Dennis Gastineau started giving blood regularly when he was in medical school in the 1970s. The $25 he received bought almost enough groceries for a week. Now, it just seems like the right thing to do. It may also be bad for his health. Gastineau, who happens to be a hematologist, is among the 2.4 million donors who risk silent damage as a result of frequent giving. U.S. government research published last year found this group iron-deficient, which can lead to fatigue, compromised mental function and eventually anemia. Now, iron levels are being examined as part of an $87.2 million study the U.S. is funding on blood donation and transfusion safety.