“BMI” is discussed
in many health & fitness articles and as important marker of health. But do
you really know what it means and how to calculate it?
The BMI, or Body Mass Index, is a number calculated by
taking your weight in kilograms and dividing it by height in meters squared. BMI
“provides a reliable indicator of body fatness for most people and is used to
screen for weight categories that may lead to health problems.”
A BMI of less than
18.5 classifies a person as underweight; between 18.5 and 24.9 as normal;
between 25 and 29.9 as overweight; 30 and 39.9 as obese and over 40 as morbidly
obese as depicted in the chart below:
You can use the
calculator below to identify your BMI:
What Does Research Say About The BMI Chart?
Being overweight or
obese at age 40 is associated with a significantly increased risk of dying
prematurely. BMI predicts diabetes as
well as a specialized blood sugar test according to another research article. BMIs over 25 are linked to heart disease and
high blood pressure.
Note: Some studies have shown
that being underweight is also a risk for premature death, sometimes even more
so than being overweight. These studies, however, did not exclude “sick”
people, like those with cancer or emphysema, or people who smoke or use
dangerous drugs, which often can cause weight loss. There are no studies to my
knowledge which show healthy people with desirable body compositions die sooner
than overweight people.
Problems With The BMI Chart
All of the BMI studies cited
above were done across large populations. Using that data and applying it to an
individual without looking at the person’s overall health is a bad idea.
BMI calculations do not consider age, gender,
frame size, muscle mass, or fat distribution. Subcutaneous fat (under the skin)
does not appear to strongly predispose to disease, whereas visceral fat (in and
between the organs) does. Therefore, someone can have a normal BMI but a high
percentage of visceral fat and be at high risk for diabetes and heart disease
but a sumo wrestler, with a high BMI but little visceral fat, will be
metabolically healthy.
Many doctors
calculate the BMI of patients and make recommendation simply based on that
number, which is a flawed approach for the reasons given above. In fact, I just
saw a patient for physique coaching who was 5’9” and weighed 185 pounds.
Despite having a 33 inch waist, 11% body fat and perfect cholesterol and blood
sugar, his primary care doctor told him he must lose 25 pounds if he didn’t
want to get diabetes and heart disease!
Better Alternatives to The BMI Chart
1) Body Fat measurement: While BMI is used as proxy for
body fat, it is a much better idea to just measure body fat. Health is
generally better with lower body fat percentages regardless of the BMI, and
this includes populations with documented diseases like diabetes. I recommend
striving for maximum muscle mass and a body fat below 15% for men and 23% for
women. These numbers are within the “fitness range” of recommended body fat.
2) Waist to hip circumference: Since it is becoming very clear that the
distribution of body fat is an important indicator is disease risk, it makes
sense to examine it. People with fat distributed around the abdomen are at
higher risk for heart disease and diabetes, so aiming to reduce the ratio is a
better idea than trying to reduce BMI.
In other words, if you just focus on
“losing weight” you may very well end up losing muscle as well and looking like
a shrunk down version of your former self, with the same risk of serious
disease as before.
As always, it is
important to look at the “big picture” rather than focusing on a particular
calculation to evaluate your chances of developing an obesity-related illness.
Think Smart
Eat and Lose Weight
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