Key takeaways
- In January 2025, a global commission of more than 50 experts, endorsed by more than 75 medical organizations, said obesity shouldn’t be diagnosed by BMI alone and split it into “clinical” obesity, where excess fat already harms organs or daily life, and “preclinical” obesity, where it doesn’t yet1.
- In U.S. survey data from 1999 to 2004, 23.5% of normal-weight adults had two or more metabolic abnormalities, while 31.7% of adults with obesity had one or none5.
- Healthy numbers can slip at any size: in one U.S. study, 52% of middle-aged adults who started with healthy blood pressure, sugar and fats developed a risk factor, and a 22-study review found 45% higher heart risk with “metabolically healthy obesity” than in healthy normal-weight people78.
- In a 2025 review of 20 studies, fit people with obesity had death rates not significantly different from fit normal-weight people, while unfit normal-weight people had about twice the risk10.
- Weight loss has specific jobs: the National Institutes of Health says modest loss may help prevent or delay type 2 diabetes, losing at least 3% to 5% may reduce liver fat, and weight loss may ease sleep apnea and arthritis symptoms13.
When researchers at Massachusetts General Hospital re-sorted 301,026 American adults using a new definition of obesity, the share with obesity jumped from 43% to 69%4.
The more telling split came next. Only about half of the people with obesity, 53%, showed signs that excess fat was already harming their organs or daily life4. Over a median of four years, that group developed diabetes at about six times the rate of people without obesity4. The rest had no statistically significant rise in their risk of death4.
Same label, very different futures. Weight and metabolic health overlap without being the same thing: some people in larger bodies have normal blood pressure, blood sugar and blood fats, and some thin people don’t5.
If a scale reading has ever left you judged, or falsely reassured, this research points to better measurements, and to the problems where losing weight does help.
A 2025 commission split obesity into two diagnoses
In January 2025, The Lancet Diabetes & Endocrinology published the work of a global commission that included people living with obesity1. Its launch release counted 56 experts, though some news reports said 5812. Francesco Rubino chaired it, and more than 75 medical organizations endorsed it1.
Its starting point: BMI works as a stand-in for whole populations, not as a measure of one person’s risk3. So the commission said clinicians should confirm excess body fat before diagnosing obesity2. That can mean BMI plus a waist-based measure, two waist-based measures, or a direct fat scan such as DEXA2. Above a BMI of 40, extra confirmation isn’t needed2.
Then it drew a line between two states:
- Clinical obesity is a chronic illness. Excess fat is already changing how organs work, or it sharply limits daily tasks such as bathing, dressing or eating1. Adult signs on the commission’s list of 18 include breathlessness, heart failure caused by obesity, and knee or hip pain and stiffness12.
- Preclinical obesity means excess fat while organs still work normally, though future risk is higher2.
The treatment goals differ. For clinical obesity, the aim is to restore function, not only to lower weight, using lifestyle care, medicine or surgery based on each person’s risks and benefits1. For preclinical obesity, the focus is counseling, monitoring and lowering risk1.
Not everyone agreed. Only about a third of commissioners first backed calling obesity a disease defined by BMI alone, and there was concern that up to 40% of people in some countries could be labeled sick overnight2. In TCTMD, a cardiology news site, Andrew Sauer said many criteria track closely with BMI anyway, while Michael Blaha called imaging-grade fat measures impractical2. Commissioner Robert Eckel countered that most criteria come from a careful medical history2.
Follow the money: Rubino has reported research grants or speaking fees from companies including Novo Nordisk, Medtronic and Eli Lilly2.
Applied to real patients, the new rules sorted risk sharply
Lindsay Fourman, Steven Grinspoon and colleagues at Massachusetts General Hospital tested the new definition on adults ages 18 to 80 in the All of Us Research Program, reporting in JAMA Network Open in October 20254:
- By BMI alone: 43% had obesity.
- By the new definition: 69% did. About 26% of all participants qualified through large waist measurements despite a lower BMI.
- Clinical obesity: 36% of everyone, 53% of those with obesity, and 54% of those 70 or older.
The researchers then followed people for a median of four years and compared them with people who had no obesity4. A hazard ratio compares rates: 2.0 means about twice the rate.
| Group | New diabetes | Heart and blood vessel events | Death |
|---|---|---|---|
| Clinical obesity | 6.11 | 5.88 | 2.71 |
| Preclinical obesity | 3.32 | 1.40 | 1.09, not significant |
The split separated higher-risk people from lower-risk ones. But “preclinical” didn’t mean risk-free: diabetes still ran about three times higher4. The summary we reviewed didn’t report absolute rates, so we can’t say how many people out of 100 these ratios represent.
