DS Math

Can anyone explain DS math to me in regards to how much we actually absorb?  I know we absorb only 20% fats, 50% protein, etc., but how do you determine how many calories you are actually getting?

 The easy answer is 2. 

Fat has 9 cal/g for normal folk.  We absorb approximately 20% or 2 calories a gram.

Protein has 4 calories per gram as do complex carbs.  Of those we absorb about 60% or a little over 2 calories per gram.

Simple carbs we get all 4 calories per gram.

Alcohol has 7 calories per gram and we get all of them.

Thus the DS food priority list: Protein, fat, complex carbs, simple carbs and, last of all, alcohol.

However, given that we malabsorb so much and our restriction at the outset is so good, there's really not much point in worrying about calories.  

Hope that helps!