Carbohydrates, commonly shortened to carbs, are prevalent in most people’s diets worldwide. Because they are so prevalent, people have many questions. What are carbs? What foods have carbs in them? Should I be concerned about carbs?

Is Sugar a Carb?

When many people talk about carbs, they typically mean bread, grains, and maybe starchy vegetables like potatoes. Undoubtedly, “low-carb” diets have conditioned us to associate these foods with carbs. But carbohydrates aren’t types of foods; they’re just a type of molecule in foods.

The carbohydrate molecules are sugars and long chains of sugars. The three main sugars in the diet are glucose, fructose, and galactose. These are like the chain links. Two sugars can link together and form a pair; many sugars can link together to make a long chain.

Glucose is a part of all starchy foods, table sugar, milk sugars, and fiber. Glucose from our diet makes it into our blood and flows to all our cells for energy. Sugar is also stored in our muscles, liver, and central nervous system for physical exercise and fasting between meals. When people with diabetes test their blood sugar level at home, they check their glucose level.

Fructose is primarily found in fruit and fruit juice but appears in many plant-based foods. We see it in table sugar, natural syrups like honey and agave, and sugar-sweetened foods. Anything with table sugar contains fructose.

The final type of sugar is galactose. This sugar is often associated with dairy because galactose ends in -lactose, but many foods contain galactose. This sugar does not trigger symptoms for those with lactose intolerance.

These three sugars require no digestion; they are absorbed in our digestive tracts. Therefore, a diet high in these three simple sugars or sugar pairs can quickly absorb and raise blood sugar. Whether simple sugars cause problems will depend on your body, the food you eat alongside the sugar, and your overall health. However, eating too much simple sugar could be a problem for anyone, especially those with diabetes and pre-diabetes.

Lactose and Other Sugar Pairs

The sugar building blocks can exist on their own. However, in our food, the sugars are often paired up. When sugars pair up, they form a chemical bond and become a new disaccharide molecule. Most of the sugar pairs include glucose.

Glucose + Glucose = Maltose or Trehalose

Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

You’ve probably heard of lactose. That’s the name for the sugar pair made from glucose and galactose. We find lactose in dairy products like milk, cheese, and yogurt.

To digest lactose, you need to make a specific enzyme. Unfortunately, some adults’ bodies can’t make the enzyme or can’t make enough of it. Without that enzyme, the lactose remains undigested until it runs into our gut bacteria. It’s common for this group of people to experience bloating, gas, diarrhea, or cramps after eating lactose. We call those who get these symptoms: lactose intolerant.

Dairy allergies are among the top food allergies in the USA however, lactose intolerance is not an allergy. The best way to manage lactose intolerance is by avoiding lactose in the diet. An alternative approach is to supplement with the enzyme lactase at every meal. Some people may even try a combination of these two strategies.

Complex Carbohydrates

Besides the sugars and the sugar pairs, there are long chains of sugars called complex carbohydrates. The building block sugars glucose, fructose, and galactose make up these chains.

Complex carbs are the most common form of carbohydrate in our food. You’ll find complex carbs in grains, bread, beans, starchy vegetables, and more. These are the things people generally know to avoid if they are eating “low-carb.” We classify the complex carbs that give us energy as starch or glycogen.

Starch is probably the most familiar complex carbohydrate. Starch is made of glucose, found in plant-based foods. Starch is sometimes one long chain of glucose (amylose), and other times it’s a chain with branches similar to a tree (amylopectin). Both are complex because they are long chains of sugars.

Glycogen is found in animal products like liver and shellfish. It has branches like amylopectin starch, although it has even more branches. Interestingly, we can make glycogen and use it as a storage form of glucose. When our body needs energy, we can pluck the glucose off the many branches of glycogen and convert them to energy.

