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Carbohydrates: Structure, Types, Functions, Deficiency Symptoms and Natural Sources

Updated on July 3, 2017

What Are Carbohydrates?

Carbohydrates are biological molecules made up of carbon, hydrogen and oxygen. They are sugars, starches and fibers found in a wide array of foods like grains, vegetables and dairy products.

Potatoes Are Rich in Carbohydrates


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Structure of Carbohydrates

The general empirical structure of carbohydrates is (CH2O)n. Their structure determines how energy is stored in bonds during photosynthesis and how breaking these bonds releases energy.

Glucose Structure

Glucose is a carbohydrate.
Glucose is a carbohydrate. | Source

Types of Carbohydrates

There are three types of carbohydrates. They are: starch, sugar and fiber.


Starch is a white substance found in plant tissues. It is an important constituent of corn, rice, beans, wheat, potatoes and many other foods. In fact it is the most common carbohydrate in various diets.


Sugar is a sweet material obtained from plants like sugar beet and sugar cane. This water-soluble crystalline carbohydrate is exemplified by sucrose and glucose. Sugars are classified as monosaccharides, disaccharides, and trisaccharides.


Monosaccharides are the simplest form of sugar; they cannot be hydrolysed to give a simpler sugar. Glucose, galactose and fructose are examples of monosaccharide.


Also known as biose and double sugar, a disaccharide is a class of sugar whose molecules contain two monosaccharide residues. Sucrose, maltose and lactose are examples of disaccharide.


Trisaccharides are oligosaccharides. A trisaccharide yields three monosaccharide molecules upon hydrolysis. Nigerotriose, isomaltotriose and melezitose are examples of trisaccharide.

Sucrose is a Disaccharide



Dietary fibers are carbohydrates that the human body cannot digest. They regulate the body's use of sugars and facilitate the movement of food through the digestive tract.

5 High-Fiber Foods

Sl No
Grams Per Cup
Split Peas
Black Beans
Lima Beans

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Functions of Carbohydrates

Carbohydrates perform many important functions in the body. Glucose provides energy to tissues in the body. Carbohydrates prevent proteins from being used as a source of energy.

Fibers Promote a Healthy Digestive System

Fibers are necessary for a healthy digestive system. They pass undigested through the small intestine and are converted into fatty acids and gases by the large intestine. These fatty acids can either be used as fuel by the large intestine or be absorbed into the blood stream. They also prevent constipation and its complications.

Brain Needs Carbohydrates

Human beings are distinguished from other animals because they have the highest development of cognitive functions, thanks to an evolved brain. Brain weighs around 1,400 g i.e., only about 2 per cent of the body weight.

Brain Has the Highest Consumption of Energy and Oxygen

Brain has the highest consumption of energy and oxygen. Normal healthy adult human brain consumes around 20% of the oxygen supply to the body and almost 20% to 30% of the energy consumption of the body. Developing brain of an infant uses almost half of the energy used by the body.

Glucose Is the Source of Energy for the Brain

The primary source of energy in the brain is glucose. Rarely, in glucose depleted situations, like fasting etc. it can use ketones to some extent for a limited period. Brain has a high rate of metabolism, using ~5.6 mg glucose per 100 g of brain tissue per minute.

Human Beings Need Carbohydrates

"Our body’s preferred source of fuel is carbohydrate, and it is especially what the brain needs to function," says Kimberly Gomer MS, RD, LDN. "When you cut out carbohydrates, you always have a diet high in fat. One can’t live on veggies and fish alone, so heavier, fattier, meats, cheese, nuts, etc. come in which negatively impacts heart health, weight, immune function, and cancer risk."

Our body’s preferred source of fuel is carbohydrate, and it is especially what the brain needs to function.

— Kimberly Gomer MS, RD, LDN

Dr Ashish Shrivastav, senior consultant neurosurgeon at Apollo Hospitals said, “Carbohydrates are the only nutrients which can match this rate of energy requirement. However, the brain prefers to get its carbohydrates from carbohydrate rich whole foods rather than simple sugars. The cognitive functions or the thinking capabilities of the brain deteriorate if the glucose levels fall in the brain. A rapid relief can be obtained from carbohydrate rich whole foods. At the same time, glucose from simple sugars causes detrimental effect to the brain. Hence, glucose from whole food carbohydrates is essential in diet but excess simple carbohydrates are harmful.”

