What substance is considered a major source of carbohydrates in diets?

Starch is considered a major source of carbohydrates in diets because it is a polysaccharide that serves as a primary form of energy storage in plants. Foods rich in starch include grains (like rice, wheat, and corn), legumes, and starchy vegetables (such as potatoes and peas). When consumed, starch is broken down into glucose, which provides energy for bodily functions and activities.

Both sugars and fiber are also important components of the carbohydrate category but serve different roles. Sugars (simple carbohydrates) provide quick energy, but they do not encompass the complexity of carbohydrates that starch represents. Fiber, on the other hand, is crucial for digestive health but is not a major energy source in the way that starch is. Thus, starch is the most significant contributor to dietary carbohydrates, making it the correct choice in this context.