Specifically, BMI is one way that healthcare professionals estimate whether you’re at a normal weight or whether you could benefit from losing a few or many pounds, according to the Centers for Disease Control and Prevention (CDC). (1) It’s also proven to be a generally reliable and easy way to estimate a person’s risk of obesity. BMI has been used to measure obesity in the United States since 1985, and the current standards were set by the National Institutes of Health in 1998, according to a May 2016 study published in the Journal of Obesity. (2) BMI is not foolproof, though. Experts today say we should think of BMI as a screening tool that can be used to determine if you’re overweight or obese, but not as an actual diagnosis. BMI doesn’t properly account for your levels of body fat or your overall health, and should be used as one piece of a series of factors your doctor uses to assess your health, according to the CDC. (1) “As with everything in healthcare, one needs to treat the patient and not just a number,” says Abraham Krikhely, MD, a bariatric and general surgeon and an assistant professor of surgery at Columbia University Medical Center in New York City.
BMI below 18.5 = UnderweightBMI 18.5 to 24.9 = Normal weightBMI 25.0 to 29.9 = OverweightBMI 30.0 and higher = ObeseBMI 40 and higher = Morbidly obese
Class 1: BMI 30–35Class 2: BMI 35–40Class 3 (sometimes called “extreme” or “severe” obesity): BMI 40 or higher
A higher BMI is linked with an increased risk of type 2 diabetes, hypertension, and cardiovascular disease compared with people of a normal weight and waist circumference, according to the National Heart, Lung, and Blood Institute. (3) Formula Weight (lb) / height (in)2 x 703 Example For a person who is 5 feet 4 inches tall and weighs 140 pounds: 140 / (64x64) x 703 = 24 BMI. Or, for those who use the metric system, it’s your weight in kilograms divided by your height in meters squared, according to the CDC. (5) Formula Your weight (kg) / height (m)2 Example For a person who is 1.7 meters tall and weighs 68 kg: 68 / (1.7 x 1.7) = 23.5 BMI. Men and women use the same formula to calculate BMI numbers, though the BMI in children requires a slightly different formula that factors in their gender and age, according to KidsHealth. (6) Kids’ BMIs are compared with those of their peers and presented as percentiles. A child is considered to be overweight if she falls at or above the 85th percentile and below the 95th percentile. Children who fall at or above the 95th percentile are obese, according to the CDC. (7) Keep in mind that BMI isn’t very accurate when a child is going through puberty and gaining weight more quickly than usual, according to KidsHealth. (6) The BMI calculator for children should be used for children ages 2 through 19. (6) Those who are 20 years or older should use the adult BMI calculator. (8) The World Health Organization defines obesity as a waist-to-hip ratio greater than 0.85 for women and .9 for men. This is calculated by dividing the waist measurement by the hip measurement measured at the widest part of the buttocks. (10) A June 2016 study published in the International Journal of Preventive Medicine found waist circumference to be a more accurate predictor of abdominal obesity compared with waist-to-hip ratio. (11) According to a July 2013 study published in the Journal of Diabetes Investigation, waist-to-height ratio and body fat percentage are two other ways to measure central obesity. (12) Ideally, your waist circumference should be less than your height, according to a past study. (13) According to Winchester Hospital, healthy body fat percentages are: (14)
Ages 20 to 39 21 to 32 percent body fat for women, or 8 to 19 percent for menAges 40 to 59 23 to 33 percent body fat for women, or 11 to 21 percent for menAges 60 to 79 24 to 35 percent body fat for women, or 13 to 24 percent for men
Yet relying on BMI alone does have some drawbacks when it comes to healthy weight measurements, though, and its accuracy has come under fire lately. BMI is not a perfect tool — and experts caution not to rely on it alone, since it doesn’t capture the full picture of one’s health. It doesn’t take a person’s sex, race, age, body composition, body fat distribution, or type of body fat into account, Schuit says. She says to view it more as a conversation starter but not as a sole deciding factor indicating that a person is unhealthy. One of the major flaws in relying on BMI solely to define obesity is there’s no way to distinguish between muscle and fat. Muscular people and athletes may qualify as obese when they’re perfectly healthy, and having a high muscle mass does not pose the same health risks as fat mass, says Ronette Lategan-Potgieter, PhD, an assistant professor of practice in health sciences at Stetson University in DeLand, Florida. A May 2016 study published in the International Journal of Obesity investigated how often BMI accurately predicts obesity. The researchers found that nearly half of people who qualified as overweight based on BMI and 29 percent of those who qualified as obese were metabolically healthy after blood pressure, triglyceride, cholesterol, glucose, insulin resistance, and C-reactive protein data (a marker of inflammation) were analyzed. Plus, more than 30 percent of people who had a healthy weight based on their BMI were cardiometabolically unhealthy. (15) The researchers concluded it’s unwise to use BMI as the key indicator of health. According to the CDC, as well as a study published in the Clinical Journal of Sport Medicine and per Harvard T.H. Chan School of Public Health, there are a number of misleading factors that suggest BMI isn’t a one-size-fits-all tool, including: (16,17,18)
Women generally have more body fat than men at the same BMI.Older adults generally have more body fat and less muscle than younger adults at the same BMI.Professional and amateur athletes may have a higher BMI because of greater muscle mass, not increased body fat.Asian people have more body fat than non-Asians.
