Staying in good shape is harder for older women than older men because women's bodies are less able to replace muscle that is lost naturally as they age, according to a new study.

The study, of 29 healthy women and men, aged 65 to 80, found that the women were less able to use protein to build muscle mass — a key difference in the way women's and men's bodies react to food. This may be caused by menopause-related hormone changes in women, said researchers from the Washington University School of Medicine in St. Louis and The University of Nottingham in the UK.

One possible culprit is decreased levels of the hormone estrogen, which is needed to maintain bone mass. The findings, published in the April issue of the Public Library of Science One, seem to fit with preliminary results showing that older women build less muscle after weight training than do older men. There seems to be no difference in muscle-building between younger women and men.

The findings of this new study suggest that it is important for older women to consume plenty of protein-rich foods such as eggs, fish, chicken and lean red meat and to do resistance training such as lifting weights, the researchers said.

"Rather than eating more, older people should focus on eating a higher proportion of protein in their everyday diet. In conjunction with resistance exercise, this should help to reduce the loss of muscle mass over time. There is also a case for the beneficial hormonal effect of limited HRT (hormone replacement therapy), although this has to be balanced against the other risks associated with such treatment," said Michael Rennie, one of the researchers and a professor of clinical physiology at the University of Nottingham.

Maintaining muscle is important because it lessens the risk of falls, one of the major causes of premature death in older adults. After age 50, people lose up to 0.4 percent of muscle mass per year.

Women are at particular risk for muscle mass decline, because they tend to have less muscle and more fat than men in early and middle age, which means they're already closer to the "danger" threshold of becoming frail when they're in their 50s and 60s, the researchers said.