Is BMI really the best indicator of a healthy weight?
BMI or Body Mass Index is one of several calculations we use to measure health but on its own, it is not the best indicator of a healthy weight.
When it was originally formulated, the purpose of the BMI was to assess sedentary individuals and to do comparative studies across large populations, so it is less accurate on an individual basis.
Despite its limitations, BMI is used widely for its simplicity because it can give a quick estimation of weight targets for those who are overweight and underweight, and for individuals who are seeking bariatric surgery who have to be above a specific BMI to qualify.
BMI should not be used for people under 18 years of age (there are specific calculators for children) or for pregnant and lactating women.
BMI is calculated by taking weight in kilograms and dividing it in by height in metres squared. The ranges of BMI are:
- Underweight: <18.5
- Normal: 18.5-24.9
- Overweight: 25-29.9
- Obese: >30
The challenge with using BMI as an absolute indicator of a healthy weight is that it only takes into account height and weight and doesn't differentiate between lean muscle mass and fat.
This becomes problematic when measuring someone with a higher muscle mass and lower body fat percentage, who may have a higher BMI and be deemed unhealthy. This is in contrast to someone with a higher body fat percentage and lower ,muscle mass who may have a lower BMI and be classified as in a healthy range.
This highlights why using BMI to assess healthy weight for individuals who are very lean or have a very muscular build may be inaccurate.
BMI is also limited because it doesn't account for age, fat distribution, gender and ethnic background. BMI in people over 65 may be inaccurate because generally as we age, our muscle mass decreases so while our weight and BMI may decrease - this does not necessarily translate into a healthier state.
Also important to note, we all store fat differently and there is increasing evidence that how our fat is distributed in our body is a more important indicator of health risk than weight alone. Specifically, fat that is stored around the waist seems to correlate to visceral fat - or fat that is around your internal organs. This is related to an increased risk of heart disease, insulin resistance and diabetes.
People of certain ethnic backgrounds such as those of South Asian descent tend to store fat around the waist despite generally having lower overall weight. So while BMI may be normal in people with a higher waist circumference, it does not identify that this person may actually be at higher risk.
For these reasons, there is an increasing emphasis on assessing health with not only BMI but by combining it with measures of waist circumference and hip circumference.
When used in combination, these measures may be more accurate and can help track changes in weight with changes in diet and exercise. Waist circumference should be <102 cm (40 in) in men and <88 cm (35 in) in women.
The waist-hip ratio should be <0.9 for men and <0.85 for women. Healthy waist circumference measures have different cut-offs for those of South Asian and Chinese descent so check with your doctor for these measures.
Obesity and its related health risks are on the rise and there is a need to find certain measures and assess healthy weight which can encourage individuals to make positive lifestyle changes.
Remember, the assessment of health risk is not defined by your BMI and waist circumference alone. Combine these measures with a healthy blood pressure and other lifestyle habits such as a healthy diet, smoking cessation and physical activity to help keep your health risks in check.
Send family doctor Sheila Wijayasinghe your questions at email@example.com. She will answer select questions, which could appear in The Globe and Mail and/or on The Globe and Mail web site. Your name will not be published if your question is chosen.
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The content provided in The Globe and Mail's Ask a Health Expert centre is for information purposes only and is neither intended to be relied upon nor to be a substitute for professional medical advice, diagnosis or treatment.
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