1.4 Weaknesses of the body mass index
BMI is just a measure of weight relative to height, and as such is very much an imperfect health metric. For example, a very muscular person may weigh more because muscle is denser than fat. As a result, they may have a higher BMI than someone who has less muscle but is the same height.
A person’s high BMI shouldn’t necessarily be cause for concern, so much as an unhealthy diet and lack of physical activity, as these factors are the leading causes of the major non-communicable diseases, such as diabetes and cardiovascular disease (World Health Organization, 2023). Researchers have also questioned the validity of BMI as a representative health measure:
Primarily derived from data obtained on Anglo-Saxon populations, the generalizability and applicability of the BMI and its cut-off points to other populations has been questioned and its sensitivity as a measure of excess fat queried.
The next activity asks you to categorise three adults based on their BMI (health and social care professionals are often required to interpret and use pre-existing data). You’ll also think more about why BMI on its own can say very little about a person’s overall health.
Activity 2 BMI categories explored
Use the BMI chart in Figure 5 to identify the category into which each of the three people listed below fit (i.e. ‘underweight’, ‘healthy weight range’, ‘overweight’, ‘obese’ or ‘severely obese’).
Here is the key to the colour coding in the chart:
- underweight = green
- healthy weight = blue
- overweight = white
- obese = orange
- severely obese = purple
If you would like to access a larger version, click on ‘Maximise’ beneath the image. There is also an accessible version available as a Word file.
Table 2 BMI data
| Name | Height (cm) | Weight (kg) |
|---|---|---|
| Kaya | 166 | 85 |
| Bradley | 158 | 86 |
| Amber | 171 | 90 |
Kaya:
Bradley:
Amber:
What can you conclude about the health of Kaya, Bradley and Amber as a result of determining their BMI?
Discussion
Each of these people fall into the obese category. Kaya and Amber each have a BMI around 31 kg/m2, while Bradley has a BMI around 34 kg/m2. Based purely on their BMIs, you might assume that they are in poor health, but this would not be accurate – each person would have been on peak form at this weight in order to compete in the 2016 Rio Olympic Games:
- Kaya Salman represented Turkey in athletics.
- Bradley Tandy represented South Africa in swimming.
- Amber Campbell represented the USA in athletics.
Very little can actually be determined about their health based on their BMIs. One would assume, by virtue of being Olympians, that they are in very good health. As such, it is fair to say that BMI is not an especially valid measure of health. But it is often used as a proxy measure, or in other words, as a ‘rough tool’ to determine whether a person is overweight or obese.
OpenLearn - Healthy living and choice (title tbc)
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