Waist circumference Fat around your waist relative to your hips increases your risk for heart disease and type 2 diabetes. As mentioned above, the waist should be 35 inches or smaller for women or 40 inches or smaller for a man in order to minimize this risk.Other risk factors for disease Healthcare professionals also consider the presence of high blood pressure (hypertension), high cholesterol, high triglycerides, high blood glucose (sugar), family history of premature heart disease, lack of physical activity, and cigarette smoking as factors that increase your risk of developing obesity-related diseases.
Schuit also says lifelong weight status and trends, body composition, lean body mass, physical activity, and diet should be taken into account as well. Body composition testing can be particularly helpful in determining where the excess weight is carried and whether it’s muscle, bone, water, or fat, Schuit says. Krikhely cautions not to simply replace BMI tracking with another metric. Instead, track several of the factors mentioned above and be sure to see a doctor to ensure he or she identifies and helps you manage any possible issues early on.
Other Ways to Measure Fat
There are other ways to measure body fat besides a BMI calculation. But they require special equipment, require training to perform, and can produce varied results depending on the equipment used and the person performing the procedure. Per the CDC, these other procedures include: (16)
Skinfold thickness measurements with calipersUnderwater weighingBioelectrical impedanceDual-energy X-ray absorptiometry (DXA)
Start by adopting a healthy diet and exercise routine. “The best diet is the one that someone can turn into a long-term lifestyle,” Schuit says. There’s no one diet that works best for everyone, but Schuit says the most successful diets generally involve vegetables making up one-third or one-half of meals, and plenty of water. Krikhely suggests meeting with a nutritionist who can outline the best approach for you. Exercise is also important. The American Heart Association recommends getting at least 150 minutes of moderate-intensity exercise or 75 minutes of vigorous aerobic activity per week. (22) If you’re just starting out, start slow with about 30 minutes a day of low-intensity exercise like walking, or break that 30 minutes into three 10-minute sessions if that seems like too much, Schuit says. You can build from there as it gets easier. If switching up your diet and exercise routine doesn’t give you the results you’re looking for, and you meet the clinical requirements, your doctor may suggest bariatric surgery. According to the Mayo Clinic, there are three types — biliopancreative diversion with duodenal switch, gastric bypass, and sleeve gastrectomy. They all involve changing your digestive system in order to promote weight loss. These surgeries are typically reserved for people who are severely overweight (with a BMI of 40 or higher) or if your BMI is between 35 and 40 and you’re also battling a weight-related health issue, such as type 2 diabetes, high blood pressure, or severe sleep apnea. (23) There are also prescription medications designed to treat obesity, either by reducing your appetite or by reducing the amount of fat absorbed by the body. According to the NIDDK, some common FDA-approved weight loss medications are: (21)
Alli or Xenical (orlistat)Qsymia (phentermine and topiramate)Contrave (bupropion-naltrexone)Victoza (liraglutide)
Medications may be an option if you have a BMI of 30 or higher or above 27, plus a weight-related health issue, Krikhely says. Keep in mind that surgery, procedures, and medication won’t magically reduce your weight. “None of these are cures — they are just treatments,” Krikhely says. “They are not replacements for a healthy lifestyle, nutrition, exercise, and activity.” Additional reporting by Anne L. Fritz.
