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Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.
What your BMI means
Once you have measured your BMI you can determine your healthy weight range. A BMI of:
Between 20 and 24.9 corresponds to the healthy weight range of young and middle-aged adults and BMI 23-28 to that of older adults.
Under 20 indicates that you are underweight.
Below 18.5 indicates that you are very underweight and possibly malnourished.
Between 25 and 29.9 indicates that you are mildly overweight.
Over 30 indicates that you are very overweight or obese.
Some exceptions to the rule
BMI does not differentiate between body fat and muscle mass. Therefore, body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight. Also, people who have a physical disability and are unable to walk, may have muscle wasting. Their BMI may be slightly lower but this does not necessarily mean they are underweight. In these instances it is important to consult a dietitian who will provide advice on the best weight to aim for.
In later life, it appears that older adults can tolerate higher BMI, with recommended cut-offs of 23-28.
A recent report from the International Diabetes Institute recommends that people who are shorter (for example Asian populations), the cut-offs for overweight and obesity may need to be lower. This is because there is an increased risk of diabetes and cardiovascular disease, which begins at a BMI as low as 23 in Asian populations. In taller Caucasian populations, this risk occurs around a BMI of 27. Data linking BMI and mortality in Asian populations, however, is currently unavailable or limited. This data is required before definitive cut-offs can be set for shorter stature populations.
Being overweight or underweight can affect your health
The link between being overweight or obese and the chance you will become ill is not definite. However, when data from a large group of people is analysed, statistically it is found that there is a greater chance of contracting various diseases if people are overweight. For example, the risk of death rises slightly (by 20-30 per cent) as BMI rises from 25 to 27. As BMI rises above 27 the risk rises more steeply, by 60 per cent.
The importance of being active
Findings from a study of 25,000 men questioned whether the problem is being overweight or sedentary. This study reported that the risk of death and disease is much lower in those who are physically fit even if overweight, than those who are unfit and in the normal weight range.
If you are overweight (BMI over 25) and physically inactive you may develop:
Cardiovascular (heart and blood circulation) disease
Gall bladder disease
High blood pressure (hypertension)
Diabetes
Osteoarthritis
Certain types of cancer, such as breast cancer.
Risks of being underweight
If you are underweight (BMI less than 20), you may develop:
Malnourishment
Compromised immune function
Respiratory disease
Tuberculosis (an infection of the lung)
Digestive (stomach) disease
Cancer