About 1 in 4 normal-weight adults had metabolic trouble
A 2008 study led by Rachel Wildman of Albert Einstein College of Medicine looked at 5,440 adults in the National Health and Nutrition Examination Survey from 1999 to 200456. The team checked six measures: blood pressure, triglycerides, fasting blood sugar, an inflammation marker called CRP, an insulin resistance score and HDL cholesterol5. Having two or more problems counted as metabolically abnormal5.
The results broke the link between size and health5:
- 23.5% of normal-weight adults were metabolically abnormal, about 16.3 million people.
- 51.3% of adults with overweight were metabolically healthy, about 35.9 million.
- 31.7% of adults with obesity were metabolically healthy, about 19.5 million.
Age made a big difference. Metabolic trouble in normal-weight adults climbed from 10.3% at ages 20 to 34 to 54.7% at ages 65 to 796. Among adults with obesity, the healthy share fell from 47.7% to 14.3%6.
These data are more than 20 years old and capture a single moment. Still, the pattern holds: in a 2016 review, normal-weight people with metabolic problems had about twice the heart risk of healthy normal-weight people8. Our insulin resistance article explains how that problem hides in lean bodies.
Healthy obesity is real, but it’s uncommon and may not last
There’s no single definition of metabolically healthy obesity9. In a 2019 review, Matthias Schulze argued that metabolic health may be temporary and that risk runs along a continuum9.
Two long U.S. studies show how rare it is, and how often healthy numbers slip. A team led by Fangjian Guo tracked 14,685 adults ages 45 to 64 and 4,990 adults ages 18 to 307. “Healthy” meant normal blood pressure, blood sugar and blood fats7.
In the middle-aged group, only 2% of all participants had healthy obesity, and 94% of people with obesity had at least one risk factor7. Among everyone who started healthy, at any weight, 52% of the middle-aged and 35% of the young adults developed one or two risk factors over follow-up7.
People with healthy numbers had similar rates of heart disease, stroke and death across weight groups, though obesity still raised diabetes risk7. Lean people with all three risk factors had about 3.6 times the heart disease risk of people with healthy obesity7.
A 2016 review of 22 studies, with Schulze as senior author, compared everyone with healthy normal-weight people8. Heart and blood vessel events ran 45% higher with healthy obesity, 107% higher with normal weight plus metabolic problems, and 131% higher with both8. No definition the team found picked out a group with obesity that had no extra risk8.
This article covers what those percentages could mean, as an illustration rather than a measured result. Picture 1,000 healthy normal-weight adults, 50 of whom have a heart attack or stroke during a study. By our math, the review’s estimates would mean about 73 among people with healthy obesity, 104 among normal-weight people with metabolic problems, and 116 among people with both.
Fitness changes the picture, but it doesn’t erase every risk
In a review in the British Journal of Sports Medicine, a team including Glenn Gaesser and Siddhartha Angadi pooled 20 long-term studies with 398,716 observations10. They compared death rates by fitness and weight group with fit, normal-weight people10.
| Group | Death from any cause, hazard ratio |
|---|---|
| Fit, overweight | 0.96, not significant |
| Fit, obesity | 1.11, not significant |
| Unfit, normal weight | 1.92 |
| Unfit, overweight | 1.82 |
| Unfit, obesity | 2.04 |
Low fitness tracked with about double the death rate at every weight. For heart deaths alone, fit people with obesity had a ratio of 1.62, but the uncertainty ran from 0.87 to 3.01, too wide to rule out real extra risk10.
The authors named the limits. The studies observed people rather than testing a treatment, 67% of participants were men, and most came from U.S. clinic groups that were mostly white and higher income10. The team reported no specific funding and no competing interests10.
Fitness also has limits. In a study of 527,662 Spanish workers, physically active men with obesity still had 4.93 times the odds of high blood pressure and 3.62 times the odds of diabetes compared with active normal-weight men11. Trends were similar in women11.
Being active helped within every weight group, but it didn’t make up for the extra weight11. One caution: people reported their own activity, and the data were a one-time snapshot11.
Weight-neutral programs: small trials, encouraging signals
Some programs focus on habits and well-being instead of the scale. In 2011, Linda Bacon and Lucy Aphramor reviewed six randomized trials, which assign people to groups by chance, testing Health at Every Size and other non-diet approaches12. They enrolled between 60 and 185 people each, by our count from the review’s table12.
Some found better blood pressure, LDL cholesterol, eating habits, activity, self-esteem, depression and body image, though some blood pressure gains didn’t beat the comparison group12. None reported adverse changes, and none found weight gain12.
