Irrespective of the fact that gains and losses in LBM have minimal effects on either metabolic rate or BF%, there are still many reasons for women to be concerned about how much LBM they are carrying either in terms of increasing it or preventing its decrease.
While this is important for all women, it becomes especially critical with aging as there is often a loss of LBM that harms both health and function. I would mention that much of this age related loss is related primarily to changing activity levels. Studies of female master's athlete show a significant retention of LBM compared to their sedentary peers.
Women have the additional factor of menopause where LBM loss accelerates significantly, especially if they decide not to go on HRT. This often sets up a vicious cycle where a loss of muscle leads to a loss of function/decrease in activity which causes more muscle to be lost.
LBM loss is, in general, not good although there are occasional situations where it may be actively desired. Outside of the normal aging process and menopause, probably the most common situation a woman will encounter where LBM loss is a risk is during a diet. Again I'm focusing only on actual muscle mass loss as the early water and glycogen loss is technically LBM.
The relative risk of LBM loss depends on a few factors. A primary one is body fat percentage with leaner dieters being at greater overall risk. Category 3 dieters may lose zero actual LBM while Category 2 dieters are at a slightly higher risk and Category 1 female dieting to the extremes the most risk.
Other factors play a role here, with the exercise program and diet playing a major role in whether or not LBM is lost. I'd point out again that women are much less likely to lose LBM than men.
There are a number of reasons to limit LBM losses while dieting. One is that even if BF% is decreasing, the loss of LBM can lead to less than hoped for visual improvements. Dieters end up being slightly smaller versions of their previous self. Maintaining LBM or even increasing it slightly has a profound impact here providing the much sought after "toned" appearance that represents sufficient muscle size coupled with a reduction in BF%. Weight training, in addition to both increasing muscle mass or preventing its loss, can indirectly improve fat loss as well. A second reason is that avoiding LBM loss while dieting helps to limit the normal drop in metabolic rate that can occur.
Additionally, it's recently been found that LBM sends a signal to the brain that can increase hunger; losing LBM while dieting results in more hunger than would otherwise occur. The main risk here is for the Category 1 dieter. So long as they are active and eating sufficient protein, there is little to no problem but I described a subgroup of lean Category 1 females who are inactive who are often attempting to lose weight. As they tend to diet without exercise (especially resistance training) or sufficient protein, they often lose LBM. This leaves them with more rebound hunger than they would otherwise experience which puts them at the risk for rebounding and ending up at a higher BF% than they started at.
There are rare occasions where LBM loss may be accepted or even desired. I mentioned one above which is the endurance athlete who may be carrying excess muscle, especially in non-relevant muscle groups, that is harming their performance. Losing that muscle reduces body weight and this often improves their performance. Outside of that there are two primary situations where actively losing LBM or at least avoiding further increases may be appropriate.
The first is in PCOS or Category 3 women (and remember that the two are often linked) who has gained a large amount of undesired LBM as they gained weight. Losing that extra LBM during a diet may be an appropriate goal either from an aesthetic point of view or to allow bodyweight to be reduced to the most significant degree.
Similarly those women with PCOS/subclinical hyperandrogenism who are not interested in sports performance (and who tend to put on LBM at a slightly faster rate than other women) may want to explicitly avoid the types of training that tend to increase LBM to the greatest degree. There are still benefits to weight training such as improved health but the program will have to be modified in this situation.
But outside of those very few exceptions, the majority of women should put at least some effort into either increasing their LBM or at least preventing its loss and this is true whether fat loss or simply general health is the goal. I'd note that the type of training that best accomplishes this turns out to be the type of training that either increases or prevents/slows the loss of bone mineral density.
In general our FitCamps are very successful with woman because by combining weight training and cardio 3 times per week, prevents losing LBM and increases fat loss.
Find out more about our camps here
The above information is taken from the The Woman's Book by Lyle Mcdonald with Eric Elms.