What Must Be Broken Down into Simple Sugars Before the Body Can Use It?

December 12, 2025 •

4 min read

According to the Mayo Clinic, carbohydrates are the body's primary fuel source, providing energy for cells, tissues, and organs. Before the body can use it, the digestive system must break down most carbohydrates, especially complex ones like starches, into their most basic units: simple sugars.

Quick Summary

Starches and other complex carbohydrates, as well as disaccharides like sucrose and lactose, must be hydrolyzed by digestive enzymes into monosaccharides like glucose and fructose to be absorbed by the body for energy.

Key Points

Complex Carbohydrates: Starches, like those in potatoes and rice, must be broken down into simple sugars before the body can use them for energy.
Disaccharides: Double sugars such as sucrose (table sugar) and lactose (milk sugar) must be split into monosaccharides by specific enzymes.
Essential Enzymes: Digestion relies on enzymes like salivary amylase, pancreatic amylase, lactase, and sucrase to perform the necessary chemical breakdowns.
Monosaccharides Are the End Product: Glucose, fructose, and galactose are the simple sugars that are small enough to be absorbed into the bloodstream.
Fiber Is Different: Dietary fiber, a type of polysaccharide, is not digested by human enzymes and is instead fermented by bacteria in the large intestine.
Digestion Rate Varies: Complex carbohydrates provide a slower, more sustained energy release compared to the rapid absorption of simple sugars.

The Chemical Necessity of Carbohydrate Breakdown

To power all its functions, the human body primarily uses glucose, a simple sugar. However, the carbohydrates we consume come in various sizes, from simple single units to long, complex chains. The digestive system is a sophisticated chemical processing plant designed to reduce these large, complex molecules into the simple sugars that are small enough to pass through the intestinal wall and into the bloodstream.

The Hierarchy of Carbohydrates

There are three main classifications of carbohydrates that dictate how they are processed by the body:

Monosaccharides (Simple Sugars): These are the most basic units of carbohydrates and include glucose, fructose, and galactose. They are already in their simplest form and do not require further digestion. They are absorbed directly into the bloodstream in the small intestine.
Disaccharides (Double Sugars): Formed from two monosaccharide units bonded together, these sugars include sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar). They must be split into their individual monosaccharides before they can be absorbed.
Polysaccharides (Complex Carbohydrates): These are long chains of monosaccharides bonded together. Starch, found in foods like potatoes, rice, and wheat, is a common example that is broken down into glucose during digestion. Dietary fiber, another type of polysaccharide, is largely indigestible by human enzymes and passes through the system to be fermented by gut bacteria.

The Enzymatic Digestive Process

The breakdown of carbohydrates begins the moment food enters your mouth. Here is a step-by-step overview of the digestion process:

Mouth: Chewing mechanically breaks down food, and salivary amylase starts chemically breaking down starches into smaller polysaccharides and maltose.
Stomach: The acidic environment of the stomach halts the activity of salivary amylase. No significant carbohydrate digestion occurs here.
Small Intestine: The main site for carbohydrate digestion. Pancreatic amylase continues to break down any remaining starches into disaccharides and smaller units. Enzymes in the brush border lining of the small intestine then perform the final breakdown:
- Sucrase breaks down sucrose into glucose and fructose.
- Lactase breaks down lactose into glucose and galactose.
- Maltase breaks down maltose into two glucose molecules.
Absorption: The resulting monosaccharides (glucose, fructose, and galactose) are absorbed through the intestinal wall and enter the bloodstream to be used for energy or stored.

Comparison: Simple vs. Complex Carbohydrate Digestion


Feature	Simple Carbohydrates (Mono- and Disaccharides)	Complex Carbohydrates (Polysaccharides)
Structure	Single or double sugar units.	Long chains of sugar units.
Digestion Speed	Rapidly digested and absorbed.	Takes longer to break down into simple sugars.
Blood Sugar Impact	Can cause a rapid spike and crash in blood sugar levels, especially processed simple carbs.	Provides a slower, more gradual rise in blood sugar due to the longer digestion time.
Nutrient Density	Often found in less nutrient-dense foods (e.g., candy), but also in whole fruits.	Typically found in nutrient-rich whole grains, legumes, and vegetables.

Indigestible Polysaccharides and the Role of Gut Bacteria

Not all carbohydrates are destined to be simple sugars for immediate use. Dietary fiber, such as cellulose and certain starches, is a polysaccharide that the human body cannot break down. This is because we lack the necessary enzymes to hydrolyze the specific glycosidic bonds. Instead, dietary fiber passes largely intact to the large intestine where it serves a crucial purpose. There, it is fermented by beneficial gut bacteria, producing short-chain fatty acids (SCFAs) that have wide-ranging physiological benefits. Fiber also promotes regular bowel movements and can help regulate blood sugar and cholesterol levels.

If disaccharides like lactose are not properly digested in the small intestine—as is the case with lactose intolerance due to insufficient lactase enzyme—they too pass into the large intestine. The fermentation by gut bacteria leads to the production of gas and other byproducts that cause symptoms like bloating and discomfort. This highlights the necessity of proper enzymatic breakdown for a smooth digestive process.

Conclusion: The Final Breakdown

Understanding what must be broken down into simple sugars before the body can use it is fundamental to grasping how our bodies derive energy from food. The digestive process is a coordinated effort of mechanical action and specialized enzymes that dismantle complex and double sugars into the absorbable, simple sugar units of glucose, fructose, and galactose. While most starches are readily digested, certain polysaccharides like dietary fiber bypass this process, instead serving as a critical nutrient source for our gut microbiome. Ultimately, the efficiency and outcome of this digestive journey significantly influence our energy levels and overall health. For further reading on the complex process of carbohydrate digestion, authoritative sources like the National Institutes of Health (NIH) provide in-depth scientific reviews.

Frequently Asked Questions

The three main types are simple sugars (monosaccharides), double sugars (disaccharides), and complex carbohydrates (polysaccharides).

The final products are monosaccharides like glucose, fructose, and galactose, which are absorbed from the small intestine into the bloodstream.

Salivary amylase, an enzyme in your saliva, begins to break down starches in the mouth, although the bulk of digestion occurs later in the small intestine.

Humans lack the specific enzymes required to break the beta-linkages that hold cellulose molecules together. It functions as dietary fiber instead.

If not broken down, disaccharides pass to the large intestine where gut bacteria ferment them, potentially causing gas, bloating, and other discomforts, as seen in lactose intolerance.

No. Simple carbohydrates are absorbed quickly, causing a rapid energy spike, while complex carbohydrates take longer to digest, providing a more gradual and sustained energy release.

While digestion begins in the mouth, the majority of carbohydrate breakdown and the subsequent absorption of simple sugars happen in the small intestine.