What is the difference between starch and sugar?

December 4, 2025 •

4 min read

Over half of the energy in the average diet comes from carbohydrates, a category that includes both starch and sugar. But what is the difference between starch and sugar, and how does this distinction impact your body's energy levels and metabolic health?

Quick Summary

Starch and sugar are both carbohydrates, but they differ significantly in their chemical complexity, digestion speed, and effect on blood glucose. Sugar is a simple carbohydrate digested quickly, while starch is a complex carbohydrate that provides sustained energy.

Key Points

Structural Difference: Sugar is a simple carbohydrate (mono/disaccharide) with one or two molecules, whereas starch is a complex carbohydrate (polysaccharide) with many molecules chained together.
Digestion Speed: Your body digests simple sugars rapidly, causing a quick energy boost, while the breakdown of complex starches takes much longer, providing sustained energy.
Blood Sugar Impact: Sugars cause a rapid, sharp rise in blood glucose and insulin levels, while starches lead to a slower, more gradual increase.
Taste and Solubility: Simple sugars taste sweet and dissolve easily in water, while starches are not sweet and are insoluble in cold water.
Health Effects: A diet with a higher proportion of complex, slow-digesting starches is generally considered healthier for long-term metabolic health than one high in refined, fast-acting sugars.
Food Sources: Sugars are abundant in fruits, honey, and processed foods, while starches are found in grains, root vegetables like potatoes, and legumes.

Understanding the Basics: Carbohydrates

At the most fundamental level, both starch and sugar are carbohydrates, one of the three macronutrients vital for life, alongside fats and proteins. All carbohydrates are made up of basic building blocks called saccharides, or sugar units. The key distinction lies in how many of these units are linked together and how they are arranged. This difference in molecular structure is what dictates how your body processes and uses them for energy.

Sugar: The Simple Carbohydrate

Sugars are considered simple carbohydrates because they are made of just one or two sugar units. Single units are called monosaccharides, such as glucose and fructose, while double units are known as disaccharides, like sucrose (table sugar) and lactose (milk sugar). This simple structure means they are very quickly broken down during digestion.

Monosaccharides: These are the most basic sugar units and include glucose, the body's primary energy source, and fructose, found in fruits.
Disaccharides: Formed by two monosaccharides joined together. Sucrose, for example, is composed of one glucose and one fructose molecule.

Because they have a simple structure, sugars are readily soluble in water and typically taste sweet. They are absorbed almost immediately into the bloodstream, causing a rapid spike in blood glucose and a subsequent insulin response.

Starch: The Complex Carbohydrate

Starch, on the other hand, is a complex carbohydrate, or polysaccharide, made up of many thousands of glucose molecules linked together in long chains. This complex structure requires your body to work much harder to break it down. Starch is the way plants store energy and is abundant in grains, potatoes, legumes, and certain vegetables.

Resistant Starch: Some starches, known as resistant starches, are not fully digested in the small intestine and function more like dietary fiber. This slows digestion even further and provides food for beneficial gut bacteria.

Unlike simple sugars, starch is generally not sweet-tasting and is insoluble in cold water. The prolonged process of breaking down these long glucose chains means that the glucose is released into the bloodstream much more gradually.

The Digestion and Absorption Process

The human digestive system handles starch and sugar differently, which explains their distinct impact on energy levels and blood sugar regulation.

How Sugar is Digested

When you consume simple sugars, your body begins processing them almost instantly. Enzymes in your digestive tract quickly break the simple bonds, allowing the monosaccharides to be absorbed into your bloodstream within minutes. This rapid absorption leads to a quick surge of energy, but it can also cause a blood sugar crash as insulin is released to move the sugar out of the blood and into cells.

How Starch is Digested

Digestion of starch begins in the mouth, where the enzyme amylase starts to break the long chains into smaller glucose units. This process continues in the small intestine, where more enzymes work to fully dismantle the complex molecules. Because of this multi-step process, it takes significantly more time for the glucose from starch to enter the bloodstream. This results in a slower, more sustained release of energy and a less dramatic spike in blood sugar.

