Beyond BMI: Understanding weight, body composition & health risks

Body mass index (BMI) has long been a standard tool to assess if someone is at a healthy weight. Despite its prevalence in health care, BMI doesn’t provide the whole picture when it comes to evaluating health risks related to weight.

To better understand your health, the nuances of body composition, fat distribution and blood work should be taken into consideration.

What is BMI, and what’s the problem with it?

Developed by a Belgian statistician looking for trends in population health — not people’s individual health — BMI is a height-to-weight ratio calculated by dividing a person’s weight in kilograms by their height in square meters.

BMI measurement is classified into the following categories for adults over 20:

• Underweight: Below 18.5
• Normal weight: 18.5–24.9
• Overweight: 25–29.9
• Obesity: 30 and above

“BMI is pretty good for what it was intended, but it’s based on European men in the 19th century, so it misclasses about a third of people, most commonly with women and different ethnicities,” explains Valerie Sutherland, MD, an obesity medicine provider with MultiCare Rockwood Weight Loss & Bariatric Surgery Center. “We know with certain ethnic backgrounds there are different BMIs that correlate with certain health risks.”

For instance, those with South Asian descent may face increased metabolic risks at a lower BMI compared to individuals of European descent.

What really matters: The type and location of weight

Another problem with BMI is that it can’t distinguish between different types of fat, tell where weight is distributed in your body or account for muscle mass.

“We address weight because of its impact on health. BMI is not very specific or precise for assessing health risk of weight, so it’s important to look at what kind of extra weight a person has,” Dr. Sutherland says. “Is it visceral or subcutaneous? Is their weight in the trunk and upper body? The impacts and risks depend on the type and distribution.”

Subcutaneous fat is typically located in the lower part of the body and is relatively harmless. This fat is stored under the skin that acts as energy reserves.

But when that storage capacity is exceeded, fat accumulates in the solid organs like the liver and pancreas. This is called visceral fat, which is carried in the trunk and upper body and linked to a variety of serious health conditions, including metabolic syndrome, cardiovascular disease and liver problems.

Body composition — how much fat, muscle and bone a person has — is also more accurate than BMI in determining health risks.

“At MultiCare, we use bioimpedance analysis to measure body composition,” Dr. Sutherland explains. “It’s obtained by standing in bare feet on a scale with handles for about 60 seconds. A measurement is obtained at baseline and then about every three months during weight loss.”

This analysis provides information on how much skeletal muscle a person has, the amount and type of fat they have, and where the fat is distributed — whether in the trunk or lower body.

Normal weight obesity and fat mass disease

A body composition analysis can also identify if you have normal weight obesity or fat mass disease. Knowing this can help with treatment.

Normal weight obesity is a condition where a person has a normal BMI but low muscle mass and high visceral fat.

“People with normal weight obesity often restrict calories and do a lot of cardio to try to keep their weight down,” Dr. Sutherland says. “But they actually need to eat more and focus on resistance training to gain muscle.”

Fat mass disease is characterized by an athletic body type, with excess weight in the form of excess muscle. This puts extra stress on their upper body, pushing on the heart and lungs, which can lead to sleep apnea or an enlarged heart.

Dr. Sutherland says people with fat mass disease tend to eat healthy foods, especially focusing on protein, but their portions are high because their body needs more fuel.

“A person with fat mass disease doesn’t need to eat so much protein and push the weights,” Dr. Sutherland notes. “They may need to reduce portion sizes and do less intense heavy weight training and do low-intensity cardio like brisk walking.”

Additional methods beyond BMI

Other measurements like waist circumference and waist-to-hip ratio can provide more information about your health. Studies show that individuals with a waist circumference greater than 35 inches for women or 40 inches for men are at a higher risk for metabolic diseases, regardless of their BMI.

“The problem with waist measurement, however, is that it can vary based on where you measure, and there is not a universal definition of where the waist is,” explains Dr. Sutherland. “And, although it might indicate excess weight in the abdomen, it doesn’t tell you what kind of fat a person has.”

Imaging and blood work also provide valuable information.

Imaging tests like abdominal ultrasounds or CT scans can help identify internal fat storage or plaque buildup in arteries, giving a clearer picture of your health.

Blood tests like lipid panels, A1c and C-reactive protein (CRP) levels can provide insights into metabolic health that BMI alone cannot.

“One of the most commonly missed diagnoses is metabolic syndrome,” Dr. Sutherland says. “Even if someone’s BMI is within the normal range, they might still have elevated blood pressure, low HDL cholesterol and high triglycerides, all markers of metabolic syndrome.”

Follow your intuition

While BMI can be a useful tool in tracking trends in population health, it doesn’t meet the mark when assessing an individual’s health risk.

If you feel like your BMI doesn’t match your health reality, talk to a health care provider, like a medical weight loss specialist, who’s equipped with the right tools and knowledge and can complete a comprehensive assessment.