Carbohydrates
Biol 135 Lecture: IV. Structure, Function and Source of Macronutrients.
Levels of Organization in Human Nutrition
One of the first things to we examine in some detail will be organic molecules, including Carbohydrates.
These are the Macronutrients required by the human body to properly maintain function, grow and
repair itself. For a brief overview of how atoms combine together to make molecules, we need to be aware
that atoms are made up of protons and neutrons (in the nucleus) and also have electrons zooming at high
speed in orbitals around the nucleus.
Figure 1.The carbon atom and the 3 sub-atomic particles that make it - protons, neutrons and electrons.
The ‘valence’ electrons are the ones that are present in the outermost electron shell. These are the ones
that will interact with other atoms and form chemical bonds! Therefore, the valance electrons are the e-‘s
that determine the chemical properties of the atoms and how they will combine with other atoms to
make molecules.
The figure above shows four fundamental atoms in biology
and nutrition (H = Hydrogen; O = Oxygen; C = Carbon and
N = Nitrogen), indicating their valence electrons. This will
determine the chemical bonds they forms (and therefore the
chemical properties). Atoms typically want to have 8 outer
shell e-s’ (or 2 in the case of Hydrogen). This allows for
optimal stability and is often referred to as the octet rule!
Below are the 4 atoms listed and the number of covalent
bonds they typically make.




