What is a Healthy Weight
How to Measure your Body Mass Index (BMI)
There are no perfect measures of overweight and obesity. Body Mass Index (BMI), which is calculated by dividing weight in kilograms by height in metres squared, is used most often – particularly in assessing overweight and obesity at the population level.At the individual level however, BMI does have some limitations in that it can be influenced by age, gender and ethnicity. Also, BMI does not distinguish fat mass from lean mass, nor does it necessarily reflect body-fat distribution.
For example, a woman 1.67m in height and weighing 65kg would have a BMI of 23.3, which falls within the healthy weight range. However, this may not be not an accurate predictor of the proportion of body fat to lean mass or fat distribution, particularly in older people or muscular individuals such as athletes, because of the differences in proportions of fat mass to lean mass and distribution of body fat.
The BMI cut-off points are based on associations between chronic disease and mortality and have been adopted for use internationally by the World Health Organisation (WHO).
The classification table below used in conjunction with a BMI calculator can assist you to assess whether you weight is in the healthy range.
Classification of Body Mass Index
Classification | BMI (kg/m2 | Risk of co-morbidities |
|---|---|---|
| Underweight | <18.50 | Low (but possibly increased risk of other clinical problems) |
| Normal range (Healthy Weight) | 18.50 - 24.99 | Average |
| Overweight: | >25.00 |  |
| Pre-obese | 25.00 - 29.99 | Increased |
| Obese class 1 | 30.00 - 34.99 | Moderate |
| Obese class 2 | 35.00 - 39.99 | Severe |
| Obese class 3 | >40.00 | Very severe |
Reproduced from: Obesity: Preventing and Managing the Global Epidemic, 2000, WHO, Geneva. Note: These BMI cut off levels may vary somewhat in different publications as they are indicative only.
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Waist circumference
A person’s waist circumference may be a better predictor of health risk than BMI. Having fat around the abdominal organs and enlarged waist circumference, regardless of your BMI, means you are more likely to develop certain obesity-related health conditions. Fat predominantly deposited around the hips and buttocks does not appear to have the same risk. Men and post menopausal women are at greater risk of excess fat in the waist region.Waist circumference for women: a waist circumference of 80cm or over indicates increased risk of obesity related health conditions. A waist circumference of 88cm or more indicates a substantially increased risk.
Waist circumference for men: a waist circumference of 94cm or over indicates increased risk of obesity related health conditions. A waist circumference of 102cm or more indicates a substantially increased risk.
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What factors may affect your weight
While there are many factors involved, an increase in body weight may result from an imbalance between energy intake (food) and energy expenditure (metabolism, thermogenesis and physical activity) over a sustained period. An increase in body weight can also result from an increase in muscle mass. This is a consideration for people who participate in regular weight training and does not reflect unhealthy weight gain.In children, there is evidence that factors early in life have the potential to contribute to the development of obesity later in life. These factors include genetics, poor foetal nutrition, low birth weight, maternal diet and absence of breastfeeding as well as levels of physical activity and diet in childhood.
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Why do energy imbalances occur?
While everyone is at risk of at least becoming overweight in the modern ‘obesogenic’ environment, particular factors influence why weight is gained. These include:- Previous history of weight loss. The effects of weight cycling – frequent large gains and losses – on long term health are unclear, but there are associations between the number of failed weight loss attempts and current body weight, as well as health risks.
- Life Stage. Weight gain is common, although not inevitable, at various life stages – for example, after pregnancy, and during menopause.
- Life events. Certain life events – such as marriage, giving up sport, and quitting smoking – can cause weight gain. Weight gain after quitting smoking can be significant (i.e. 5 kg in the first year). For this reason, instituting a weight management plan at the time of quitting may help reduce the weight gain that normally occurs after quitting.
- Family, work and social environments. Can influence weight gain and the inability to lose weight.
- Genetic influences. Genetic predisposition can influence the amount and rate at which weight is gained and lost.
- Stress. May need to be considered as a factor that can cause either weight gain or weight loss, depending on the person’s reaction to stress.
- Medical conditions. Certain medical conditions, for example, hypothyroidrism, are known causes of overweight.
- Medical treatments. Prescription medications can exacerbate weight gain (in particular, benzodiazepines, corticsteroids, anti-psychotics, tricyclic antidepressants, anti-epileptics, sulphonylureas, and insulin).