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Last updated January 30, 2020. 7 minute read

BMI and obesity—the implications of a body mass index >30

BMI is the abbreviation for body mass index, a simple calculation used by healthcare providers to determine a person’s risk for weight-related health issues. A person with a BMI of 30 or above is considered to be obese.

Meagan Morris Photo Written by Meagan Morris
Reviewed by Tzvi Doron, DO

Obesity. It’s a word that gets thrown around a lot when we talk about weight. Most of us have at least a vague idea that it refers to having more fat on the body than is optimal for health. But who—or what—decides that a number on the scale classifies a person’s weight as obese instead of as overweight or normal?

While obesity might seem like an arbitrary concept based on personal views and preferences, there is a medically accepted way to determine where a person falls on the healthy weight spectrum: BMI. 

If you’ve been told that your BMI falls under the obese category, but aren’t sure what that means, read on. We’ll get into the details about obesity and BMI—and what it could mean for your overall health.

Vitals

  • BMI is the abbreviation for body mass index, a simple calculation used by healthcare providers to determine a person’s risk for weight-related health issues.
  • BMI is calculated using a person’s height and current weight.
  • A person with a BMI of 30 or above is considered to be obese.
  • It’s a surface-level measurement, meaning it doesn’t take other factors—like muscle mass, age, and genetics—into account, which is why some think it’s a flawed way of determining health.

What is BMI?

According to the U.S. Centers for Disease Control and Prevention, BMI—short for body mass index—is a quick-and-easy screening tool often used by healthcare providers to classify adiposity in children, teenagers, and adults.

Put more bluntly, it’s an index of a person’s fatness (Nutall, 2015). It’s calculated with a formula using a person’s height and weight, meaning it doesn’t take anything else into account (like muscle mass, age, ethnic background, or personal habits. 

BMI is broken down into four main categories: underweight, normal, overweight, and obese. A normal—or healthy—BMI is anything that falls between 18.5 and 24.9, while an underweight BMI is 18.5 and under, and an overweight BMI is 25 to 29.9.

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What is an obese BMI?

One out of every three people in the U.S. is obese, meaning they have a BMI of 30 or above. (NIH, 2017).

BMIs in the obese range are broken down further into three separate classes:

  • Class 1: BMI is 30 to 34.9
  • Class 2: BMI is 35 to 39.9
  • Class 3: BMI is 40 or above

Class 3 obesity is sometimes referred to as morbid, extreme, or severe obesity.

To put this into perspective, a person who is 5-feet, 9-inches tall who weighs 203 lbs or more is considered to be obese, while a weight of over 271 is Class 3 obesity with a BMI of 40 (CDC, 2017).

Health risks associated with high BMI

Having a high BMI is related to a number of serious—and potentially fatal—health problems.

Heart disease

Heart disease is a catch-all term that refers to all diseases that affect the heart, like peripheral artery disease, coronary artery disease (which causes heart attacks), and arrhythmia. Having a high BMI can not only increase the risk of developing heart disease, but it also can develop at a younger age (Khan, 2018).

Stroke

Strokes happen when the brain’s blood supply is cut off, resulting in brain damage, disability, weakened muscles, and death. Obesity is shown to increase the risk of stroke by 64 percent (Strazzullo, 2010).

Type 2 diabetes

Obesity increases the risk of developing type 2 diabetes. However, losing 10 percent of body weight can potentially reverse it. One study found that nine out of 10 people who dropped that much weight within five years of diagnosis were able to put it into remission (Dambha-Miller, 2019).

High blood pressure

High blood pressure (hypertension) is the pressure that blood has on the inner walls of the arteries. More pressure means that the heart has to work harder to pump blood through. One 44-year study found that obesity accounted for 26 percent of hypertension cases in men and 28 percent in women (Encyclopedia of Epidemiology n.d.).

Sleep apnea

Sleep apnea is a serious condition where breathing stops and starts during the night because the upper airway is blocked. Obesity can increase the risk of sleep apnea because it adds pressure to the neck. But like type 2 diabetes, a 10 to 15 percent reduction in weight is shown to cut sleep apnea severity by 50 percent (Schwartz, 2008).

Other considerations to determine if a person is overweight

While BMI is useful as an at-a-glance screening tool for health, it’s often criticized because it can’t give an accurate picture of the actual amount of body fat on a person—and it can’t capture the amount of fat on different parts of the body (Nuttall, 2015).

That’s why some healthcare professionals prefer other screening tools, alone or in addition to BMI, to determine a person’s risk for disease. 

Waist-to-hip ratio

While BMI uses height and weight to calculate BMI, a waist-to-hip ratio (WHR) uses a comparison of waist size and hip size. Having a large amount of excess fat in the waist, as opposed to the hips, is a risk factor for developing cardiovascular disease. One study of 15,000 people found that WHR was linked to an increased risk of death, even for people with a normal BMI (Sahakyan, 2015).

Having a WHR over 1.0 is the highest risk area. Women should aim to have a WHR of 0.85 or less, while men should have a 0.9 or less (WHO, 2008).

Waist-to-height ratio

Like WHR, waist-to-height ratio (WHtR) calculates the risk of health problems using waist circumference in relation to height. A meta-analysis of studies involving 300,000 people found that WHtR is a better predictor of diabetes, hypertension, strokes, and heart attacks than BMI (Ashwell, 2012).

A healthy WHtR is one where a person’s waist is less than half of their height. For example, a person who is 60 inches tall should have a waist no larger than 30 inches to be considered normal.

Body fat percentage

Body fat percentage can be calculated using a number of different methods, including skinfold caliper tests, hydrostatic weighing, and DEXA scans. Unlike BMI, body fat testing methods allow for a more accurate measurement of the actual amount of fat on a person’s body. This is helpful for athletes who have high BMIs but are actually healthy because of the low amount of body fat and higher percentage of muscle mass they carry. It’s also helpful for those who may have normal BMIs but have low muscle mass. In these people, BMI actually underestimates adiposity.

Ideally, men between the ages of 20 and 39 should have 8 to 19 percent body fat, while women in that same age group should aim for somewhere between 20 to 32 percent body fat (Beth Israel Lahey Health Winchester Hospital, n.d.).

That’s not to say that people with an obese BMI are the only ones facing health risks. Having an underweight BMI (18.5 or below) can also lead to health problems, including cardiovascular disease, malnutrition, decrease immune system function, infertility, and osteoporosis (Dobner, 2016; Lim, 2016; Mori, 2016; Park, 2017).

How do I determine my BMI?

You can calculate your BMI by dividing your weight in pounds by height in inches squared (CDC, n.d.)

Want to avoid math? You can use online tools—like this BMI calculator from the National Heart, Lung, and Blood Institute—to find your BMI. Charts plotted by height and weight can also clue you into your BMI.

It’s a good idea to speak to your healthcare provider if your BMI falls into the obese range. He or she can evaluate your disease risk factors, diet, and physical activity and work with you to figure out a sustainable method of weight loss to get to a normal weight range—and your risk of developing health issues from obesity.