FYI, malabsorbtion would make ou lose weight. You should be checked out if you have abnormal weight gan or loss.
Food is absorbed in the stomach. IBS is a functional disorder mainly of the sigmoid colon. The food has already been digested.
IBS and diet
"There is no evidence that digestion of food is different in those with IBS compared to those without IBS. Diet, food and eating do not cause IBS. However, muscles and nerves are over-reactive in IBS. This can cause the bowel to over-respond to stimuli. Even a normal event such as the act of eating itself, and not a particular food, may aggravate symptoms at times. Eating releases hormones that stimulate the gut."
I am not a doctor. All information I present is for educational purposes only and should not be subsituted for the advise of a qualified health care provider.
Please make sure you have your symptoms diagnosed by a medical practitioner or a doctor.