H= 1
O= 2
N= 3
C=4
Our first area of focus is how atoms combine to make
molecules. So we need to know the basics about chemical
bonds in the body, so that we can better understand the
molecules we are about to examine.
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Atoms >Molecules> Organelles > Cells > Tissues > Organs > Organ Systems > Organism
Atoms (e.g. C, H, O) form chemical bonds to become larger Molecules (e.g. H2O, C6H12O6) and there are
three ways that atoms can create chemical bonds that bind them together.
Three types of chemical bonds:
1. Covalent bonds – a sharing of electrons between, strong bonds.
Non-Polar: equal sharing
of electrons. e.g. C=C
Polar: unequal sharing of
electrons. e.g. H2O
2. Ionic bonds – complete transfer of electrons, relatively weak bond (crystals strong), break in water.
When substances that are held together with ionic bonds are placed in water they become ions, which are
charged particles (atoms or molecules). They dissociated in water to form electrolytes in solution.
Examples are Na+, K+, Cl-, Ca2+, OH-, Mg2+, HCO3-, H2PO33. Hydrogen bonds – attractive forces between H atoms and O or N atoms. Very weak but important.
Polar molecules, such as water, have a weak, partial negative charge at oxygen region of the molecule
because it is more electronegative than H and pulls the e-s closer to. The H region consequently has a
partial positive, as thee-s spend less time with the H atoms. The saying “opposites attract” is true for water
molecules, as H-Bonds form between the partially negative O and partially positive H atoms, generating
small but important binding forces.
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A. Carbohydrates
Carbohydrates (also called saccharides) are molecular compounds made from just three elements:
Carbon, Hydrogen and Oxygen (CHO). Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose)
are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large
(polysaccharides such as starch, glycogen and cellulose) and have different functions and impacts on
human health.
Carbohydrates are:
 A source of energy for the body e.g. Glucose and a store of energy, e.g. Starch in plants.
 Building blocks for polysaccharides (giant carbohydrates), e.g. Cellulose in plants and glycogen in
the human body.
 Components of other molecules e.g. DNA, RNA, glycolipids, glycoproteins, ATP.
Monosaccharides
Monosaccharides are the simplest carbohydrates and are often called single sugars. They are the building
blocks from which all bigger carbohydrates are made.
Monosaccharides have the general molecular formula (CH2O)n, where n can be 3, 5 or 6. They can be
classified according to the number of carbon atoms in a molecule:
A) n = 3 trioses, e.g. glyceraldehyde
B) n = 5 pentoses, e.g. ribose and deoxyribose ('pent' indicates 5)
C) n = 6 hexoses, e.g. fructose, glucose and galactose ('hex' indicates) 6
A) In a Triose monosaccharide, or simple sugar, it containing three carbon atoms with chemical formula
C3H6O3. There are only three possible trioses: L- or D-Glyceraldehyde (both aldo-trioses because the
carbonyl group is at the end of the chain) and dihydroxyacetone, a ketotriose because the carbonyl group
is the center the chain. The trioses are important in cellular respiration, during glycolysis, Fructose-1,6diphosphate is broken down into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. The
important cellular respiration molecules Pyruvic acid and Lactic acid are later derived from these
molecules.
The monosaccharides contain a single aldehyde or ketone functional group (see figures below). They are
subdivided into two class’s
aldoses and ketoses
on the basis of whether they are aldehydes or
ketones. They are also classified as a triose, tetrose, pentose, or hexose on the basis of whether they
contain three, four, five, or six carbon atoms.
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With only one exception, the monosaccharides are optically active compounds. Although both D and L
isomers are possible, most of the monosaccharides found in nature are in the D configuration. Structures
for the D and L isomer of the simplest aldose, glyceraldehyde, are shown below.
D-Glyceraldehyde L-Glyceraldehyde
There is more than one molecule with the molecular formula C5H10O5 and more than one with the
molecular formula C6H12O6. Molecules that have the same molecular formula but different structural
formulae are called structural isomers.
Benedict's Test - What is a Non-reducing Sugar? See separate notes on details of Benedict's Test.
Briefly, sugars can be classified as either reducing or non-reducing based on their ability to reduce copper
(II) ions to copper (I) ions during the Benedict's Test.
Non-reducing sugars do not contain an aldehyde group - the reducing species. Reducing sugars are
simple, disaccharide sugars. Sucrose is the most common disaccharide non-reducing sugar. One test for
the presence of many simple carbohydrates is to use Benedict's reagent. It turns from turquoise to yellow
or orange when it reacts with reducing sugars. These are simple carbohydrates with unbound aldehyde or
ketone groups. Benedict's reagent is also used to test for an important reducing sugar: glucose. This
information is included here because a Benedict's Test is used to determine what type of sugar a substance
has in it.
Benedict's & Glucose:
Positive Control.
Benedict's & Water:
Negative Control.
Benedict's & Unknown:
Trace (No sig reducing
sugars)
Benedict's & Unknown:
Positive (Some reducing
sugars).
Figure 2. Performing the “Benedict’s Test” on food samples to measure the type of simple sugar food contains.
B) The Pentoses are monosaccharides that contain five carbon atoms, along with a combination of other
elements. It also contains 10 hydrogen and five oxygen atoms. The most relevant and famous are ribose
and deoxyribose, though there are a number of other types of pentoses (e.g., xylose, ribulose, and
arabinose).
The ribose unit forms part of a nucleotide of Ribonucleic Acid (RNA) and the deoxyribose unit forms
part of the nucleotide of Deoxyribonucleic Acid (DNA). Nucleotides form
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Although Nucleotides are organic molecules that serve as the monomers, or subunits, of nucleic acids like
DNA and RNA, they are not directly involved in human nutrition in terms or energy yielding organic
molecules in the diet. It is worth knowing that the building blocks of nucleic acids, nucleotides are
