Elderly and vegetarian men are being warned about the effect of low protein diets on their sex lives in later years.

Researchers say people who do not eat enough protein are at risk of low testosterone levels which can cause a decline in sexual function as well as muscle loss, reduced red blood cells and damage to bones.

People who do not eat meat are particularly at risk, because animal products are good sources of high biological value proteins.

As well as vegetarians and vegans who choose not to eat animal products, the elderly are known to eat less meat because of loss of appetite in later years and difficulty with chewing.

Dr Christopher Longcope, of the University of Massachusetts medical school, says his latest findings mean the elderly in particular should be counselled about the importance of eating enough protein.

He found that low protein diets lead to increases in sex hormone-binding globulin in older men, which reduces the availability of testosterone.

Sex hormone levels

Dr Longcope and colleagues looked at globulin levels in 1,552 men aged between 40 and 70. Protein and fibre intake levels were significant contributors to sex hormone levels, as were age and body mass, they said in The Journal of Clinical Endocrinology and Metabolism.

He said: "The decrease in bioavailable testosterone can then result in declines in sexual function and red cell mass, and contribute to the loss of bone density."

And high protein diets could increase the available testosterone, though further research into this areas was needed, he said.

Denise Parish, a dietician at the University Hospital of Wales in Cardiff, said it was known that low protein diets caused muscle loss, but the link with testosterone had not previously been made.

She added: "In the elderly population, people tend to have lower protein intake because of poor denticulation, which means they can't eat as much meat, and poorer appetite. The same is the case for vegetarians and vegans."

She said foods such as textured vegetable proteins, soya, pulses and rice and bread were good providers of protein for these groups of people.

A spokeswoman for the Vegan Society said there was not usually a problem of low protein intake for vegans, who do not eat meat or dairy products.

"It is just sensible to eat a varied diet," she said. "As long as you are doing that, you will be fine."