Fruits, vegetables and cereals or more scientifically, foods with low GI is desirable. Foods with high GI, like white sugar or juices, are not the correct sources of carbohydrates.

In type 2 diabetes, there is a situation of “deprivation in the presence of excess”. The glucose level remains high outside the cell but low inside it. Thus, the cells, especially neurones/brain cells cannot function properly.

There is a unique metabolism in different types of bodily organs. Contrary to brain, the muscles require more proteins. The fat in our diet has been chastised a lot in recent times as being “Bad”. But fat is essential for many functions of our body.

The membrane covering the nerves is made from fatty material. So, as always, there are no absolute positives or negatives. Carbohydrates are necessary for our body, especially for our brain, but in right quantity and of the right quality ( i.e. from whole foods and of low GI).

Clinical nutritionist Tanu Arora explained, “ Carbohydrates are one of the most important nutrients needed for healthy brains functioning. They are the main source of energy for brain. When carbs are eaten they are eventually digested and broken down into smaller sugars molecules called glucose. Carbohydrates significantly affect the mood and behaviour, they are equally important for mental health also. People who are on high fat, low carbohydrate diet for a year had more anxiety, depression, and anger than people on a low fat, high carbohydrate diet. Without sufficient glucose, the central nervous system suffers, which may cause dizziness or mental and physical weakness. If the body has insufficient carbohydrates, body consume protein for its energy source, which ultimately leads to burden on kidney also. People who don’t consume enough carbohydrate may also suffer from insufficient fibre, which may cause digestive problems and constipation”.

Carbohydrates play an essential role in many molecular processes in cells, from detecting pathogenic agents to cellular communication or anti-inflammatory response.

It is more important to eat some carbohydrates at breakfast, because the brain needs fuel right away, and carbohydrate is the best source.

— Andrew Weil

Written by Allan Borushek, this a simple, safe, practical and effective guide helps you lose weight and to prevent obesity.

Deficiency Symptoms

Diets that reduce carbohydrates have been around for a while, they are popular due to diets like Keto and Paleo. However, as tempting as they may sound, harmfil things can happen to your body if you cut back too drastically on carbohydrates.

If you do not consume enough carbohydrates, your blood sugar drops. "This can actually cause us to seek out more carbohydrates and sugar for instant energy," says Sarah Schlichter, MPH, RDN, LDN.

"You may also feel tired, sluggish and lethargic," said Schlicter. "Protein and fat take longer to absorb and digest, and don't have the same 'instant energy' function as carbohydrates."

"When you cut back on your carbohydrate intake, you are cutting back on your fiber intake, and that can leave you backed up," says Brigitte Zeitlin, MPH, RD, CDN. "Constipation can also cause bloating, abdominal cramps, and gas."

"When our blood sugar levels drop too low, we can frequently get headaches," said Zeitlin. "If you are restricting your carb intake, and feeling headachy, this is your body’s way of telling you it needs more carbs."

Bad breath is a known side effect of low-carb diet. When you reduce carbohydrate intake, your body is deprived of glucose and, therefore, it begins breaking down fats for energy. "As this happens, chemicals called ketones (responsible for the name: ketogenic diet) are released, which lead to bad breath and odor," said Rachel Berman, RD. Nausea is another sign that you are not getting enough carbohydrates.

Those who are replacing carbohydrates with animal protein may be inviting trouble. "All animal fats will raise non-HDL, (the dangerous cholesterol) which can lead to heart disease and increased risk of atherosclerosis," says Gomer.

To obtain all the nutrients you need, adhering to a balanced diet is important. If you are not consuming enough carbohydrates, you may be low on some essential nutrients. "You may be missing vitamins including the B’s and antioxidants and phytonutrients that come along with the healthy carbohydrates," says Gomer.

Food affects more than just your physical body — it affects your brain and mental state also. "Our body’s preferred source of fuel is carbohydrate, and it is especially what the brain needs to function," says Gomer. "Hence when you cut out carbohydrates, you feel a brain fog."

If you are not consuming enough carbohydrates, you will realize that you are always hungry. "Our bodies are smart and realize that just vegetables and protein will not completely satisfy us," says Schlichter. "We'll often be in search of some sort of carbohydrates to provide the missing link."

Carbohydrates promote the production of disease-fighting antibodies. If you are eating less than 100g of carbohydrates a day, you are likely to become ill often.

