There is growing evidence that oestrogen and oestrogen replacement therapy for post menopausal women may have a role in reducing the risk of developing Alzheimer's

Dr Richard Harvey, Alzheimer's Society

Researchers from the University of Alabama at Birmingham examined the impact of certain types of phytoestrogen found in soy, known as isoflavones.

The study was carried out on aged female monkeys who had their ovaries removed to mimic the effects of going through the menopause.

A total of 45 monkeys were fed one of three diets:

• soy with isoflavones
• soy without isoflavones
• Premarin, a commonly prescribed oestrogen replacement for women

Brain tissue

Scientists then examined the brain tissue of three monkeys in each group for some of the protein changes that are markers of Alzheimer's disease.

Lead researcher Dr Helen Kim said: "Consistent with our expectation that the soy phytoestrogens would have 'beneficial' effects, we found fewer Alzheimer's disease-linked tau protein changes in the brains of monkeys that received soy isoflavones."

Improperly functioning tau proteins in the brains of people with Alzheimer's disease can cause the skeletal structure of nerve cells to collapse.

The researchers also found that monkeys fed on Premarin showed less signs of tau protein malfunction.

Dr Kim suspects that isoflavones and real oestrogens, such as those in Premarin, may both be beneficial for the brain.

But she believes they work through different mechanisms.

A health claim approved for food labels by the US Food and Drug Administration states that a daily diet containing 25 grams - two to three servings - of whole soy protein is enough to reduce the risk of heart disease.

Dr Kim said: "I would expect the same levels of soy intake to translate to protection in the brain."

As the population ages, Alzheimer's is becoming a growing a problem. It is estimated that nearly half of all people aged over 85 suffer from the condition.

Dr Richard Harvey, director of research, Alzheimer's Society, told BBC News Online: "This research needs to be interpreted very cautiously because it is animal based and there were only a small number of animals involved in the study.

"At this stage nothing from this study could be generalised to humans.

"However, there is growing evidence that oestrogen and oestrogen replacement therapy for post menopausal women may have a role in reducing the risk of developing Alzheimer's disease in later life.

"There are a number of large on-going clinical trials of HRT looking at this question and we wait with anticipation for the outcome of these studies."

The research was presented at a meeting of the American Chemical Society.