Doctors are praising a new diet craze for its benefits for helping the heart and controlling diabetes.
Apples have a low GI value
Low glycaemic index diets involve replacing high GI foods potatoes and white bread with low GI options such as apples, pasta and beans.
A Hammersmith Hospitals NHS Trust team found eating just one extra low GI item per meal cut blood sugar levels, the British Journal of Nutrition said.
Experts agreed such diets could have an impact.
The low GI diet has been dubbed the "Atkins alternative" and has been recommended by a host of celebrities.
THE GI INDEX
Low GI foods - Apples, oranges, pasta, beans, lentils and porridge
High GI foods - French fries, parsnips, watermelons, white bread and potatoes
Although the British Dietetic Association has warned against over-reliance on such diets, pointing out ice cream has a low GI value, and the key to losing weight was cutting down on calories.
The Hammersmith team measured the blood sugar levels of nine people on normal diets, and then put them on a low GI diet involving replacing one low GI item per meal for two weeks.
When the readings were taken again eight out of nine subjects had lower blood sugar readings.
Dr Gary Frost, head of nutrition and dietetics at the trust, said: "The scientific benefits of low GI diets are becoming increasingly clear, and this new research is a further indicator of the potential that including these foods in a balanced diet has.
"What is interesting is that replacing just one item per meal has this beneficial effect on blood sugar levels, putting people at lower risk of metabolic syndrome, a condition which can increase the risk of heart disease and diabetes."
And Dr Frost said the low GI diet had the potential to have a huge impact because of its simplicity.
"Radical changes to diet and increases in exercise levels have a big effect on an individual's diabetes risk, but in today's western society achieving this on a population-wide basis poses an enormous and probably unrealistic challenges."
A spokeswoman for Diabetes UK said the low GI diet was the only one the charity recommended.
"It is a good way of controlling blood sugar levels, but it should form part of a balanced diet."
And diabetes dietician Paul McArdle, of the Eastern Birmingham Primary Care Trust, said: "The GI index is a useful concept. It should not be taken out of context, but diabetes dieticians are increasingly referring to the GI index."
Sir Alexander Macara, of the National Heart Forum, said the study confirmed what was already known.
"For busy people it points to an easier way to reduce the risk of disease by recognising the value of avoiding high levels of sugar in the bloodstream.
"The GI diet reinforces the old message that it's wise to consume lower sugar foods.
"Interestingly, given a choice many people now do eat wholemeal bread and pasta rather than potatoes, for example, which maybe a sign that the healthy eating message is getting through."