People who are obese and have normal blood pressure, cholesterol and blood-sugar readings will still be unhealthy and die sooner compared with people who have a normal body weight, according to researchers.
A study, published in the Annals of Internal Medicine, found "healthy obesity" was a myth.
"Healthy obesity" or "benign obesity" is a term that has been applied to describe a subset of individuals who are defined as obese based on their BMI but who do not have the metabolic abnormalities typically seen in obesity (such as increased blood pressure, increased blood glucose and abnormal lipid profile), according to Ravi Retnakaran, an endocrinologist and co-author of the study.
People with a BMI of 30 to 40 are considered to be obese. But while health problems associated with being obese such as an increased risk of Type 2 diabetes and cardiovascular disease are well documented, not all obese people show unhealthy signs that would suggest a problem.
The researchers wanted to consider whether these people were actually at any more risk of death or heart problems. Their meta-analysis used data on more than 60,000 people across three weight categories - normal, overweight and obese - in eight studies carried out over the last decade. While metabolically healthy obese people showed a similar risk of problems compared to those with normal weight, this wasn't the case when the researchers looked only at studies that had ten years of follow up. Study participants in all weight categories with unhealthy metabolisms showed an increased risk of mortality and cardiovascular problems.
"The main finding is that metabolically healthy obese individuals are indeed at increased risk for death and cardiovascular events over the long term as compared with metabolically healthy normal-weight individuals," Retnakaran said. "These data suggest that increased body weight is not a benign condition even in the absence of metabolic abnormalities."
It is not clear why some obese people have normal metabolism or why some people with more normal BMI show problems. Of more than 60,000 people considered in the study, 8.9% had metabolically healthy obesity while 6% had metabolically unhealthy normal weight. Retnakaran said it was possible that abnormal metabolic features that weren't detected in the study developed over time in some people.
BMI is a calculation used to work out a person's mass by dividing weight in kilogrammes by height in metres. The study suggests that BMI alone is not a sufficient measure for working out who is at risk of dying or developing cardiovascular disease.
"It is important to consider both BMI and metabolic status for estimating long-term risks of these outcomes," Retnakaran said.
James Hill and Holly Wyatt of the University of Colorado say research was strong evidence that "healthy obesity" was a myth. In an accompanying editorial, they said:
The review casts doubt that any obese persons have no long-term risk for cardio-metabolic disease. Obesity affects almost all aspects of human function and physiology. Obesity also increases risk for Type 2 diabetes, kidney disease and some types of cancer. It is linked to orthopaedic problems, reproductive problems, depression, asthma, sleep apnea, renal disease, back pain, skin infections and cognitive decline.
Obesity produces social stigma and overall reduced quality of life. It would be a mistake to label obese persons as healthy on the basis of only the presence or absence of risk factors for cardio-metabolic disease.
The study authors said doctors needed to be aware that a healthy metabolism in someone who was obese did not necessarily mean they were safer in the long term.