In Bacon’s own 2005 trial of 78 women with obesity who were long-term dieters, 8% dropped out of the weight-neutral program, compared with 42% of the diet group12. The weight-neutral group improved LDL cholesterol and systolic blood pressure12.
Weigh the source. Both authors are weight-neutral practitioners who sometimes earn fees for speaking and writing about it, which they disclosed, and the trials were small12. They called for larger, more diverse studies12.
That approach matches what many natural-health readers already value: real food, movement you enjoy, rest and respect for your body. Those habits pay off whether or not the scale moves, and they work alongside medical care.
When losing weight treats a specific problem
For some conditions, weight loss itself is the treatment. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) spells out where13:
- Type 2 diabetes: Nearly 9 in 10 people with type 2 diabetes have overweight or obesity. Modest weight loss may help prevent or delay it; our article on realistic goals covers the landmark trial behind that advice.
- Fatty liver disease: Losing at least 3% to 5% may shrink liver fat.
- Sleep apnea: Fat around the neck can narrow the airway, and weight loss may ease sleep apnea or even resolve it.
- Osteoarthritis: Weight loss may take pressure off the knees, hips and lower back, and may improve symptoms.
A later article in this series covers what keeps weight off, including medicines and surgery.
How you lose weight matters. NIDDK warns that losing weight quickly raises the risk of gallstones, and it advises talking with a health care professional about losing weight safely13. If you take medicine for blood sugar or blood pressure, bring it up then, and don’t change a dose on your own.
Beyond the scale: your metabolic check
This check is our own, built from the sources cited. Use it to guide conversations with your clinician, not to diagnose yourself.
| Measure | A healthy target, per the source | What it tells you that weight can’t |
|---|---|---|
| Blood pressure | Under 120/80 earns the top score in the heart-health measure Healthy People 2030 uses14 | Strain on your arteries; metabolic problems clustered in about 1 in 4 normal-weight adults5 |
| A1C, a blood sugar test | Under 5.7%, used as a stand-in for fasting blood sugar under 100 mg/dL14 | Early blood sugar trouble, which can start at any size |
| Non-HDL cholesterol | Under 130 mg/dL14 | The blood fats that build up in arteries |
| Waist-to-height ratio | Keep your waist under half your height, for people with a BMI under 3515 | Where fat sits; large waists explained most of the new obesity cases in the U.S. study4 |
| Weekly activity | 150 minutes or more of moderate activity14 | Fitness tracked with survival at every weight10 |
| Breathing, joints and daily tasks | Breathlessness, knee or hip pain and stiffness, or trouble bathing or dressing are clinical obesity signs1 | Whether excess fat is already affecting how your body works |
| Breathing during sleep | Snoring or pauses in breathing can signal sleep apnea13 | A problem weight loss may ease13 |
| Liver | Ask whether liver tests fit your risks; fatty liver is linked to insulin resistance13 | Fat inside the liver, which a scale can’t see; our metabolic panel article covers liver tests |
If measuring your body stirs up old struggles with food, let your clinician take the measurements. The National Alliance for Eating Disorders helpline, 1-866-662-1235, offers weekday support and referrals from licensed therapists.
How strong is the evidence?
| Claim | Evidence level | What the research shows |
|---|---|---|
| Weight and metabolic health often diverge | National survey and cohort data45 | |
| Metabolic problems raise heart risk even at a normal weight | About twice the risk in a 22-study review8; lean adults with three risk factors fared worst7 | |
| Healthy obesity carries no extra long-term risk | 45% higher heart risk in a 22-study review; one U.S. cohort found extra diabetes but not heart risk, and healthy numbers often slipped at any weight78 | |
| Fitness offsets much of the death risk tied to higher weight | Observational, and mostly in men10 | |
| Exercise cancels obesity’s link to high blood pressure and diabetes | A one-time snapshot of 527,662 workers11 | |
| Weight-neutral programs improve health markers and well-being | Six small trials12 | |
| The clinical versus preclinical split predicts who gets sick | One large U.S. study over four years4 | |
| Modest weight loss helps prevent or delay type 2 diabetes in people at risk | NIH summary13 |
Questions readers ask
Can you be healthy at a higher weight?
Many people are, at least for now: about a third of adults with obesity had one metabolic problem or none5. But healthy numbers often slip over time, at any size, so regular checks make sense79.
My BMI is normal. Do I still need these checks?
Yes. Nearly 1 in 4 normal-weight adults had two or more metabolic problems in national data, and older age, inactivity and a larger waist were linked to that pattern5.
Will the new definition label me with a disease?
