Differences in the way male and female bodies metabolize food means older women do not use protein as effectively to maintain muscle, the research found.
Nottingham and Washington researchers studied 29 men and women aged 65 to 80.
Women over 65 eat protein-rich foods like meat and eggs and do resistance exercise, they write in PLOS One.
The researchers found that after resistance exercise like lifting weights, women did not build up muscle as their male counterparts did.
The male body, it appeared, was able to store protein in the muscle and use this to make them stronger.
The researchers, from the University of Nottingham in the U.K. and Washington University School of Medicine in the U.S., speculated that the inability of the female body to perform the same function as effectively was linked to the hormonal changes of the menopause.
Rather than eating more, older people should focus on eating a higher proportion of protein
Oestrogen, which declines during this period, is known to help maintain bone mass and may perform a similar role in the preservation of muscle.
Studies of younger men and women have found little difference in the way the body builds up muscle, suggesting the changes seen in this research do not kick in until the menopause.
"Nobody has ever discovered any mechanistic differences between men and women in muscle loss before," said Michael Rennie, professor of clinical physiology at the University of Nottingham.
"This is a significant finding for the maintenance of better health in old age and reducing demands on the National Health Service."
From the age of 50 onwards, people lose up to 0.4% of muscle mass every year.
This can make them less mobile and at a higher risk of a life-threatening fall. At present, half of all elderly people who suffer a serious fall die within two years.
Women are seen as being at particular risk as even by early middle-age, they tend to have more fat and less muscle than men of the same age.
"We know that women tend to have less muscle bulk than men as they enter old age so the advice to eat more protein is very sensible indeed," British Dietetic Association spokesman Jackie Lowdon said.
"Many elderly people subsist on toast and biscuits -- food that is easy to make -- and there needs to be a much greater focus than there is at present on improving the diets of those who are already vulnerable."
From the March 31, 2008, Prepared Foods e-Flash