Eating black soya beans could lower fat and cholesterol levels and may help prevent diabetes, a study suggests.
Abdominal fat increases the risk of type 2 diabetes
Yellow soya is already known to lower cholesterol, but black soya is used in traditional oriental medicine as a treatment for diabetes.
The Korean study found rats who got 10% of their energy from black soya gained half the weight of those who had none, Chemistry and Industry reports.
UK diabetes experts warned black soya alone would not prevent the condition.
In the study, also reported in the Journal of the Science of Food and Agriculture, the researchers from Hanyang University, Seoul, fed 32 rats a high-fat diet.
The animals were divided into four groups. One group was given no black soya protein while the other groups derived either two, six or 10% of their energy from the food.
After 28 days, it was found that the animals which ate the most black soya had gained half as much weight as those who had had none.
The group who had eaten most black soya also levels of total blood cholesterol that were 25% lower and of LDL "bad" cholesterol that were 60% lower than those who ate none.
The researchers, led by Shin Joung Rho, said the study showed that eating black soya prevented weight gain and improved cholesterol levels, but did not suggest why the food might have the effects.
But David Bender, of the Royal Free and University College Medical School, London, suggested the protein might affect fat metabolism in the liver and fatty tissue, reducing synthesis of new fatty acids and cholesterol.
He said: "The key problem in type 2 diabetes is impairment of insulin action, mainly as a result of excess abdominal adipose tissue - so loss of weight often improves glycaemic control."
But he said eating black soya was unlikely to be a useful treatment for obesity.
"If food intake is greater than energy expenditure then the excess will have to be stored somehow, and that will be storage as fat - even if black soya is inhibiting fatty acid synthesis to some extent."
Libby Dowling of Diabetes UK said: "The results of this study are interesting.
"However more research needs to be done to determine if the results found in rats would be replicated in humans.
"Weight is certainly a key issue when it comes to type 2 diabetes - 80% of people are overweight when diagnosed with the condition - and keeping the right weight for your height is important in helping to reduce your risk of developing type 2 diabetes."
She added: "Black soya beans are low in fat, high in soluble fibre and a good source of protein but they alone will not stop someone from developing diabetes.
"Regular physical activity and a healthy, balanced diet are proven to be very effective in reducing the risk of developing the condition."