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-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 fiber), 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); 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 (per kg body weight per day, ACSM/AND/DC):

• 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.

The Acceptable Macronutrient Distribution Range (AMDR) for carbohydrate is 45–65% of total calories, with the minimum to fuel the brain set near 130 g/day (RDA).

• 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.
• AMDR 45–65% of calories; RDA minimum ~130 g/day for brain glucose.

• glycogen — the stored form that fuels exercise.
• fiber — the non-digestible complex carbohydrate.
• added-sugars — limiting the simple-sugar fraction.
• carbohydrate-timing / endurance-fueling — using carbs around training.
• lactic-acid-system — carbohydrate breakdown for ATP.