Body Mass Index (BMI) is a numerical value that you can calculate using your height and weight. According to Statistics Canada, BMI = [(weight in lbs.)/(height in inches)2] x 703. For example, if your weight is 140 lbs. and your height is 5 feet, 4 inches, or 64 inches, your BMI = [140/(64)2] x 703 = 24.03, which is in the normal range. BMI is a reliable indicator of health and is used to classify people into underweight, normal weight, overweight and obese categories.
BMI Categories
Statistics Canada uses BMI levels to classify people into six categories. The normal BMI range is between 18.5 and 24.9 and people in this range have the lowest risk of health issues. People with a BMI less than 18.5 are considered underweight and have a higher risk of developing health problems. The same is true for people with a BMI between 25.0 and 29.9, which is the overweight range. The three obese classes fall into the BMI range of 30.0 to 40.0 and over. In the obese category, health risks are higher and increase in proportion to BMI.
BMI for Children and Teens
The interpretation of BMI for growing children and teenagers is slightly different, because height and weight fluctuate with growth. Also, healthy weight ranges for children and teens are different for each month of age. According to the Centers for Disease Control and Prevention (CDC), the BMI for children and teens is interpreted by using percentiles that consider age and sex. Body fat changes with age and differs between boys and girls. This is different from adults who have stopped growing and experience an increase in BMI primarily due to added body fat.
Health Risks Indicated by High or Low BMI Values
People who have a BMI greater than 25 and are physically inactive have an increased risk of developing certain cancers such as breast cancer and colon cancer. Risk of cardiovascular disease, hypertension, diabetes and osteoarthritis are also higher in overweight and obese people. Underweight people may be malnourished and may have a weaker immune system. There is also an increased risk of cancer, fractures and problems with the respiratory and digestive systems.
Exceptions to Consider
A BMI number may not be a reliable indicator of body fat content in some cases. Because the calculation only considers the weight of a person, a BMI number does not distinguish between unhealthy fat and healthy muscle. Body builders and athletes who have muscular bodies may have high BMIs, but this is attributable to muscle mass. Similarly, pregnant women may have high BMIs, but it does not mean that they are unhealthy. Elderly people and people who have disabilities that restrict movement may have a low BMI, but this is not indicative of health risks.
Drawbacks of BMI
Measures such as waist or hip circumference, skin fold thickness and ratio of body fat are better indicators of health risks than BMI, says the National Obesity Observatory (NOO). While BMI is a reasonable indicator of body fat content, it may not provide an accurate indication of a person's health status. Factors such as ethnic origin, puberty and amount of muscle mass can cause an inaccurate interpretation of BMI numbers. BMI also does not consider body shape or the distribution of fat in the body.
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