Dietary Guidelines for Americans
The 2010 edition "provides authoritative advice for people two years and older about how good dietary habits can promote health and reduce risk for major chronic diseases." This is a joint publication from the Department of Human Services (HHS) and the US Department of Agriculture (USDA).
Healthy People 2020
From the Office of Disease Prevention and Health Promotion of the U.S. Department of Health and Human Services, the full-text is available at this site. There are also updates.
Office of Dietary Supplements
This is a collaboration between the NIH Office of Dietary Supplements and the National Library of Medicine. It allows users to limit the over 20 million citations in PubMed to those that are dietary supplement-related. There are many other useful resources linked from here, including the Computer Access to Research on Dietary Supplements (CARDS) database of current and ongoing dietary supplement research.