The study, which appears in JAMA Internal Medicine, found that even in twin pairs with a BMI difference of 7.0 or more, and where the heavier twin had a BMI of 30.0 or more, the risk of heart attack or death was not greater in heavier sets of twins.
Specifically, twins with the higher BMI there were 203 heart attacks (5%) and 550 deaths (13.6%) during the follow-up period.
Twins with a lower BMI recorded 209 heart attacks (5.2%) and 633 deaths (15.6%) during the same period.
However, in the total sample of twins, the risk of diabetes was greater in the heavier twins.
The study stands out against conventional thinking that has shown obesity to be a major risk factor for cardiovascular disease and death.
These latest results also suggest that making lifestyle changes to lose weight and lower the risk of death and heart attack, may not be as effective as first thought.
Independent of genes
Researchers from Umeå University in Sweden looked at health data from 4,046 identical twin pairs. These twins were specifically selected for their differences in BMI.
Twins were taken from the Swedish Twin Registry, and the ages ranged from 42-92. Median age was 57.5.
The study was conducted from March 1998, to January 2003, with follow-ups scheduled within a ten-year period until December 2013. Here, twins were compared in the number of heart attack and type 2 diabetes cases.
"What the study does show is that there's a strong association between obesity and diabetes,” said says Peter Nordström, researcher at Umeå University.
“This leads us to conclude that weight reduction interventions can be more effective against diabetes than when it comes to reducing the risk of heart attack and mortality."
The findings suggest that the relationship between obesity, cardiovascular disease (CVD) and death may be more complex than initially thought.
Other studies seem to suggest the same. One study recorded higher rates of death in normal-weight (BMI of 18.5 to 24.9) individuals compared with overweight individuals.
A higher death risk has also been identified in normal-weight patients with CVD and patients with type 2 diabetes than in overweight or obese individuals with these diseases.
The Swedish team pointed to a number of explanations to try to explain these results. These included selection bias. Additionally, genetic factors shared by obesity and CVD may have accounted for the link.
The self-reporting of weight and height was also suggested. However, a previous assessment of data from the twin registry was confident that the use of this data would probably not affect the results.
“With all studies of this nature, we have to be careful not to imply that something causes a condition just because there’s a relationship between the two, or to oversimplify complex risk calculations,” said Dr Elizabeth Robertson, director of Research, Diabetes UK.
“While we are very much focused on promoting weight loss and preventing weight gain as a means to reduce the prevalence of Type 2 diabetes, we believe that further research is needed before we send a potentially confusing message that the same isn’t required to protect against cardiovascular disease or mortality.”
Source: JAMA Internal Medicine
Published online ahead of print, doi:10.1001/jamainternmed.2016.4104
“Risks of myocardial infarction, death, and diabetes in identical twin pairs with different body mass indexes.”
Authors: Peter Nordström et al