composed of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate
group.
C) The Hexoses –there are 3 very important monosaccharides in human nutrition, each contain six carbon
atoms:
1. Glucose
2. Galactose
3. Fructose
1. Glucose
Glucose is the most important carbohydrate fuel in human cells. Normally, blood glucose levels or
concentrations in the blood, range from 70 to 100 mg/dL (overnight fasting), and can be as high as 150
mg/dL (after a meal). The small size and solubility in water of glucose molecules allows it to pass
through the cell membranes into the cell. Energy is released when the molecules are metabolized
(C6H12O6 + 6O2
6CO2 + 6H2O). This is part of the process of cellular respiration.
There are two forms of the cyclic ‘ring’ glucose molecule: α-glucose and β-glucose.
Figure 3. Comparison of the linear and “ring” structures of glucose and the alpha and beat glucose molecules.
Note that it is a seemingly small difference in the orientation of the OH group between the α and β
glucose molecules. This has an important impact when forming long polymers (many many glucose
molecules connected in a long chain), creating important dietary differences between starch and
cellulose.
Glucose is a very important energy molecule in the body and is the preferred energy source of the neurons
in the brain. Glucose is also called blood sugar, as it circulates in the blood, and relies on the enzymes
glucokinase or hexokinase to initiate metabolism. Your body processes most carbohydrates you eat into
glucose, either to be used immediately for energy or to be stored in muscle cells or the liver as glycogen
for later use. Unlike fructose, insulin is secreted primarily in response to elevated blood concentrations of
glucose, and insulin facilitates the entry of glucose into cells.
glucokinase
Glucose
Glucose-6-Phosphate
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2. Galactose
Galactose molecules look very similar to glucose molecules. They can also exist in α and β forms.
Galactose reacts with glucose to make the disaccharide lactose.
However, glucose and galactose cannot be easily converted into one another. Galactose cannot play the
same part in respiration as glucose. This comparison of glucose and galactose shows why the precise
arrangement of atoms in a molecule is so important, displayed in the structural formula below.
3. Fructose – often called ‘fruit sugar’, it is one of the three dietary monosaccharides, along with glucose
and galactose, that are absorbed directly into the bloodstream during digestion is a simple ketonic
monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide
sucrose.
Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses
(reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a
six-atom ring. Fructose reacts with glucose to make the disaccharide sucrose.
Figure 4. In terms of structure, glucose and fructose are different enough so that they are process and used by the
human body very differently. Glucose can be used by every cell, but it is mostly the liver which must process
fructose. If the fructose load is high in the diet, this will tax the liver and can cause serious liver damage.
Different sugars don’t all taste the same. Some taste more or less sweet than each other. If the sweetness
of sucrose, the sugar with which most people are the most familiar, is arbitrarily assigned a sweetness of
100%, then here’s how other common sugars compare:
Sugar Sweetness
fructose 173%
sucrose 100%
glucose 74%
maltose 33%
galactose 33%
lactose 16%
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High-Fructose Corn Syrup
High-fructose corn syrup (HFCS) is widely used, relatively cheaper to produce, and backed by an army of
corn refiners. It began to saturate the commercial processed “food producers” market in the mid 1970’s
and continued to make great leaps in the 1980’s and 1990 (see Fig 5.). But the tide may be turning. The
original argument to counter health concerns about HFCS was that is all natural and really no different to
sucrose. Both true statement’s – this fact is that too much of any refined sugar is bad for you, whether its
sucrose or HFCS. In addition, Heroine and Morphine are all natural too! All are highly addictive.
History of HFCS
Invented by research duo Marshall and Kooi in 1957, high fructose corn syrup purged glucose from the
old corn syrup formula and went on a profitable journey into the American market. The rise of highfructose corn syrup in our taste and consciousness was aided by the federal subsidies on corn refiners,
surplus in corn supply, and commercial interests. Since fructose is much sweeter than glucose, the slightly
greater proportion of fructose in HFCS made it appealing initially, as it was initially thought that less of it
would be needed to reach the same sweetness. What actually occurred was a heavier dosing of
‘sweetness’ to commercially produced foods, which not surprisingly changed the American palate to
become much more accustomed to overly sweet food, especially beverages.
High fructose corn syrup (HFCS) is liquid and contains 24% water, compared to sucrose (table sugar)
which is dry and granulated. HFCS 55 (≈55% fructose if water were removed) is mostly used in soft
drinks; HFCS 42 is used in beverages, processed foods, cereals, and baked goods. There is HFCS 90
(90% fructose), yielding excessive amounts of fructose, which can be very harmful. HFCS 90 is rarely
used, and then only in tiny amounts due to its extreme sweetness.
High Fructose Corn Syrup vs. Regular Sugar
There are only tiny differences between the most common type of high fructose corn syrup (HFCS 55)
and regular sugar. The fructose and glucose in high fructose corn syrup are not bound together like in
granulated sugar (sucrose), but instead they “float” around separately next to each other. In our digestive
system, sugar is broken down into fructose and glucose, so corn syrup and sugar end up looking exactly
the same, with slight variations in the proportions of fructose to glucose. Gram for gram, HFCS 55 has
slightly higher levels of fructose than regular sugar. The difference is very small and not particularly
relevant from a health perspective.
Figure 5. This graph shows the usage of various sweeteners in the U.S.A over the recent past (last 45 years).
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HFCS makes Diabetes Worse and promotes other Diseases

