ObesityObesity is a condition of excess body weight measured by the body mass index (BMI). Obesity is caused by an imbalance between caloric intake and expenditure, or, in other words, eating more food than your body can use. It usually results from a diet high in saturated fats, refined carbohydrates and salts, and low levels of physical exercise.
The body mass index is a measure of the body fat in adult Caucasians. To calculate your BMI, you need to know your height in metres and weight in kilograms. Your BMI is calculated by dividing your weight by the square of your height.
The normal BMI range for adult Caucasians is 18–24.9. A person with a BMI in the range 25–29 is classified as overweight, while one exceeding 30 is classified as obese. When assessing health risks, obese individuals are often further classified as:

  • Severely obese or class 1 obese (BMI 30–34.9)
  • Morbidly obese or class 2 obese (BMI 35–39.9)
  • Super obese or class 3 obese (BMI > 40.2)
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Obesity is now considered an epidemic in many Western countries. In Europe, America and Australia, more than half of all adults and children are overweight or obese. More than 25% of Australian adults are obese, and about 60% are classified as overweight or obese.
A person’s lifestyle and background influences their risk of becoming obese. People who have a low level of education, watch a lot of television and/or have low levels of physical activity are more likely to be obese.
There are many health risks associated with being overweight or obese. They include an increased risk of developing a variety of cancers. This risk rises with your BMI.


Cancer is a commonly occurring condition of unregulated cell growth. Normal human cells regulate their replication and divide only when new cells are needed (i.e. the number of cells dying and emerging are equal). Cancerous cells, on the other hand, continually replicate without structure or organisation. The excessive cancer cells invade healthy human organs and form a tumour.
Benign tumours are generally not life threatening as they do not invade the surrounding tissues and organs and are not considered cancers. Malignant tumours on the other hand, can move between different sites in the body (e.g. the breast to the lung), and are classified as cancers. Cancers can be classified as low, intermediate and high grade depending on the size of the tumour and degree of similarity between cancerous and normal cells. These influence the number of new cancer cells forming and the chances of successful treatment.
Some individuals may be predisposed to cancer because of their genes. However, lifestyle factors and exposure to carcinogens cause most cancers. Cigarette smoking is the behaviour most commonly associated with cancer, although in recent years the associations between obesity and cancer have received increasing attention. Obesity is now the second leading cause of cancer following smoking.

The link between cancer and obesity

There is evidence that the more your BMI increases, the greater your risk of developing some cancers. For example, the risk of colorectal cancers increases by 7% for every 2 kg/m2 increase in BMI. The cancers associated with obesity include cancers of the:

While it is clear that obesity increases the risk of cancer, scientists are still investigating why this is the case. What is currently known is that the relationships between obesity and cancer are complex and depend on the type of cancer.
Being obese changes hormone levels in the body. This is thought to underpin the increased risk of breast, colon, endometrial, kidney and oesophageal cancers. The risk of gallbladder cancer is likely associated with the increased risk of gallstones in obese patients; gastrointestinal conditions associated with obesity (e.g. reflux) are the most likely explanation for obese patients’ increased risk of gastrointestinal cancers. Obese individuals are also more susceptible to inflammatory conditions, which are associated with an increased risk of cancer.

The benefits of maintaining a healthy weight

The old saying “prevention is better than cure” certainly applies to obesity. While it is likely that obese people who reduce weight will also reduce their cancer risk, scientists are still determining the extent to which this is true.
Currently there is evidence that weight loss has a positive effect on the risk of endometrial and breast cancers. There is also evidence suggesting that losing weight can increase the likelihood of cancer treatment being successful, and reduce the risk of cancer recurring after treatment.
Due to the many known health benefits of weight loss in obese individuals (e.g. reduced risk of diabetes) and the likely benefit in reducing cancer risk, weight loss is an extremely important goal for overweight and obese people wishing to improve their health.


Once obese, many individuals remain obese for a long time. Many diet and exercise interventions have proven effective in promoting weight loss in the short term; however, long term weight loss is more difficult to achieve. This is evidenced by the increasing prevalence of obesity in both developed and developing countries. In Australia, almost 60% of the adult population is either overweight or obese, and the prevalence of obesity is increasing – the prevalence of obesity was 2.5 times higher in 1999 than in 1980.

Strategies for losing weight

Support from health professionals, family and friends
Overweight and obesityLosing weight often requires professional help. If you are obese and want help developing a weight loss program, talk to you doctor, who can help you or refer you to somebody else who can.
In most cases, health professionals will provide you support and information to help you lose weight. This will include recommending diet and exercise strategies, and helping you monitor how well they work. It is important that you limit stress, boredom and frustration when trying to lose weight, as these can cause you to eat unnecessarily.
If you are having problems sticking to your diet or exercise schedule, a health practitioner can be a good source of support. Make sure you also have support in your living environment. Keep unhealthy foods out of sight (they’re much less tempting when you can’t see them) and encourage relatives or friends to join you in exercising. Eating habits develop over a long period of time and can be difficult to change, so support from friends, family and health professionals is important.
If you are supporting a family member or friend in losing weight, remember to be tactful.

