What can weight tell you about your metabolic health?

The “obesity = dangerous” message is everywhere. Headline-making studies underscore the link between excess fat and more than 200 chronic diseases, and ads for the new wave of weight-loss agents—GLP-1 receptor agonist drugs—flood social media. 

This idea is particularly embedded in discussions of metabolic health, as obesity increases the likelihood of insulin resistance, prediabetes, and metabolic syndrome, a cluster of conditions that elevate diabetes, stroke, and cardiovascular disease risk. 

At the same time, some people with obesity never go on to develop these conditions, while some people at a “healthy” weight—and even those on the thinner end of the spectrum—do. So, to what degree is our weight really indicative of our metabolic health?

As you’ll learn below, the answer is complex. To connect the dots between weight and metabolic health, it helps to understand the current flaws in measuring obesity as well as what the research shows about this relationship. While it would be foolish to ignore our weight completely, this biomarker alone doesn’t provide the full picture of our health.

Defining a “healthy” weight isn’t simple

When we talk about weight and health, we’re most often talking about obesity. More than 40 percent of Americans live with obesity. Officially recognized as a disease by the American Medical Association since 2013, obesity is linked with medical conditions affecting nearly every part of the body, from head (dementia, stroke, depression) to toe (Type 2 diabetes, osteoarthritis, gout).

Historically, determining the cutoffs between healthy weight, overweight, and obesity has been based on a standardized body fat measurement known as body mass index (BMI). To calculate this, you divide your weight in kilograms by the square of your height in meters.

 BMI = body weight (in kilograms) ÷ height (in meters) squared

• ≥30 = obesity
• 25.0 to ＜30 = overweight
• 18.5 to ＜25= healthy weight
• ＜18.5 = underweight

 According to BMI, someone who is 5’3” and weighs 170 pounds has obesity (BMI = 30.1), as does someone standing 5’10” and weighing 210 pounds (BMI = 30.1).

 When the Centers for Disease Control and Prevention says that as of 2020, an estimated 42 percent of U.S. adults have obesity and an additional 31 percent are overweight, those numbers are BMI-based.

The problems with BMI

BMI has long been used in obesity screening. Yet, it is also widely recognized as a flawed metric, especially when assessing metabolic health at the individual level.

First, BMI doesn’t measure body fat percentage. Instead, it relies on the number on the scale, which can’t distinguish fat from metabolically favorable muscle. (A heavily muscular professional athlete with an excellent cardiometabolic profile could easily fall into the obesity category.) Nor does it differentiate between subcutaneous and visceral fat or factor in body fat distribution, i.e., where one carries their weight.

These omissions are noteworthy, as body fat percentage and distribution are more accurate at assessing and predicting cardiometabolic risk than BMI.

Body fat distribution, in particular, is paramount. Fat accumulating at the waistline increases cardiometabolic risk far more than fat stored around the hips and legs. Doctors refer to people who carry most of their excess weight in the abdominal area as having “central obesity.” This is the more concerning pattern of body fat distribution, tightly correlated with insulin resistance, Type 2 diabetes, heart and liver disease, and other chronic conditions.

For this reason, many experts prefer using waist-to-hip ratio (WHR) for diagnosing obesity. To calculate this, you divide your waist size by your hip size:

WHR = waist circumference (measured at the smallest point, in inches) ÷ hip circumference (measured around the widest part of the buttocks)

The World Health Organization considers men with a WHR greater than 0.90 and women with a WHR greater than 0.85 to have central obesity, though others have slightly lower cutoffs. In a 2023 JAMA Network Open study examining the measurements of nearly 390,000 adults, WHR trumped BMI in predicting all-cause mortality, as well as predicting risk of death due to cardiovascular disease-related health conditions like hypertension. 

 Other methods doctors use to assess body composition involve visualizing the fat in your body, while BMI and WHR are generalized proxies. These tools may be more expensive and include: 

• Computed tomography (CT)
• Magnetic resonance imaging (MRI)
• Dual x-ray absorptiometry (DEXA)

A final and significant BMI shortcoming is its basis on height and weight statistics gathered from European white men in the 1830s, which has drawn criticism for being racially biased. We know, for example, that since non-Hispanic African Americans generally have more muscle mass and lower body fat percentages than Caucasians and Mexican Americans, they’re prone to being misdiagnosed as having obesity based on BMI alone. Misdiagnosis can not only have emotional and social consequences but could impact medical care.

Because of the problems extrapolating standard BMI to people of color, Fatima Cody Stanford, MD, MPH, an obesity medicine physician-scientist at Harvard Medical School, has proposed a revised BMI chart that corrects for race and ethnicity, adjusting the BMI obesity cutoff down for black men and up for women. (It’s been shown that black women don’t experience the same uptick in mortality risk as their BMI rises, even up to a BMI of 37.)