Everybody is so afraid of carbs—they talk about them in a negative way, like, ‘Carbs make me fat or bloated’—but just pick the right one and have a little bit,” the illustrious dancer told Us Weekly. “I think when you deprive yourself, that’s when you crave pasta and pizza.... Carbohydrates make my brain click on and everything else is able to function. I thought they were my enemy, but they’re actually my best friend.

— Julianne Alexandra Hough, American dancer, singer, and actress

10 Natural Sources of Carbohydrates

Sl No
Serving Size
No of Carbohydrates
1 apple
1 banana
Black beans (half cup)
Garbanzo beans (half cup)
Green beans (half cup)
Pinto beans (half cup)
Cantaloupe (1 wedge)
Carrot (half cup)
Corn (half cup)
Green peas (half cup)

Make the Most of Carbohydrates

The quantity of carbohydrate in the food is less important than the type of carbohydrate in the diet. Healthy, whole grains like whole wheat bread, rye, barley and quinoa are better than highly refined white bread or French fries.

There is a great deal of misunderstanding about carbohydrates. Remember that it is more important to get carbohydrates from healthy foods than to follow a strict diet limiting carb intake.

Begin your day with whole grains. Choose a cereal that has at least four grams of fiber and not more than eight grams of sugar per serving. Consume whole grain breads for lunch.

Try a whole grain in salad form (like quinoa). Go for whole fruits instead of juices. Fruits usually have more fiber and less sugar than juices. Carbohydrates are not contained in patatoes only. Beans are excellent sources of slowly digested carbs. Legumes and beans are rich in proteins too.

Carbohydrate Synthesis

Carbohydrates play an essential role in many molecular processes in the cell, from detecting pathogenic agents to cellular communication or anti-inflammatory response. Therefore there is a great interest in the design of medicines that copy the structure of carbohydrates, as they would be easily assimilated and recognized by a part of the biological receptor system.

Carbohydrate synthesis is important for the development of diagnostic tests, vaccines and new drugs. In a study, research scientists have synthesized carbohydrates with enzymes through a reaction that was not much studied so far on these biomolecules and which creates few by-products.

Research scientists have found a way to modify glycosidase, an enzyme that usually degrades carbohydrates, so that it acts reversely and synthesizes carbohydrates. Glycosidase catalyses the hydrolysis in glycosidic bonds.

"Moreover, this synthesis has been reached with a new reaction that had not been observed in glycosidase yet. This is a variation from a reaction called front-face or SNi-like" said Carme Rovira. "The advantage of this reaction in glycosidase is that it makes the synthesis in a clean way, without almost causing hydrolysis residues and using economically viable substrates (sugar)." "Basically, we used our experience with enzymes that synthesize carbohydrates to apply the reaction to one side in other enzymes with more biotechnological possibilities."

The study was co-led by a team of the Department of Inorganic and Organic Chemistry and the Institute of Theoretical and Computational Chemistry of the UB (IQTCUB), together with a research group of biological chemistry of the University of Oxford.

Basically, we used our experience with enzymes that synthesize carbohydrates to apply the reaction to one side in other enzymes with more biotechnological possibilities.

— Carme Rovira, ICREA researcher from the IQTCUB


  • Carbohydrates comprise of carbon, hydrogen and oxygen.
  • General empirical structure of carbohydrates is (CH2O)n.
  • Starch, sugar and fiber are three types of carbohydrates.
  • Fibers regulate the body's use of sugars and facilitate the movement of food through the digestive tract.
  • Dietary fibers promote a healthy digestive system.
  • Quinoa is a healthy carbohydrates food.

There are plenty of commercial diets that advocate cutting carbohydrates for instant weight loss and as a result, many Americans have become accustomed to counting the carbohydrate grams in the foods they eat and buy. The problem is our bodies need carbohydrate to function properly. So instead of avoiding this essential nutrient all together, make your carbohydrate work for you buy choosing foods that provide the nutrition needed for a healthy life.

— Taryn Palmer RDN, LD Magic Valley YMCA dietitian


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    • jimmyglaughlin profile image

      Jim Laughlin 9 months ago from Connecticut

      Carbs, carbs, carbs! My doctor wants me to lower my carb intake. I love bread and pasta! Argh, what to do?

      Good Hub!

    • srirad0675 profile image

      Srikanth R 9 months ago

      Most welcome!

    • ChitrangadaSharan profile image

      Chitrangada Sharan 9 months ago from New Delhi, India

      Very nice and informative hub about carbohydrates! The table about natural resources is very helpful. I learnt something new from your this well presented hub.

      Thanks for sharing!