HEALTH

What is BMI, is it right for me?

February 10, 2017 by John Quinn - Product Owner - mPort

Every body is a work in progress, right? And whether you’re overweight or healthy, your BMI, or Body Mass Index, is a good indicator of where to start on your health journey.

What is BMI?

The Body Mass Index (BMI) is a method used by healthcare professionals; it uses a person’s weight and height in order to calculate whether the person is healthy, overweight or obese. It is simply a measure of a person’s thickness or thinness, and is calculated by dividing weight by the square of height as follows:

 

BMI = Weight (kg) / Height (m)2

BMI value Classification
BMI < 18.5 Underweight
18.5 <= BMI < 25 Healthy
25  <= BMI <  30 Overweight
30 <= BMI Obese

Classification defined by World Health Organization (WHO) in 1997 and published in 2000.

 

Ancel Keys is first attributed with describing “body mass index” as the ratio of human body weight to squared height. He published a paper in the July 1972 edition of the Journal of Chronic Diseases, arguing that BMI was “…if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity”.

What are the limitations of BMI?

There is considerable research showing that BMI correlates well to body fatness in large groups of people, but it is less reliable for any one individual and is at best an approximation of your total body fat.

Central obesity refers to the amount of fat specifically around your middle. Certain studies have concluded that a better indicator of health is to measure a person’s central obesity rather than BMI. These studies have concluded that people with central obesity have a worse long-term survival rate than people who are classed as overweight or obese according to their BMI alone. The logic surrounding this argument is that central obesity indicates that an individual’s fat is primarily surrounding their middle where vital organs are located. BMI only uses weight and cannot determine where this fat is located, or whether the weight is fat or muscle. Consider a body builder; their BMI would indicate that they are overweight or obese.

Often BMI is used as an initial screening tool to identify people who may have health problems associated with being under or overweight, but it is not a diagnostic tool. Other measures could then be used to see whether or not that individual had any health issues that may be related to their weight. If you don’t think BMI is accurate or relevant for you, then it’s best not to use it. If you are concerned about your weight, it’s a good idea to raise this with your GP, or see a dietitian or nutritionist, to discuss your individual situation.

Some Exceptions to the BMI rule

BMI does not differentiate between body fat and muscle mass. This means there are some exceptions to the guidelines, including:

Body builders

Body builders and people who have a lot of muscle bulk will have a high BMI, but are not overweight.

Physical disability

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 helpful advice.

Height

BMI is not totally independent of height. It tends to overestimate obesity among shorter people and underestimate it among taller people. Therefore, BMI should not be used as a guide for adults who are very short (less than 150 cm) or very tall (more than 190 cm).

Ethnicity

Body composition varies for people of different ethnicities. Asians and Indians, for example, have more body fat at any given BMI compared to people of European descent. Therefore, the cut-offs for overweight and obesity may need to be lower for these populations. This is because an increased risk of diabetes and cardiovascular disease begins at a BMI as low as 23 in Asian populations. Some populations have equivalent risks at a higher BMI, such as people of Torres Strait Islander and Maori origin.

Elderly

For elderly people over the age of 70, general health status may be more important than being mildly overweight. Some researchers have suggested that a BMI range of 22-26 is more desirable for the elderly.

Pregnancy

It’s normal for pregnant women to gradually gain weight as your baby grows and your body develops extra tissue.

Eating disorders

It is also not an accurate indicator for people with eating disorders like anorexia nervosa or people with extreme obesity.

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Sources

  1. World Health Organization: http://www.who.int/mediacentre/factsheets/fs311/en/
  2. (US) National Heart, Lung & Blood Institute (NHLBI): http://www.nhlbi.nih.gov/
  3. 12WBT: https://www.12wbt.com/weight-loss
  4. Department of Health & Human Services, State Government of Victoria, Australia: https://www.betterhealth.vic.gov.au/health/healthyliving/body-mass-index-bmi
  5. Australian Healthy Food Guide: http://www.healthyfoodguide.com.au/resources/bmi-calculator
  6. https://www.healio.com – Waist circumference superior to other fat measures for NALFD risk (Non-alcoholic fatty liver disease and cardiovascular risk)