Ruby,
The DS is usually done in two parts only on those who are super morbidly obese and have a very large hanging pannus. The stomach transection alone will cause restriction of eating and that will cause weight loss. Without the malabsorptive component, however, regain would be an issue in time. My understanding is that the restrictive component (stomach reduction) leads to weight loss, and the malabsorptive portion leads to maintenance.
I know that I, personally, have an eating capacity after almost four years that I would most certainly be gaining weight without the malabsorption, unless I ate perfectly, all the time. (If I could do that, I wouldn't have needed WLS in the first place).
There are studies available on
www.duodenalswitch.com, as well as side by side comparisons of available weight loss surgeries and other hopefully useful information.
Hope this helps,
Dawna