Carbohydrates – Anatomy, Types, Structure, Functions

Carbohydrates – Anatomy, Types, Structure, Functions

Carbohydrates are one of the three macronutrients in the human diet, along with protein and fat. These molecules contain carbon, hydrogen, and oxygen atoms. Carbohydrates play an important role in the human body. They act as an energy source, help control blood glucose and insulin metabolism, participate in cholesterol and triglyceride metabolism, and help with fermentation. The digestive tract begins to break down carbohydrates into glucose, which is used for energy, upon consumption. Any extra glucose in the bloodstream is stored in the liver and muscle tissue until further energy is needed. Carbohydrates is an umbrella term that encompasses sugar, fruits, vegetables, fibers, and legumes. While there are numerous divisions of carbohydrates, the human diet benefits mostly from a certain subset.


Monosaccharide: The most basic, fundamental unit of a carbohydrate. These are simple sugars with the general chemical structure of C6H12O6.

  • Examples: glucose, galactose, fructose

Disaccharide: Compound sugars containing two monosaccharides with the elimination of a water molecule with the general chemical structure C12H22O11

  • Examples: sucrose, lactose

Oligosaccharide: The polymer contains three to ten monosaccharides

  • Examples: maltodextrins, raffinose

Polysaccharides: Polymers containing long chains of monosaccharides connected through glycosidic bonds

  • Examples: amylose, cellulose


Simple Carbohydrates: One or two sugars (monosaccharides or disaccharides) combined in a simple chemical structure. These easily are utilized for energy, causing a rapid rise in blood sugar and insulin secretion from the pancreas.

  • Examples: fructose, lactose, maltose, sucrose, glucose, galactose, ribose
  • Foods: candy, carbonated beverages, corn syrup, fruit juice, honey, table sugar

Complex Carbohydrates: Three or more sugars (oligosaccharides or polysaccharides) bonded together in a more complex chemical structure. These take longer to digest and therefore have a more gradual effect on the increase in blood sugar.

  • Examples: cellobiose, rutinulose, amylose, cellulose, dextrin
  • Foods: apples, broccoli, lentils, spinach, unrefined whole grains, brown rice

Starches: Complex carbohydrates contain a large number of glucose molecules. Plants produce these polysaccharides.

  • Examples include potatoes, chickpeas, pasta, and wheat.

Fiber: Non-digestible complex carbohydrates that encourage healthy bacterial growth in the colon and act as a bulking agent, easing defecation. The main components include cellulose, hemicellulose, and pectin.

  • Insoluble: Absorbs water in the intestines, thereby softening and bulking the stool. Benefits include regularity of bowel movements and a decreased risk of diverticulosis. Examples: brans, seeds, vegetables, brown rice, potato skins.
  • Soluble: Helps decrease blood cholesterol and LDL levels, reduces straining with defecation, and blunts postprandial blood glucose levels. Examples are fleshy fruit, oats, broccoli, and dried beans

There are three macronutrients—carbohydrates (4 kcal/gm), fat (9 kcal/gm), and protein (4 kcal/gm) found in food.  Therefore, studies have defined low carbohydrate as a percent of daily macronutrient intake or total daily carbohydrate load.  We will define it here as:

  • Very low-carbohydrate (< 10% carbohydrates) or 20-50 gm/day
  • Low-carbohydrate (<26% carbohydrates) or less than < 130 gm/day
  • Moderate-carbohydrate (26%-44%)
  • High-carbohydrate (45% or greater)

For reference, the Institute of Medicine proposes Americans obtain 45%-65% of calories from carbohydrates.  This article will review the evidence and effectiveness of low-carb approaches in clinical medicine.

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Carbohydrate Molecules

Carbohydrates are essential macromolecules that are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.


Key Points

Monosaccharides are simple sugars made up of three to seven carbons, and they can exist as a linear chain or as ring-shaped molecules.

Glucose, galactose, and fructose are monosaccharide isomers, which means they all have the same chemical formula but differ structurally and chemically.

