Obesity questions and answers

Obesity is defined as an excessively high amount of body fat (adipose tissue) in relation to lean body mass. Obesity is associated with a substantially increased risk of a number of health conditions.

Why are we concerned about obesity?

There is evidence that obese children and adults are at greater risk of short-term and long-term health consequences.

Obese children are likely to be obese into adulthood and to have abnormal lipid profiles, impaired glucose tolerance and high blood pressure at a younger age. Obesity in children is also associated with musculoskeletal problems, asthma and psychological problems including body dissatisfaction, poor self esteem, depression and other mental health problems. Obesity is also associated with a long list of adult health conditions including Type 2 diabetes, ischaemic heart disease (IHD), stroke, several common cancers, osteoarthritis, sleep apnoea and reproductive abnormalities.

The impact of excess body weight on these diseases operates, at least in part, through its effects on insulin resistance, blood glucose, blood lipids and blood pressure. It is important to note that although BMI cut-offs have been used to define overweight and obesity, the risk of disease increases as BMI increases in all population groups, even those within the ‘normal’ range.

What causes overweight and obesity?

Overweight and obesity are the result of a positive energy balance – that is, a long term excess of energy intake (food and beverage consumption) over energy expenditure (basal metabolic rate, physical activity).

Although some people are more genetically susceptible to weight gain than others, the rapid increase in the prevalence of obesity in recent years has occurred too quickly to be explained by genetic changes and most experts believe it is due to living in an increasingly ‘obesogenic’ environment – one that promotes over-consumption of food and drinks and limits opportunities for physical activity.

How is obesity measured?

Body mass index (BMI) is a commonly used measure to classify underweight, overweight and obesity in both children and adults. BMI is a measure of weight adjusted for height and is calculated by dividing weight in kilograms by height in metres squared (kg/m2).

International cut-off points for adults aged 18 years and over
Classification BMI score (kg/m2) Risk of co-morbidity (multiple diseases)
Underweight < 18.50 Low risk (but risk of other clinical problems increased)
Normal range 18.50–24.99 Average risk
Overweight 25.00–29.99 Increased risk
Obese:

Obese (class I)

Obese (class II)

Obese (class III)

≥ 30.00

30.00–34.99

35.00–39.99

≥ 40.00

High risk

Moderate risk

Severe risk

Very severe risk

For children aged 2−17 years, BMI cut-off points developed by the International Taskforce on Obesity (IOTF) are used to define thinness, overweight and obesity. The IOTF BMI cut-off points are sex and age-specific, and have been designed to coincide with the WHO BMI cut-off points for adults at age 18 years.

International Taskforce on Obesity BMI cut-offs for overweight and obesity in children and young people (2–18 years)
Age Thinness Overweight Obese
  Males Females Males Females Males Females
2 15.14 14.83 18.41 18.02 20.09 19.81
2.5 14.92 14.63 18.13 17.76 19.80 19.55
3 14.74 14.47 17.89 17.56 19.57 19.36
3.5 14.57 14.32 17.69 17.40 19.39 19.23
4 14.43 14.19 17.55 17.28 19.29 19.15
4.5 14.31 14.06 17.47 17.19 19.26 19.12
5 14.21 13.94 17.42 17.15 19.30 19.17
5.5 14.13 13.86 17.45 17.20 19.47 19.34
6 14.07 13.82 17.55 17.34 19.78 19.65
6.5 14.04 13.85 17.71 17.53 20.23 20.08
7 14.04 13.86 17.92 17.75 20.63 20.51
7.5 14.08 13.93 18.16 18.03 21.09 21.01
8 14.15 14.02 18.44 18.35 21.60 21.57
8.5 14.24 14.14 18.76 18.69 22.17 22.18
9 14.35 14.28 19.10 19.07 22.77 22.81
9.5 14.49 14.43 19.46 19.45 23.39 23.46
10 14.64 14.61 19.84 19.86 24.00 24.11
10.5 14.80 14.81 20.20 20.29 24.57 24.77
11 14.97 15.05 20.55 20.74 25.10 25.42
11.5 15.16 15.32 20.89 21.20 25.58 26.05
12 15.35 15.62 21.22 21.68 26.02 26.67
12.5 15.58 15.93 21.56 22.14 26.43 27.24
13 15.84 16.26 21.91 22.58 26.84 27.76
13.5 16.12 16.57 22.27 22.98 27.25 28.20
14 16.41 16.88 22.62 23.34 27.63 28.57
14.5 16.69 17.18 22.96 23.66 27.98 28.87
15 16.98 17.45 23.29 23.94 28.30 29.11
15.5 17.26 17.69 23.60 24.17 28.60 29.29
16 17.54 17.91 23.90 24.37 28.88 29.43
16.5 17.80 18.09 24.19 24.54 29.14 29.56
17 18.05 18.25 24.46 24.70 29.41 29.69
17.5 18.28 18.38 24.73 24.85 29.70 29.84
18 18.50 18.50 25.00 25.00 30.00 30.00

Since BMI does not distinguish between weight associated with muscle and weight associated with fat, it provides only a crude measure of body fatness in individuals. If you are concerned about your BMI please see your health professional.

The above BMI cut-offs apply to all ethnic groups.

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