Carbohydrate Metabolism Basics

Carbohydrate Structure and Types

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen atoms in the ratio CH₂O. Dietary carbohydrates range from simple sugars to complex polymers of glucose. The body can only directly utilize glucose and a few other simple sugars; all dietary carbohydrates must be broken down into basic units during digestion.

Simple carbohydrates include monosaccharides (single sugar units) like glucose and fructose, and disaccharides (two sugar units) like sucrose and lactose. Complex carbohydrates include polysaccharides such as starch (plant storage) and glycogen (animal storage), as well as fiber, which provides structural support to plants.

Digestion and Glucose Absorption

Carbohydrate digestion begins in the mouth with salivary amylase, an enzyme that breaks down starch into smaller glucose chains. Digestion continues in the small intestine where additional enzymes break carbohydrates into glucose units. Glucose is then absorbed through the intestinal epithelium into the bloodstream.

The rate of glucose absorption varies based on carbohydrate type, food preparation, food matrix, individual digestive factors, and concurrent food consumption. Whole grains and foods containing fiber typically produce slower glucose absorption, while simple sugars and refined carbohydrates produce more rapid glucose spikes. This variation affects blood glucose patterns and insulin responses.

Glucose Regulation

Once absorbed, glucose enters the bloodstream, raising blood glucose concentration. The body maintains relatively stable blood glucose levels (approximately 70-100 mg/dL fasting) through hormonal regulation. Elevated blood glucose stimulates the pancreas to release insulin, a hormone that facilitates glucose uptake into cells and lowers blood glucose concentration. When blood glucose drops, the pancreas reduces insulin secretion and increases glucagon secretion, stimulating the liver to release stored glucose.

Glycolysis

Glycolysis is the metabolic pathway that converts glucose into pyruvate, a three-carbon compound. This process occurs in the cell cytoplasm and does not require oxygen (is anaerobic). Glycolysis involves ten enzymatic steps and results in the net production of two ATP molecules (the primary energy currency of cells) and two NADH molecules (electron carriers). The pyruvate produced can then enter the citric acid cycle for further energy extraction or be converted to other molecules including lactate, alanine, or acetyl-CoA.

The Citric Acid Cycle

When oxygen is available, pyruvate enters the mitochondria and is converted into acetyl-CoA, a two-carbon unit that enters the citric acid cycle (also called the Krebs cycle or TCA cycle). The citric acid cycle is a series of eight enzymatic reactions that completely oxidizes acetyl-CoA and extracts energy. This process generates electron carriers (NADH and FADH₂) that are utilized in oxidative phosphorylation and produces ATP directly.

Oxidative Phosphorylation

The electron carriers generated in glycolysis and the citric acid cycle (NADH and FADH₂) transfer electrons to the electron transport chain, a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move through the chain, energy is released and used to pump protons into the intermembrane space, creating an electrochemical gradient. ATP synthase utilizes this gradient to phosphorylate ADP into ATP. A single glucose molecule can ultimately generate approximately 30-32 ATP molecules through complete oxidation.

Anaerobic Metabolism

During high-intensity exercise when oxygen availability may be limited, pyruvate is converted into lactate rather than entering the citric acid cycle. Lactate is released into the bloodstream and can be utilized as fuel by other tissues or converted back to glucose in the liver. This allows continued ATP production during intense activity, though at lower efficiency than aerobic metabolism.

Glucose Storage

Excess glucose that is not immediately needed for energy can be stored as glycogen, a polymer of glucose units. The liver and muscles are primary storage sites. Liver glycogen is broken down and released as glucose to maintain blood glucose levels during fasting. Muscle glycogen serves as a local fuel source for muscle contraction and is not released into the bloodstream. Glycogen storage capacity is limited; once these stores are full, excess carbohydrates are converted to fat.

Gluconeogenesis

During fasting or intense prolonged activity, when blood glucose drops and glycogen stores are depleted, the liver synthesizes new glucose through gluconeogenesis. This process converts non-carbohydrate substrates including lactate, amino acids, and glycerol into glucose to maintain blood glucose levels and provide fuel for the brain and other glucose-dependent tissues.