If you have ever been overweight, even if you haven't, you are undoubtedly familiar with the term body mass index, or BMI. Also known as the Quetelet index, it was invented in the 19th century by a Belgian mathematician named Adolphe Quetelet, and has been widely used since the 1950s as a way of measuring whether an individual is at a healthy weight for his height. While the concept of BMI remains in debate, maintaining a healthy BMI is always considered a wise idea.
BMI math
Your BMI is a simple calculation, equal to your weight in kilograms divided by your height in meters squared. In June 1998, the National Heart, Lung and Blood Institute published its "Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults." It labeled various BMI ranges, from underweight (BMI less than 20) to morbidly obese (BMI greater than 35). Those with a BMI over 25 are considered "overweight" and are thought to be at higher risk for cardiovascular diseases such as heart disease.
What BMI doesn't measure
Unfortunately, BMI does not take into account various factors that make up an individual's weight, such as body fat percentage, muscle mass, bone structure or age. Therefore it is possible for individuals who are naturally more muscular or larger-boned to have a BMI in the "overweight" or "obese" range, even though they are a perfectly healthy weight for their height. Professional athletes and bodybuilders often fall into this category, but it can also apply to nonathletes.
BMI: a big fat lie?
A 2006 Mayo Clinic study examined the theory that those with a BMI over 25 were more susceptible to heart disease by examining the records of over 250,000 patients, separated into five groups: underweight (BMI less than 20), normal weight (BMI 20-25), overweight (BMI 25-30), obese (BMI 30-35) and morbidly obese (BMI 35+). Contrary to expectations, the patients in the overweight and obese categories had the lowest mortality rates and the lowest incidence of heart disease. The study did not offer any real explanations but stated that the inability of BMI to differentiate between lean and fat body mass may have played a role.
Implications
According to a more recent study by the Mayo Clinic, this is precisely the reason that using BMI alone as a measure of healthy weight is at best inaccurate, at worst, dangerous. The components missing from BMI calculations mean that it is possible for people with a "normal" BMI to have an unhealthy level of body fat, a condition known as "normal weight obesity." Researchers now believe that body fat percentage and fat distribution may be more accurate indicators than BMI of a person's risk of heart disease and other weight-related health problems.
How to measure body fat
Since body fat percentage is key in determining a healthy weight, learning your body fat percentage is a good idea. There are a number of ways to measure your body fat percentage, including calipers, hydrostatic weighing (immersion in water) and home body fat scales. The only truly accurate method is hydrostatic weighing, but even an estimate may be helpful in determining your weight or fat loss needs. However, these methods don't tell you anything about fat distribution.
BMI replaced?
Researchers have determined that abdominal fat is the culprit in most obesity-related ailments like heart disease; it puts undue stress on your organs, including your heart. Therefore, instead of concentrating on BMI or even overall body fat percentage, medical professionals are now more concerned with two specific measurements: waist and hips. Your waist circumference and the ratio of your waist to your hips can help doctors determine how your weight is distributed and whether you are carrying a dangerous amount of abdominal fat.
How to test yourself
Measure your hips and your waist, and divide your waist measurement by your hip measurement. For women, the ratio of waist to hips should be less than .85 (.7 is optimal, both for health and aesthetic reasons); for men, .9 is optimal. This means that a woman with a 30 inch waist would have an optimal hip measurement of approximately 42-43 inches. Additionally, waist measurement alone plays a factor. A recent study published in the New England Journal of Medicine states that a waist measurement greater than 47 inches for men or 39 inches for women almost doubles the risk of death from heart disease.
What does this mean for you?
Unfortunately, this news does not mean that you can ignore your BMI entirely, nor does it mean that having plenty of "junk in your trunk" means that you're not at risk for heart disease. Doctors continue to recommend that people aim for a BMI in the normal (BMI 20-25) range, although they readily admit that some may not be able to get their weight that low, even with proper diet and exercise. For those with slightly above normal BMI or normal BMI but a high body fat percentage, resistance training is recommended to build muscle and decrease overall body fat percentage. People with high waist-to-hip ratios are encouraged to eat a high-fiber diet and strive to lower their weight and body fat percentage. Those with a BMI over 40 are considered extremely high risk and are often told to consider bariatric surgery to reduce their weight and risk of heart disease.
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