Foods with Carbs


Serving Size

Total Carb (g)

Simple + Complex Carb (g)

Apple 1 medium (3″ diameter) 25.1 20.7
Banana 1 medium (7″-7 ⅞” long) 27.0 23.9
Watermelon 1 cup diced 11.5 10.9
Grapes 1 cup 15.8 15.0
Strawberries 1 cup, halves 11.7 8.7
Avocado, California 1 medium 11.8 2.6
Carrot 1 medium 5.8 4.1
Sweet Potato, no skin ½ cup, mashed 29.1 25.0
Potato, boiled ½ cup, mashed 18.4 16.8
Broccoli, boiled ½ cup chopped 5.6 3.0
White bread 2 slices 24.8 23.6
Wheat Bread 2 slices 23.8 22.9
White Rice 1 cup 41.2 39.8
Brown Rice 1 cup 45.8 42.3
Oats, cooked 1 cup 31.8 4.0
Corn, yellow 1 cup 123.0 110.9
Pasta, spaghetti 1 cup, cooked 43.2 40.7
Quinoa 1 cup, cooked 39.4 34.2
Wheaties ¾ cup 21.8 19.1
Black Beans 1 cup 40.8 25.8
Lentils, cooked 1 cup 39.9 24.3
Low-Fat Yogurt, fruit w/10 g protein 1 cup 46.7 46.7
Skim Milk 1 cup 12.0 12.0
2% Milk 1 cup 12.3 12.3
Whole Milk 1 cup 12.8 12.8
Chips Ahoy, original 3 cookies 22.0 21.0
Coke-Cola 20 oz bottle 65.0 65.0
Table Sugar 1 tsp 4.0 4.0
Honey 1 tbsp 17.3 17.3

Carbs & Energy

Carbohydrates are one of our key sources of energy. Calories are a measurement of energy, so more calories means more energy. We use that energy to move and complete many other biological processes.

The body uses energy for:

    • temperature regulation
    • digestion
    • brain functioning
    • cellular functioning
    • hormonal responses

Carbohydrates are one of our fundamental sources of energy. That’s because certain parts of our bodies prioritize using carbs for energy instead of fat or protein. So even if we aren’t eating carbohydrates, our bodies will produce them using pieces of fat, specific proteins, and lactate.

One organ that prioritizes carbs is our brain. Even in periods of starvation, our brains will choose to use carbohydrates for energy. In this scenario, the rest of the body will utilize another substance for energy called ketones. A high level of ketones in the blood is dangerous, which is why a diet that increases ketone production involves close monitoring of ketone levels.

One way to understand the energy provided by carbohydrates, fats, and proteins is to think about fueling your car. In this example, carbs are fuel to your body like gasoline is fuel to your car. Fuel is necessary to energize the car and keep it running. However, your car doesn’t get a turbo boost by overfilling the tank, and typically you won’t either. So, like the excess fuel spilling onto the ground, extra food must go somewhere. That excess ends up in fat tissue.

Carbs aren’t very dense in calories. We get about four calories for every gram of carbohydrate in our diet. That’s less than half of the energy we get from a gram of fat. However, it is easier to overeat when we concentrate on carbs as table sugar. It’s even easier when those sugars are in a beverage.

Digesting & Absorbing Carbs

We begin digesting our carbs in the mouth. Not just by chewing but by mixing the chewed food with our saliva. Our saliva contains enzymes that specifically target carbohydrates. When we chew, our salivary glands can secrete saliva more efficiently. We mix in the enzymes with the chewed-up food, helping us digest.

When we swallow food, it travels to the stomach, where stomach acid deactivates the carb-specific enzymes. The stomach is a focal point for protein digestion, not carbohydrates. However, once the food leaves the stomach and enters the intestine, the pancreas secretes more enzymes that can break down the carbs.

Carbs continue to break down as the food moves further through the digestive tract. Bacteria is much more abundant in the large intestine, also known as the colon. The bacteria will consume any undigested, unabsorbed carbs, producing helpful products as well as disruptive products like gas. Individuals respond differently to carbs, specifically fiber, because their digestive bacteria are different. Those with symptoms like gas may need time for their bacteria to balance out. If diet and time don’t work, other solutions may be necessary.

The two most common issues with carbohydrate digestion are lactose intolerance and fiber issues. While lactose intolerance is a lifelong problem with digestion, issues with fiber can improve over time. Slowly eating more fiber and working with a professional can help mitigate symptoms.

