While dieting it is important to maintain not only protein intake but mineral and vitamin intake.  There are two classes of vitamins water soluble and fat soluble.  Water soluble vitamins like vit C taken in high doses are excreted in the urine and if you take high amounts the excess is not stored for a long period.  Fat soluble vitamins are stored in fat cells and in the liver.  Taking too much fat soluble vitamins can be unhealthy and unnecessary. 

Most people who have normal gastrointestinal tracts only need to take a multivitamin with iron and a calcium with vit d supplement usually 600mg tablets.  There are many good cheap vitamins available. I don't have any favorites.  If you have had gastric bypass surgery you need to take extra vitamins everyday and it is important to have vit B12, vit D, Iron and Vit A levels measured at least 2 times during the first year after surgery and than every year after that once per year.

When you take fat soluble vitamins it is important to have some fat in the food taken at the same time.  This will help with absorption. 

There have been changes in the amount of Vit D that people should have stored.  The recommendation is to take two calcium and vit D supplements 600mg per day.  It is also important to get sun exposure even during the winter.