Carbohydrates, or saccharides, are biomolecules. The four major classes of biomolecules are carbohydrates, proteins, nucleotides, and lipids. Carbohydrates are the most abundant of the four.

Also known as “carbs,” carbohydrates have several roles in living organisms, including energy transportation. They are also structural components of plants and insects.

Carbohydrate derivatives are involved in reproduction, the immune system, the development of disease, and blood clotting.

Fast facts on carbohydrates

  • “Saccharide” is another word for “carbohydrate.”
  • Foods high in carbohydrates include bread, pasta, beans, potatoes, rice, and cereals.
  • One gram of carbohydrate contains approximately 4 kilocalories
  • High glycemic index (GI) carbohydrates quickly enter the bloodstream as glucose
  • Switching to a low-GI diet improves the chance of a healthy weight and lifestyle

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Sources of carbohydrate include whole grains, fruit, and vegetables.

Carbohydrates, also known as saccharides or carbs, are sugars or starches. They are a major food source and a key form of energy for most organisms.

They consist of carbon, hydrogen, and oxygen atoms.

Two basic compounds make up carbohydrates:

Aldehydes: These are double-bonded carbon and oxygen atoms, plus a hydrogen atom.

Ketones: These are double-bonded carbon and oxygen atoms, plus two additional carbon atoms.

Carbs can combine together to form polymers, or chains.

These polymers can function as:

  • long-term food storage molecules
  • protective membranes for organisms and cells
  • the main structural support for plants

Most organic matter on earth is made up of carbohydrates. They are involved in many aspects of life.

There are various types of carbohydrate. They include monosaccharides, disaccharides, and polysaccharides.


This is the smallest possible sugar unit. Examples include glucose, galactose, or fructose. Glucose is a major source of energy for a cell. “Blood sugar” means “glucose in the blood.”

In human nutrition, these include:

  • galactose, most readily available in milk and dairy products
  • fructose, mostly in vegetables and fruit


Disaccharides are two monosaccharide molecules bonded together, for example, lactose, maltose, and sucrose.

Bonding one glucose molecule with a galactose molecule produces lactose. Lactose is commonly found in milk.

Bonding one glucose molecule with a fructose molecule, produces a sucrose molecule.

Sucrose is found in table sugar. It is often results from photosynthesis, when sunlight absorbed by chlorophyll reacts with other compounds in plants.


Different polysaccharides act as food stores in plants and animals. They also play a structural role in the plant cell wall and the tough outer skeleton of insects.

Polysaccharides are a chain of two or more monosaccharides.

The chain may be:

  • branched, so that the molecule looks like a tree with branches and twigs
  • unbranched, where the molecule is a straight line

Polysaccharide molecule chains may consist of hundreds or thousands of monosaccharides.

Glycogen is a polysaccharide that humans and animals store in the liver and muscles.

Starches are glucose polymers that are made up of amylose and amylopectin. Rich sources include potatoes, rice, and wheat. Starches are not water soluble. Humans and animals digest them using amylase enzymes.

Cellulose is one of the main structural constituents of plants. Wood, paper, and cotton are mostly made of cellulose.

You may have heard about simple and complex carbohydrates.

Monosaccharides and disaccharides are simple carbohydrates, and polysaccharides are complex.

Simple carbohydrates are sugars. They consist of just one or two molecules.They provide a rapid source of energy, but the consumer soon feels hungry again. Examples include white bread, sugars, and candies.

Complex carbohydrates consist of long chains of sugar molecules. Wholegrains and foods that still have their fiber in are complex carbs. They tend to fill you up for longer, and they are considered more healthful, as they contain more vitamins, minerals, and fiber. Examples include fruits, vegetables, pulses, and wholemeal pasta.

Bread, pasta, beans, potatoes, bran, rice, and cereals are carbohydrate-rich foods. Most carbohydrate-rich foods have a high starch content. Carbohydrates are the most common source of energy for most organisms, including humans.

We could get all our energy from fats and proteins if we had to. One gram of carbohydrate contains approximately 4 kilocalories (kcal), the same amount as protein. One gram of fat contains around 9 kcal.

However, carbohydrates have other important functions:

  • the brain needs carbohydrates, specifically glucose, because neurons cannot burn fat
  • dietary fiber is made of polysaccharides that our bodies do not digest

The United States (U.S.) Dietary Guidelines 2015-2020 recommend obtaining 45 to 65 percent of energy needs from carbohydrates, and a maximum of 10 percent should come from simple carbohydrates, in other words, glucose and simple sugars.

Every couple of decades, some “breakthrough” appears, and people are advised to “avoid all fats,” or “avoid carbs.”

Carbohydrates have been, and will continue to be, an essential part of any human dietary requirement.

Carbs and obesity

Some argue that the global rise in obesity is linked to a high intake of carbs. However, a number of factors contribute to this problem:

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Simple carbohydrates, such as sugar, increase the risk of obesity.

