Know Your Numbers: How BMI Impacts Your Health
BMI is Body Mass Index. It is a calculation that uses a person’s height and weight to identify a weight category for that individual. BMI can be used as a simple measurement to track your weight, correlate health risk and when losing weight, can indicate improved health.
It is determined by taking a person’s weight in pounds and dividing that number by the person’s height in inches, squared. The BMI calculation is: weight (pounds) / height (inches) x height (inches) x 703.
For example, a person weighing 200 pounds and standing 70” tall would have a BMI of 200/70x70x703 = 28.7. There are many calculators available online, like this one.
That number correlates to one of the weight categories assigned to various ranges of BMIs. There are four main categories: underweight, normal weight, overweight, and obese. The goal is the “normal weight” meaning a BMI that falls in the range of 18.5 to 24.9 based on the above calculation.
Generally, BMI below 18.5 and above 25 is associated with increased health risks. As BMI increases from overweight into the higher categories of obesity, health risk increases proportionally.
- underweight BMI <18.5
- normal weight BMI 18.5-24.9
- overweight BMI 25-29.9
- obesity class I BMI 30 – 35
- obesity class II BMI 35-40
- obesity class III BMI >40
Associated health conditions
Increasing BMI generally is associated with worsening of medical conditions linked to being overweight. Those include:
- high blood pressure (hypertension)
- high cholesterol (hyperlipidemia)
- type II diabetes
- coronary artery disease
- sleep apnea
- degenerative joint disease
- early mortality
As BMI increases, the ability to treat these illnesses becomes more difficult and responsiveness to treatment typically diminishes. Also, the incidence of certain types of cancers increases as BMI increases.
Can you have a high BMI and still be "healthy"?
There are exceptions to the rule that higher BMI is associated with increased health risk. Since BMI is calculated simply by weight and height, individuals who have a higher degree of muscle mass accounting for a larger percentage than normal of their total weight may have a higher BMI that is not associated with greater health risk. It is often helpful to obtain body fat percent measurements when using BMI to make health related decisions. This additional information can help in interpreting the significance of an elevated BMI number.
When exercising and especially when trying to build muscle mass, it is helpful to test and monitor body fat percent along with BMI. Body fat percent can be determined easily by an exercise physiologist, health educator or trainer through a skin fold test using calipers or by using an electronic digital scale that measures body fat though electrical impedance.
What happens when you lower your BMI?
The good news is those who have high BMIs can reduce their risk of developing dangerous health conditions by losing weight, and ultimately lowering BMI. It also can lead to an improvement in those conditions associated with being overweight. Hypertension, hyperlipidemia, cardiovascular disease, sleep apnea, degenerative joint disease, pulmonary conditions and others can be significantly improved through weight loss and the resulting lower BMI.
An excellent example is type II diabetes. This serious condition often requires medications and sometimes insulin to control. There is frequently a connection to BMI and being overweight with the condition – as weight increases, so does the need for more medication and insulin. When an individual with diabetes loses weight and diminishes their BMI, frequently there is marked improvement in their diabetes. This can result in the need for less insulin or medication, and sometimes it may even be stopped.
Overall, the use of BMI and body fat percent measures along with monitoring weight, exercise fitness, respiratory rate, blood pressure, HbA1c (the test for diabetes), serum cholesterol levels and others can all play an important role in helping your healthcare provider and you ensure optimal health throughout your entire life.
About the Author:
Vincent Pera, MD
Dr. Vincent Pera is medical director and program director for the Center for Weight and Wellness.
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