Most people believe that when you lose weight (calorie deficit) you are loseing a mixture of fat and muscle (lean tissue). The fat being good to lose and the muscle being bad to lose. The theory being that when losing weight, you will always lose some percentage of fat and lean tissue. So some people will do resistance training to try to spare muscle loss during weight loss. But what if you could determine a caloric deficit that would let you lose only fat and not lean tissue?
Here's an interesting study on how to maximize fat loss while minimizing muscle loss when dieting: Alpert SS. A limit on the energy transfer rate from the human fat store in hypophagia. J Theor Biol. 2005 Mar 7;233(1):1-13.
http://www.ncbi.nlm.nih.gov/pubmed/15615615Basically, the researcher found that the most energy you can get from a pound of fat in a day is ~30 calories (thats rounding off and adding a conservative buffer) when you are in a calorie deficit mode. Supposedly, as long as you stay within this 30 calories per pound of fat in a day, you won't be burning any muscle tissue. Once you go beyond the 30 calories a day per pound of fat, your weight loss will be from lean tissue.
So for example, lets say you have 50 lbs of fat on your body. To maximize fat loss without losing lean tissue the maximum calorie deficit you would want is 30 calories x 50 lbs of fat or 1500 calories a day deficit.
As long as your calorie deficit is no more than 1500 calories a day then you will have maximum fat loss with no lean tissue loss.
So if your usually eating 3000 calories a day, you can go down to a 1500 calorie a day diet and spare lean tissue loss.
Of course, the lower your fat level goes the less calorie deficit you can have per day without losing lean tissue. So you need to recalulate your max calorie deficit every few weeks or so.
And yes calorie deficit can be from calorie restriction (i.e., diet), or exercise, or neat, or thermogenics or any combination of these.
To my mind its an interesting idea, but I wonder if the body is so simple and clean cut on when it chooses to use fat or lean tissue calories during a calorie deficit.
Hopefully someone can comment on this theoretical study...