BMI, Waist Circumference, or Waist-To-Hip Ratio?

Which Is Best for Assessing Cardiac Risk?

Woman's waist

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Almost everyone knows by now that being overweight or obese substantially increases your risk of developing cardiovascular disease, including coronary artery disease (CAD)heart attack, and stroke. For this reason, doctors should formally assess a person’s weight status any time they are estimating their overall cardiovascular risk.

However, researchers don’t always agree which method is best for quantifying whether an individual is “too” overweight. The three most commonly used measures are BMI (body mass index), waist circumference, and waist-to-hip ratio. But is one better than the others?

The BMI

The measure most commonly used to assess weight-related risk is BMI, a ratio calculated from your weight and height. Specifically, your BMI equals your body (in kilograms) divided by your height squared (in meters).

A “normal” BMI is 18.5 - 24.9 kg/m2). A BMI of 25 - 29.9 kg/m2 is considered overweight, from 30 - 34.9 kg/m2 is obese, and 35 kg/m2 or higher is very obese. BMI calculators are easy to use (all you need is your height and weight) and are readily available online. (Here’s one from the NIH.)

The BMI is useful because this measurement has been employed in numerous clinical studies, so lots of analysis has been done with the BMI measure. In fact, the formal definitions of “overweight,” “obese” and “very obese” were themselves based on these BMI studies.

However, BMI is not always accurate in every individual. It overestimates body fat in people with a lot of muscle mass and tends to underestimate it in elderly people (who often lose muscle mass).

Waist Circumference

The idea of using waist circumference as a risk predictor stems from the fact that abdominal obesity (accumulation of fatty tissue in the belly) is generally thought to be “worse” than accumulating fat elsewhere (such as the buttocks or thighs). This is because abdominal obesity correlates with an increased risk for not only cardiovascular disease, but also metabolic syndromehypertension, and diabetes.

Studies have shown that a waist circumference of 40 inches or more (102 cm) in men, and of 35 inches or more (88 cm) in women, is associated with elevated cardiovascular risk.

Waist-to-Hip Ratio

The waist-to-hip ratio is another way of assessing abdominal obesity, and studies have confirmed that this measure correlates with cardiovascular risk. To calculate your waist-to-hip ratio, measure both your waist and hip circumferences, then divide the waist measurement by the hip measurement. In women, the ratio should be 0.8 or less, and in men, it should be 1.0 or less. (This means that in women the waist should be narrower than the hips, and in men, the waist should be narrower or the same as the hips.)

The waist-to-hip ratio is helpful because in smaller people waist circumference alone may underestimate risk. By comparing waist circumference to hip circumference, you can get a better indication of abdominal obesity.

Which Measurement Is Better at Predicting Risk?

There is no definitive answer to this question.

BMI is certainly the “standard” measure of obesity, in that it is the measure recommended by the NIH, the American Heart Association, the American College of Cardiology, and The Obesity Society. These recommendations, again, are based on the large body of research that has used BMI to predict cardiovascular outcomes.

However, it is important to realize that, while BMI is quite good at predicting overall risk in large populations, it might not be a particularly accurate measure for a given individual. Also, it does not specifically take into account the degree of abdominal obesity a person may have.

Several studies have suggested that a measure of abdominal girth can be more accurate than BMI in predicting heart disease. In particular, while BMI is a predictor of heart attack, it is a relatively weak predictor when other risk factors (such as diabetes, smoking, cholesterol, diet, activity, and hypertension) are taken into account. In contrast, some studies have shown an elevated waist-to-hip ratio to be a strong predictor of heart disease, even after the statistics were adjusted for these other risk factors.

The Bottom Line

Many doctors are now relying on a combination of measures to advise patients on their weight-related risk. If your BMI is 35 or higher, that’s pretty much all you need to know. And if your BMI is 30 - 35, unless you are a bodybuilder or other type of muscular athlete, you are almost certainly too fat. But if you’re in the “overweight” category, knowing your waist circumference or your waist-to-hip ratio can tell you something important, since abdominal obesity is bad for you even if your overall weight is not outlandishly high.

One advantage of the waist-to-hip ratio is that you can assess it yourself, without formally measuring anything, in the privacy of your own home. Just strip down to your skivvies and look at yourself in the mirror, both head-on and in profile. If your waist in either dimension is bigger than your hips, you’re busted, and the excess poundage you’re carrying around in your abdomen is contributing to your overall cardiovascular risk. To reduce that risk, your weight is something you will need to address.

A Word From Verywell

Being overweight is an important risk factor for cardiovascular disease and metabolic conditions such as diabetes. The question of how best to measure whether we weigh “too much” is a good one, but in most cases, it’s not too difficult for us to figure out.

For people whose BMI is quite elevated (over 30 kg/m2), that’s usually the only measure you need to know to conclude that obesity is posing a significant risk. But for people whose BMI is in the 25 - 30 kg/m2 range, a measurement of abdominal obesity can be quite helpful in determining whether excess fat is contributing to their risk.

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