Contribution of Carbohydrates in total Dietary consumption. Carbohydrates are the main source of energy in the diet (55-75%) for most people. Grain products, tubers, roots and some fruits are rich in complex carbohydrates. Generally, they need to be cooked before they are fully digestible. Sugars usually increase the acceptability and energy density of the diet and total sugar intake is often inversely related to total fat intake. Moderate intakes of sugar are compatible with a varied and nutritious diet.