2.1 Carbohydrates: Structure, Function, and Metabolism

Carbohydrate Basics and Structure

Carbohydrates are your body's primary energy source, fueling everything from brain function to physical activity. They come in forms ranging from simple sugars to complex chains, and understanding the differences between those forms is key to understanding how they affect your nutrition and health.

Beyond energy, carbs regulate blood sugar, spare protein for other jobs, and support digestive health through fiber. How your body breaks down and absorbs carbs directly shapes their impact on blood sugar and overall health.

Types of Carbohydrates

Carbohydrates are classified by how many sugar units they contain and how those units are linked together.

Monosaccharides are the simplest form: single sugar units that can't be broken down further. The three you need to know are glucose (the body's preferred fuel), fructose (found in fruit and honey), and galactose (part of milk sugar).

Disaccharides are two monosaccharides bonded together. Your body has to split them apart before it can absorb them.

• Sucrose = glucose + fructose (table sugar)
• Lactose = glucose + galactose (milk sugar)
• Maltose = glucose + glucose (found in germinating grains)

Polysaccharides are long chains of monosaccharides, sometimes hundreds or thousands of units long. These are the complex carbs.

• Starch is how plants store energy. It comes in two forms: amylose (linear chains) and amylopectin (branched chains). Foods like potatoes, rice, and bread are rich in starch.
• Glycogen is how animals (including you) store energy, mainly in the liver and muscles. It's highly branched, which allows for rapid breakdown when you need quick energy.
• Fiber is plant-based carbohydrate that human enzymes can't digest. Soluble fiber (found in oats, beans, and fruits) dissolves in water and helps manage cholesterol. Insoluble fiber (found in whole grains and vegetables) adds bulk to stool and promotes regular digestion.

What makes these types structurally different comes down to the number of sugar units, the types of bonds connecting them, and how much branching occurs in the chain.

Functions of Carbohydrates

• Energy provision: Carbs are the body's preferred fuel, providing 4 kcal per gram. They're broken down quickly, making them especially important during high-intensity activity.
• Blood glucose regulation: Your body works to keep blood sugar within a stable range. Carb intake directly influences this balance.
• Protein sparing: When you eat enough carbs for energy, your body doesn't need to break down protein for fuel. That frees protein up for building and repairing tissues.
• Central nervous system function: Your brain runs almost exclusively on glucose, using about 120 grams per day. This is why very low blood sugar can cause confusion and dizziness.
• Dietary fiber benefits: Fiber promotes digestive health, helps manage blood cholesterol levels, and contributes to feelings of fullness after meals.
• Glycoprotein formation: Carbohydrates attach to proteins on cell surfaces to form glycoproteins, which play roles in cell recognition and immune function.

Carbohydrate Digestion and Absorption

Carb digestion is a step-by-step process that breaks polysaccharides and disaccharides down into monosaccharides, the only form your body can absorb.

1. Mouth: Salivary amylase begins breaking starch into smaller fragments as you chew. This is why a cracker starts to taste slightly sweet if you hold it in your mouth.

2. Stomach: Very little carb digestion happens here. The acidic environment inactivates salivary amylase.

3. Small intestine: This is where most digestion occurs. Pancreatic amylase continues breaking starch into maltose and short chains. Then brush border enzymes (maltase, sucrase, lactase) on the intestinal lining split disaccharides into individual monosaccharides.

4. Absorption: Monosaccharides cross the intestinal wall into the bloodstream.

   • Glucose and galactose are absorbed via sodium-dependent active transport (they need energy and a sodium co-transporter to get across).
   • Fructose is absorbed via facilitated diffusion (it crosses through a carrier protein but doesn't require energy).

5. Transport: All absorbed monosaccharides travel to the liver through the portal vein.

6. Storage: The liver converts galactose and fructose into glucose. Excess glucose is stored as glycogen in the liver and muscles. Once glycogen stores are full, additional excess is converted to fat.

Glycemic Index and Blood Sugar

The glycemic index (GI) ranks foods on a scale of 0 to 100 based on how quickly they raise blood glucose compared to pure glucose (which scores 100).

• Low GI: below 55 (e.g., most beans, non-starchy vegetables, steel-cut oats)
• Medium GI: 56 to 69 (e.g., brown rice, whole wheat bread)
• High GI: 70 and above (e.g., white bread, sugary cereals, white potatoes)

Several factors affect a food's GI: the degree of processing, fiber content, and whether fat or protein is eaten alongside it. A whole apple has a lower GI than apple juice because the fiber slows digestion.

High-GI foods cause rapid blood glucose spikes followed by sharp drops. Low-GI foods produce a more gradual, sustained rise. This distinction matters for diabetes management, weight control, and cardiovascular health.

Glycemic load (GL) gives a more complete picture because it accounts for both the GI and the amount of carbohydrate in a typical serving:

$GL = \frac{GI \times \text{grams of carbohydrate per serving}}{100}$

For example, watermelon has a high GI (around 76) but relatively little carbohydrate per serving, so its GL is low. This makes GL a more practical tool for evaluating how a real portion of food will affect your blood sugar.