Carbohydrates have become surprisingly divisive. Some people swear by them, others swear against them. But it is important to understand that carbohydrates are a diverse group of compounds that have a multitude of effects in the body. Thus, trying to make blanket statements about carbohydrates is probably not a good idea.

Carbohydrates are named because they are hydrated (as in water, H₂O) carbon. Below is the formula showing how carbon dioxide (CO₂) and water (H₂O) are used to make carbohydrates (CH₂O)_n and oxygen (O₂). The “n” after the carbohydrate in the formula indicates that the chemical formula is repeated an unknown number of times, but that for every carbon and oxygen, there will always be two hydrogens.

CO₂ + H₂O –> (CH₂O)_n + O₂

Carbohydrates are produced by plants through a process known as photosynthesis. In this process, plants use the energy from photons of light to synthesize carbohydrates. The formula for this reaction looks like this:

6CO₂ + 6H₂O + Light –> C₆H₁₂O₆ + 6O₂

There are many different types of carbohydrates as shown in the figure below. The first way that carbohydrates can be divided is into simple, complex, and sugar alcohols. As the names imply, complex carbohydrates contain more sugar units, while simple carbohydrates contain either 1 or 2 sugars. In the next sections, you will learn more about the different forms of carbohydrates.

Figure 2.11 The different forms of carbohydrates

Subsections:

2.11 Simple Carbohydrates

2.12 Alternative Sweeteners

2.13 Oligosaccharides

2.14 Polysaccharides

No References

2.11 Simple Carbohydrates

As shown in the figure below, simple carbohydrates can be further divided into monosaccharides and disaccharides. Mono- means one, thus monosaccharides contain one sugar. Di- means two, thus disaccharides contain 2 sugar units.

Figure 2.111 Overview of Carbohydrates

Monosaccharides

The 3 monosaccharides are: glucose, fructose and galactose. Notice that all are 6-carbon sugars (hexoses). However, fructose has a five member ring, while glucose and galactose have 6 member rings. Also notice that the only structural difference between glucose and galactose is the position of the alcohol (OH) group that is shown in red.

Figure 2.112 The 3 monosaccharides

Glucose – Product of photosynthesis, major source of energy in our bodies

Fructose – Commonly found in fruits and used commercially in many beverages

Galactose – Not normally found in nature alone, normally found in the disaccharide lactose

Disaccharides

Disaccharides are produced from 2 monosaccharides. The commonly occurring disaccharides are:

Maltose (glucose + glucose, aka malt sugar) – seldom found in foods, present in alcoholic beverages and barley

Sucrose (glucose + fructose, aka table sugar) – only made by plants.

Lactose (galactose + glucose, aka milk sugar) – primary milk sugar

The different disaccharides and the monosaccharides components are illustrated below.

Figure 2.113 The 3 disaccharides

Each of these disaccharides contains glucose and all the reactions are dehydration reactions. Also notice the difference in the bond structures. Maltose and sucrose have alpha-bonds, which are depicted as v-shaped above. You might hear the term glycosidic used in some places to describe bonds between sugars. A glycoside is a sugar, so glycosidic is referring to a sugar bond. Lactose, on the other hand, contains a beta-bond. We need a special enzyme, lactase, to break this bond, and the absence of lactase activity leads to lactose intolerance.

High-Fructose Corn Syrup

Food manufacturers are always searching for cheaper ways to produce their food. One method that has been popular is the use of high-fructose corn syrup as an alternative to sucrose. High-fructose corn syrup contains either 42 or 55% fructose, which is similar to sucrose¹. Nevertheless, because an increase in high-fructose corn syrup consumption (see figure below) has coincided with the increase in obesity in the U.S., there is a lot of controversy surrounding its use.

Figure 2.114 U.S. per capita sugar and sweetener consumption²

Opponents claim that high-fructose corn syrup is contributing to the rise in obesity rates. As a result, some manufactures have started releasing products made with natural sugar. You can read about this trend in the following New York Times article in the link below. Also, manufacturers tried to rebrand high-fructose corn syrup as corn sugar to get around the negative perception of the name. But the FDA rejected the Corn Refiners Association request to change the name officially to corn sugar as described in the second link. The last link is a video made by the American Chemical Society that gives some background on how HFCS is produced and how it compares to sucrose.