Fiber and the Microbiome

There are some carbohydrate chains that we can’t digest. However, bacteria love some of these chains. For example, beans have complex carbohydrates that we don’t digest. The bacteria eat these carb chains and produce gas, hence the connection between beans and gas.

One common type of fiber is cellulose, a carb that humans cannot digest. Again, bacteria end up eating the undigested fiber as food. Although cellulose isn’t strongly associated with causing gas, it can cause other gastrointestinal irritation, especially when changing to a high-fiber diet too quickly.

The bacteria in our gut can create more waste products than gas. However, some of the waste products are beneficial to us. For example, a bacterial colony can produce vitamins and healthy short-chain fatty acids depending on the species and variants. This is one reason why fiber is so vital to our health.

Maintaining a healthy microbiome is like having a supplement production chain inside your body.

In addition to producing healthy compounds, the microbiome can protect us from infectious pathogens. Healthy microbiomes can trap pathogens and signal to the immune system that there’s a foreign invader. On a bigger scale, the healthy microbiome naturally creates an environment in the intestines that is difficult for many pathogenic bacteria to survive. So, if you support your microbiome, you also support your immune system.

When Carbs Become Fat

What is the connection between carbs and the accumulation of body fat? Most simply, it’s when we over-consume foods. This is not only because of fatty foods; it can be the overconsumption of carbs, fats, protein, and alcohol because our bodies can convert any of these substances into fat molecules.

Our bodies constantly burn energy. The speed at which we burn energy changes throughout the day. Still, when we eat a meal, we’ll have excess energy available. The body can store the excess energy from carbohydrates as glycogen or fat molecules. We can only store so much energy as glycogen. However, we can store an unlimited amount of fat.

Here’s how it works:

    1. After eating, the cells in our body will turn carbs into energy . . . until the energy stores are replenished.
    2. Extra carbs will be sent to storage in the liver and the muscles as glycogen . . . until the storage limit is reached.
    3. If there are even more excess carbs, the liver can use some extra energy to turn them into saturated fat molecules. Since the cells are already energized, the fat will be transported and stored in our fat tissue.

In short, an excess of carbohydrates/sugars from a meal or snack will energize us, fill up carbohydrate stores, and convert to fat for storage in fat tissue (adipose). That’s how sugar, bread, pasta, potatoes, etc., can become fat.

Remember, this is only a problem when we’re out of balance and when we consume more than we need. It could be a slight excess over a long period or a huge meal; either can cause the body to convert carbs into fat. Learning to pay attention to hunger and fullness cues is often the first step to balanced intake.

Keto, Low-Carb, & No-Carb Diets

Eating low-carb diets for health has been around for a surprisingly long time. The same can be said about the Ketogenic diet, a diet pattern that causes the body to use ketones for energy. Many use these diets for weight loss, but it’s not a guaranteed result.

Our body needs energy in the form of calories to sustain itself. Cutting out some or all carbohydrates leaves a deficit in energy intake, which leads to weight loss. However, if a person eats enough protein and fat to replace the energy lost from carbs, they won’t lose any weight. It all depends on the energy balance in the body.

Some innate characteristics of no-carb keto diets do support weight loss. One key factor is the difficulty of eating enough protein and fat to replace the cut carbs. Many people will have to double the protein and fat they usually eat to sustain body weight. This can be difficult because protein and fat are very filling. This effect can make no-carb keto diets easier for someone who usually feels hungry on a weight-loss diet.

So, are low-carb diets going to help you lose weight? Keto? Only if you’re not consuming enough calories to sustain your body weight.

Notice: compared to other common diets, there can be greater risks when eating a ketogenic diet. Certain people are at greater risks than others. It’s important to know your individualized risks and needs in order to manage the diet in a safe manner.

Even without carbs, our body still uses sugar for energy. Like making fats out of carbohydrates, we can make carbohydrates out of fats, protein, and other substances. If we don’t eat carbs, we will make glucose to refill our glucose stores. In the end, those glucose stores help balance our blood sugar despite eating no sugars. The body will always attempt to find balance.

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I am a licensed, registered dietitian and nutritionist who helps people change their lives.

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