What to do if i am wondering what people who are allergic to shellfish lack in there diet ?

Don't Worry: People who are allergic to shellfish, do not generally develop nutritional/dietary deficiencies as a result. The most common important nutrient that shellfish are rich in is iodine which is included in table salt in many countries. You can get nutrients from other foods. Since your question is not clear, let me also state that shellfish allergy is not caused by a dietary deficiency either.