7 Reasons Why BMI Is Not an Accurate Predictor of Health

Maintaining a healthy weight can be a challenge, and knowing what weight is healthy can be too. Most people rely on the body mass index, or BMI, which measures our weight in relation to our height. If you've been to a doctor, you’re likely familiar with BMI. You will be surprised to know that this form of measurement originated in the 1800s as a way to categorize someone into a range of 'underweight,' 'normal weight,' 'overweight' or 'obese,'. However, researchers and physicians are beginning to realize that those weight classifications may not be as helpful as once thought.

BMI is a measurement that takes into account your age, sex, height, and weight to produce a calculation. This calculation is a measurement of your body size and can be used to determine how your body weight is related to your height. It is not a diagnostic tool nor is it a measurement of body fat percentage. A high BMI may be an indicator of high body fat, but it doesn't necessarily mean that a person is overweight or obese. And it alone is not a direct indicator of health. In some populations, BMI has been found to be a fairly reliable indicator of body fat measures. But the calculation is less effective in other groups, such as bodybuilders and older adults. There are other methods that are more accurate in estimating body fat. So, what’s a normal BMI?

A normal BMI is between18.5 and 25; a person with a BMI between 25 and 30 is considered overweight; and a person with a BMI over 30 is considered obese. A person is considered underweight if the BMI is less than 18.5. As with most measures of health, BMI is not a perfect test. For example, results can be thrown off by pregnancy or high muscle mass, and it may not be a good measure of health for children or the elderly. So then, why does BMI matter?

In general, the higher your BMI, the higher the risk of developing a range of conditions linked with excess weight, including diabetes, arthritis, liver disease and several types of cancer. According to the WHO, nearly three million people die yearly worldwide due to being overweight or obese. In addition, independent of any particular disease, people with high BMIs often report feeling better, both physically and psychologically, once they lose excess weight. Now let’s see why BMI is not an accurate predictor of health:

1.It does not measure body fat percentage: Body fat percentage or BFP is the percent of your body that is fat tissue compared to your total body mass. It is normally measured with skinfold calipers, bioelectrical impedance, or most accurately through a DXA X-ray Scan. One of the main issues with BMI is that it cannot account for the difference between muscle and fat. Because muscle tissue is denser than fat, many athletes and bodybuilders are considered overweight according to BMI despite being in peak athletic health. Body fat percentage will give a better assessment of health because the disease risk is more correlated with body fat rather than body weight.

2.Doesn’t consider other factors of health: BMI only answers yes or no regarding whether a person is of normal weight. It does not take in any context of their age, sex, genetics, lifestyle, medical history, or other factors. Relying only on BMI may miss other important measurements of health, such as cholesterol, blood sugar, heart rate, blood pressure, and inflammation levels. It may overestimate or underestimate a person’s true health. What’s more, despite men and women’s varying body compositions — with men having more muscle mass and less fat — BMI uses the same calculation for both groups. Plus, as a person ages, their body naturally increases in fat mass and declines in muscle mass. Numerous studies have shown that a higher BMI of 23–29.9 in older adults can be protective against early death and disease. Finally, by simply using BMI to determine a person’s health, it ignores other aspects of health, such as mental well-being and complicated sociological factors, such as income, access to affordable and nutritious food, food skills and knowledge, and living environment.

3.It isn’t Optimal for very tall or very short people: BMI is designed specifically to provide accurate data on body weight and obesity in large samples of the general population. However, its disadvantage is that it may not provide accurate data for people who are very tall or very short. The formula used to calculate BMI doesn’t take into the extra skeletal weight of extremely tall people. This means that tall, fit and healthy people often get BMI results that place them in the “overweight” or “obese” BMI ranges. For example, a man measuring in at 6’6” with a weight of 230 lb. scores 26.6 using BMI — enough for him to be considered overweight. And for short-statured people, the opposite is true. The current BMI scale often divides the weight by too much for shorter people, giving these people the impression they’re thinner than they really are.

4.It Misses normal weight obesity: Because BMI is simply a measure of your weight versus your height, it doesn't take into account where that weight comes from -- lean tissue or fat. For this reason, you might have a normal "healthy" weight, according to your BMI, but still face health risks due to excess body fat. For example, excess abdominal fat that pushes your waistline to larger than 35 inches for women and 40 inches for men ups your risk of obesity-related diseases, regardless of your BMI. And normal weight obesity -- which happens when you have extra body fat but are not overweight, according to BMI -- increases your blood lipid and blood pressure levels. This increases your risk of heart disease.

5.It may not reflect positive change: BMI is a broad number that doesn't accurately reflect changes in behavior, which could be improving your health. People with a high BMI who are physically active are at lower risk for many health problems than people with a high BMI who are sedentary. For example, physical activity correlates with reduced risk of coronary heart disease and early death, regardless of your weight. People who adopt a healthier lifestyle by exercising more and choosing healthier foods over junk food may not lose weight if they haven't reduced their calories significantly. They are healthier, but BMI doesn't change because their weight has remained stable. If they rely on BMI as the only marker of their health, their new habits don't seem to be doing much good. Even if you lose weight, your BMI may not change noticeably. Losing 5 to 10 percent of your weight can lead to positive benefits, such as decreased blood pressure and cholesterol. High levels of bad cholesterol can be really bad for your health. You can bring its levels back to normal by eating the right kinds of foods. Learn more about them by watching “8 Foods That Can Lower Your Cholesterol Level” Now, back to reasons why BMI is not an accurate predictor of health may not be relevant for all populations:

6.Despite the wide use of BMI among all adults, it may not accurately reflect the health of certain racial and ethnic populations. For example, numerous studies have shown that people of Asian descent have an increased risk of chronic disease at lower BMI cut-off points, compared with white people. In fact, the World Health Organization developed Asian-Pacific BMI guidelines, which provide alternative BMI cut-off points. Numerous studies have shown that these alternative cut-off points better identify health risk among Asian populations. Also, Black people may be misclassified as overweight despite having lower fat mass and higher muscle mass. This may suggest that chronic disease risk occurs at a higher BMI cut-off point, compared with other races, especially among Black women. In fact, one study found that Black women were considered metabolically healthy at cut-off points higher than people who are not Black. This further puts into question the usefulness of BMI for all racial groups.

7.Finally, relying only on BMI ignores the cultural importance of body size to different groups. In some cultures, higher fat mass is viewed as healthier and more desirable. Healthcare providers should consider what “health” means to each patient individually. May lead to weight bias: It’s expected that a medical professional uses their best judgment, meaning they would take the BMI result and consider their patient as a unique individual. However, some health professionals only use BMI to measure a person’s health before providing medical recommendations. This can lead to weight bias and poor-quality healthcare. Those with higher BMIs more often report that their physicians only focus on their BMI, even if their appointment was for an unrelated concern. Often, serious medical issues go unnoticed or are incorrectly seen as weight-related problems. In fact, studies have shown that the higher a person’s BMI, the less likely they are to attend regular health checkups. This is due to fear of being judged, distrust of the healthcare provider, or a previous negative experience, which can lead to late diagnoses, treatment, and care. While BMI may not be an accurate predictor of health, being overweight can sure lead to a lot of health problems.