - NHLBI NIH. Accessed July 20 2014. link here
- Kilojoules and Calories | Better Health Channel. Accessed July 20 2014. link here
- Obesity | Better Health Channel. Accessed July 20 2014. link here
- Obesity and Overweight for Professionals: Adult: Causes - DNPAO - CDC. Accessed July 20 2014. link here
- Statistics c=AU; o=Commonwealth of Australia; ou=Australian Bureau of. Chapter - Overweight and Obesity. Accessed July 20 2014. link here
FAQ Frequently asked questions
What is obesity?
Obesity is 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.
What is the difference between being overweight and obese?
The body mass index (BMI) and waist circumference are used to classify the weight of adults into healthy, overweight and obese. The BMI is calculated by dividing weight (in kilograms) by height (in metres), squared. For a healthy …
What is the body mass index?
The Body Mass Index (BMI) is a tool used to classify weight in adults into healthy, overweight and obese. BMI is calculated by dividing your weight (in kilograms) by your height (in metres), squared.
Is the body mass index (BMI) completely accurate?
No, the BMI is a only an approximate guide and it is not equally 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). BMI can …
What causes obesity?
Weight gain that can lead to obesity is caused by consuming more energy through food and drink than your body can use. Some medical conditions and medications can contribute to weight gain.
What are the health consequences of obesity?
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 extra load and stress on important structures such as the …
What medications can cause obesity?
Some medications have been shown to increase 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 …
Why does obesity cause health problems?
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 extra load and stress on important structures such as the joints, …
How can I lose weight?
Losing weight can be challenging, and it's 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 …