What Are Carbohydrates? Fiber, Starch, and Sugar Explained
What Are Carbohydrates? Fiber, Starch, and Sugar Explained
If carbohydrates feel confusing, you are not alone. They are often oversimplified, misunderstood, or talked about as if they are optional. In reality, carbohydrates are one of the six classes of essential nutrients and play a central role in how the body functions every single day.
This post builds on the earlier discussions about carbohydrates versus sugar and the overview of essential nutrients. Here, the goal is to back up, break things down further, and explain what carbohydrates actually are, where they come from, how the body uses them, and why cutting them too low can have consequences.
What Are Carbohydrates, Chemically Speaking?
Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen. You will often see this abbreviated as C-H-O. That chemistry matters because it explains how carbohydrates provide energy. [Note: In this context, the term organic in the world of chemistry means something that contains carbon.]
Carbohydrates provide 4 calories per gram. Calories are simply a unit of energy. They are not a moral judgment, a scorecard, or a measure of willpower. Calories represent the energy the body gets from food and uses to fuel basic functions, movement, thinking, digestion, and everything else that keeps us alive.
To put this into perspective, a teaspoon of table sugar contains about 4 grams of carbohydrate. Since carbohydrates provide 4 calories per gram, that teaspoon provides about 16 calories. The same math applies whether the carbohydrate comes from sugar, starch, or fiber that contributes calories. For example, say a serving of plain Greek yogurt with 7 grams of carbohydrate has 28 calories from carbs (7 grams x 4 calories per gram = 28 calories), and three strawberries have about 3 grams of carbohydrates, so that would be 12 calories from carbs (3 grams x 4 calories per gram = 12 calories). Of course, the yogurt and strawberries will vary in their amount, and the yogurt also has protein and maybe fat, but you get the idea here. And, we’ll get to fiber in another post.
For comparison:
Carbohydrates provide 4 calories per gram
Protein provides 4 calories per gram
Fat provides 9 calories per gram
Alcohol also provides calories, but it is not considered an essential nutrient.
Where Do Carbohydrates Come From?
Carbohydrates come from plant foods and milk. ALL plant foods are foods that come from a plant. There is no exception here.
Plants make carbohydrates through photosynthesis. They take carbon dioxide, water, and sunlight and convert them into carbohydrates. That is why any food of plant origin contains carbohydrates. Fruits, vegetables, grains, beans, lentils, nuts, seeds, and foods made from them all contain carbohydrates in some form. Some have more, some have less. Some include protein and fat, such as nuts and seeds, while others do not. Or at least not significant amounts, like fruits and vegetables.
Milk also contains carbohydrates in the form of lactose. Lactose is the natural sugar found in the milk of lactating mammals, including humans. Cow’s milk, yogurt, and other dairy foods contain carbohydrates because milk contains lactose. As noted previously, lactose is made from the two monosaccharides glucose and galactose.
Foods that do not contain carbohydrates include meat, poultry, seafood, and eggs. These foods are made up of protein and fat, along with vitamins and minerals, but they do not provide carbohydrates.
This is an important point. Unless someone is eating zero plant foods and zero dairy foods, they are consuming carbohydrates. Carbohydrates are not limited to bread, pasta, rice, or potatoes. They are far more widespread than that. So, saying, “I’m avoiding carbs, so I will just have a salad,” is not avoiding carbs. In fact, it is likely mostly carbohydrates.
The Three Types of Carbohydrates
Carbohydrates are divided into three categories: starch, fiber, and sugar. All three are carbohydrates, but they behave differently in the body. Keep in mind that carbohydrates are a nutrient and not a food group.
Starch
Starch is made up of long chains of glucose molecules. It is found primarily in grains, beans, lentils, peas, potatoes, corn, and other starchy vegetables.
During digestion, starch is broken down into individual glucose molecules. This process takes time, which is why starch-containing foods tend to provide longer-lasting energy compared to foods that contain mostly sugar.