Changing your diet

Consuming too many calories is the main cause of obesity. Developing and maintaining a healthy, balanced diet is key to reducing and maintaining a normal weight (i.e. BMI 18–24.9).
While there are many popular diets which can rapidly reduce weight in the short term, these are generally ineffective in reducing weight in the long term. For sustained weight loss, healthy eating habits must be developed and maintained. One important aspect of ensuring the development of long term healthy eating habits is developing a lifestyle which enables healthy, fresh food to be accessed and prepared for consumption. For many people, accessing fresh food is difficult because they do not have enough money, are too busy to cook it, or are limited by their location. If you are too busy to cook involved meals, it may be a good idea to become familiar with some healthy, fast recipes. If money is an issue, you may wish to start a vegetable garden, or learn some new recipes which are less expensive.
There are a few key components of a well balanced diet. As you attempt to lose weight, you should:

  1. Eat more fruit and vegetables: Eat a piece of fruit instead of sweets, chips or other snacks. Try to eat at least 2 pieces of fruit and 5 servings of vegetables or legumes (e.g. beans) per day.
  2. Reduce intake of fats, particularly saturated fats: Use vegetable or fish oils instead of animal fats. Eat lean meat (e.g. chicken breast, fish) instead of fatty meats (eg. beef). Trim fat off meat. Grill or boil food instead of frying it. Substitute full fat products with low fat (e.g. dairy). Avoid high fat meals with low nutritional content (e.g. chips, hamburgers, fried chicken).
  3. Limit consumption of refined carbohydrates and salt: Limit the amount of sweets, biscuits, cakes and potato chips you consume. Try dried or fresh fruits and vegetables, wholewheat biscuits and breads, low fat yoghurt, seeds, and legumes as alternative healthy snacks.
  4. Eat fresh rather than processed foods: Fresh foods contain more vitamins and minerals than processed foods, and often less calories, so are a vital part of a healthy diet. While fresh, uncooked vegetables are the best source of vitamins and minerals, cooked, frozen and dried fruits and vegetables also provide good sources.
  5. Eat from the five food groups: The body requires a range of different nutrients and micronutrients to maintain an optimum metabolic balance. It is therefore important to eat from all the food groups.


ExerciseIt is generally recommended that individuals engage in moderate exercise (e.g. brisk walking) for at least thirty minutes (preferably for one hour) a day to help maintain a healthy weight. More vigorous exercise (e.g. jogging) several times per week can further reduce cancer risk. Daily exercise can be obtained in a single session or in bits and pieces throughout the day. There are lots of ways to “Find 30” minutes of exercise in little bits throughout the day – you might do some heavy chores around the house, dig the garden or play games with the kids or pets.
If you are obese, the additional weight you carry puts a lot of pressure on your muscles and joints, and vigorous exercise can be dangerous. It is best to start with moderate exercise and increase gradually. Before you start exercising, speak to a health professional who will be able to advise you which exercises are suitable and how long you should exercise for each day.
Walking is one of the best forms of exercise, so try to fit walking into your daily routine, whether it’s to the shops or around the park, up the stairs or for a Sunday outing.
Depending on where you live and your financial situation, you may wish to visit a gym or swimming pool. You may even want one-on-one support from a personal trainer. Keep in mind that this is a fairly expensive option, and a friend who will support you and make exercise fun is often just as good.

Treatment with medications

There is a range of medicines which can enhance the benefits of healthy eating and exercise for obese people trying to lose weight. These should only be tried if exercise and diet modifications fail to result in sufficient weight loss. Weight loss medications are effective in addition to, rather than instead of, long term diet and exercise changes.
There are two types of medications that can assist in the treatment of obesity: appetite suppressants (which make you fell less hungry) and fat absorption inhibitors (which reduce the amount of fat your body absorbs). These medicines are only available on prescription in Australia, so talk to your doctor about whether they are suitable for helping you lose weight.

Surgical treatments

Surgery to treat obesity is generally only performed on individuals 18–55 years of age who have been obese for more than five years and who have a BMI > 40 kg/m2. As with medications, it is important to remember that surgical treatments are most effective when strict diet and exercise patterns are included.
Although infrequently performed, surgery is a more effective intervention than prescribing medications for reducing the weight of super obese (class 3 obese) individuals.

Top tips for preventing cancer

Obesity increases the risk of cancer, but obesity is a preventable and treatable disease. To reduce the risk of cancer associated with obesity, an individual should maintain a BMI of 18–24.9. To maintain or reduce to a healthy weight it is important to:

  • Exercise for at least 30 minutes each day
  • Eat lots of fresh food, especially fruits and vegetables
  • Limit the amount of fat, salt and refined sugar you consume

If you are obese and having difficulty losing weight:

  • Talk to your doctor about whether or not taking medications will help you
  • Talk to your doctor about whether or not surgery is an appropriate strategy

Remember, family and friends can support you and encourage you to exercise and eat healthy food.

More information

Obesity and weight loss
For more information on obesity, health and social issues, and methods of weight loss, as well as some useful tools, see
Obesity and Weight Loss.
Living with obesity
For more information on living with obesity, including discussing obesity with friends or loved ones, bullying and obesity in children, obesity and its cost on the workplace and links between obesity and pain, sexuality, fertility and depression, see Living with Obesity.



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