Some of the standard guidelines for supplementation per the Crohn's and Colitis Foundation of America are below:
Should supplemental vitamins be taken? If so, which ones?
...vitamin B-12 is absorbed in the lower ileum. That means that people who have ileitis (Crohn's disease that affects the ileum) or those who have undergone small bowel surgery may have a vitamin B-12 deficiency because they are unable to absorb enough of this vitamin from their diet or from oral supplements. To correct this deficiency (which can be determined by measuring the amount of this vitamin in the blood), a monthly intramuscular injection of vitamin B-12 may be required. Folic acid (another B vitamin) deficiency is also quite common in patients who are on the drug sulfasalazine. They should take a folate tablet, 1 mg daily, as a supplement. For most people with chronic IBD, it is worthwhile to take a multivitamin preparation regularly. If you suffer from maldigestion or have undergone intestinal surgery, other vitamins-particularly vitamin D-may be required. Affecting as many as 68 percent of people, vitamin D deficiency is one of the most common nutritional deficiencies seen in association with Crohn's disease. Vitamin D is essential for good bone formation and for the metabolism of calcium. Supplementation of this vitamin should be in the range of 800 I.U./day, especially in the non-sunny areas of the country, and particularly for those with active disease. Together with vitamins A, E, and K, vitamin D is a fat-soluble vitamin; these tend to be less easily absorbed than water-soluble vitamins. Consequently, they may be absorbed better in liquid rather than pill form.
Are any special minerals recommended?
...iron deficiency is fairly common in people with ulcerative colitis and Crohn's colitis and less common in those with small intestine disease. It results from blood loss following inflammation and ulceration of the colon. Blood iron levels are easily measured, and if a deficiency is found (otherwise known as anemia), oral iron tablets or liquid may be given. The usual dose is between 8 to 27 mg, taken one to three times a day-depending on the extent of the deficiency and the patient's tolerance. Oral iron turns the stool black, which can be mistaken for intestinal bleeding. Other mineral deficiencies include potassium and magnesium. People may develop potassium deficiencies with diarrhea or vomiting, or as a result of prednisone therapy. Potassium supplements are available in tablet and other forms. Oral supplements of magnesium oxide may prove necessary for people who have magnesium deficiency caused by chronic diarrhea or extensive small intestinal disease, or those who have had substantial lengths of intestine removed through surgery...