A body mass index (BMI) of between 20.5 and 29.9 is classed as overweight, and a BMI or 30 or above is obese.
Obesity commonly entails a higher risk of cardiovascular disease (CVD), diabetes, high blood pressure, or hypertension, and high cholesterol. Together, these are known as metabolic syndrome. However, not everyone with a high BMI has these complications, and not everyone has all four of them.
Some studies suggest that up to 35 percent of people with obesity may be metabolically healthy.
Why some have metabolic problems and others do not is probably related to healthy lifestyle habits, and whether or not they are likely to cause weight loss.
What is metabolically healthy obesity?
Not all people with a high BMI are prone to metabolic disorders.
The first question is how to decide what is metabolically health obesity (MHO).Obesity is normally classified by BMI, but BMI is not always an accurate way of classifying. A person may have a high BMI but be muscular and physically fit. BMI is a calculation, and it cannot distinguish between fat and lean tissue.
Similarly, there is not one single definition of metabolic disorder. Some physicians will diagnose the disorder by one or two factors, say, CVD and hypertension, while others will need to see all four to diagnose metabolic disorder.
If the criteria used for diagnosis requires the person to have all four factors, fewer people will be diagnosed with obesity and metabolic disorder. Many people with obesity have one, but not multiple, metabolic problems.
However, if all patients with borderline hypertension are classified as having metabolic disorder, the number of diagnoses will be higher.
Findings of a study of 6,000 Americans aged 18 to 65 years over a 9-year period found that only 1.3 percent of people with obesity had no metabolic issues.
A 2014 study comparing 10 cohorts in different countries found that the percentage of those who were obese and were metabolically healthy ranged from 24 percent to 65 percent for women and from 43 percent to 78 percent for men.
Criteria and causes
There are currently no criteria for identifying MHO.
In a 2013 study, researchers noted that: "No universally accepted criteria exist to define metabolically healthy obesity."
They suggested that possible factors for determining MHO could be:
- waist circumference
- blood pressure, cholesterol, and blood sugar levels
- insulin resistance
- physical fitness
One study, published in The Journal of Clinical Endocrinology & Metabolism, notes that people with MHO are more likely to have lower levels of inflammation than those who are not healthy. However, other findings have countered this.
Studies have suggested that some proteins may protect the body from the harmful effects of obesity. More research is necessary to evaluate the effectiveness of these mechanisms in humans.
Some researchers have suggested that not all fat is hazardous. Having different kinds of fat might make a difference. Subcutaneous fat, the layer of fat that is under the skin, may not be a risk factor for metabolic disease, but fat in or around the liver and the heart can ultimately be fatal.
One team found that metabolically healthy people burn fat more effectively than those with metabolic problems, such as type 2 diabetes. This indicates a kind of vicious cycle in which the levels of health in the healthy person continue to decline.
A lack of physical activity, combined with a high calorie, high sugar diet, can also lead to a low level of fat burning, or oxidation.
Obesity does not mean that a person has an unhealthy lifestyle. A person with obesity who is physically active and who makes healthy food choices may continue to derive the same benefits as people who are not obese.
One study reports that, regardless of BMI, when an overweight or obese person had four healthy habits, they had the same mortality risk as those who were slimmer. The four habits were being a non-smoker, moderate alcohol intake, 30 minutes of exercise daily, and eating 5 or more servings of vegetables and fruits daily.
Exercise is key to managing the health risks of obesity.
A 2015 study suggests that people with MHO may have different clinical characteristics than people with obesity and metabolic disorder. Those with MHO tended to be younger, female, were more likely to exercise and were less likely to smoke or drink heavily.
A study published in February 2017, found a further distinction between people with MHO and those with metabolically abnormal obesity (MAO) when it came to sleep quality.
They found women with MHO had regular sleep disturbances, but they did not have problems with sleeping duration or with overall sleep quality, compared with those who have MAO.
The researchers call for a closer look at links between sleep quality and cardiometabolic health, and whether or not sleep is a factor, or perhaps an indicator, of MHO.
It should be noted that metabolic disorder does not only affect people with obesity. A person does not have to be obese or maintain an unhealthy lifestyle to have diabetes and cardiovascular disease.
People with MHO appear to have a lower risk of health problems.
In 2016, researchers found that older people with MHO were not more likely to develop cardiovascular disease. However, they also found a strong link between metabolic disorder CVD, and a link between high BMI and CVD.
Researchers Prof. Matthias Schulze, of the German Institute of Human Nutrition in Nuthetal, and Prof. Norbert Stefan, of the University of Tübingen, German have suggested the need for a better understanding of which individuals with obesity are susceptible to complications. This, they say, would be helpful in treating and preventing health problems.
"The health consequences of obesity are well documented. In particular, the worldwide increase in the incidence of type 2 diabetes, cardiovascular disease, and several types of cancer is thought to be largely attributed to the obesity epidemic. Therefore, prevention and treatment of obesity to reduce risk of chronic diseases at the population and individual level are crucial."
Prof. Matthias Schulze and Prof. Norbert Stefan
The researchers suggest that "Potentially, scarce resources can be more effectively used if tailored towards the metabolic profile of an obese individual; some prevention and treatment strategies can be very expensive and time-consuming."
The need for criteria
The scientists note that the following barriers remain:
- a lack of standard criteria to define MHO
- limited understanding of the biological mechanisms behind it
Removing these barriers, they say, could help target treatment more effectively. However, they point out that people with MHO may have other reasons for seeking treatment.
More research is needed to clearly understand MHO.
A clearer definition of MHO could lead to a better understanding of the link between obesity, inflammation, and metabolic health.
It could contribute toward the development of drugs that protect against illnesses to which many people with obesity are susceptible.
Elsewhere, Prof. Frank Hu and colleagues suggest that distinguishing people with MHO from people with obesity plus metabolic disorder could lead to more appropriate and cost-effective forms of treatment.
It could, for example, reduce the need for intrusive weight loss interventions, such as bariatric surgery.
The need for action
However, Jennifer Kuk and researchers from York University in Toronto, Canada, said in 2009 that whether a person with obesity is diagnosed as metabolically healthy or not, they still need treatment.
"It's important to note that metabolically-normal obesity is an extremely rare subtype, but when it does occur, treatment is absolutely necessary."
Jennifer Kuk, an Assistant Professor in York's School of Kinesiology & Health Science
Doctors also warn that while a person may be healthy at one time with obesity, they may still develop problems later. Health status changes over time, and obesity could be a major factor in the development of disease in the long-term.
Also, metabolism is not the only aspect of health that obesity can affect. Problems with knee and hip joints, respiratory problems, and sleep apnea are all potential results of obesity, as are several types of cancer.