These include:

  • a reduction in physical activity
  • a higher consumption of junk food
  • a higher consumption of food additives, such as coloring, taste enhancers, and artificial emulsifiers
  • fewer hours sleep each night
  • a rise in living standards

Stress may also be a factor. One study found that the molecule neuropeptide Y (NPY), which the body releases when stressed, can “unlock” Y2 receptors in the body’s fat cells, stimulating the cells to grow in size and number.

Rapidly developing countries, such as China, India, Brazil, and Mexico, are seeing a rise in obesity, as living standards and dietary habits change.

When these populations were leaner, their diets were more carb-heavy than they are now. They also consumed more natural produce and less junk food, were more physically active, and slept longer each night.

What about diet foods?

Many promoters of high or low carb diets promote branded and processed products to as weight-loss aids, such as nutritional bars, powders. These often contain colorings, artificial sweeteners, emulsifiers, and other additives, similar to junk foods.

If consumers of these products remain physically inactive, they may see some temporary weight loss, but when they quit the diet, the weight will go back on.

When a person consumes carbohydrates, the digestive system breaks some of them down into glucose.This glucose enters the blood and raises blood sugar, or glucose, levels. When blood glucose levels rise, beta cells in the pancreas release insulin.

Insulin is a hormone that makes our cells absorb blood sugar for energy or storage. As the cells absorb the blood sugar, blood sugar levels start to drop.

When blood sugar levels drop below a certain point, alpha cells in the pancreas release glucagon. Glucagon is a hormone that makes the liver release glycogen, a sugar stored in the liver.

In short, insulin and glucagon help maintain regular levels of blood glucose in cells, especially the brain cells. Insulin brings excess blood glucose levels down, while glucagon brings levels back up when they are too low.

If blood glucose levels rise too rapidly, too often, the cells can eventually become faulty and not respond properly to insulin’s instructions. Over time, the cells need more insulin to react. We call this insulin resistance.

After producing high levels of insulin for many years, the beta cells in the pancreas can wear out. Insulin production drops. Eventually it can stop altogether.

Effects of insulin resistance

Insulin resistance can lead to a wide range of health problems, including:

This is known as metabolic syndrome, and it is linked to type 2 diabetes.

Reducing the risk of metabolic syndrome

Long-term blood sugar control reduces the chances of developing metabolic syndrome.

Ways of doing this include:

  • consuming natural carbohydrates
  • good sleeping habits
  • regular exercise

The carbohydrates in fruits and vegetables, legumes, whole grains, and so on, tend to enter the bloodstream more slowly compared with the carbohydrates in processed foods.

The carbohydrates in junk and processed foods and drinks can cause a person to feel hungry again more quickly, because they cause glucose and insulin production levels to spike quickly. Natural foods that contain carbohydrates are less likely to do this.

The so-called Mediterranean diet is high in carbohydrates from natural sources plus a moderate amount of animal or fish protein.

This has a lower impact on insulin requirements and subsequent health problems, compared with the standard American diet.

Carbohydrates are needed for good health. Those that come from natural, unprocessed foods, such as fruit, vegetables, legumes, whole grains, and some cereals also contain essential vitamins, minerals, fiber, and key phytonutrients.

Carbohydrates that raise blood sugar quickly are said to be high on the glycemic index (GI), while those with a gentler effect on blood sugar levels have a lower GI score.

Carbohydrates enter the bloodstream as glucose at different rates.

  • High-GI carbs enter the bloodstream quickly as glucose
  • Low-GI carbs enter slowly, because they take longer to digest and break down

In the long term, low-GI foods, together with exercise and regular sleep, are better for maintaining health and body weight.

Low GI carbohydrates are linked to:

  • less weight gain
  • better control of diabetes and blood sugar
  • healthier blood cholesterol levels
  • lower risk of heart disease
  • better appetite control
  • enhance physical endurance

One factor that increases the GI score of a food is the milling and grinding process, which often leaves no more than starchy endosperm, or the inner part of the seed or grain. This is mainly starch.

This process also eliminates other nutrients, such as minerals, vitamins, and dietary fibers.

To follow a low-GI diet, eat more unrefined foods, such as:

  • oats, barley, or bran for breakfast, the less refined, the better
  • wholegrain bread
  • brown rice
  • plenty of fresh fruit and vegetables
  • fresh, whole fruit instead of juice
  • whole grain pasta
  • salads and raw vegetables

Junk foods, processed foods, and foods with too many additives should be avoided.

We need carbohydrates for health, but they must be the right kind of carbohydrate.

Following a well-balanced diet that includes unprocessed carbohydrates, and getting enough sleep and physical activity is more likely to lead to good health and an appropriate body weight than focusing on or eliminating a particular nutrient.