We often discuss the main culprits of mucus production, which are milk/dairy products - pasteurized and homogonized ones, anway - and weak digestion overall, but there are some nutritional deficiencies that will cause the body to produce more mucus in order to protect vital tissues until the nutrients are there to repair them properly. Mucus is used by the body a lot like mechanics use WD40 for cars. Mucus membranes, other tissues and joints that are properly lubricated last longer, get damaged less.
So if you tend to produce too much mucus such as runny noses, wet phlegmy coughing, or in the bowels, chances are you are deficient in Vitamin A, beta-carotene, iodine, and Vitamin C, all essential skin foods.
If you see a lot of mucus coming out in enemas, it may be that you have parasites, lesions (open sores like in Crohn's Disease), diverticulitis or mucoid placque, and the intestines are trying to protect their delicate tissues from these irritating factors that tend to compromise the skin, and to keep toxins from entering the bloodstream.
If you have a dry, hacking cough, then you've overdosed on Vitamin A somehow. The most common (and practically the ONLY way) to overdose on A is to take vitamin supplements rather than cod liver oil or cleansing carrot salad.
Hope that helps someone! :)