These dynamics help explain why BMI, long used in obesity screening at the population level to provide a reasonably accurate snapshot of health risk, isn’t always spot-on at the individual level.

What do we know about weight-related health risks?

Despite its flaws, many studies examining the interplay between weight and metabolic health use BMI. According to this work, the central issue between obesity and poor health is insulin resistance.

When you eat, your blood glucose levels rise. In response, the pancreas secretes the hormone insulin, which ushers glucose out of your blood and into cells to be utilized immediately (for energy) or stored as fat for future use. After the temporary insulin rise and the ensuing drop in blood glucose, the pancreas stops secreting its star hormone, and insulin levels return to baseline.

 However, several factors can undermine this process, including poor nutrition. (Poor sleep and excess stress are other culprits.)

Consider the standard American diet (SAD), featuring nutrient-poor carbohydrates like white bread and other refined grains, candy, soda, and other ultra-processed foods. Besides providing a surplus of calories—a known obesity driver—these foods are quickly digested. When consumed throughout the day, as they often are, they create a cycle of repeated blood sugar and insulin surges that eventually cause cells to become “numb,” or resistant, to insulin’s effects. This phenomenon is called insulin resistance. The body (specifically, cells in the muscles, fat, and liver, which are intimately connected with the pancreas) has become so insulin resistant that it can no longer control blood glucose levels, which soar dangerously high. 

The other major cause of insulin resistance is obesity. Although fat was considered inert storage tissue, it’s now recognized as a metabolic organ, producing and secreting hormones that help promote further weight gain (by stimulating hunger) and chemical messengers called cytokines that increase inflammation and perpetuate insulin resistance. 

A type of fat called visceral adipose tissue (VAT, or visceral fat) is especially dangerous. Compared to subcutaneous fat—the kind you can see and pinch—VAT lurks deep below the skin, enveloping the stomach, liver, kidneys, and intestines. VAT is far more hormonally and metabolically active than subcutaneous fat, churning out more inflammatory, insulin resistance-fueling cytokines. This creates a dangerous cycle.

The combination of obesity and insulin resistance has been tied to the following medical conditions:

• Type 2 diabetes and prediabetes: Type 2 diabetes is a chronic disease characterized by dangerously high blood glucose levels. It is an extreme form of insulin resistance. Prediabetes is an intermediate stage between normal blood sugar regulation and Type 2 diabetes, characterized by blood sugar levels that are higher than normal but not high enough to qualify as Type 2 diabetes. Based on data from 2001-2016, between 30 and 50 percent of people with Type 2 diabetes can attribute their glucose dysregulation to obesity, and between 80 and 90 percent of people with Type 2 diabetes have obesity or are overweight.
• Metabolic syndrome: Having three or more of the following risk factors indicates metabolic syndrome: abdominal obesity (extra fat in the stomach versus hips), high blood sugar, high blood pressure (hypertension), high triglycerides, and low HDL cholesterol (the “good” kind). This elevates risk for heart disease, Type 2 diabetes, and other health conditions.
• Heart disease: People with insulin resistance and obesity are more prone to atherosclerosis (a chronic inflammatory condition in which unhealthy fats and waxy cholesterol accumulate inside blood vessels, impeding blood flow), heart disease (including heart attack and stroke), hypertension, and other heart conditions.
• Cognitive dysfunction and dementia: Chronically elevated blood sugar levels and insulin resistance create oxidative stress and inflammation in the brain, which are both linked with cognitive decline. The relationship between Alzheimer’s disease and poorly controlled blood sugar levels is strong enough that some experts now refer to Alzheimer’s as “Type 3 diabetes”. Additionally, just as the fat and cholesterol accumulation in cardiac blood vessels can hinder blood flow to the heart, it can block blood flow to the brain, increasing vascular dementia risk.
• Cancer: Forty percent of cancers diagnosed in the United States can be caused by, or associated with, overweight and obesity, including those of the breast (in post-menopausal women, specifically), colon and rectum, and thyroid, liver, pancreas, and ovaries. Chronic inflammation and insulin resistance caused by obesity can pave the way toward genetic mutations that allow abnormal cells to turn cancerous. 

 Other chronic diseases thought to be strongly influenced by the metabolic, inflammatory, and structural complications of obesity include, but are not limited to:

• Non-alcoholic fatty liver disease
• Sleep apnea
• Gastroesophageal reflux disease (acid reflux)
• Osteoarthritis
• Sexual dysfunction
• Urinary incontinence

Exceptions to the rule: skinny fat and metabolically healthy obesity

Despite the robust, research-backed association between obesity and these metabolic abnormalities, not everyone who develops these conditions falls into a high BMI category, and not everyone with obesity goes on to experience these health challenges. 

