Obesity has a greater impact on the blood pressure of teenage girls than on teenage boys, a US study has suggested. High blood pressure is a risk factor for heart disease and stroke in later life. The study of 1,700 teenagers, presented to the American Physiological Society conference, found girls had three times the risk of higher blood pressure. A British Heart Foundation spokeswoman said a third of young people in the UK were overweight or obese. The teenagers, aged between 13 and 17 had their blood pressure measured as part of school district health surveys and health checks. Their body mass index (BMI) - a measure of weight/height ratio - was also recorded. There are two types of blood pressure which are measured. Diastolic pressure - the lower number in a reading - measures the force on the arteries between heartbeats. Systolic blood pressure, represented by the top number in a blood pressure reading, is the amount of force that blood exerts on blood vessel walls when the heart beats. High systolic measurements indicate risk for heart disease and stroke. It was found obese boys were 3.5 times more likely to develop elevated systolic blood pressure than non-obese boys. But similarly obese girls were nine times more likely to develop elevated systolic blood pressure than their non-obese peers.