Many of us have a vague idea that we could stand to lose a few pounds. We get bombarded with weight-loss messages all day long, and we’re urged to do it for both our health and to attain a certain standard of beauty that favors lean figures.
But who actually needs to lose weight, and what exactly does it mean to be at a healthy weight? It’s not as simple as just stepping on the scale and seeing a magic number.
Measurements of a Healthy Weight
Discussions of a healthy weight often reference the BMI or Body Mass Index as a standard of judging what’s healthy or not. “BMI is currently used by medical professionals as a quick assessment to whether or not a person has a healthy weight,” says Antonette Hardie, a registered dietitian at the Ohio State University Wexner Medical Center in Columbus.
“BMI is thought to provide medical professionals with a sense of whether or not an individual is at risk for certain obesity-related chronic diseases, such as heart disease, high blood pressure, diabetes and other comorbidities,” she adds.
The reason why boils down to fat. The BMI is a quick and dirty way of calculating whether someone’s weight is out of proportion to their height. When that happens, it’s often assumed that the person is carrying around excess fat – also called adipose tissue – which can elevate risk for several chronic conditions.
Carrying excess white fat – cells that are stored under the skin or around the organs may be dangerous. This is especially true when the excess fat accumulates around the midsection where the vital organs reside. Too much belly fat has been associated with an increased risk of heart disease, diabetes, cancer and dementia. That’s because these fat stores are metabolically active, meaning that the fat functions like an endocrine organ, secreting hormones that influence how your body functions.
But even thin people who still conform to the BMI scale can be carrying around too much belly fat, which can increase their risk of chronic illness. It’s all about where the fat is stored in the body and how it alters the way the metabolism works. Metabolism is the process by which your body converts calories from the food you eat into energy to power your activities and bodily functions each day. If you consume more calories than you need to meet those needs, your body will store the excess as fat. And that’s the issue when it comes to being overweight.
The BMI is intended to help doctors spot patients who might need to shed some excess fat. Brenda Braslow, a registered dietitian nutritionist and certified diabetes care and education specialist with MyNetDiary, a calorie and exercise tracker launched in 2007, explains that the BMI is “primarily meant to be used as a screening tool to assess weight status.”
Calculating Your BMI
You determine your BMI with a relatively simple calculation that divides your weight in kilograms by your height in meters squared: BMI = weight (kg)/height (m2). The National Institutes of Health also provides a free BMI calculator online.
You can use the resulting number to see which BMI category you fall into:
- A BMI of less than 18.5 means you’re underweight.
- A BMI of between 18.5 and 24.9 means you’re normal weight.
- A BMI of 25 to 29.9 means you’re overweight.
- A BMI of 30 or greater means you’re obese.
According to the BMI scale:
- A 5’8” tall person who weighs 150 pounds has a BMI of 22.8 and is considered normal weight.
- A 5’2” tall person at that same 150 pounds has a BMI of 27.4 and is in the middle of the overweight category.
The BMI does not consider the body type of either individual or whether their weight is mostly muscle or fat. Indeed, many Olympic athletes – who we tend to think of as the fittest of the fit – would register as overweight or obese according to the BMI scale because they have a larger proportion of strong muscles, which is denser than fat.
The BMI’s Limitations
Hardie notes that the BMI equation was “created by a mathematician in the 1800s based off the average white man.” As such, it’s simply “a math equation of weight and height.” This means it falls woefully short in adequately addressing certain health questions related to weight.
BMI doesn’t take into consideration several important determinants of health including:
Because these factors are overlooked by the BMI scale, Hardie says it’s a “completely inadequate and outdated measure of someone’s health. BMI tells us what ‘category’ we fit into based off our weight and height. It can be an indicator of possibly being at an increased risk for developing obesity-related disease such as hypertension, heart disease or diabetes.” But a person’s BMI is “not an all-encompassing picture of health and wellness and should be used in conjunction with other measures of health, such as lab tests, body fat percentage and waist circumference,” Hardie says.
Using these multiple measures can help develop a far more accurate view of a person’s overall metabolic health than a simple calculation of height and weight.
Braslow notes that health determinants such as body composition and bone density can differ greatly from person to person based on sex, genetics and age. These variations can have a big impact on what’s actually considered healthy weight. For example, “a woman’s healthy weight will be lighter than a man’s because of the difference in muscle mass. Women have less muscle mass than men.”
Finding Your Optimal Weight
Braslow agrees that the BMI is just one tool in the arsenal that health care providers and dietitians should use to help guide patients to their ideal weight. “Measuring an individual’s metabolic health ideally should include a combination of indicators.”
There are many other methods to determining healthy weight, including:
- Other equations. Weight assessment equations such as Harris-Benedict, which takes into account age, height, weight and gender, or Mifflin-St Jeor that takes into account those factors and basal metabolic rate, or the amount of calories your body burns just to stay alive, provide a more nuanced view.
- Waist circumference measurements. This method can help determine “metabolic health because extra abdominal fat increases the risk for diseases like heart disease and diabetes,” Braslow says. “Risk increases with a waist size greater than 35 inches for women and 40 inches for men.”
- Other health measurements. Braslow says “a more comprehensive evaluation of a person’s metabolic health looks at an individual’s current health status by considering measures like blood pressure, blood cholesterol levels, blood glucose, pulse and oxygenation.” Basic blood work can provide your doctor with a lot more detail about your health and whether you’re metabolically healthy and at a healthy weight than the BMI scale.
- Body composition checks. There are some specific ways (with calipers that pinch a bit of fat on the arm or belly or via a special submersion test) to get a detailed understanding of your body composition and the ratio of fat you’re carrying around. This information can provide more insight as to whether you’ve got too much fat – which could elevate risk for certain conditions. For instance, some people may be tipping into the overweight category because they have larger bones or big muscles – not too much fat. “The BMI can be way off the mark for assessing an athlete’s weight status,” Braslow says.
- Personal and family medical history. “Personal medical conditions and family history of conditions are factors to determine disease risk,” Braslow notes, and understanding what conditions you may be more likely to develop can help guide your health journey as you age. “For example, a woman weighing 150 pounds with extra body fat in hips and thighs with no medical conditions, healthy blood pressure, cholesterol and other vitals may be advised that her weight is just fine. Another woman weighing 150 pounds may have extra central body fat and have diabetes and high blood pressure and may be advised to lose 10 pounds for improved health. Ideally, weight should be assessed in a comprehensive, personalized way.”
With all that in mind, if the next time you visit the doctor you’re told that you need to lose weight for your health, ask for more detail about why that recommendation is being made. If the advice is based purely on your BMI, see if you can have another assessment made.
“BMI should never be interpreted as gospel truth. A health care provider should not use it as a strict weight assessment tool without further exploration of an individual’s weight and health status,” Braslow says.