It’s well known that a high body mass index (BMI) has negative effects on our health, increasing the risk of several diseases. This is concerning since nearly one-third of the world’s population is now classified as overweight or obese.
One disease strongly linked to obesity is cardiovascular disease (such as heart disease and stroke) which, according to the World Health Organization, is the leading cause of death worldwide.
Obesity is a complex condition, and can be influenced both by genetics and a person’s lifestyle and environment. The latter can include air pollution, smoking, diet, physical activity, medications, education, socioeconomic status, and mental health, for example. This means obesity can result from a wide variety of factors – and notably, that different causes of obesity may have different consequences for our health.
In a recent study, we examined the link between obesity and cardiovascular disease – in particular, whether the risk differs if obesity is driven by genetic predisposition or mainly by other factors, such as environment or lifestyle.
We used data from the Swedish Twin Registry, a nationwide register that includes virtually all twins born in Sweden since the early 1900s. We included more than 15,000 twins born before 1959, gathering information about their BMI and whether they had a genetic predisposition to a high or low BMI.
We also used data from national medical registries to establish the incidence of cardiovascular disease. We were then able to see how cardiovascular disease risk changed depending on where people sat on the spectrum of genetic predisposition and BMI.
Obesity driven by genetic factors less harmful
We found the risk of cardiovascular disease was on average 76% higher in people with obesity, compared with those of a healthy weight. However, this link was twice as strong in those with a genetic predisposition to having a low BMI – meaning their obesity resulted from other factors such as environment or lifestyle – compared with those genetically predisposed to having a high BMI.
Previous research has similarly shown that people with obesity due mainly to environmental factors have a higher risk of dementia, diabetes and heart problems, and earlier death in general than those with obesity due to genetic factors.
Based on our findings and other research, it appears that it’s not high BMI in itself that increases the risk of disease. Rather, obesity affects our health differently depending on what caused it. We don’t yet understand why this is, but it may be that the body can better cope with obesity in people who are genetically predisposed to having a higher weight.
As a first step in understanding why we see differences between genetically and environmentally influenced obesity, we are currently examining differences between these two groups in their metabolic health – for example, cholesterol levels, blood sugar and blood pressure.
The drawbacks of focusing on BMI
A healthy lifestyle and healthy weight are always worth striving for. But our findings, as well as other studies, indicate that obesity is less harmful when it’s due to genetic predisposition to a higher BMI than when it’s mainly due to other factors.
Unfortunately, it’s difficult to know whether you have a genetic predisposition to a higher BMI. We can look around us in our family to get an idea, but it’s important to remember that we also share many environmental and lifestyle factors with our families.
A healthy BMI when you’re young could safeguard your heart for later life
While BMI is a useful tool for ascertaining weight in population studies, it’s a limited measure of health. Obesity is a complex and stigmatised disease that can have many different causes, not captured by measuring BMI.
Our results provide further evidence that the effects of obesity on health differ from one person to the next. A healthy lifestyle is important regardless of BMI, but we are all built differently. While diet and exercise may work well for some to maintain a healthy weight, this won’t be enough for everyone.
In the first instance, we need to better understand obesity and its effects on health. After that, we hope that society can move away from focusing mainly on BMI – and instead consider what has caused the obesity, and what can be done to reduce the risk of poor health in each person.
Ida Karlsson receives funding from the Strategic Research Program in Epidemiology at Karolinska Institutet; the Swedish Research Council for Health, Working Life and Welfare (2018-01201 and 2022-00672); Loo and Hans Osterman’s Foundation (2022-01222); Foundation for Geriatric Diseases at Karolinska Institutet (2022-01296); Karolinska Institutet Research Foundation (2022-01718). Ida Karlsson has also received payment via a Promising Researcher Award from The Nordic Gerontological Federation.
Elsa Ojalehto does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.