BMI is a popular method of measurement for physical condition by medical staff and the general public. It has been statisically proven as a relatively reliable indicator of health risk in general but it has its limitations.
Advantages of BMI
a) Non-invasive procedure
An underrated aspect of the calculation of BMI is that it is non-invasive. People tend to favour tests or procedures which rarely trouble them or make them feel ill at ease. For instance, skin fold tests require a practitioner to pinch a subject's body fat in up to 13 places. This can be rather embarrassing for some, regardless of whether or not they are overweight.
b) Simple, quick, effective and applies to all
The calculation of BMI is the same for both male or female, adult or otherwise. It requires only quick measurement of both the height and weight, followed by a simple calculation which can be computed manually on paper or through the internet.
c) Population assessment of overweight and obesity
Since calculation requires only height and weight, it is easy to use for clinicians and for the general public. The use of BMI allows people to compare their own weight status to that of the general population. It is a good indicator of a person's well-being since he/she is also able to analyze the risks involved relating to their own respective BMI.
Limitations of BMI
a) BMI is unable to discriminate between body fat and muscle.
A body builder may find himself as overweight in a BMI scale when in reality, he is muscular. BMI is not a direct measure of body fatness and that BMI is calculated from an individual's weight which includes both muscle and fat. For example, highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness. Body builders with higher muscle mass with height around six feet (1.82m) and weighing approximately 250 pounds (113kg) may find themselves to be classified under obese in the BMI. In short, BMI is unable to distinguish between body fat and muscle.
b) Interpretation of BMI is different for children and adults
Although the BMI number is calculated the same way for children and adults, the criteria used to interpret the meaning of the BMI number for children and teens are different from those used for adults. For children and teens, BMI age- and sex-specific percentiles are used for two reasons:
- The amount of body fat changes with age.
- The amount of body fat differs between girls and boys.
Because of these factors, the interpretation of BMI is both age- and sex-specific for children and teens. Specific growth charts take into account these differences and allow translation of a BMI number into a percentile for a child's sex and age. For adults, on the other hand, BMI is interpreted through categories that are not dependent on sex or age.
c) Asians versus Caucasians
In general, Asian populations have a higher percentage of body fat at a lower BMI compared to Caucasians. Generally, for the same BMI their body fat percentage was 3-5% points higher compared to Caucasians. The high body fat percentage at low BMI for Asians can be partly explained by differences in body build. For example, differences in trunk-to-leg-length ratio and differences in slenderness. Differences in muscle mass may also contribute to the cause. Thus, the relationship between percentage of body fat and BMI is ethnic-specific. Hence, universal BMI cut-off points are not appropriate to be used as comparison.
In March 2005, BMI cut-off levels for Singapore were even revised based on the findings from local studies and the recommendations from the WHO Expert Consultation in Singapore.
| BMI (kg/m²) for Adults | Health Risk |
| 27.5 and above | High Risk |
| 23 – 27.4 | Moderate Risk |
| 18.5 – 22.9 | Low Risk (healthy range) |
| Below 18.5 | Risk of nutritional deficiency diseases and osteoporosis |
Despite the limitations of the calculation of BMI, it remains a fairly reliable indicator of body fatness for most people. Additionally, BMI is inexpensive, non-invasive relatively simple for screening weight categories that may lead to health problems. It allows people to follow up with a suitable diet plan or an exercise regime to fit with their respective health risks.
It is also important for people to keep in mind that the BMI and the health risks differ with ethinicity, age, sex as well as physical condition. The best way to a healthier lifestyle is by combining personal responsibility and consultation with certified medical practitioners.
