People with a non-O blood group have a slightly increased risk of heart attack and stroke, research suggests. Scientists say it could be because higher levels of a blood-clotting protein is present in people with A, B and AB blood. The findings could help doctors better understand who is at risk of developing heart disease, the researchers said. But a heart charity said people should focus on giving up smoking and eating healthily to reduce their risk. The research, presented at the European Society of Cardiology congress, analysed studies involving 1.3m people.