It rests on a database composed of the USDA Standard Reference and information from food companies. As the info on the tool indicates, "the data should not be taken as an exact representation of the nutrient content of your diet. The numbers used for analysis are averages and there is great variance in nutrient content for each food item in the database. The nutrient data may not be complete for every food listed in NAT as there are still gaps in our knowledge of food composition."
Even with those caveats, nutrition uber-geeks may like having this in their toolkit!
The link is here: http://bit.ly/15Km5bY
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