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Published on January 18, 2016

BMI may not tell you all you need to know about weightBMI may not tell you all you need to know about weight

Weight loss was the third most popular New Year’s resolution this year, according to ABC News. But determining how much you should actually weigh is not that simple.

The most common ways to find your ideal weight include measuring your Body Mass Index, Waist-Hip Ratio and Waist to Height Ratio. Each has pros and cons and the best answer is probably a combination of more than one, according to Miguel Prieto, MD, an internal medicine physician with Emerald Physicians in Bourne.

“The Body Mass Indicator is based on your height and your weight,” Dr. Prieto said. “It’s pretty useful when it comes to doing medical research, however it is not very specific. You can have people with normal BMIs who are actually at a higher risk for cardiovascular disease or diabetes. Or people with a higher BMI may not be at risk if they are very muscular.”

Currently doctors automatically check height and weight with every visit, so a patient’s BMI is easy to calculate. Additionally, websites like the Centers for Disease Control and Prevention have BMI calculators.

Once you get that number, the CDC classifies BMI the following way:

  • Below 18.5 = underweight
  • Between 18.5 and 24.9 = normal
  • Between 25.0 and 29.9 = overweight
  • 0 and above = obese

In each category there is quite a range of weights. For example, the normal weight range for a 5-foot, 5-inch woman is between 111 and 150 pounds. That’s a pretty big span, so this is when knowing your frame size comes into play. If you have a measuring tape, there are simple calculators that use wrist measurement to determine your frame.

An informal way to check frame size is to wrap your thumb and middle finger around your wrist, just above the wrist bone. If your fingers overlap, you have a small frame. If they just meet, you have a medium frame. If they don’t meet you have a large frame. Once you know your frame size, it tells you which end of the “normal” spectrum of weight you should aim for on the BMI chart.

The problem with BMI, as Dr. Prieto pointed out, is that it doesn’t take into account waist size or muscle tone. It underestimates the amount of body fat in overweight people and overestimates it in muscular people, because people who have developed muscles simply weigh more. An Olympic athlete could have a higher BMI than someone who never exercises, so the non-athlete could very well look healthier on paper if the BMI alone is used as a measure.

To account for those differences another common measurement is waist circumference. Waist circumference coupled with BMI is much more specific and sensitive for prevention of diseases, Dr. Prieto explained.

“Unfortunately in primary care, we don’t measure waist circumference on a regular routine basis,” he said. “That might change, though, because studies show that it is a very good tool.”

Waist-Hip Ratio (WHR) is one way to use waist circumference to determine health. To calculate your Waist-Hip Ratio, measure the smallest circumference of your waist and the widest part of your hips. You then divide your waist measurement by your hip measurement.

The Centers for Disease Control and Prevention has concluded that WHR is a better predictor than BMI when it comes to the risk of diabetes and hypertension. The CDC considers a ratio of .90 or less to be safe for men and a ratio of .80 to be safe for women. Both genders are considered to be at risk for heart disease if their WHR is over 1.0.

In 2012, Dr. Margaret Ashwell and a team at the British Nutrition Foundation did a study that showed that the best – and simplest – ratio was the Waist to Height Ratio (WHtR). Instead of looking at complicated charts with meaningless numbers, the WHtR has one simple rule. For an optimum healthy weight, your waist circumference should be less than half your height.

Dr. Ashwell’s study analyzed several studies involving approximately 300,000 people. She found that a good WHtR was better at preventing health risks including diabetes, hypertension, stroke, dyslipidemia and cardiovascular disease than the BMI.

Regardless of what tool you use, living a healthier lifestyle will help you live longer and better, said Dr. Prieto.

“In our culture we have an obsession about weight. When you actually implement the lifestyle changes that we recommend, the pursuit of the optimal weight becomes secondary,” he said.

“If you are eating foods like fresh vegetables and fruit and you’re eating foods that are really low in processed starches and fats and you’re exercising 150 minutes a week or more at 70 percent of your heartrate that decreases your risk for Alzheimer’s by 45 percent to 54 percent. For me that’s more important than trying to chase a number.”

Note: As of 5/1/2019, Emerald Physicians joined Medical Affiliates of Cape Cod (MACC), a division of primary and specialty care physicians from Cape Cod Healthcare.