The following Federal Government resources provide reliable, science based information on nutrition and physical activity, as well as an evolving array of tools to facilitate Americans adoption of healthy choices:

Dietary Guidelines for Americans
The New Food Guide Pyramid -plate
Physical activity guideline for Americans
Nutrition information
USDA nutrition & food information center
Food and drug administration
National Institute of Health
Let's Move (Michelle Obama)
Healthy People
US National physical activity plan
Nutrition website run by dietitians
The Academy of Dietitetics and Nutrition
Academy For Eating Disorders:   

Food fun and news

American Academy of Pediatrics