Technically, any given reader of this blog could have one of two primary goals. The first would be to gain muscle. While the process may be difficult in terms of what is required in terms of training or nutrition, changes here are relatively simple in the sense that gaining muscle will cause muscle to be gained. Losses of body fat may alter how someone looks for any given muscle mass but cannot increase the total amount of muscle mass that someone is carrying. Generally speaking, here the goal is to gain muscle without gaining excessive amounts of fat. In contrast, since it represents the ratio of fat to total bodyweight, gaining or losing LBM or fat can impact on overall BF% although in slightly different ways.
If someone gains LBM without gaining fat, BF% will go down as they are now heavier with the same total amount of fat (only the BF% has changed). Even if some fat is gained, so long as more LBM is gained than fat, BF% still goes down. If fat is lost, BF% goes down almost without exception and in this case a woman will have less total pounds/kg of fat. If fat is lost with no muscle loss, BF% will go down. Even if some muscle is lost (and women are less likely to have this occur), so long as more fat than muscle is lost, BF% will go down. Finally, if someone gains LBM while losing fat, BF% will go down.
This works identically in reverse in terms of BF% going up and body composition worsening. If LBM is lost with no change in total fat, BF% will go up due to fat making up a larger proportion of total weight. If fat is gained either without a change in LBM or in excess of LBM gains, BF% will also go up. In the case where LBM is lost while fat is gained (as occurs in some diseases, with certain drugs and with some types of birth control), BF% will go up and this may be true even if weight doesn't change.
Since it is rare outside of a few specific situations for someone to want to increase their BF%, and since dieting is a far more prevalent goal, I will be focusing primarily on lowering BF% here. And the reason that I am discussing this is due to an often heard suggestion that, rather than dieting and focusing on fat loss per se, the goal should be to focus on increasing LBM.
This idea generally revolves around two primary claims. The first is the fact that muscle burns calories even at rest and that increasing the amount of LBM will raise metabolic rate. Old studies suggested that a single pound of muscle could burn 40-50 calories per day but this is drastically incorrect with the real value being closer to 6 cal/lb or 2.7 cal/kg. For perspective, a pound of fat burns roughly 2 cal/lb or 0.9 cal/kg meaning that three pounds of fat will burn the same number of calories as one pound of muscle.
The consequence of this is that only the most extreme gains in muscle mass have even the potential to raise energy expenditure meaningfully. Consider that, over the first 6-7 months of training might gain 3-4 pounds of muscle which amounts to 18-24 calories extra per day burned. Two different studies have found that women show perhaps a 30 calorie per day increase in resting metabolic rate when they gain 4.5 pounds (2 kg) of muscle over 12-24 weeks. A gain of 10 lbs of LBM has the potential to burn 60 calories per day and a massive 20 pound gain in muscle might burn an additional 120 calories per day.
Every bit adds up but, in the short- term especially, gains in LBM have no meaningful impact on energy expenditure. It does take energy to synthesize muscle but even there the relatively slow rate of muscle gain in women makes this fairly insignificant. It takes roughly 2,700 calories to synthesize one pound of muscle so a woman gaining one pound of muscle per month might burn ~100 calories extra per day. Any actual increase in energy expenditure from the process of gaining muscle will primarily come from the training involved but long- term increase in metabolic rate from muscle gain are more or less irrelevant under all but the most extreme circumstances.
The second idea behind gaining LBM to lower BF% revolves around the mathematical fact that BF% will go down if pure LBM is gained and body weight goes up. As described above, here the total amount of fat a woman is carrying will not change but, since her total weight has increased, the relative percentage of fat will go down. While this is certainly true, as I'll show in the chart below, the effect of gaining LBM pales in comparison to the process of losing fat in terms of its impact on BF%.
I will be starting with a sample dieter who weighs 150 lbs with a BF% of 22%. The first calculation in the last blog can be used to determine that she has 117 lbs of LBM and 33 lbs of fat. All I will be doing below is to manipulate the amounts of LBM, fat or both and recalculating BF% (using the second equation from the previous blog) for each change. All I've done here is recalculate BF% by dividing the total weight by the total amount of fat. I will be making one simplifying assumption which is that 100% fat is being lost or 100% LBM is being gained. While this isn't always the case, it makes the math simpler and the differences in the results don't change that meaningfully without that assumption.
First I'll look at moderate changes of either a 5 lb gain in LBM or a 5 lb loss of fat with no other change. I'll also look at what happens if someone gains 5 lbs of LBM while losing 5 lbs of fat (this isn't common and I'm showing it mainly to make a point). I'll also look at the extremes of gaining 20 lbs of LBM (roughly a woman's maximum potential) or losing the same 20 pounds of fat. Finally, just for illustration, I'll show a 10 lb loss of fat.
The primary message of the above chart is that, in every case, compared to gaining LBM, losing the same amount of fat has a far more pronounced effect on lowering BF%. In the first case, gaining 5 lbs of LBM only lowers BF% by 0.7% while losing the same 5 lbs of fat lowers it by 2.7%, nearly four times as much. Gaining 5 lbs of LBM while losing 5 lbs of fat generates a larger result (3.3% vs. 2.7%, basically the individual results added together) but the major effect is still from losing fat. At the extremes, gaining 20 pounds of LBM only reduces body fat by 2.6%, almost the same as losing only 5 pounds of fat. But losing 20 pounds of fat reduces BF% by 12% from 22% to 10% (the lower limits of what a woman might achieve). Even a 10 pound fat loss causes over twice the reduction in BF% (- 5.6% vs. -2.6%) than gaining 20 pounds of muscle. Half as much fat loss as LBM gain has twice the impact on BF%.
Hopefully the above shows that the impact on BF% by gaining muscle isn't even close to that of actually losing fat. The fact that even smaller amounts of fat loss have a greater effect than enormous gains in LBM shows that; when the numbers are equal, fat loss may have four times the overall effect. Even if this weren't the case, there is an additional issue that must be considered which is the time frames involved.
Because in almost all situations, the fact is that gaining muscle is a grindingly slow process, even more so for women than men. In contrast, fat loss can occur relatively quickly. So while gaining even 5 pounds of muscle might take 6 months of effort (and might come with a small amounts of fat gain), that same 5 pound fat loss might take only 5-10 weeks. As the numbers get larger, so do the differences. A 10 pound gain in muscle might take a woman a year or more. The same 10 pounds of fat loss might take 10- 20 weeks. Gaining the extreme of 20 lbs of muscle is a career goal for most women and might take 3+ years if it is achieved at all. That same 20 lb fat loss might take 6 months for a lean female and less than that for someone in my Category 2 or 3. Even if it took a full year to lose that 20 lbs of fat, it's still one third of the time it would take to gain the same amount of muscle with far greater impact on BF%.
I'll finish this blog by noting that everything discussed above works the same in the opposite direction. That is, losing LBM tends to have a relatively small overall impact on increasing BF% while fat gain always has the much more profound effect. Without putting it in chart form, if the sample dieter loses 5 lbs of LBM with no change in her total fat her BF% only increases from 22% to 22.7%. If instead she gained 5 pounds of fat with no change in LBM, her BF% would increase from 22% to 24.5%. Regardless, increasing LBM or preventing it's decrease is still important for other reasons, discussed in the next blog, which covers gaining and losing LBM.
The above information is taken from the The Woman's Book by Lyle Mcdonald with Eric Elms.