Researchers in Nottingham now can put their fingers on why exactly even the most well-built of people tend to loose their muscles and develop thinner arms and legs as they get older.
As age catches up, it becomes harder to keep our muscles healthy-they get smaller, which decreases strength and increases the likelihood of falls and fractures.
The researchers have already shown that when older people eat, they cannot make muscle as fast as the young, and now they have found that the suppression of muscle breakdown, which also happens during feeding, is blunted with age.
But the team think that weight training may "rejuvenate" muscle blood flow, and help retain muscle for older people.
The study's results may explain the ongoing loss of muscle in older people- when they eat they do not build enough muscle with the protein in food and also, the insulin (a hormone released during a meal) fails to shut down the muscle breakdown that rises between meals and overnight.
Normally, in young people, insulin acts to slow muscle breakdown.
These problems could be a result of a failure to deliver nutrients and hormones to muscle because of a poorer blood supply.
In the study, the researchers compared one group of people in their late 60s to a group of 25-year-olds, with equal numbers of men and women.
Professor Rennie said: "The results were clear. The younger people's muscles were able to use insulin we gave to stop the muscle breakdown, which had increased during the night. The muscles in the older people could not."
"In the course of our tests, we also noticed that the blood flow in the leg was greater in the younger people than the older ones. This set us thinking: maybe the rate of supply of nutrients and hormones is lower in the older people? This could explain the wasting we see," he added.
Later, Beth Phillips, a PhD student working with Rennie, confirmed the blunting effect of age on leg blood flow after feeding, with and without exercise.
The team predicted that weight training would reduce this blunting.
"Indeed, she found that three sessions a week over 20 weeks 'rejuvenated' the leg blood flow responses of the older people. They became identical to those in the young," said Rennie.
The study has been published in The American Journal of Clinical Nutrition.