References & Links

1. http://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm324856.htm

2.http://www.foodnavigator-usa.com/Markets/The-changing-American-diet-consumption-of-corn-based-sweeteners-drops

Links

Not familiar with Ring structures, see how glucose forms a ring – http://en.wikipedia.org/wiki/File:Glucose_Fisher_to_Haworth.gif

Sugar is back on labels, this time as a selling point – http://www.nytimes.com/2009/03/21/dining/21sugar.html?_r=1&ref=nutrition

No new name for high-fructose corn syrup – http://well.blogs.nytimes.com/2012/05/31/no-new-name-for-high-fructose-corn-syrup/?_r=0

Video

Sugar vs. High Fructose Corn Syrup – What’s the Difference? – https://www.youtube.com/watch?v=fXMvregmU1g

2.12 Sugar Alcohols (Polyols, Sugar Replacers)

Sugar(s) can provide a lot of calories and contribute to tooth decay. Thus there are many other compounds that are used as alternatives to sugar that have been developed or discovered. We will first consider sugar alcohols and then the alternative sweeteners in subsequent sections.

Below you can see the structure of three common sugar alcohols: xylitol, sorbitol, and mannitol.

Figure 2.121 Structure of three commonly used sugar alcohols: xylitol, sorbitol, and mannitol^1-3

Remember that alcohol subgroups are (OH), and you can see many of them in these structures.

Sugar alcohols are also known as “sugar replacers”, because some in the public might get confused by the name sugar alcohol. Some might think a sugar alcohol is a sweet alcoholic beverage. Another name for them is nutritive sweeteners, which indicates that they do provide calories. Sugar alcohols are nearly as sweet as sucrose but only provide approximately half the calories as shown below. The name polyols also seems to be increasingly used to describe these compounds.

Table 2.121 Relative sweetness of monosaccharides, disaccharides, and sugar alcohols^4,5

*Differs based on a person’s lactase activity

Sugars are fermented by bacteria on the surfaces of teeth. This results in a decreased pH (higher acidity) that leads to tooth decay and, potentially, cavity formation. The major advantage of sugar alcohols over sugars is that sugar alcohols are not fermented by bacteria on the tooth surface. There is a nice picture of this process in the link below as well as a video explaining the process of tooth decay.

While not a sugar alcohol, tagatose is very similar to sugar alcohols. Tagatose is an isomer of fructose, that provides a small amount of energy (1.5 kcal/g). 80% of tagatose reaches the large intestine, where it is fermented by bacteria, meaning it has a prebiotic-type effect⁴. Notice the similarity in structure of tagatose to sugar alcohols, the only difference being a ketone (=O) instead of an alcohol (OH) group.

Figure 2.122 Structure of tagatose¹¹

References & Links

1. https://pubchem.ncbi.nlm.nih.gov/compound/xylitol#section=Top

2. https://pubchem.ncbi.nlm.nih.gov/compound/D-Sorbitol#section=Top

3. https://pubchem.ncbi.nlm.nih.gov/compound/D-mannitol#section=Top

4. Wardlaw GM, Hampl J. (2006) Perspectives in nutrition. New York, NY: McGraw-Hill.

5. Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.

6. http://en.wikipedia.org/wiki/File:Tagatose.png

Link

Sugar and Dental Caries – http://www.asu.edu/courses/css335/caries.htm

Video

Tooth Decay – http://www.youtube.com/watch?v=_oIlv59bTL4

2.13 Alternative Sweeteners

Alternative sweeteners are simply alternatives to sucrose and other mono- and disaccharides that provide sweetness. Many have been developed to provide zero-calorie or low calorie sweetening for foods and drinks.

Because many of these provide little to no calories, these sweeteners are also referred to as non-nutritive sweeteners (FDA is using high-intensity sweeteners to describe these products³). Aside from tagatose (described in sugar alcohol section), all of the sweeteners on the list below meet this criteria. Aspartame does provide calories, but because it is far sweeter than sugar, the small amount used does not contribute meaningful calories to a person’s diet. Until the FDA allowed the use of stevia, this collection of sweeteners were commonly referred to as artificial sweeteners because they were synthetically or artificially produced. However, with stevia, the descriptor artificial can no longer be used to describe these sweeteners. More recently, Luo Han Guo Fruit extracts have also been allowed to be used as another high-intensity sweetener that is not synthesized or artificially produced. The table in the link below summarizes the characteristics of the FDA approved high-intensity sweeteners.

