In a world filled with nutritional advice and health claims, it’s easy to feel overwhelmed by conflicting messages about body fat, obesity and weight-related health risks.
But is all fat created equal?
Researchers across Mass General Brigham are here to break down common myths about weight and body fat and share science-backed insights into obesity-related health risks based on their recent research.
FACT: BMI is Not a Perfect Measure of Body Fat-Related Health
Body Mass Index (BMI), which is a measure of body fat based on a person’s weight and height, has traditionally been used to assess weight status and categorize individuals as underweight, normal weight, overweight and obese.
However, BMI alone doesn’t tell the full story.
A new study published in JAMA Network Open by a team of Mass General Brigham researchers led by Lindsay Fourman, MD, and Steven Grinspoon, MD, found that using a new definition proposed by the Lancet Commission earlier this year that incorporates BMI along with other metrics, the number of US adults in a nationally representative study who could be classified as having obesity jumped from 40% to nearly 70%.
The new guidelines combine BMI with other measures such as waist circumference, weight-to-height ratio and waist-to-hip ratio. This combination approach helps to identify people at increased risk of health complications related to excess fat, even in the absence of a high BMI.
The researchers analyzed data from more than 300,000 participants in the All of US Study and found that obesity rates were particularly high in adults over 70, with almost 80% in that group now meeting the criteria for obesity.
Fact: BMI is Not a Perfect Measure of Body Fat-Related Health
It turns out that a high BMI is not always indicative of increased health risks.
In fact, some studies have found a “paradox,” in which some people with higher BMI have lower than expected rates of obesity-realted conditions such as diabetes, hypertension and abnormal lipids.
This can create confusion about how to assess risk and treat people with obesity.
One possible reason for this paradox is that BMI isn’t a perfect measure of body fat. It doesn’t account for differences in muscle mass, fat distribution and other important factors (see myths #1 and #4 for more details).
So, researchers are now looking at other ways to measure body fat (called adiposity-related anthropometrics) across diverse populations.
These alternative measures could help improve:
- Our understanding of how body fat affects health outcomes
- How we diagnose and manage obesity in people at high risk
A recent study in JACC from a team of Mass General Brigham researchers led by John Ostrominski, MD, and Muthiah Vaduganathan, MD, MPH, found that even people with a “normal” BMI (below 30) often had excess abdominal fat, which was linked to serious heart problems, including heart failure, atrial fibrillation and cardiovascular death. This was especially common among older adults and women.
The researchers also looked at how the medication finerenone, which is used to treat heart and kidney disease, worked for people with different body fat patterns.
They found that finerenone helped reduce the risk of heart failure and heart-related death in all groups, no matter their BMI or waist size.
People with more body fat saw even greater benefits, showing that this treatment may be especially helpful for those with higher levels of abdominal fat.
Fact: Obesity is a complex biological process.
Obesity is more than just excess weight—it involves complex biological signals happening inside the body.
In a recent study published in Cell Genomics, researchers from Mass General Brigham led by Emeli Chatterjee, PhD, and Saumya Das, MD, PhD, looked at tiny particles in the blood called extracellular vesicles (EVs).
EVs are like small packages released by cells that carry important information. Inside these packages is RNA, a molecule that helps cells follow instructions from our genes to do their jobs.
By studying the RNA inside EVs, researchers found patterns that show how fat tissue behaves differently in people with obesity compared to those with leaner body types.
The RNAs inside EVs that reflected those present in fat cells were linked to genes involved in metabolism and type 2 diabetes.
This means obesity affects the body at a deeper, cellular level—not just through visible weight gain.
Blood tests that measure these EVs could be used to better understand, diagnose, and treat obesity-related conditions, and understand how popular weight loss drugs alter the biology of fat cells.
Fact: Where Your Body Fat is Distributed Tells a More Accurate Picture of Your Health
Traditional tools for assessing obesity risk such as BMI don’t tell the full story because they don’t show where fat is stored or how much muscle a person has.
A recent study published in Annals of Internal Medicine by an MGB research team led by Matthias Jung, MD, and Vineet K. Raghu, PhD, shows that an artificial intelligence (AI) tool can quickly analyze body scans to measure fat and muscle in just three minutes.
This approach could help identify people who are at higher risk for body fat-related health issues but may not be flagged by standard methods.
The researchers used data from over 33,000 adults who had full-body MRI scans and were followed for more than four years.
Researchers found that certain types of fat—especially fat around the organs and inside muscles—were strongly linked to future risk of diabetes and heart disease.
In men, having less muscle was also linked to higher risk.
With more research, this AI tool could be used to screen patients using routine scans already done in hospitals, helping doctors catch health risks earlier and offer more targeted prevention strategies.
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