BMI, or body mass index, is a standard measurement that health care professionals use to decide whether a patient is a healthy body weight. BMI compares an individual’s height to their body weight. More specifically, to calculate your BMI, you divide your weight in kilograms by the square of your height in meters. If you don’t want to do the metric conversions, you can use one of the BMI calculators, widely available online. The value that you get is your BMI. Once you have this value, you can look at a chart to determine whether you’re within your ideal body weight range. Here’s how to interpret the results:
· Underweight Less than 18.5
· Recommended 18.6 to 24.9
· Overweight 25.0 to 29.9
· Obese 30 or greater
How much emphasis should you place on this number? Calculate your BMI, if you’re interested, but don’t place too much importance on it. A growing number of health experts believe that BMI is not a good representation of ideal body weight or health. Here’s why.
Healthy Body Weight: BMI Doesn’t Take Gender into Account
If a man and woman are the same height and weight, they have the same calculated BMI. The problem is women have a higher body fat percentage than men. On average, a man and woman of similar height and weight might have a body fat percentage that differs by 10% or more. Based on BMI, a man with 15% body fat and a woman with 30% body fat are at similar risk based on BMI. However, a body fat percentage of 30% or above in a woman is higher than what would theoretically be ideal for health.
In fact, a study published in the American Journal of Clinic Nutrition, after following more than 1,600 people, concluded that body fat percentage is a better indicator of future weight-related health problems than BMI.
Healthy Body Weight: BMI Doesn’t Take into Account WHERE You Store Fat
As discussed, two people with different body fat percentages can have the same BMI, as long as their height and weight are the same. BMI as a measurement also doesn’t take into account body fat distribution. Studies now show that where you carry your body fat matters and you’re at higher risk of health problems if you store a higher proportion of body fat around your waist and tummy.
In contrast, fat stored around the hips and thighs doesn’t appear to carry the same risk. In fact, deep tummy fat is a marker of insulin resistance and poor metabolic health. So, two people can have the same BMI, yet one will be at higher risk of type 2 diabetes and heart disease because of where they store their fat.
Healthy Body Weight: It Doesn’t Take Into Account Bone Mass
Bone is denser than muscle and muscle is denser than fat. So, if you have large, strong bones, your BMI will be higher than someone with a small bones or a lower bone density, assuming everything else is equal. However, having strong bones is a benefit in terms of future health risks. You’re less likely to suffer a debilitating hip fracture if you have a higher bone density.
Healthy Body Weight: BMI Doesn’t Consider Age
When you calculate the BMI of an older adult, they may fall into the normal category yet be sarcopenic with significant loss of muscle tissue. Their BMI is normal because it only takes into consideration total body weight and height. Yet, a sarcopenic man or woman might be overweight or obese in terms of body fat percentage alone but their BMI doesn’t reflect it. Older people with normal BMIs may still have excess body fat and loss of muscle tissue that places them at risk.
Healthy Body Weight: It Doesn’t Take into Account Ethnicity
Some ethnicities have naturally smaller frames. For example, a male or female of Japanese ethnicity could have a much smaller frame than a Caucasian, Hispanic, or African American. So, if a Japanese male or female has the same BMI as a Caucasian one, they may have significantly more body fat to achieve that BMI as their bone and frame size are smaller.
Healthy Body Weight: BMI Isn’t Reliable for Athletes
What if you strength train and increase your muscle mass? You’re strong, toned, and exceptionally fit. Yet, when your doctor measures your BMI, you fall smack dab into the overweight category. This scenario isn’t uncommon and it reveals another weakness of BMI as a measure. Any time your weight increases, whether through gains in muscle or gains in body fat, your BMI goes up. As an article in New Scientist pointed out, some of the Olympic level athletes fall into the overweight category. So, BMI is particularly unreliable for men and women who are highly fit and athletically trained.
The Bottom Line
All in all, BMI is a rather crude measure – it doesn’t distinguish between bone, muscle, and fat and, instead, weights them all equally. We know in terms of health risk, they’re not the same. Plus, a BMI measurement gives no indication of body fat distribution.
So, why do health care professionals use it? There are some studies showing that BMI correlates with health outcomes. Plus, calculating a person’s BMI is quick and easy, unlike measuring body fat percentage. At the very least, it’s important to get a waistline measurement to screen for excess visceral fat. Studies show that a waist size greater than 35 inches in females and 40 inches in males is linked with a higher risk of cardiovascular disease and type 2 diabetes.
Also, what you can measure with a scale, calipers, and a tape measure is only one factor. It’s also important to follow other parameters that impact your risk of chronic health problems, including blood pressure, lipid profile, and blood sugar.
The take-home message? Don’t let your BMI measurement give you a false sense of security if it’s normal or make you feel overweight if it’s too high and you’re very fit. Health care professionals will likely continue to use BMI as a measurement into the foreseeable future – but make sure you know your other parameters, including your waist size, body fat percentage, blood pressure, blood sugar, and lipids. BMI isn’t all it’s cracked up to be.
Science Daily. “BMI Not a Good Measure of Healthy Body Weight, Researchers Argue”
WebMD. “Body Fat Measurement: Percentage Vs. Body Mass”
National Heart, Lung, and Blood Institute. “Assessing Your Weight and Health Risk”
New Scientist. “Overweight Olympians: Guess the BMI of top athletes”
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