DS math
Malabsorption is not surgery specific it is based on the length of the bypass limb of the intestines. There is nothing magic about the lower part of a DS. The reason it is often said that there is a greater malabsorptive component to the DS is because the DS bypass channel USED to be a standard 400 cm. Many DS surgeons now use lifestyle questions regarding pre surgery lifestyles and post surgery expectations, muscle mass and tone, length of time that weight gain occurred (i.e. overweight since early childhood, since pregnancy, since accident or traumatic event, since reaching middle age etc) to determine how long to make the bypass channel. Mine is 150 cm which is short for a DS. Actually, it is right on the border between a proximal RNY (low malabsorption rate) and a distal RNY (high malabsorption rate).
Basically, regardless of surgery type it goes as follows (well actually between RNY and DS, there is no malabsorptive potential in either VSG or LapBand)
RNY or DS Bypass Channel Length (this is the length of the bypassed limb NOT the common channel length)
<100 cm bypass = very low malabsorptive potential
100 - 150 cm bypass = low malabsorptive potential
150 - 250 cm bypass = moderate malabsorptive potential
250 - 350 cm bypass = high malabsorptive potential
350 cm or greater = very high malabsorptive potential
Water, Vitamins, minerals, sugars, protein, fat, amino acids, and alcohols are absorbed through the walls of the intestinal tract. The more of the intestines that are bypassed the less opportunity for those items to be absorbed through the intestinal wall. This I know as fact.
As food moves along it's content is absorbed into the walls of the intestine. Because certain elements absorb at different rates, different length bypasses reduce your ability to absorb that component. Allow me to explain.
Water and alcohol is absorbed almost instantly (alcohol notably faster). Interestingly, with an RNY you actually get drunker quicker as the full force of the alcohol is immediately placed into the intestines (the most absorptive path for alcohol to reach the bloodstream) with a DS or non op, a larger percentage of the alcohol is absorbed in the stomach (a much slower absorptive path to the bloodstream) and thus there is no significant change in rate of intoxication with a DS due to the retention of the pyloric valve to hold it in the stomach for a longer period.
Sugars are the second fastest absorption rate. DS and RNY alike will absorb nearly all the calories from a serving rich in sugars. This is because you could have a common channel (the non bypassed or 'normal' section of your intestines) that was 100 cm or 500 cm and it wouldn't matter. Most if not all of the sugar will be absorbed within the first 75 cm. RNY has an advantage where this is concerned due to dumping. Some see dumping as a 'side effect' but it is considered by many to be a blessing.
To understand why this is an advantage we must clarify what dumping is: In an unaltered stomach (or any stomach where the pyloric valve remains in tact such as the DS or VSG) sugar rich foods mix with digestive juices and a large percentage roughly 50-60% are absorbed through the walls of the stomach which is again a much slower path to the bloodstream. The remaining 40% have started to break down into much simpler molecules before passing into the intestines. In an RNY, the sugar laden food passes directly into the intestines after only a very small percentage is absorbed through the stomach (typically <10 %).
Our intestines are not used to processing raw (in this case raw refers to completely un broken-down sugar molecules) and when that sugar hits the walls of the intestine, an endothermic reaction occurs. The intestinal walls cannot handle the complex sugar molecules and as they absorb the sugars they draw water from surrounding tissue at an amazing rate to mix with / dilute the sugar. This basically causes a hypoglycemic reaction in patients that experience dumping. This is what causes sweating, light headedness etc. It occurs less in well hydrated people and approximately 40% of all RNY patients exhibit no outward symptoms when this process occurs (however the process of drawing water from surrounding tissue does still occur whether you exhibit ill effects from the process or not).
It is interesting to note that while many say DSers cannot experience dumping, that is not entirely true. I have experienced dumping on numerous occasions, usually with liquid sugars such as milkshakes. While DSers rarely dump from such items as cookies or cakes, it is possible for DSers to dump.
In my "said all that to say this..." explanation, the reason dumping benefits RNY patients is since many of them dump, most RNYers avoid sugar rich products that are quickly and completely absorbed by both RNY and DS patients alike regardless of channel length.