Disaccharides form when two monosaccharides undergo a dehydration reaction (a condensation reaction); they are held together by a covalent bond.

Sucrose (table sugar) is the most common disaccharide, which is composed of the monomers glucose and fructose.

A polysaccharide is a long chain of monosaccharides linked by glycosidic bonds; the chain may be branched or unbranched and can contain many types of monosaccharides.

Key Terms

  • isomer: Any of two or more compounds with the same molecular formula but with different structure.
  • dehydration reaction: A chemical reaction in which two molecules are covalently linked in a reaction that generates H2O as a second product.
  • biopolymer: Any macromolecule of a living organism that is formed from the polymerization of smaller entities; a polymer that occurs in a living organism or results from life.

Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. Therefore, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. The origin of the term “carbohydrate” is based on its components: carbon (“carbo”) and water (“hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.


Monosaccharides (mono- = “one”; saccharine = “sweet”) are simple sugars. In monosaccharides, the number of carbons usually ranges from three to seven. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R’), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms.


Monosaccharides: Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.

Common Monosaccharides

Glucose (C6H12O6) is a common monosaccharide and an important source of energy. During cellular respiration, energy is released from glucose and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose, in turn, is used for energy requirements for the plant.

Galactose (a milk sugar) and fructose (found in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and stereochemically. This makes them different molecules despite sharing the same atoms in the same proportions, and they are all isomers of one another, or isomeric monosaccharides. Glucose and galactose are aldoses, and fructose is a ketose.


Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond. Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type.


Disaccharides: Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose.

Common Disaccharides

Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.


A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

Plants are able to synthesize glucose, and the excess glucose is stored as starch in different plant parts, including roots and seeds. Starch is the stored form of sugars in plants and is made up of glucose monomers that are joined by α1-4 or 1-6 glycosidic bonds. The starch in the seeds provides food for the embryo as it germinates while the starch that is consumed by humans is broken down by enzymes into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

Common Polysaccharides

Glycogen is the storage form of glucose in humans and other vertebrates. It is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose and provides structural support to the cell. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds. Every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells.


Polysaccharides: In cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.

Carbohydrate Function

Carbohydrates serve various functions in different animals. Arthropods have an outer skeleton, the exoskeleton, which protects their internal body parts. This exoskeleton is made of chitin, which is polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls.

Importance of Carbohydrates

Carbohydrates are a major class of biological macromolecules that are an essential part of our diet and provide energy to the body.


Key Points

Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is found in many basic foods.

Carbohydrates contain soluble and insoluble elements; the insoluble part is known as fiber, which promotes regular bowel movement, regulates the rate of consumption of blood glucose, and also helps to remove excess cholesterol from the body.

As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces ATP, the energy currency of the cell.

Since carbohydrates are an important part of human nutrition, eliminating them from the diet is not the best way to lose weight.

Key Terms

  • carbohydrate: A sugar, starch, or cellulose that is a food source of energy for an animal or plant; a saccharide.
  • glucose: a simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism
  • ATP: A nucleotide that occurs in muscle tissue, and is used as a source of energy in cellular reactions, and in the synthesis of nucleic acids. ATP is the abbreviation for adenosine triphosphate.

Benefits of Carbohydrates

Biological macromolecules are large molecules that are necessary for life and are built from smaller organic molecules. One major class of biological macromolecules is carbohydrates, which are further divided into three subtypes: monosaccharides, disaccharides, and polysaccharides. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all-natural sources of carbohydrates. Importantly, carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many basic foods.


Carbohydrates: Carbohydrates are biological macromolecules that are further divided into three subtypes: monosaccharides, disaccharides, and polysaccharides. Like all macromolecules, carbohydrates are necessary for life and are built from smaller organic molecules.

Carbohydrates in Nutrition

Carbohydrates have been a controversial topic within the diet world. People trying to lose weight often avoid carbs, and some diets completely forbid carbohydrate consumption, claiming that a low-carb diet helps people to lose weight faster. However, carbohydrates have been an important part of the human diet for thousands of years; artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas.