Saccharin

Saccharin is the oldest of the artificial sweeteners. However, it should be noted that both sweet and bitter taste receptors are triggered by it, so for some people it has an aftertaste that is offputting^4,5. It has been found that this bitter or metallic flavor can sometimes be masked by mixing alternative sweeteners⁶.

Figure 2.131 Structure of saccharin⁷

Aspartame

Aspartame is made up of 2 amino acids (phenylalanine and aspartate) and a methyl (CH₃) group. The compound is broken down during digestion into the individual amino acids. This is why it provides 4 kcal/g, just like protein⁴. However, it is still considered noncaloric because it is so sweet that we use very small amounts that don’t provide any meaningful caloric value. Because it can be broken down to phenylalanine, products that contain aspartame contain the following message: “Phenylketonurics: Contains phenylalanine.” Phenylketonuria (PKU) will be covered in greater detail in section 2.25. When heated, aspartame breaks down and loses its sweet flavor¹.

Figure 2.132 Structure of aspartame⁸

Neotame

Neotame is like aspartame version 2.0. Neotame is structurally identical to aspartame except that it contains an additional side group (bottom of figure below, which is flipped backwards to make it easier to compare their structures). While this looks like a minor difference, it has profound effects on the properties of neotame. Neotame is much sweeter than aspartame and is heat-stable. It can still be broken down to phenylalanine, but such small amounts are used that it is not a concern for those with PKU^1,4.

Figure 2.133 Structure of neotame⁹

Advantame

The newest, sweetest alternative sweetener approved by the FDA in 2014 is advantame. It is heat-stable and does not have a trade name yet³. Notice it also has a similar structure to aspartame and neotame. Like Neotame it can broken down to phenylalanine, but such small amounts are used that it is not a concern for those with PKU. However, it has a much higher acceptable daily intake than Neotame⁴, meaning there is less concern about adverse effects from consuming too much.

Figure 2.134 Structure of advantame¹⁰

Acesulfame-Potassium (K)

Acesulfame-potassium (K) is not digested or absorbed, therefore it provides no energy or potassium to the body¹. It is a heat-stable alternative sweetener.

Figure 2.135 Structure of acesulfame-potassium (K)¹¹

Sucralose

Sucralose is structurally identical to sucrose except that 3 of the alcohol groups (OH) are replaced by chlorine molecules (Cl). This small change causes sucralose to not be digested and as such is excreted in feces^1,4. It is a heat-stable alternative sweetener.

Figure 2.136 Structure of sucralose¹²

Stevia

Stevia is derived from a South American shrub, with the leaves being the sweet part. The components responsible for this sweet taste are a group of compounds known as steviol glycosides. The structure of steviol is shown below.

Figure 2.137 Structure of steviol¹³

The term glycoside means that there are sugar molecules bonded to steviol. The two predominant steviol glycosides are stevioside and rebaudioside A. The structure of these two steviol glycosides are very similar¹⁴. The structure of stevioside is shown below as an example.

Figure 2.138 Structure of stevioside¹⁵

The common name for a sweetener containing primarily rebaudioside A is rebiana. Stevia sweeteners had been marketed as a natural alternative sweeteners, something that has been stopped by lawsuits as described in the following link.

Stevia is a heat-stable alternative sweetener.

Luo Han Guo Fruit Extracts

Luo Han Guo (aka Siraitia grosvenrii Swingle, monk) fruit extracts are a newer, natural heat-stable alternative sweetener option derived from a native Chinese fruit. These extracts are sweet because of the mogrosides that they contain³. The structure of a mogroside is shown below.

Figure 2.139 Structure of a mogroside¹⁶

References & Links

1. Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.

2. http://www.fda.gov/AboutFDA/Transparency/Basics/ucm214865.htm

3. http://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm397725.htm

4. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.

5. Fernstrom JD, Munger SD, Sclafani A, de Araujo IE, Roberts A, Molinary S (2012) Mechanisms for Sweetness. J Nutr 142 (6) 1134S–1141S.

6. Behrens M, Blank K, Meyerhof W. (2017) Blends of Non-caloric Sweeteners Saccharin and Cyclamate Show Reduced Off-Taste due to TAS2R Bitter Receptor Inhibition. Cell Chem Biol 24 (10) 1199-1204.

