Carbohydrates are the primary energy source for the human body, providing 4 kcal per gram. They are organic compounds composed of carbon, hydrogen, and oxygen and are classified by chemical structure (simple vs complex) and digestibility.
Chemical Classification
Monosaccharides are single sugar units. Glucose (dextrose) is the primary fuel for all cells and the preferred energy source for the brain. Fructose is the sweetest natural sugar, found in fruits and honey. Galactose is part of lactose (milk sugar). These monosaccharides are absorbed directly into the bloodstream.
Disaccharides are two sugar units linked by glycosidic bonds. Sucrose (glucose + fructose) is common table sugar and is widely used as a sweetener. Lactose (glucose + galactose) is the primary carbohydrate in milk. Maltose (glucose + glucose) is produced during starch digestion and fermentation. Disaccharides must be hydrolyzed by specific brush border enzymes in the small intestine before absorption.
Oligosaccharides contain 3–10 monosaccharide units. Raffinose, stachyose, and verbascose are found in legumes and are fermented by gut bacteria, producing gas. Fructooligosaccharides (FOS) are prebiotics that stimulate beneficial gut bacteria.
Polysaccharides contain many monosaccharide units. Starch is the storage form of carbohydrates in plants, consisting of amylose (linear) and amylopectin (branched). Glycogen is the storage form in animals, stored in the liver and skeletal muscle. Dietary fiber — cellulose (insoluble), hemicellulose, pectin, beta-glucans (soluble) — is resistant to human digestive enzymes.
Digestion and Absorption
Carbohydrate digestion begins in the mouth with salivary alpha-amylase, which hydrolyzes starch into shorter polysaccharides and maltose. In the small intestine, pancreatic alpha-amylase continues starch digestion. The brush border enzymes (maltase, sucrase, lactase, isomaltase) hydrolyze disaccharides into monosaccharides at the enterocyte surface. Glucose and galactose are absorbed via SGLT1 (sodium-dependent transporter). Fructose is absorbed via GLUT5 (facilitated diffusion). All monosaccharides enter the portal circulation and travel to the liver.
Lactose intolerance results from lactase deficiency, leading to undigested lactose reaching the colon, where it is fermented by bacteria, producing gas, bloating, abdominal pain, and diarrhea. Primary lactose intolerance is genetically programmed (lactase activity declines after weaning in most populations). Secondary lactose intolerance occurs after gastrointestinal injury. Prevalence is highest in Asian, African, and Native American populations.
Metabolic Pathways
Glycolysis converts glucose to pyruvate in the cytoplasm, producing 2 ATP per glucose molecule. Under aerobic conditions, pyruvate enters the mitochondria and is converted to acetyl-CoA, which enters the Krebs cycle, generating ATP through oxidative phosphorylation (total yield: approximately 32 ATP per glucose). Under anaerobic conditions, pyruvate is converted to lactate.
Glycogenesis is the synthesis of glycogen from glucose, stimulated by insulin after meals. Glycogenolysis is the breakdown of glycogen to glucose, stimulated by glucagon and epinephrine during fasting or exercise. Liver glycogen maintains blood glucose; muscle glycogen fuels contraction.
Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (lactate, amino acids, glycerol) during prolonged fasting or starvation. It occurs primarily in the liver.
Glycemic Response
The glycemic index (GI) classifies carbohydrate-containing foods by their effect on blood glucose. High-GI foods (white bread, white rice, sugary drinks, potatoes) cause rapid glucose spikes. Low-GI foods (legumes, whole grains, most vegetables, intact fruits) produce gradual, sustained glucose rises. Glycemic load (GL) adjusts GI for the amount of carbohydrate per serving (GL = GI × grams carbohydrate / 100).
Lowering dietary GI or GL may improve glycemic control in diabetes, reduce postprandial hyperglycemia, and modestly improve cardiovascular risk factors. However, overall diet quality — fiber content, processing, nutrient density — is more important than GI alone.
Dietary Recommendations
The Dietary Guidelines for Americans recommend that carbohydrates provide 45–65% of total daily calories. Added sugars should be limited to less than 10% of total calories. Dietary fiber intake should be 25–30 g per day for adults. Most Americans consume insufficient fiber (average 15 g/day).
Food Sources
Nutrient-rich carbohydrate sources: whole grains (oats, quinoa, brown rice, whole wheat, barley), legumes (beans, lentils, chickpeas, peas), vegetables (starchy — potatoes, sweet potatoes, corn; non-starchy — leafy greens, broccoli, peppers), fruits (berries, apples, citrus, melon, bananas), and dairy (milk, yogurt). These provide not only carbohydrates but also fiber, vitamins, minerals, and phytochemicals.
Refined and processed carbohydrates: white bread, white rice, refined pasta, pastries, cookies, chips, and sugary cereals. These are low in fiber and nutrients and are rapidly digested.
Summary
Carbohydrates are essential macronutrients providing energy and structural roles. They are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Digestion begins in the mouth and is completed in the small intestine. Metabolic pathways — glycolysis, glycogenesis, glycogenolysis, gluconeogenesis — maintain glucose homeostasis. Complex, fiber-rich carbohydrates from whole foods are superior to refined carbohydrates for health. Added sugars should be limited.