Carbohydrates should be supplemented with proteins, vitamins, and fats to be part of a well-balanced diet. Calorie-wise, a gram of carbohydrate provides 4.3 Kcal. In comparison, fats provide 9 Kcal/g, a less desirable ratio. Carbohydrates contain soluble and insoluble elements; the insoluble part is known as fiber, which is mostly cellulose. Fiber has many uses; it promotes regular bowel movement by adding bulk, and it regulates the rate of consumption of blood glucose. Fiber also helps to remove excess cholesterol from the body. Fiber binds and attaches to the cholesterol in the small intestine and prevents the cholesterol particles from entering the bloodstream. Then cholesterol exits the body via the feces. Fiber-rich diets also have a protective role in reducing the occurrence of colon cancer. In addition, a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces adenosine triphosphate (ATP), the energy currency of the cell. Without the consumption of carbohydrates, the availability of “instant energy” would be reduced. Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercises and plenty of water, is the more sensible way to lose weight.

Diet Recommendations

In its 2002 report (), the Institute of Medicine (IOM) established an RDA for carbohydrates of 130 g/d for adults and children aged ≥1 y. This value is based on the number of sugars and starches required to provide the brain with an adequate supply of glucose. The IOM set an acceptable macronutrient distribution range (AMDR) for carbohydrates of 45–65% of total calories. Thus, current dietary guidance recommends the consumption of carbohydrate-containing foods, including grains, vegetables, fruits, pulses, nuts, seeds, and milk products. Carbohydrate foods are an important source of fiber and other nutrients.

Sugars and starches provide glucose, the main energy source for the brain, central nervous system, and RBCs. Glucose also can be stored as glycogen (animal starch) in the liver and muscle or, like all excess calories in the body, converted to body fat. Dietary fibers are non-digestible forms of carbohydrates. Dietary fiber is intrinsic and intact in plants, helps provide satiety, and promotes healthy laxation. Diets high in fiber reduce the risk of coronary heart disease, diabetes, obesity, and other chronic diseases.

The energy value of digestible carbohydrates is generally accepted as 4 kcal/g for both sugars and starches. Fermentation of fiber in the gut will produce SCFAs that contribute calories, generally estimated to be ∼2 kcal/g. Few studies have linked carbohydrates to obesity. Indeed, observational data generally report that higher carbohydrate intake is linked to lower body weight.

In its 2002 report, the IOM set an Adequate Intake (AI) value for fiber of 14 g of fiber per 1000 kcal (). This value is derived from data on the relation of fiber consumption and coronary heart disease risk, although the IOM also considered the evidence for fiber decreasing the risk of chronic disease and other health-related conditions. Consequently, the IOM fiber recommendations are highest for populations who consume the most calories, namely young men. Fiber recommendations are lower for women and the elderly. The use of this method for determining recommended fiber intake for children is problematic (e.g., intake of 19 g of fiber is recommended for 2-y-old children, an implausible number). Past recommendations for children were based on the “age plus 5” rule (e.g., a child aged 2 y should consume 7 g of fiber/d) ().

Dietary fiber is listed on the Nutrition Facts panel, and 25 g of dietary fiber is the currently recommended amount in a 2000-kcal diet. Manufacturers are allowed to call a food a “good source of fiber” if it contains 10% of the recommended amount (2.5 g/serving) and an “excellent source of fiber” if the food contains 20% of the recommended amount (5 g/serving). Dietary fiber on food labels includes both dietary fiber and functional fiber.

Food Sources

Vegetables, fruits, whole grains, milk, and milk products are the major food sources of carbohydrates. Grains and certain vegetables including corn and potatoes are rich in starch, whereas sweet potatoes are mostly sucrose, not starch. Fruits and dark-green vegetables contain little or no starch but provide sugars and dietary fiber. Marriott et al. () examined the intake of added sugars and selected nutrients from 2003–2006 NHANES data. Thirteen percent of the population had an added-sugars intake of >25% of calories. Higher added-sugars intakes were associated with higher proportions of individuals with nutrient intakes below the Estimated Average Requirement (EAR), but the overall high-calorie content and low quality of the U.S. diet remained the predominant issue.