I am going to mention vitamins now as some are absorbed relatively fast and some are absorbed very slowly depending on whether they are water soluble or fat soluble. I do not know the mode in which all are absorbed, but Vitamin A and D are typically fat soluble which is why DSers have to supplement Allergen A & D (a water soluble version of those vitamins). An interesting note here is that unlike most vitamins, the B vitamin family is almost completely absorbed through the walls of the stomach in a stomach that contains a pyloric valve (i.e. non-op, DS, LapBand or VSG) which is why RNYers need to supplement B12 frequently. Another interesting note is that some distal RNY patients have reported much better health and reduced hair loss by supplementing Alergen A & D to their diet. This is only necessary by surgeons in distal RNYs and DS, but if you are experiencing weird side effects, it can't hurt to try water soluble versions of any vitamin available in that form.
Protein and amino acids are the next most readily absorbed substances. It takes much longer (many times longer) to absorb than sugars, alcohol, water/ water soluble vitamins. For this reason, the longer your bypass, the less protein you have consumed will be absorbed. While there is no hard figure on it because everyone absorbs differently, generally in long limb distal RNYs (400 cm) and DS, some people absorb as little as 30% of the protein they consume. This is why DS surgeons recommend post op DSers shoot for 100g of protein, figuring that if they are one of the people who absorb protein poorly, by ingesting 100g they will absorb at least 30g. This is an area that I feel long limb distal RNY surgeons fail to set a high enough goal. It stands to reason that if it is accurate that some 400cm DSers absorb as little as 30% of the protein they consume, then a 400cm distal RNY could potentially be absorbing protein at that same rate. Most RNY surgeons recommend 50-70g protein per day regardless of the length they made the common channel which leaves a person who has been consuming 70 grams of protein actually absorbing 21g per day. If you are experiencing weakness, fatigue, hair loss, or slow healing wounds, and you had a distal RNY, I would recommend you boost your protein up to 100g per day and would be willing to bet it alleviates your symptoms.
Minerals are the next most readily absorbed substance; or more accurately very slowly absorbed substance. This is why most DS and RNYers have to supplement roughly 3 times the recommended daily allowance of calcium citrate to maintain a healthy level and would require well over nine times as much if we were using calcium carbonate. Iron falls into this category but oddly, some DSers readily absorb it and some need to supplement. This makes me believe that it is not simply a matter of absorbing it through the stomach. As far as I know, most RNYers have to supplement iron. I honestly don't know and could use some help on this one.
The slowest to absorb is fat. This is why a DS patient can eat a diet that is comprised as 50% fat without gaining weight or calories or absorbing it into the bloodstream. The fat requires prolonged contact with the surfaces of the intestinal wall to be absorbed. DS patients typically have very short common channels (as do long limb distal RNYs) and thus the very limited amount of time the fats have in contact with the intestinal wall results in a very very low absorption rate, though some is absorbed. The crux here is that while most of it isn't absorbed, the unabsorbed fat is deposited directly into the stool causing loose stools to diarrhea depending on how much fat was consumed or more accurately... depending on how much fat was not absorbed. Currently DS surgeons (mine in particular) are studying the effects of supplementing omega 3 fatty acids in DS patients as while very little fat is absorbed it is also believed that it is logical to assume that very little omega 3 fatty acids are absorbed as well. Currently there has not been a documented study published regarding this hypothesis. If you are a DS or a long limb distal RNY, I personally recommend supplementing Omega 3 fatty acid into your diet, just to be safe. It can't hurt.
Now that I have said all that and was it ever a mouthful, here comes the twist. Human bodies are remarkable items that have the uncanny ability to adapt. This is why it may require eating 100g of protein to maintain a daily absorption rate of 30g at 6 months out and then suddenly, for no explanation you find that a year later you chart the same results in your blood tests even though you have slacking and only actively consuming 70g on a regular basis. This doesn't happen with everyone and truly is a per individual occurrence that seems to have very little rhyme or reason. There are also changes that a body simply can't overcome.
Hope this answers your question 
