Why does cystic fibrosis cause a person to be malnourished?

Part of this question includes that the thick mucus produced by cystic fibrosis sometimes blocks the pancreatic duct that allows pancreatic juice to enter the small intestine.

In addition to the information noted in Pigley's answer, because of the pancreatic deficiencies we also have trouble absorbing fat soluble vitamins - A, D, E and K specifically which then has impact to our overall malnourishment. With vitamin D not absorbing well it means we also do not absorb calcium well, which leads to osteoporosis. We are usually on a more stringent vitamin therapy than most people, which often includes pre-natal vitamins and higher dosage levels.

With the malnourishment our bodies have a more difficult time fighting off lung infections, which in turn make us lose our appetite and lose weight, which then leads to further malnourishment and lung infections. It can be a rather vicious cycle.

Most of us take pancreatic enzymes when we eat which helps with the nutrient absorption. We eat frequently and large quantities of food, along with high calorie 'milkshakes'.

Not all of us have the same level of impact to our digestive systems due to the number of gene mutations that result in CF. Some of us require few if enzymes and I have known CF'ers that were on the 'chunky' side but by and large most of us are very skinny.

the pancreatic juice contains trypsin, pancreatic lipase and pancreatic amylase. the bile duct also connects to the pancreatic duct. if all these enzymes and bile cannot reach the duodenum, digestion of food will definitely be affected. the insoluble, big food molecules cannot diffuse into the bloodstream in the small intestine. thus, the patient will become malnourished.