The body mass index (BMI) is a method of assessing whether a person is a healthy weight in relation to their height. Some experts call it the Quetelet index. Using the person’s height and weight, BMI indicates whether an individual is underweight, a healthy weight, overweight, or obese. BMI has become the standard method for assessing body weight in many countries across the world. However, there are some instances where using BMI may not be appropriate or accurate.

How is BMI Calculated?

To calculate a person’s body mass index, they will need their height in meters and their body weight in kilograms. Their weight is divided by their height squared to arrive at a numerical value. There are many online tools that can calculate BMI easily.

BMI Thresholds

Once a person knows their BMI, they can use it to find out into which weight category they fall. Different countries have slightly different thresholds for these categories. In the USA, anybody with a BMI less than 18.5 is considered underweight. A BMI between 18.5 and 24.9 is healthy. People with a BMI between 25-29.9 are in the overweight range. Anyone with a BMI over 30 is classified, by this standard, as obese.

Why is BMI Helpful?

BMI is popular as a method of assessing body weight because it is generally good at predicting a person’s risk of weight-related disease. It’s easy to measure and can be calculated in the doctor’s office. Another advantage of using BMI measurements is that they are easy for people to understand. Although BMI is not always the best way of telling whether a person is a healthy weight, it is generally accurate for the majority of the population.

Limitations of the Body Mass Index

Some experts criticize the blanket use of the body mass index as the only measure of body weight because it has certain limitations. For example, a BMI measurement cannot discriminate between whether a person’s weight is because of fat or muscle. For this reason, very muscular individuals or athletes may be classed as overweight or even obese despite having no increased risk of disease. Another criticism of BMI is that it fails to take a person’s body shape into account when arriving at a measurement. For instance, a person with a small body frame could be carrying considerably more body fat than a person with a larger frame but of the same height and weight. BMI cannot be used to accurately assess the weight of pregnant women.

Risks Associated with High BMI

In many instances, having a BMI of 25 or higher increases a person’s risk of developing cardiovascular disease and high blood pressure. Overweight and obese individuals are also more likely to get osteoarthritis, gallbladder disease, and type 2 diabetes. Some types of cancer are more common in people with a high BMI. These include colon and breast cancers. A high BMI is also associated with certain mental health conditions including depression and anxiety.

Risks Associated with Low BMI

If a person has a body mass index lower than 18.5, this could indicate they are underweight enough to be malnourished. Malnutrition is associated with various health problems. Some people with a low BMI may lack vital vitamins and minerals, which often leads to deficiencies. Certain factors can raise one’s risk of having a low BMI, including low immunity, respiratory and digestive diseases, some cancers, and osteoporosis (weakened bones).

BMI and Disability

For some people with physical disabilities, BMI is not an accurate way to assess whether or not they are a healthy weight. In particular, people who are unable to walk are likely to experience muscle wastage. Because muscle weighs more than fat, this lowers their body weight and causes a lower BMI value, although they may not actually be underweight.

BMI and Ethnicity

Some ethnic groups require revised body mass index guidelines. For example, people from some parts of Asia have a higher amount of body fat at a given BMI than other people with the same BMI value. This means that they are more likely to develop health problems associated with high BMI at a lower body weight than other populations. Conversely, some populations are not at increased risk of developing weight-related health issues until they reach a higher BMI than other populations. This is true of those of Torres Strait Islander or Maori heritage.

Measuring BMI in Children

It is difficult to generate BMI cut-offs for children because they are growing all the time and their body fat levels are constantly changing. For this reason, BMI is not used to assess whether children are at a healthy weight in the same way it is for adults. Childhood BMI measurements are adjusted to take into account the child’s age and their sex. Experts can generate BMI measurements for children over the age of two. However, these numbers are less accurate than for adults and people outside the medical field should not draw conclusions from them.

Alternatives to BMI

While body mass index is a useful tool for assessing whether a person is a healthy weight, other methods can replace BMI if it isn’t appropriate. For instance, athletes and people with physical disabilities may require different or supplementary measurements. Determining a person’s body fat percentage is one way to identify obesity. Body fat calipers and special machines can take this measurement. Checking a person’s waist-to-height ratio is another way of judging obesity, and some research suggests that this is a more accurate way than BMI of predicting the risk of diabetes and cardiovascular disease.