Sugar
Sugars are smaller carbohydrate units. They include glucose, fructose, and galactose, as well as combinations of these sugars like sucrose and lactose. See the post Carbohydrates vs Sugar: What’s the Difference and Why It Matters.
Sugars occur naturally in foods such as fruit, milk, and yogurt. They can also be added during processing or preparation. Regardless of the source, sugar is still a carbohydrate and still provides energy. See the post Natural vs Added Sugars for a more detailed explanation.
During digestion, sugars are broken down into their individual monosaccharides. Fructose and galactose are converted into glucose in the body so they can be used for energy.
Fiber
Fiber is also a carbohydrate, but it behaves differently. Humans cannot digest fiber into glucose. Instead, fiber passes through the digestive tract, where it plays important roles in digestion, gut health, blood sugar regulation, and cholesterol management.
Fiber deserves its own deeper discussion, which will be coming next. For now, it is important to know that fiber comes from plant foods and that processing can reduce fiber content.
What Do Carbohydrates Do in the Body?
Why do we need carbohydrates? Remember, they are one of the six essential nutrients. Carbohydrates provide energy for all cells, with a special role for the brain and muscles. Glucose is the primary fuel source for the brain, red blood cells, and many tissues.
The body stores carbohydrates as glycogen in the liver and muscles. Liver glycogen helps maintain blood sugar between meals. Muscle glycogen provides fuel during physical activity.
However, the body can only store a limited amount of glycogen. Once those stores are filled, additional carbohydrates must be used for immediate energy or stored for later use. Because glycogen stores are limited, carbohydrates need to be replenished regularly, often daily.
Digestion, Blood Sugar, and Hormones
When carbohydrates are digested, starch and sugars are broken down into glucose. Glucose enters the bloodstream, raising blood sugar levels.
Insulin is the hormone that not only allows glucose to move from the bloodstream into cells, where it can be used for energy or stored as glycogen. It is absolutely needed. This is precisely what insulin is supposed to do. Without insulin, glucose would remain in the blood, and cells would be unable to access their primary fuel. This would not be good. We would have difficulty functioning.
Glucagon, another hormone, works alongside insulin but facilitates the release of glucose into the cells. When blood sugar drops, glucagon signals the liver to release stored glucose into the bloodstream.
Insulin is often blamed for fat storage or weight gain, but this is an oversimplification. Insulin’s job is to manage energy use and storage. Weight gain occurs when total energy intake consistently exceeds energy needs, regardless of whether the calories come from carbohydrates, fat, or protein. In other words, excess calories, irrespective of the source, are stored as fat or the storage form of energy.
How Much Carbohydrate Do We Need?
The minimum recommended carbohydrate intake is 130 grams per day, based on the brain's glucose needs. Most people will need more than this, especially if they are physically active.
Getting too few carbohydrates can lead to low energy, reduced exercise performance, difficulty concentrating, and increased reliance on protein and fat for energy.
If we consistently take in less, our body will break down protein to make glucose, and most people don’t want that. In fact, I have yet to meet someone who wants that. Some fat will be used, but not as much as people would like or as much as they think. That just isn’t how it works.
Getting too many carbohydrates leads to the same outcome as getting too many calories from any source. Excess energy intake is stored, regardless of whether it comes from carbohydrates, fat, or protein.
One thing that often confuses people, or they don’t know about, is the relationship between carbohydrates and water. For every gram of carbohydrate stored as glycogen, the body stores about 3 to 4 grams of water. When carbohydrate intake drops significantly, glycogen and its associated water are lost. When carbohydrate intake increases, glycogen and water are restored. This explains much of the rapid weight change people notice with low-carbohydrate diets and when carbohydrates are reintroduced.
What’s Next: Fiber
Carbohydrates are often reduced to a single idea, but they are far more complex and far more important than that. Understanding starch, sugar, and fiber helps create a clearer picture of how carbohydrates support health.
Next, the focus will shift to fiber, why it matters, how much we need, and how to get enough without turning eating into a math problem.