Take the approximately 30 million Americans who have “normal weight obesity” (NWO). Despite having a “healthy” BMI, they have a high body fat percentage (defined in the linked study as roughly 20-30 percent for men and 30-40 percent for women). And since many medical professionals rely on BMI to dictate which patients they target for cardiometabolic screening, these “skinny fat” patients may not get the treatments they need to avoid health risks. 

In a related phenomenon, metabolic obesity with normal body weight (MONW), a person has a “healthy” weight and BMI but carries enough fat in their abdominal area to significantly raise their cardiometabolic risk. Those of South Asian descent, for instance, tend to have different fat distribution (with higher amounts of abdominal fat) than non-Hispanic whites and are more likely to develop Type 2 diabetes and high cholesterol when at a “healthy” BMI. As such, traditional BMI cutoffs miss many South Asians at risk for metabolic disease. About 30 percent of the global population has MONW.

On the flip side of the coin is a concept called metabolically healthy obesity (MHO). No universally accepted definition of MHO exists, but most MHO studies use the absence of metabolic syndrome (or, more specifically, having no more than two metabolic syndrome components). Others base MHO on a test called HOMA-IR (homeostatic model assessment of insulin resistance), which measures fasting insulin and glucose to assess insulin sensitivity, or the degree of insulin resistance someone has.

Depending on the criteria used to define obesity, studies suggest that anywhere from 15 percent to 40 percent of Americans with obesity have favorable metabolic profiles. In a 2021 study examining data from the Third National Health and Nutrition Examination Survey—a nationwide probability sample of more than 12,000 Americans—researchers concluded that 41 percent of participants with obesity had MHO, using a three-pronged definition of metabolic health:

• Systolic blood pressure less than 130 mmHg with no use of blood pressure-lowering medication
• WHR less than 0.95 for women and less than 1.03 for men
• No Type 2 diabetes

 But when the researchers re-examined the data using three different MHO definitions (which included or excluded people with varying metabolic syndrome indicators, for instance), the number of MHO people went as low as 9.9 percent (for the strictest definition) and up to close to 47 percent (for the more lax definition, which allowed for up to two metabolic syndrome indicators.)

When compared with the traditional person with obesity (metabolically unhealthy obesity, or MUO), people with MHO have relatively reduced rates of insulin resistance and other comorbidities linked with excess weight.

How can these people be metabolically healthy? The mechanisms remain unclear, but when compared to those with MUO, people with MHO appear to have:

• Less VAT (particularly in the liver) and more subcutaneous leg fat
• Higher insulin sensitivity
• Less systemic inflammation

Although MHO seems favorable, some researchers claim that “people reported as having MHO are not metabolically healthy, but simply have fewer metabolic abnormalities than those with metabolically unhealthy obesity.” And long-term studies following MHO people suggest it’s a transient state, with between one-third and one-half converting to metabolically unhealthy obesity within 10 years. In a 2024 International Journal of Obesity study of 786 Swedish twins, 67 percent of those with MHO at age 50 to 64 developed metabolic syndrome by age 80.

It’s possible that genetics may play a role in MHO, particularly in respect to who stores their fat where. (Most MHO people have a gynoid, or “pear”, shape, not an android, or “apple”, shape.) Hormonal status matters, too; estrogen is protective against visceral fat storage.

 Lifestyle plays a smaller role. Studies indicate that people with MHO generally have higher physical activity levels, and exercise improves insulin sensitivity. (Hence the “fat-but-fit” phrase used to describe some people with MHO, like certain competitive athletes and sumo wrestlers.) Diet and sleep may also predispose people to MHO, though research has been mixed.

A more balanced approach to metabolic healthcare

Research shows that weight is a relevant factor in metabolic health. Still, BMI is just one data point that’s better understood when you also factor in:

• WHR and waist circumference
• Body composition to determine the percentage of adipose tissue and its distribution in the body
• Metabolic syndrome criteria, including cholesterol, blood pressure, blood sugar, and triglycerides, and HOMA-IR
• Lifestyle habits like exercise, diet, and microbiome health
• Mental health (Obesity and depression often go hand-in-hand and insulin resistance can trigger changes in brain physiology) 

Clearly, metabolic health isn’t easily defined, and weight’s relationship with overall health has yet to be unanimously agreed upon. What does seem to be cemented is the importance of basing one’s diet on whole, nutrient-dense foods; getting plenty of exercise; managing one’s metabolic risk factors, including insulin sensitivity and visceral fat levels; and prioritizing supportive lifestyle habits like stress management and adequate sleep. Ask your healthcare professional what blood tests they recommend, and ask them to help you track your metabolic health metrics. Lastly, approach BMI with a dose of healthy skepticism and remember that metabolic health is more than just a number.