Are You Really Too Fat To Be Fit? Here's How To Tell

If you live in the United States, probably every time you go to the doctor, the physician's assistant will ask you your height and put you on the scales. Your height and weight are used to calculate your body mass index, or BMI. Your doctor assess how well you are doing with the management of your weight, or more often, to tell you that you may have lost weight but you are still obese.

Body mass index was invented by a Belgian mathematician named Lambert Adolphe Jacques Quetelet 200 years ago. In the 1970's, an American scientist named Ancel Keys, who also gave the world K-rations (the tiny packages of dry battlefield food American soldiers in World War II mostly hated with passion) and the cholesterol theory of heart disease, convinced the scientific world that BMI is a good proxy for body fat percentage. About the same time doctors started measuring cholesterol levels as an indicator of heart health, doctors started using BMI to diagnose obesity.

What's the Problem with BMI?

BMI is computed by a simple mathematical formula that attempts to account for both height and weight. Instead of computing kilos of weight per meters of height, or pounds of weight per inch of height, the formula for BMI divides weight by height squared (adjusting for US units by multiplying that result by a factor of 703). 

The reason Quetelet used the square of the height measurement rather than the height measurement itself was that as you get taller, your other proportions don't increase as much. If you are 20 cm taller than someone else, you aren't necessarily 20 cm wider and longer if you lie on your back or on your side. Quetelet's index was a relatively easy way to come up with a number that gives an indication of how fat (or, in Quetelet's time, how emaciated) you are.

But BMI doesn't take into account how dense your bones are, or how well muscled you are, or your age, race, or gender. If you work out to grow big muscles, you will have a higher BMI that may cause you to be labeled as obese. And if you have cancer and your muscles are wasting away, your BMI may suggest your health is improving.

Waistline Is Not an Especially Useful Measurement, Either

BMI, of course, is not the only way doctors detect obesity.

See Also: New Obesity Scale Developed: Is Body Adiposity Index (BAI) Better than Body Mass Index (BMI)?

Men who have a waist size greater than 102 cm (40") and women who have a waist size greater than 88 cm (35"), in the United States, are assumed to be overfat.

But waist size varies with fluid retention, with genetics (some people pack on the weight on their hips or thighs), and with indigestion. 

Throw Away The Scales To Measure Cardiovascular Risk

So if BMI isn't a good measurement of metabolic health, and waistline isn't a good measurement of metabolic health, what is?

Dr. Margaret Ashwell, a nutritionist and former science director of the British Nutrition Foundation, says the most useful measurement is the ratio of waistline to height, which is usually abbreviated WHtR. If the ratio of your waistline to your height in less than 0.5, then chances are than you don't have a high risk of cardiovascular disease.

This means that if you are 2 meters (200 cm) tall (you're really tall, but very convenient for our example), but your waistline was 95 cm, your WHtR of 95 cm divided by 200 cm or 0.475 indicates you are not overfat, and you are not at increased risk of heart disease. However, if you are 2 meters (200 cm) tall, and your waistline is 105 cm, then your WHtR of 105 divided by 200 or 0.525 indicates you are at elevated risk of heart disease.

And it isn't necessary for the patient to get on the scales to do the calculation.

Most People Who Have High BMI or High WHtR Aren't Sick

Just having a high BMI or a high WHtR doesn't mean you are doomed to have heart disease. In fact, before the age of 50, most people who have high BMI or high WHtR don't have any metabolic symptoms of heart disease at all. After the age of 50, however, it might be time to start getting back on the scales again.

A study in Ireland called the Cork and Kerry Diabetes and Heart Disease Study tracked the health of 2047 people aged 50 to 69. After adjusting for alcohol use, smoking or not smoking, exercising or not exercising, age, and gender, the Cork and Kerry Diabetes and Heart Disease Study found that it was people who had both high BMI and high WHtR who were most likely to have significant metabolic problems.

See Also: Being Overweight And Obesity Increase Risk For Heart Attack

But when people in the study had both high BMI and high WHtR:

• 17% were diagnosed as prediabetic,
• 34% were found to have multiple lab tests indicating heart disease risk factors,
• 55% were found to have insulin resistance, and
• 81% were found to have high blood pressure.

All of these correctable problems contribute to heart disease. If you have high BMI and high WHtR, and you're over 50, sorry, you need to see your doctor every 3 months to make sure cardiovascular problems are kept under control and to make sure you aren't developing type 2 diabetes. (Testing your HbA1C gives an early warning signal of the appearance of diabetes, as does testing postprandial, or after-meal, blood sugar levels.) But if the issue is just your weight, or just your BMI, the need for medical followup isn't as urgent.

We don't recommend that anyone stay obese when it's possible to lose weight. But if you can't lose weight, maybe you can at least stay active enough to stay just a little trimmer. And that might make the critical difference in your heart health.