All the nutritional and food composition data stems from the National Nutrient Database for Standard Reference of the USDA (United States Department of Agriculture).
The exact reference: US Department of Agriculture, Agricultural Research Service, Nutrient Data Laboratory. USDA National Nutrient Database for Standard Reference, Release 28. Version Current: September 2015.
You can find more information about the database on the USDA website: https://ndb.nal.usda.gov/ndb/.
About interpretations of the data and calculations, see further reading