7.https://en.wikipedia.org/wiki/Saccharin#/media/File:Saccharin.svg

8. http://en.wikipedia.org/wiki/Aspartame

9. http://en.wikipedia.org/wiki/File:Neotame.png

10. http://en.wikipedia.org/wiki/File:Advantame.svg

11. http://en.wikipedia.org/wiki/File:AcesulfameK.svg

12. http://en.wikipedia.org/wiki/File:Sucralose2.svg

13. http://en.wikipedia.org/wiki/File:Steviol.svg

14. Carakostas MC, Curry LL, Boileau AC, Brusick DJ. (2008) Overview: The history, technical function and safety of rebaudioside A, a naturally occurring steviol glycoside, for use in food and beverages. Food and Chemical Toxicology 46 Suppl 7: S1.

15. http://en.wikipedia.org/wiki/File:Steviosid.svg

16. http://en.wikipedia.org/wiki/File:Mogroside_II_E.gif

Links

FDA High-Intensity sweeteners – http://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm397725.htm

What is natural and who decides? – http://www.nutraingredients-usa.com/Markets/Pure-Via-to-settle-class-action-suit-over-natural-claims

2.14 Oligosaccharides

Within complex carbohydrates, there are oligosaccharides and polysaccharides. Oligosaccharides (oligo means few) are composed of 3-10 sugar units and polysaccharides contain greater than 10 sugar units.

Figure 2.141 Overview of carbohydrates

Raffinose and stachyose are the most common oligosaccharides. They are found in legumes, onions, broccoli, cabbage, and whole wheat¹. The link below shows the raffinose and stachyose content of some plant foods.

The structures of the two oligosaccharides are shown below.

Figure 2.142 Structure of raffinose²

Figure 2.143 Structure of stachyose³

Our digestive system lacks the enzymes necessary to digest these alpha 1-6 glycosidic bonds found in oligosaccharides. As a result, the oligosaccharides are not digested and reach the colon where they are fermented by the bacteria there. Gas is produced as a byproduct of this bacteria fermentation that can lead to flatulence. To combat this problem, Beano® is a popular product that contains an enzyme (alpha-galactosidase) to break down oligosaccharides, thereby preventing them from being used to produce gas.

References & Links

1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.

2. http://en.wikipedia.org/wiki/File:Raffinose.png

3. http://en.wikipedia.org/wiki/File:Stachyose.png

Videos

Raffinose and stachyose content of foods – http://books.google.com/books?id=LTGFV2NOySYC&pg=PA374&lpg=PA374&dq=raffinose+and+stachyose+content+of+vegetables&source=bl&ots=X4Dr7jWmwL&sig=CJFvhAIysSZCP2SOy_MqhfoVYQQ&hl=en&ei=TSRITdTfLNH0gAfB2MX_BQ&sa=X&oi=book_result&ct=result&resnum=6&ved=0CD0Q6AEwBQ#v=onepage&q=raffinose%20and%20stachyose%20content%20of%20vegetables&f=false

Beano’s University of Gas – http://beano.com.cn/university-of-gas#

2.15 Polysaccharides

Poly means “many” and thus polysaccharides are made up of many monosaccharides (>10). There are 3 main classes of polysaccharides: starch, glycogen, and most fibers. The following sections will describe the structural similarities and differences between the 3 classes of polysaccharides that are divided in the figure below.

Subsections:

2.151 Starch

2.152 Glycogen

2.153 Fiber

2.151 Starch

Starch is the storage form of glucose in plants. There are two forms of starch: amylose and amylopectin. Structurally they differ in that amylose is a linear polysaccharide, whereas amylopectin is branched. The linear portion of both amylose and amylopectin contains alpha 1-4 glycosidic bonds, while the branches of amylopectin are made up of alpha 1-6 glycosidic bonds.

Figure 2.1511 Structure of amylose

Figure 2.1512 Structure of amylopectin

Amylopectin is more common than amylose (4:1 ratio on average) in starch^1,2. Some starchy foods include grains, root crops, tubers, and legumes.

References & Links

1. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.

2. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.

2.152 Glycogen

Glycogen is similar to starch in that it is a storage form of glucose. Glycogen, however, is the carbohydrate storage form in animals, rather than plants. It is even more highly branched than amylopectin, as shown below.

Figure 2.1521 Structure of glycogen

Like amylopectin, the branch points of glycogen are alpha 1-6 glycosidic bonds, while the linear bonds are alpha 1-4 bonds, as shown below.

