Carbohydrates — Types, Glycemic Index, and Requirements

Carbohydrates provide 4 kcal per gram and are the preferred fuel for moderate-to-high-intensity exercise (see Lactic Acid (Glycolytic) System). They are classified by structure:

• Simple carbohydrates (sugars): monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose). Found naturally in fruit and milk, and added to processed foods.
• Complex carbohydrates: starches and dietary fiber (see Dietary Fiber and Gut Health), found in grains, legumes, and vegetables. Starches digest to glucose; fiber largely resists digestion.

Dietary carbohydrate is stored as glycogen in muscle and liver (see Glycogen — Stored Carbohydrate Fuel); excess beyond storage capacity and energy needs can be converted to fat.

Glycemic index (GI) ranks how quickly a carbohydrate raises blood glucose relative to a reference. High-GI foods (white bread, sugary drinks) spike glucose fast; low-GI foods (legumes, most whole fruit, oats) raise it gradually. Glycemic load adjusts GI for typical portion size. GI is useful context — high-GI carbs suit rapid glycogen refueling after endurance exercise, while lower-GI, fiber-rich choices favor satiety and steadier energy — but whole-diet quality matters more than any single food's GI.

Requirements for athletes (per kg body weight per day; AND/DC/ACSM joint position, 2016):

• Light/skill-based activity: 3–5 g/kg/day.
• Moderate exercise ~1 h/day: 5–7 g/kg/day.
• Moderate-to-high intensity, 1–3 h/day: 6–10 g/kg/day.
• Extreme endurance, >4–5 h/day: 8–12 g/kg/day.

General-population reference values. The Dietary Reference Intakes set the Acceptable Macronutrient Distribution Range (AMDR) for carbohydrate at 45–65% of total calories and the adult RDA at 130 g/day (175 g/day in pregnancy, 210 g/day in lactation). The Dietary Guidelines for Americans 2025–2030 give no carbohydrate percentage: they ask for 2–4 servings/day of fibre-rich whole grains, a significant cut in highly processed refined carbohydrates (white bread, packaged breakfast foods, flour tortillas, crackers), and note that some people with certain chronic diseases may do better on a lower-carbohydrate diet worked out with their health care professional (see Dietary Guidelines for Americans — Overview). The prior 2020–2025 edition used the AMDR and the 130 g RDA in its nutritional goals.

• 4 kcal/g; primary fuel for moderate-to-high-intensity work.
• Simple (sugars) vs. complex (starch + fiber); stored as glycogen.
• GI/glycemic load describe blood-glucose response; whole-diet quality matters more.
• Activity-scaled needs: 3–5 (light), 5–7 (moderate 1 h), 6–10 (1–3 h), 8–12 (extreme) g/kg/day.
• DRIs: AMDR 45–65% of calories; adult RDA 130 g/day (175 g pregnancy, 210 g lactation).
• DGA 2025–2030: no % target; 2–4 servings/day whole grains; cut refined carbohydrates.