Diabetics should be checking their blood sugar levels more regularly to reduce the risk of heart disease and strokes, US research shows.
Diabetics tend to check their blood sugar levels twice a day
People with diabetes type I and II tend to check their levels twice a day by testing blood by pricking their finger.
But the Johns Hopkins University study said diabetics should test more often to make sure the levels do not exceed a set limit for a sustained period.
Researchers pooled data from 13 earlier studies, involving 10,000 people.
Diabetics are advised to make sure their blood sugar levels stay between 4mmol/l (millimoles per litre) and 7mmol/l (80 to 120 milligrams per decilitre in US measurements) but the levels can vary between readings.
The study said diabetics that test more regularly should make sure their levels do not exceed 8mmol/l (150 mg/dl) for a long period.
If the blood sugar levels exceeded the limit, diabetics were increasing their chances of developing cardiovascular disease, including heart disease and stokes, it warned.
The report, published in the Annals of Internal Medicine, also urged diabetics to have their long-term blood sugar levels monitored.
Diabetics can have an HbA1c test from their GP which can give an average blood sugar level for the previous few months by reading the glycated haemoglobin rate.
Researchers found that for every 1% rise in glycated haemoglobin in people with diabetes type II there was an 18% increase in getting cardiovascular disease.
For diabetes type I, there was a 15% rise in risk - although the study said the estimate was not as accurate as for type II as researchers looked at fewer studies.
The recommended glycated haemoglobin rate was 7%.
Report author Sherita Golden, assistant professor of medicine and epidemiology at the university's school of medicine in Baltimore, said diabetics needed to pay attention to blood sugar levels as well as cholesterol and blood pressure.
"The relationship between blood sugar levels in people with diabetes and whether this increases their risk of developing heart disease has remained unclear until now.
"People living with diabetes are twice as likely to die from cardiovascular disease compared to those without diabetes.
"As a result, many people living with diabetes monitor their health for well-known risk factors for heart disease, such as obesity, cholesterol levels and blood pressure - but the big unknown has been the role of blood sugar levels in managing their risk of developing cardiovascular disease.
"Our research suggests that management of blood sugar levels is still a key part of cardiovascular disease prevention in diabetes."
He admitted keeping within the report's limits would be tough.
Among the 13 studies analysed by the team was the UK Prospective Diabetes Study, which was published in 1998 and was the first time blood sugar level and cardiovascular disease was linked.
About 1.4 million people have diabetes in the UK although another 1m are thought to be diabetic but not aware of it.
A spokesman for the charity Diabetes UK agreed some diabetics should be testing themselves more.
"For some diabetics more regular more testing would be helpful but it is not necessarily needed for everyone.
"And some do not always know what to do. People need to be provided with information about why they are testing and what they should do about their test results.
"There is also some suggestion people have struggled to get hold of the necessary equipment to test blood sugar levels.
"We have heard some PCTs are reluctant to provide the glucose testing strips for measuring blood sugar levels on prescription - that is worrying."