Figure 2.1522 Structure of glycogen¹

The advantage of glycogen’s highly branched structure is that the multiple ends (shown in red above) are where enzymes start to cleave off glucose molecules. As a result, with many ends available, it can provide glucose much more quickly to the body than it could if it was a linear molecule like amylose with only two ends. We consume almost no glycogen, because it is rapidly broken down by enzymes in animals after slaughter².

References & Links

1. http://en.wikipedia.org/wiki/File:Glycogen.png

2. Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.

2.153 Fiber

The simplest definition of fiber is indigestible matter. Indigestible means that it survives digestion in the small intestine and reaches the large intestine.

There are 3 major fiber classifications¹:

Dietary Fiber – nondigestible carbohydrates and lignin that are intrinsic and intact in plants

Functional Fiber – isolated, nondigestible carbohydrates that have beneficial physiological effects in humans

Total Fiber – dietary fiber + functional fiber

The differences between dietary and functional fiber are compared in the table below:

Table 2.1531 Differences between dietary fiber and functional fiber

Dietary fiber is always intact in plants, whereas functional fiber can be isolated, extracted or synthesized. Functional fiber is only carbohydrates, while dietary fiber also includes lignins. Functional fiber can be from plants or animals, while dietary fiber is only from plants. Functional fiber must be proven to have a physiological benefit, while dietary fiber does not.

Polysaccharide fiber differs from other polysaccharides in that it contains beta-glycosidic bonds (as opposed to alpha-glycosidic bonds). To illustrate these differences, consider the structural differences between amylose and cellulose (type of fiber). Both are linear chains of glucose, the only difference is that amylose has alpha-glycosidic bonds, while cellulose has beta-glycosidic bonds as shown below.

Figure 2.1531 Structures of amylose and cellulose

The beta-bonds in fiber cannot be broken down by the digestive enzymes in the small intestine so they continue into the large intestine.

Fiber can be classified by its physical properties. In the past, fibers were commonly referred to as soluble and insoluble. This classification distinguished whether the fiber was soluble in water. However, this classification is being phased out in the nutrition community. Instead, most fibers that would have been classified as insoluble fiber are now referred to as nonfermentable and/or nonviscous and soluble fiber as fermentable, and/or viscous because these better describe the fiber’s characteristics². Fermentable refers to whether the bacteria in the colon can ferment or degrade the fiber into short chain fatty acids and gas. Viscous refers to the capacity of certain fibers to form a thick gel-like consistency. The following table lists some of the common types of fiber and provides a brief description about each.

Table 2.1532 Common types of nonfermentable, nonviscous (insoluble) fiber

Table 2.1533 Common types of fermentable, viscous (soluble) fiber

The following table gives the percentage of total dietary fiber in 5 foods.

Table 2.1534 Total dietary fiber (as percent of sample weight)³

The table below shows the amount of nonfermentable, nonviscous fiber in these same five foods.

Table 2.1535 Nonviscous fiber (as percent of sample weight)³

The table below shows the amount of fermentable, viscous fiber in these same five foods.

Table 2.1536 Viscous Fiber (as percent of sample weight)³

tr = trace amounts

Foods that are good sources of non fermentable, non viscous fiber include whole wheat, whole grain cereals, broccoli, and other vegetables. This type of fiber is believed to decrease the risk of constipation and colon cancer, because it increases stool bulk and reduces transit time⁴. This reduced transit time theoretically means shorter exposure to consumed carcinogens in the intestine, and thus lower cancer risk.

Fermentable, viscous fiber can be found in oats, rice, psyllium seeds, soy, and some fruits. This type of fiber is believed to decrease blood cholesterol and sugar levels, thus also lowering the risk of heart disease and diabetes, respectively⁴. Its viscous nature slows the absorption of glucose preventing blood glucose from spiking after consuming carbohydrates. It lowers blood cholesterol levels primarily by binding bile acids, which are made from cholesterol, and causing them to be excreted. As such, more cholesterol is used to synthesize new bile acids.

References & Links

1. DRI Book –  [Anonymous]. (2005) Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. Washington, D.C.: The National Academies Press. https://www.nap.edu/read/10490/chapter/9

2. Dietary Reference Intakes: Proposed Definition of Dietary Fiber Food and Nutrition Board. 2001 https://www.nap.edu/read/10161/chapter/3

3. Marlett JA. (1992) Content and composition of dietary fiber in 117 frequently consumed foods. J Am Diet Assoc 92: 175-186.

4. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.