Most carbohydrates are naturally occurring in plant-based foods, such as grains. Food manufacturers also add carbohydrates to processed foods in the form of starch or added sugar. Common sources of naturally occurring carbohydrates include: 1  Fruits. 2  Vegetables. 3  Milk. 4  Nuts. 5  Grains.6  Seeds.7  Legumes.ood manufacturers also add carbohydrates to processed foods in the form of starch or added sugar. Common sources of naturally occurring carbohydrates include: 1  Fruits. 2  Vegetables. 3  Milk. 4  Nuts. 5  Grains. 6  Seeds. 7  Legumes.