BMI Defined
Your BMI. You probably hear it when you go to the doctor’s office, or you hear about BMI in the news when news outlets discuss health issues and obesity.
But what exactly is BMI, and why does it matter?
What does it mean and what can it tell you about your health?
BMI stands for body mass index. Put simply, it’s your weight in relation to your height. More specifically, your BMI is your weight in kilograms divided by your height in inches, squared.
Why is BMI Used?
Using BMI helps your health care provider to assess your nutritional status but also to help determine if your weight is normal for your age and if it poses any risks to your health. Just taking a weight measurement is not a complete enough picture, since all people come in a variety of different shapes and sizes.
But more complex measuring tactics like hydrostatic body fat tests, DEXA scans and other modes used to assess body composition are both more expensive and more invasive.
So instead, your doctor probably relies on BMI as a useful proxy for your nutrition status and approximate body fat level. It is not a true measurement of body fat, but it is a useful approximate for many people. This tool helps your doctor determine if you are underweight, overweight, or at just the right weight for your age.
BMI Categories
For adult men and women, the standards for BMI are as follows. A BMI under 18.5 is considered underweight, a BMI of 18.5 to 24.9 is considered normal weight, a BMI of 25-29.9 is considered overweight, a BMI of 30-34.9 is considered Obese Class 1, a BMI of 35-39.9 is considered Obese Class 2 and a BMI over 40 is considered obese Class 3.
BMI for Children
The BMI classifications are slightly different for children. A child’s weight is calculated based on their age and growth curve. Because children are different shapes and sizes throughout their different phases of development, appropriate BMI for children may differ throughout their lifespan. You can find some more information regarding BMI, including charts and percentiles for kids, here. Of course, you can defer any questions to a medical professional.
BMI Calculator
You can calculate your BMI. The equation is: BMI= KG/M². KG= Your Weight in Kilograms. If you live in the USA and are used to hearing your weight in pounds, you can find your weight in kilograms by dividing your weight in pounds by 2.2. M²= Meters. You will want to convert your height to meters squared. If you do not feel like doing the calculations and just want to determine your BMI by typing in your height and weight, you can do that here.
Limitations
Using BMI or Body Mass Index can have a number of limitations. It can be inaccurate when used to assess your body fat and potential health status. BMI does not distinguish between muscle and fat, different bone structures, activity level, fat distribution, sex, or other factors.
It is simply a comparison of your weight to your height, which does not paint a complete picture of your body composition.
BMI has been criticized because it does not account for the difference between fat mass and muscle mass. For very muscular individuals, their BMI may overestimate their body fat percentage. This may cause people with normal or even low body fat to be labeled as overweight or even over fat. In contrast, BMI also indicates very little about a person’s overall fitness or health level. A person might have a very low BMI and may fall into a low risk category. However, if this person has a poor diet, and low activity level, they might have more health risks than someone with a higher BMI that is actually in better shape.
Fat Distribution and Metabolic Health
BMI or Body Mass Index also does not make distinctions based on your fat distribution that might tell a more complete picture of your metabolic health. For example, two people of the same BMI might have a similar body fat percentage, but one may carry more fat in their midsection while another person carries more fat in their lower body. Carrying more fat near your midsection, which is called an android fat distribution, may indicate more unhealthy visceral fat, which can more negatively affect your metabolic health. In contrast, carrying more fat in your legs and lower body and less near your internal organs, called a gynoid fat distribution, may indicate a healthier fat distribution.
Bone Structure
Within a population, you will see people with thicker bone structures and those with thinner structures. The BMI scale does not account for bone structure or thickness. Some races have thicker bones and this might cause these people to have higher BMIs. White people and Asians tend to have smaller bone structures. In general, the BMI scale will be less accurate for people with bigger or smaller bone structures than average.
Sex
The BMI scale also does not make a distinction between weights for men and women. Men tend to have more muscle mass, thicker bones and joints and an overall larger bone structure than women. They also tend to weigh more. Women tend to have more body fat and less muscle than men at any given height. The BMI may classify many otherwise healthy men as overweight just because they carry more muscle or have a larger bone structure.
Body Fat
As discussed, BMI does have its limitations and might not be the most accurate way to gauge a person’s body fat. Getting a body fat scan scan can give you a much more accurate picture of your overall body composition. Good methods to estimate your body fat percentage include hydrostatic weighing, Dexa Scans, and calibers. Using calibers is actually the most effective way to determine body fat.
BMI and Waist Circumference
Measuring your waist circumference in addition to your BMI might give more information about your health than either method alone. For men, a waist circumference over 40 inches might indicate poor metabolic health. For women, a waist circumference over 35 inches might indicate poor metabolic health.
BMI Usefulness
Despite its problems, BMI is still a relatively useful tool to assess health risk. The BMI stratification system is not based on arbitrary numbers, but instead a level of health and mortality risk associated with each category. Indeed, those who are underweight or extremely obese have the greatest health risks, followed by Obese Class 2, Obese Class 1 and those in the overweight category. Those in the Normal group typically have the lowest risk of health problems and weight-related mortality, however, there is some research that being moderately overweight might serve as a protective measure in old age. Still, this assessment of health risk is based on a population level, so some individual differences may present.
A Useful, but Incomplete Tool
While BMI does not reveal everything about a person’s weight, body fat, activity or fitness level and nutrition status, it is a fairly useful tool for large sample sizes. Coupled with other standard measurements, you can learn a bit more about your body fat and potential health status. As always, refer to your doctor for more questions.
Disclaimer: This article is for informational purposes only and does not substitute for medical advice. Please direct any further questions about BMI, body fat percentage or your weight and health to your doctor or other health care provider.
