This was written by Diana Cox. (Permission granted from author.) It is a great explanation of DS Math concerning calories absorbed.
You may be wondering how DSers can eat 3000+ calories per day and not gain weight. Some people also seem to have difficulty understanding why our arteries aren't clogging up, since most of us eat about 50% of our calories as fat. And my personal favorite is the accusations that we are eating like pigs at a trough or somesuch. For those of you who are genuinely interested, this is how the DS works.
I (and many if not most DS post-ops over 2 years out) eat about 3000 cal/day most days. But we don't absorb that much, of course. This is the "magic" and the pleasure of how the DS works, and why it works. The mathematics of malabsorption look something like this:
I always get about 100 g of protein/day (and NONE of it makes me puke or foam or gets stuck, and I have NEVER had protein shake -- since surgery, I have eaten all my protein as FOOD). 100 g of protein x 4 cal/g = 400 cal eaten; x 0.6 (60% of protein absorbed) = 240 cal from protein absorbed.
Since I eat at least 50% of my calories as fat (full fat mayo to my heart's content; butter with my seafood and veggies; full fat salad dressing, and lots of it, with my salads and veggie dips; nice marbled meats including steak, bacon and crispy chicken skin), let's call it 1530 calories which equals 170 g of fat (fat is 9 cal/g) to make the math easier. But I absorb only 20% of the fat I eat, for only 306 cal from fat absorbed.
So, fat + protein eaten = 1930 calories of the 3000 I eat per day. That leaves 1070 cal or 267.5 g of carbs (4 cal/g). But I only absorb about 60% of complex carbs. I have to account for sugar first, because I do absorb 100% of sugars -- I would guestimate that I eat about 50 g of sugar/day x 4 cal/g for 200 calories from sugar absorbed; of the remaining 870 calories of complex carbs x 0.6 (60% absorbed) = 522 cal from complex carbs absorbed.
So, my ABSORBED calories from 3000 ingested calories (eating 100 g of protein, about 50% fat and the rest carbs) is:
240 cal protein
306 cal fat
200 cal sugars
522 cal complex carbs
1268 calories absorbed per day from eating 3000 cal
And that is why I (and other DSers) can eat like that, not exercise, and still be either maintaining or losing weight. It doesn't suck!
To be clear, I am NOT advocating not exercising! Everyone needs exercise for cardiovascular health, bone health, looking toned, etc. My point is, exercise for losing or maintaining weight loss isn't as necessary for most DSers.
As for arteries clogging from all that fat -- what doesn't get absorbed doesn't get to my arteries, of course! My total cholesterol is 167, my HDLs are 53, and my LDLs are 95. My triglycerides are 95. All of which numbers are EXCELLENT -- my doctor wishes her levels were that good. My CRP (an indicator of inflammation and thus cardiovascular risk, especially in women), which was a dangerous 10 times normal pre-op, and which was not helped much by statins, is now completely normal with NO statins. My cardiologist released me -- there is no need to see him anymore! (A personal note: My cholesterol was 189 prior to my DS. My current cholesterol is 108. I don't know the specific individual numbers, but my doctor said they were well within range.)
Note that contrary to unfounded accusations, I don't eat huge amounts of food at a time (nor do most of us) -- I do eat smaller, higher calorie meals (full of yummy fat!), and I eat 4-6 times/day. I didn't binge pre-op, and I don't binge now. I usually get at least a snack if not an entire second meal out of a restaurant meal, which for me is generally sharing a couple of appetizers with my husband, sometimes a piece or two of bread with LOTS of butter, an entree of my own, and sharing a dessert, along with a drink (Mai Tai!). I usually take home some of one of the appetizers, and half of my main course, and finish it off before bed or for breakfast the next day.
I didn't have a major sweet tooth pre-op, but I eat even fewer sweets now -- the DS changes metabolism to correct insulin resistance, and after 2-4 bites of something sweet, I am generally done. I am satisfied with less sweets than I was pre-op -- sweets taste DELICIOUS for a couple of bites, and then taste overly sweet. I get both the pleasure of having sweets with no repercussions, and no fear of overindulgence. Sometimes, when I feel inclined to indulge but find myself physically not wanting more, this is almost amusingly annoying. No willpower needed!
By the way, based on studies on people with short bowel syndrome, intestinal adaptation is generally complete by about 2 years out. The statistics of weight loss maintenance over 10-15 years with the DS supports the theory that the same is true for DSers -- if there was continued adaptation of the intestines after two years, there would likely be significant and ongoing weight gain in long term DSers, which there generally isn't.