The US Department of Agriculture has an extensive database of the nutrient constituents of over 8,000 foods. The amount of protein, carbohydrates, fibre and fat in food can be accessed as well as the individual fatty acids, amino acids, minerals and vitamins.
The Food and Nutrient Information Center provides links to the Dietary Reference Intake tables that are developed by the Institute of Medicine’s Food and Nutrition Board.
There are also many other resources including research data and information regarding the food intake of US citizens.