Comparing Starch and Sugar

To provide a clearer picture, here is a comparison table outlining the key differences between starch and sugar.


Feature	Starch	Sugar
Chemical Structure	Complex carbohydrate (Polysaccharide), long chains of glucose molecules	Simple carbohydrate (Monosaccharide or Disaccharide), one or two sugar units
Taste	Not sweet	Sweet
Solubility	Insoluble in cold water	Readily soluble in water
Digestion Speed	Slow and gradual	Fast and immediate
Energy Release	Sustained energy	Quick burst of energy
Blood Sugar Impact	Gradual increase	Rapid spike and crash
Primary Function	Energy storage in plants	Instant energy source for organisms
Food Examples	Potatoes, rice, wheat, corn, legumes	Fruits, honey, table sugar, milk

Practical Health Implications

Understanding the contrast between starch and sugar is crucial for making informed dietary choices. The glycemic index (GI) is a tool that ranks carbohydrates based on how quickly they raise blood sugar levels, and it helps illustrate this difference. Foods high in simple sugars typically have a high GI, while complex carbohydrates, especially whole-grain varieties that contain fiber, have a lower GI because they are digested more slowly.

For Diabetics: For individuals with diabetes, managing blood sugar levels is a primary concern. Opting for complex carbohydrates and starches that are digested slowly can help prevent the dramatic fluctuations in blood sugar associated with simple sugars.
For Weight Management: The rapid blood sugar spike from simple sugars can lead to overeating and weight gain. Complex carbohydrates provide a feeling of fullness and sustained energy, which helps regulate appetite. Research has also explored how different types of starches and sugars affect obesity and metabolic health.

For a deeper dive into how different carbohydrate choices can affect health outcomes, the Cleveland Clinic offers a useful overview of the function and types of carbohydrates.

Sources of Starch and Sugar in the Diet

Foods Rich in Starch:

Grains (bread, pasta, oats, rice)
Starchy vegetables (potatoes, corn, peas)
Legumes (beans, lentils)

Foods Rich in Sugar:

Fruits (contain natural fructose)
Dairy products (contain natural lactose)
Processed foods and beverages (soda, candy, pastries) with added sugars

Conclusion

In summary, while both starch and sugar are classified as carbohydrates and provide energy, their distinct molecular structures lead to vastly different effects on the body. Sugars are simple, fast-acting carbs that provide a quick energy boost but can disrupt blood sugar regulation. Starch is a complex, slow-digesting carb that offers a more stable and sustained energy release. For optimal metabolic health, it is generally recommended to prioritize complex carbohydrates and minimize intake of refined and added simple sugars, though both have a place in a balanced diet.

Frequently Asked Questions

Yes, fundamentally. Starch is a polysaccharide, which means it's a long chain of many glucose (a simple sugar) molecules linked together. Your body breaks down this starch into individual glucose units during digestion.

For most people, complex carbohydrates like starch are healthier than simple sugars, especially refined ones. The slower digestion of starch leads to more stable blood sugar levels, while the rapid spike from sugar can be less healthy in large quantities.

The body begins breaking down starch with the enzyme amylase in the mouth, continuing the complex process in the small intestine. Simple sugars, by contrast, are absorbed into the bloodstream almost immediately because they require minimal breakdown.

In terms of calories per gram, they are the same (4 calories per gram). However, because starch is digested more slowly, the energy is released over a longer period, while the energy from sugar is released very quickly.

Sweet potatoes contain complex carbohydrates (starches) and fiber, which slow down the digestive process and lead to a gradual increase in blood sugar. A candy bar, rich in simple sugars, provides a rapid blood sugar spike.

Like anything, moderation is key. A diet high in refined, processed starches (like white bread and pasta) can still cause blood sugar issues. A diet high in whole-food starches, like whole grains and legumes, is generally beneficial due to the presence of fiber and nutrients.

Natural sugars are those naturally present in foods like fruits and milk, which also contain fiber and other nutrients. Added sugars are those added to foods and drinks during processing. While chemically similar, natural sugars are part of a more nutritious package, whereas added sugars can be a source of empty calories.