BMI stands for Body Mass Index, which is a measurement used to assess a person’s body weight in relation to their height. It is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared. Originally, it was based on large populations of white men (like everything else) but has since been applied to individual white men and women of all ages and races. Let’s talk about why BMI is used and why it’s not always the best metric to use to make health-related conclusions.
BMI Use
The primary use of BMI was and continues to be to classify people as either underweight, normal weight or overweight because of the potential health risks associated with each. The result is able to reveal the risk of:
- Coronary heart disease
- Hypertension
- Osteoarthritis
- Sleep apnea and other respiratory conditions
- Stroke
- Type 2 diabetes
- Cancer
For those classified as underweight or under the normal range, the potential health conditions include:
- Cardiovascular disease
- Depression
- Difficulty getting pregnant
- Dry skin
- Hair loss
- Irregular menstrual cycles
- Nutrient deficiencies
- Osteoporosis
- Reduced immune system function
Limitations of BMI
Muscle mass
BMI is widely used because of how simple and cheap it is. All it accounts for is height and weight. This is too simplistic and fails to account for a key metric like how much of your weight is muscle mass. When you have more muscle, your BMI will be higher even if you’re fit and healthy.
Bone density
BMI also fails to factor in bone density. Some people have denser bones and larger frames than others. People with larger frames than others will have higher BMIs and even be classified as overweight even though they may be perfectly healthy.
Age
BMI was initially used on adult men, not children or the elderly which is why it can be inaccurate. In children, factors like puberty can change the link between BMI and body fat. While it generally is a good predictor of excess body fat in obese children, elevated BMI levels can be caused by an increase in either fat or non-fat mass. Children also don’t yet have fully developed bones and muscles. Conversely, elderly people have less muscle mass than the average.
Lifestyle factors
BMI does not consider lifestyle factors like diet, physical activity, and other health behaviours which have a significant effect on health.
Sex
BMI is highly ineffective when it comes to women. Fat accumulation and distribution vary in men and women especially as they age. Studies found that women over 50 had a higher prevalence of obesity than men. This physiological difference in body fat is not properly measured by BMI.
Ethnicity
BMI may not be equally applicable to all ethnic and racial groups due to differences in body composition and fat distribution. People from different ethnic groups may have a higher BMI but still, be healthy.
Different ethnicities have disparities in fat distribution, muscularity, bone mass and leg length. The differences in body fat, for example, are the result of genetically determined differences, body composition and metabolism as well as a variety of risk factors caused by social and environmental factors.
They may also have different health risks associated with the same BMI level. For example, individuals of Asian descent at the same BMI level as white people are twice as likely to develop type 2 Diabetes. Another example is how a BMI over 30 in white people is linked to a higher risk of developing type 2 diabetes than for black people. BMI fails to account for these ethnic differences leading to inaccurate results.
How to use BMI
Originally, BMI was designed to be used on large groups of white men, not on individuals or on women and people of other races. This is why it is wildly inaccurate. When you consider how insurance companies use it to determine coverage and rates and how unfair the outcomes are, you see how important it is to consider adopting another metric for determining an individual’s health status.
BMI should be part of the bigger picture when considering a person’s health, not used in isolation. It could be used among with:
- The ratio of body fat to lean mass
- Waist circumference
- Neck circumference
- Waist-to-hip ratio
- Cholesterol levels
- Stress levels
- Sleep habits
- Blood pressure
- Family medical history
- Diet
- Physical activity levels
- Habits like drinking and smoking
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