Scientists believe they have found a way to enable the elderly to maintain muscle.
Maintaining muscle mass is easier when young
Muscle is constantly being built and broken down, which works to maintain a balance in young adults.
But as people age, the breakdown process is more successful than the muscle-building action.
However French researchers, writing in the Journal of Physiology, say adding the amino acid leucine to old people's diets could help them keep muscle.
Leucine is found in meat, soy beans and legumes, among other foods.
Once adults reach 40, they start to lose between 0.5 and 2% of their muscle each year.
The team from the Human Nutrition Research Centre of Auvergne, in Clermont-Ferrand, France looked at the behaviour of proteins in muscle.
As in all mammalian tissues, proteins are created (synthesised) from amino acids and digested (degraded) by enzymes.
Straight after a meal, the rate of synthesis doubles, prompted by the arrival of a large amount of amino acids.
The rate of the breakdown of protein is highest in-between meals.
The difference between the two rates determines how much protein remains in the muscle.
But, in older animals - and, it is believed, humans - the amino acid stimulus prompting synthesis is less effective, and the process slows down.
However, the breakdown of proteins is not, leaving older animals with less protein than their younger counterparts.
The researchers compared protein breakdown in young (eight-month-old) and old (22-month) rats.
They discovered that the slow down in degradation that normally follows a meal does not occur in old animals, so there is excessive breakdown.
But when the scientists boosted levels of one amino acid, leucine, the balance of synthesis and breakdown was restored.
The team, led by Dr Didier Attaix, suggest the protein processing imbalance which comes with age results from defects in the complex machinery that breaks down muscle protein, and that leucine supplementation can fully restore correct function.
He said: "Preventing muscle wasting is a major socio-economic and public health issue, that we may be able to combat with a leucine-rich diet."
Dr Michael Rennie from the University of Nottingham Medical School at Derby told the BBC News website more research into the finding was needed.
But he said older people could make changes to their diet now which could help them maintain muscle.
"If they don't, they can fall over more easily; they can trip down stairs or fall in the bath."
Dr Rennie said older people could act now, even before further research had been carried out.
"Leucine is most abundant in meat, so it makes sense in terms of protein synthesis to eat meat.
"As people get older, they tend to need to eat less. But people should maintain their protein intake as they age."