A one-unit increase in body mass index raises the risk of heart failure by a fifth, a major study has found.
The research, involving almost 200,000 participants, confirmed a genetic link between the chances of developing the condition and weight.
Body mass index (BMI) is the standard measurement used to relate a person's weight and height.
It is calculated by dividing weight in kilograms by height in metres squared. A person of normal weight has a BMI of between 18.5 and 24.9, while 25 to 29.9 is defined as overweight and 30 or greater is classed as obese.
The new study looked at links with the FTO gene which regulates appetite and influences BMI.
Each copy of a common variant of FTO raises BMI by an average of 0.3 to 0.4 units.
While obesity is known to increase the risk of heart and artery disease, experts have not been sure whether it contributes directly or acts as a marker for some other underlying cause.
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The new research established a direct connection between the FTO gene, obesity, and heart failure.
It showed that if BMI goes up by just one unit, this translates to an average 20% increased risk of heart failure, which occurs when the heart is too weak to pump blood efficiently around the body.
The study, published in the online journal Public Library of Science Medicine, also confirmed that obesity led to higher insulin values, higher blood pressure, worse cholesterol levels, increased inflammation markers, and a greater risk of diabetes.
Lead scientist Dr Tove Fall, from Uppsala University in Sweden, said: "We knew already that obesity and cardiovascular disease often occur together.
"However, it has been hard to determine whether increased BMI as such is dangerous.
"In this study we found that individuals with gene variants that lead to increased body mass index also had an increased risk of heart failure and diabetes.
"The risk of developing diabetes was greater than was previously thought."
Co-author Professor Erik Ingelsson, also from Uppsala University, said: "We can now confirm what many people have long believed, that increased BMI contributes to the development of heart failure.
"We also found that overweight causes increases in liver enzymes. This knowledge is important, as it strengthens the evidence that forceful societal measures need to be taken to counteract the epidemic of obesity and its consequences."
Raised BMI was also causally linked to high blood pressure, itself a risk factor for cardiovascular disease.