Dietary fiber intake was particularly low in their analysis. With the exception of older women (≥51 y), only 0–5% of individuals in all other life-stage groups had fiber intakes meeting or exceeding the AI (). Fiber intake is closely linked to calorie intake. Thus, recommendations to reduce calorie intake will make increasing fiber intake particularly challenging.

It is advisable to select foods high in dietary fiber, including whole-grain bread and cereals, legumes, vegetables, and fruits, whenever possible. Typically, vegetables and fruits are not the most concentrated fiber sources, but these are important foods to encourage because they contribute important micronutrients. Similarly, milk and milk products, which contain lactose, generally do not contain fiber but these, too, are important because they contribute calcium, vitamin D, and protein to the diet.


Carbohydrates are an important part of a nutritional diet. The healthiest sources include complex carbohydrates because of their blunted effects on blood glucose. These options include unprocessed whole grains, vegetables, fruits, and legumes. While simple carbohydrates are acceptable in small amounts, white bread, sodas, pastries, and other highly processed foods are less nutritious and cause a sharp increase in blood glucose. Healthy adult diets should include 45% to 65% carbohydrates as part of the daily intake, equaling about 200 g to 300 g per day. Carbohydrates contain about 4 kcal/ gram (17 kJ/g). Fiber is an important carbohydrate as well. Healthy adults should consume about 30 g per day of fiber, as it is found to reduce the risk of coronary heart disease, strokes, and digestive issues.

A glycemic index is a tool used to track carbohydrates and their individual effects on blood sugar. This scale ranks carbohydrates from 0 to 100 based on how rapidly the rise in blood glucose occurs upon consumption. Low glycemic foods (55 or less) produce a gradual increase in blood sugar. These foods include steel-cut oatmeal, oat bran, muesli, sweet potatoes, peas, legumes, most fruits, non-starchy vegetables. Medium glycemic foods (56 to 69) include quick oats, brown rice, and whole-wheat bread. High glycemic foods (70 to 100) increase the risk of type 2 diabetes, heart disease, obesity, and ovulatory infertility. These foods include white bread, corn flakes, white potatoes, pretzels, rice cakes, and popcorn.

Initiation of a Low-Carb Lifestyle

After a shared decision-making process with the patient, there are numerous ways to start a patient on a low-carb diet. Low-carb nutrition may be advisable for those who desire healthy or athletic performance, weight loss, improvement of glycemic control for type 1 or 2 diabetes, or for a seizure disorder.

  • First, an understanding of what macronutrients are and their relation to food is a critical part of counseling.
  • Secondly, determine the patient’s desire for either small steps or a rapid induction phase through motivational interviewing and S.M.A.R.T goal setting.
  • Limitation of added sugar (sucrose) and refined carbohydrates is critical in the overall improvement of food quality and will generally reach a moderate carbohydrate (< 45% carbohydrates) level.
  • A way to initiate low-carb is through a rapid induction phase of 2 to 4 weeks, with 20 to 50 gms of carbohydrates to induce nutritional ketosis. Ad libitum vegetables that grow above the ground and are lower in carbohydrate content are encouraged. Additionally, carbs should be limited to those found in whole, unprocessed food.
  • Finally, after the induction phase, depending on goals, patients can remain in the keto phase or slowly add healthy carbohydrates from whole, unprocessed vegetables, and low-glycemic, high fiber fruit (i.e., berries).

Maintenance of a Low-Carb Lifestyle

If limited initially or during the induction phase, full-fat dairy, legumes, and whole grains can also be added during this maintenance phase as long as goals are maintained and tolerated without any hypersensitivity or an adverse response. The lifelong maintenance phase can then continue in accordance with patient preference. Periodic monitoring of cardiovascular risk markers and control of cardiometabolic disease should also be a priority. Those with type 2 diabetes require close monitoring for hypoglycemia, and reduction of insulin or hypoglycemic medications are prudent with rapid reductions in fasting glucose.



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