An Email Response that Might Help Some ...

I just answered an email that I got and as I was writing it, I thought that for some it might be helpful. So I'm sharing here:

Hello, you asked me why it's a bad thing to lose lean body mass. vs other kinds of body mass.

Think of it as though the lean body mass (your bones, organs blood, and muscle tissue, etc.) are your engine. They all take energy to "run" every minute of the day. The fat stored on your body is like your fuel tank, there to provide energy (fuel) for the operation of your body. With that in mind, think about what kind of engine would burn through a fuel tank quicker -- a fuel efficient little Hyundai engine or a big truck engine? Of course the bigger engine would consume the fuel faster, and that's exactly how our bodies work. People with large muscle mass (in addition to their bones, organs, etc.) burn through much more energy (calories) than people with lesser muscle mass. In addition to the other positive reasons for wanting strong and flexible muscles, burning through extra fat is another great benefit. It's bad to lose this lean muscle because doing so will reduce our caloric need and slow our weight loss. Lean muscle mass loss also has consequences down the road.

Imagine that two identical 165 pound (30% body fat or 49.5 pounds of fat) women go on two different weight loss programs. Woman A is on a low calorie diet and is walking but isn't doing any weight training. Woman B is on a low calorie diet (although she can eat a bit more than woman A) and she is walking AND doing resistance training (weight lifting). After two weeks, woman A may lose 4 pounds, and about 1 pound of it will be fat. Woman B may lose only 3 pounds but 2.5 will be fat. Continue this out with examples from real clients I've worked with (the first number is her new weight and after the slash is the number of fat pounds she lost):

Woman A Woman B
Week 2 161/1 162/2.5
Week 4 156/1.5 158/3.1
Week 6 152/.9 155/2.0
Week 8 151/.25 152/1.5
Week 10 149/0 149/1.8

At the end of ten weeks, Woman A has lost 12 pounds, as has Woman B.
But their body composition has changed.
Woman A has lost 3.65 pounds of fat, which means that the rest of the 8.35 pounds were lean muscle mass.
Woman B has lost 10.9 pounds of fat, which means that the rest of her 1.1 pounds lost were lean muscle mass.
Woman A now has 45.85 pounds of body fat. At 149 pounds, that makes her 30.8% body fat.
Woman B now has 38.6 pounds of body fat. At 149 pounds, that makes her 25.9% body fat.

Woman A is now actually FATTER (30.8% body fat compared to her starting body fat of 30%).
Woman B is now healthier at only 25.9% body fat.

But think about the "engine". Woman A's engine weighs 103.1 pounds. Woman B's engine now weighs 110.4 pounds. Which engine is going to need more calories? Of course, woman B is going to burn more calories. That means two things. It'll be much easier for her to continue to lose weight if she so desires. It also means that if she's intending to maintain that weight, she'll be able to eat more food and STAY at that weight. Conversely, that means that Woman A is going to gain weight back much faster if she overeats much. And she won't be able to eat as much as Woman B, so in practical terms, it's going to be less likely she'll stick with it for life because she'll now have to get by on fewer calories for life.

You also asked why the body picks and chooses between lean body mass and body fat? And you wondered whether our body fat is there for the sole purpose of getting us through a "famine" (or low calorie diet in today's terms).

Our body does indeed store fat for reserves. And it knows when it should hang onto that fat. At heavier weights, the body has little to worry about (it has plenty of fat stores left!) so it tosses the fat overboard with abandon. Think of how fast the folks on Biggest Loser lose weight. But as it nears it's goal it starts to get a little nervous about maybe running out. That gas tank is mighty low even though you're looking good! As such, it becomes more important for you to eat, as you grow increasingly near your weigh goal, more of your exercise calories so your body feels comfortable that you're eating enough and it doesn't need to worry about the low "reserves." It doesn't worry that there is a famine around the corner! Eat too little, and you'll trigger a metabolic slowdown. Not a big deal for those with much to lose, but a big issue for those nearing their weight goals.

Hope that helps you out some ... let me know if I failed to answer any part of your question.
Good luck!