Being overweight makes your heart larger and heavier, raising the risk of suffering a heart attack or stroke.
The first study of its kind in healthy people has found putting on weight can add up to eight grams to the heart, and increase its volume by almost five per cent.
A bigger, heavier heart raises the risk of conditions from an irregular heartbeat to a heart attack, and the organ should in fact grow smaller with age.
Researchers at Queen Mary University of London and the University of Oxford examined MRI scans for 4,561 people from the UK Biobank database.
They found an increase of 4.3 in BMI, which could take someone from a healthy weight to the brink of obesity, made their heart substantially heavier.
It matters because a larger heart stretches its upper chambers, disrupting the electrical signal needed to keep it beating regularly.
While people have long known they risk heart disease from overeating because it hikes their cholesterol and blood pressure, this is the first evidence of changes to the structure of the heart itself.
Professor Steffen Petersen, lead author at QMUL, said: ‘We all know that our lifestyle has a big impact on our heart health – particularly if we’re overweight or obese. But researchers haven’t fully understood how exactly the two things are linked.
‘With this research, we’ve helped to show how an unhealthy lifestyle increases your risk of heart disease.’
Soaring rates of irregular heartbeats
The results suggest Britain’s obesity crisis may be contributing to soaring rates of atrial fibrillation – or irregular heartbeat.
This raises people’s risk of stroke, because the atria, which are the two upper chambers of the four chambers in the heart, fail to pump properly.
Professor Petersen said: ‘We believe the increase in the left atrium’s size stretches it so that it does not play its part in conducting an electrical impulse from the sinus node, the pacemaker of the heart, to the node which causes the heart to beat.’
The researchers looked at a number of lifestyle factors which could affect the heart’s size, which were blood pressure, smoking, exercise, cholesterol, alcohol intake, diabetes and people’s weight, measured using BMI.
The structure of the heart
Body weight had the biggest consistent impact on the structure of the heart, according to the study published in the journal PLoS One.
A BMI increase of 4.3 increased the weight of the left side of the heart by 8.3 per cent, which was calculated using measurements of the heart muscle taken from the patient scans.
This weight bracket would include people who go from a healthy BMI of 25 to one of 29.3, which is in the overweight category and close to the obesity threshold of 30.
Men in this group would add around eight grams to their heart, which weighs an average of 64 to 141 grams, as a result.
Women, whose heart weighs up to 93 grams on average would see theirs increase by around six grams. The volume increase was estimated at around 8ml for men and about 6.5ml for women.
The research was funded by the British Heart Foundation, whose medical director Professor Sir Nilesh Samani said: ‘This research shows the silent impact of being overweight and having high blood pressure on the structure and function of the heart, which over time may lead to heart disease and heart failure.
‘The important message is that these are things we have the power to change before they result in irreversible heart damage.’
Avoidable lifestyle factors
It is estimated more than a quarter of the risk of having a heart attack comes from avoidable lifestyle factors looked at in the study.
Professor Francesco Cappuccio, professor of cardiovascular medicine and epidemiology at the University of Warwick and President of the British & Irish Hypertension Society, said the study under-represented ethnic minorities and smokers.
She added: ‘Despite these inherent limitations, this is an important study that reinforces the concept that the prevention and control of hypertension, high cholesterol and diabetes remain the most important modifiable factors for the prevention of heart damage.’