Swedish scientists have identified a substance found in broccoli that can reduce blood sugar levels in people with type 2 diabetes.

Scientists have found another reason why people should be eating their broccoli, saying it can help people with diabetes.

A study published in journal Science Translational Medicine found the green vegetable contains an antioxidant known as sulforaphane that significantly lowered blood sugar levels.

Commonly prescribed drug metformin works to control blood sugar levels but does have gastric side-effects and can't be taken by those with poor kidney function.

To find alternative treatments, researchers at the University of Gothenburg in Sweden turned to broccoli after mice studies showed sulforaphane resulted in a 23 per cent drop in blood sugar levels in just four weeks.

The 12-week randomised placebo-controlled trial involved nearly 100 people with type 2 diabetes who were given a powdered broccoli sprout extract - said to contain about 100 times the amount of sulforaphane found in broccoli.

According to the researchers, sulforaphane had the greatest impact on the obese participants.

In the group that took sulforaphane every morning for 12 weeks, the patients with poor glucose control and a body mass index at 30 or above had significantly lower blood sugar levels than the placebo group.

Lead researcher Assistant Professor Anders Rosengren says there is strong indications that broccoli can provide a valuable supplement to existing diabetes medication.

"Sulforaphane targets a central mechanism in type 2 diabetes and has a mild side-effect profile. As functional food, it can reach the patients faster than a medication, and it is also an interesting concept from a diabetes perspective where diet is central," said Ass Professor Rosengren.

About four to five kilograms of broccoli is needed to provide the required daily dose of sulforaphane.

The plan is to have a supplement available within two years.