Fat mobilization, Maximum Daily Deficit and Starvation Mode

Sorry for the long mathy post.
There's a lot of confusion about deficits, starvation mode and eating back exercise calories, so I'm going to try to clear a few things up.

First of all, go and read SHBoss1673's extremely excellent post about starvation mode [here](http://www.myfitnesspal.com/topics/show/710742--starvation-mode-exercise-calories-dillema?hl=understanding+starvation&page=1#posts-10438841)

The deficit you can run is based on how well your body can mobilize fat for a fuel source. The less fat you have the less mobile it becomes and the less of a deficit you can maintain. [Source](http://www.ncbi.nlm.nih.gov/pubmed/15615615 )

The basic math is as follows:
Free Fat Mass (FFM) = total fat mass (TFM) - essential fat mass (EFM)
TFM = Body Weight * Fat%
EFM = (Lean Body Mass [LBM])*0.03 for men or (Lean Body Mass)*0.12 for women
LBM = Body Weight - TFM

Maximum Daily Fat Mobilization = 2.5grams*FFM
Maximum Daily Deficit (MDD) = 2.5grams*FFM*9calories/gram

This is the largest deficit that you can maintain per day and theoretically have all of the deficit be supplied by fat oxidation.

So, let's say you're a 30 year old woman, 5'5'' tall and 180 lbs with 40% body fat. Your total daily energy expenditure is about 2200 calories (lightly active).
BW = 180 lbs
LBM = 180 - 180*.4 = 108 lbs
TFM = 180*.4 = 72 lbs
EFM = 108*.12 = 13 lbs
FFM = 59 lbs
MDD = 59 lbs * 2.5g/lb * 9 cal/g = 1327 calories per day

Now let's say that you eat 1200 calories per day and do no additional exercise. This gives you a 1000 cal/day deficit which is less than your MDD, so you're fine for now. Most of your weight loss will be from fat stores since your body can effectively mobilize enough fat per day to fuel your energy needs. However, if you burn 500 calories per day from exercise, your TDEE increases to 2700 calories and now if you only eat 1200 calories you have a deficit of 1500 calories per day. Your body cannot mobilize this much fat per day, so the energy has to come from somewhere else. The first thing that happens is your body uses up its glycogen supplies (we have about 2400 calories of glycogen in our muscles and liver). It will take about 2 weeks at this kind of deficit for your body to exhaust all of its glycogen stores. During this period you'll feel fine though you may find your self tiring more quickly during workouts. Eventually, all the glycogen will be gone and your body won't be able to make any more. This is when things go downhill. Now to properly run itself, the body has to start relying on some tricks to get by. This is effectively where the dreaded "starvation mode" begins, though in the first stages its not all that terrible. The first thing your body does is it effectively turns down the thermostat (metabolism) to save energy. This is a hormonal process that results in a decrease to the resting metabolic rate. Concurrently, it increases its use of triglycerides and muscle protein for energy, resulting in muscle catabolism and a further decrease in BMR. If this goes on long enough, your body will start turning off "non-essential" functions such as reproductive functions, skin, nail and tooth enamel production, decreasing digestive function and impaired cognition. It takes quite a while for this kind of stuff to happen and generally, the first measures employed by the body are enough to stave off starvation.

Now most of this is pretty extreme and won't happen to the average person (plus it takes a long time and willful disregard of your body's distress signals), but it is one of the things that contributes to plateaus and diet failure followed by regaining the lost weight plus some. A month at too high of a deficit can lead to a relatively significant decrease in metabolic function which can be fairly hard to undo.