Only if excess fat is already affecting your organs or daily life; otherwise the commission’s term is preclinical obesity1. Its report is an expert proposal endorsed by more than 75 organizations, and critics have challenged parts of it12. Ask your clinician which definition they use.
Is “fat but fit” a myth?
It’s half right. Fit people with obesity had death rates close to those of fit normal-weight people, and low fitness tracked with about double the death rate at every weight10. But activity didn’t cancel obesity’s link to high blood pressure or diabetes in a study of half a million workers11.
The bottom line
Your weight is one clue about your metabolic health, not the verdict. Many people in larger bodies have healthy blood pressure, blood sugar and blood fats, and many thin people don’t. A 2025 commission built that idea into a definition that asks whether excess fat is harming the body.
Healthy numbers at a higher weight can change, so keep checking them. Fitness, good food and rest help at every size. And when weight drives a specific problem, such as diabetes risk, fatty liver, sleep apnea or sore knees, weight loss is a treatment to discuss with your clinician.
Keep exploring: metabolic syndrome criteria and limits of BMI.
Related reading
About this article
This article was researched using NIH and federal health pages, a NICE guideline, university releases, peer-reviewed studies and reviews, and professional news reports on the 2025 obesity commission, all listed below. It was reviewed in October 2026. It is general education and not medical, legal or financial advice.
Sources
- King’s College London. “Global Commission proposes major overhaul of obesity diagnosis.” News release, January 15, 2025. kcl.ac.uk
- TCTMD (Cardiovascular Research Foundation). “Global Commission Proposes ‘Radical’ Overhaul of Obesity Diagnosis.” January 2025. tctmd.com
- PACE-CME. “Reframing obesity: updated definition and diagnostic criteria for clinical and preclinical obesity” (summary of Rubino F, Cummings DE, Eckel RH, et al. “Definition and diagnostic criteria of clinical obesity.” Lancet Diabetes & Endocrinology, 2025). pace-cme.org
- Mass General Brigham, Advances in Motion. Summary of Fourman LT, Grinspoon SK, et al. “Implications of a New Obesity Definition Among the All of Us Cohort.” JAMA Network Open, October 2025. Posted January 20, 2026. pubmed.ncbi.nlm.nih.gov
- Wildman RP, Muntner P, Reynolds K, et al. “The Obese Without Cardiometabolic Risk Factor Clustering and the Normal Weight With Cardiometabolic Risk Factor Clustering: Prevalence and Correlates of 2 Phenotypes Among the US Population (NHANES 1999-2004).” Archives of Internal Medicine, 2008;168(15):1617–1624. pubmed.ncbi.nlm.nih.gov
- Albert Einstein College of Medicine. “A Person’s Weight Often Doesn’t Jibe With Heart-Disease Risk.” News release, August 11, 2008. einsteinmed.edu
- Lewis K. “What happens to patients with metabolically healthy obesity over time?” Journal of Clinical Outcomes Management (MDedge), 2016, on Guo F, Garvey WT. Obesity, 2016;24:516–525. mdedge.com
- Eckel N, Meidtner K, Kalle-Uhlmann T, Stefan N, Schulze MB. “Metabolically healthy obesity and cardiovascular events: A systematic review and meta-analysis.” European Journal of Preventive Cardiology, 2016;23(9):956–966 (abstract record). nutrition-evidence.com
- Schulze MB. “Metabolic health in normal-weight and obese individuals.” Diabetologia, 2019;62(4):558–566 (abstract record). nutrition-evidence.com
- Weeldreyer NR, De Guzman JC, Paterson C, Allen JD, Gaesser GA, Angadi SS. “Cardiorespiratory fitness, body mass index and mortality: a systematic review and meta-analysis.” British Journal of Sports Medicine, 2025;59(5):339. pubmed.ncbi.nlm.nih.gov
- MDedge. “Large study weighs in on ‘fat but fit’ paradox.” January 2021, on Valenzuela PL, Lucia A, et al. European Journal of Preventive Cardiology, 2021. mdedge.com
- Bacon L, Aphramor L. “Weight Science: Evaluating the Evidence for a Paradigm Shift.” Nutrition Journal, 2011;10:9. pmc.ncbi.nlm.nih.gov
- National Institute of Diabetes and Digestive and Kidney Diseases. “Health Risks of Overweight & Obesity.” NIH, last reviewed May 2023. niddk.nih.gov
- Office of Disease Prevention and Health Promotion. “Improve cardiovascular health in adults — HDS-01: Data Methodology and Measurement.” Healthy People 2030, HHS. origin.health.gov
- National Institute for Health and Care Excellence (NICE). “Overweight and obesity management” (NG246). January 14, 2025. nice.org.uk