Contributes to fat deposits in your liver, increasing buildup of lipoproteins.

Leads to plaque buildup and narrowing of blood vessels. And speeds-up aging process.

HFCS consumption worsens diabetes mellitus, which is mainly rooted in insulin problems.

Metabolic Syndrome – is a host of deleterious conditions that can be caused by consuming too
much HFCS-containing drinks and other foods.

Damage to your Immune System – asthma, food allergies, multiple sclerosis and other immune
system problems are triggered by use of sugars such as HFCS.

Mercury (extremely toxic) is found in samples of commercial HFCS from the processing
Metabolic Syndrome – is a condition when an individual has a group biochemical and physiological
abnormalities that create the state of numerous metabolic factors existing in one individual which
generally include:
1.
2.
3.
4.
5.
6.
High Blood Pressure
Abdominal Fat
High Blood Triglyceride levels
High Uric Acid levels
Insulin Resistance
State of Chronic Inflammation
These are the 6 major f metabolic actors that that can potentially lead to chronic disease states, most
notably are the significantly elevated risks of
 Cardiovascular Disease
 Diabetes Mellitus Type II
* Est. over 50 million Americans have this Metabolic Syndrome condition.
There is a lot of evidence to support the growing hypothesis that there is a causal link between the over
consumption of high-fructose corn syrup (HFCS) and metabolic syndrome.
Figure 6. The very real health dangers of Metabolic Syndrome
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Disaccharides
Monosaccharides are rare in nature. Most sugars found in nature are disaccharides, which form when two
monosaccharides react. A condensation or dehydration synthesis reaction takes place, by removing a
water molecule and releasing it to form a covalent bond. This process requires energy and a glycosidic
(covalent) bond forms as a consequence and holds the two monosaccharide units together.
The three most important disaccharides in human nutrition are maltose, lactose and sucrose. They are
formed from the appropriate monosaccharides (see below). Lactose and maltose are reducing sugars.
Sucrose is a non-reducing sugar.
Disaccharide
Monosaccharides____
1.Maltose
from
α-glucose + α-glucose
2.α-Lactose*
from
α-glucose + β-galactose
3.Sucrose
from
α-glucose + α-fructose
*Lactose also exists in a beta form, which is made from β-galactose and β-glucose
Disaccharides are soluble in water (meaning they are polar or hydrophilic), however when consumed in
the diet they are too big to pass through the cell membrane by diffusion. They must be broken down in the
small intestine during digestion to give the smaller monosaccharides that are able to pass into the blood
stream and through cell membranes into cells.
The basic catabolic process of the important dietary disaccharides is:
C12H22O11 + H2O
C6H12O6 + C6H12O6 + Energy
This is a hydrolysis reaction and releases energy. This is what occurs in the gastrointestinal (G.I.) tract
when we ingest macronutrients and break them down.
The opposite (or reverse) of this is a condensation reaction, called dehydration synthesis reaction, and
requires energy input. This is what predominantly occurs during tissue growth and repair.
The basic anabolic process of the important nutritional disaccharides is:
C6H12O6 + C6H12O6 + Energy
C12H22O11 + H2O
In the body, monosaccharides are used very quickly by cells. However, a cell may not need all the
energy immediately and it may need to store it. Monosaccharides are converted into disaccharides in the
cell by dehydration synthesis reactions. Further dehydration synthesis reactions result in the formation
of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell.
These large molecules made by the body can be broken down again by hydrolysis into monosaccharides
when energy is needed by the cell.
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1. Maltose– is made from two glucose molecules combining to form the disaccharide maltose, sometimes
referred to as ‘malt’ or ‘grain’ sugar - as it is present in germinating grain (e.g., barley are used in making
grain alcohol). It is the least common disaccharide in nature.
Malted Barley: Beer is basically made from water, malted barley and hops (for the bitter taste).
Barley grains must be "malted" before using in the brewing process, this involves bringing the grain to the
point of its highest possible starch content by allowing it to begin to sprout roots and take the first step to
becoming a photosynthesizing plant. Then, by heating the grain to a high enough temperature, it stops
growth but allows an important natural enzyme diastase to remain active. This enzyme can convert starch
quite easily into the disaccharide called Maltose. This sugar is then fermented or metabolized by the
yeasts to create carbon dioxide and ethyl alcohol.
2. Lactose is sometimes referred to a ‘milk sugar, it is a disaccharide sugar derived from galactose and
glucose that is found in milk. Lactose makes up around 2–8% of milk (by weight),[3] although the
amount varies among species and individuals, and milk with a reduced amount of lactose also exists. It is
extracted from sweet or sour whey
Lactose intolerance is the inability of adults to digest lactose (‘milk sugar’). This is a consequence of
lactase deficiency, which is the enzyme that breaks the glycoside bond between glucose and galactose.
The cause may be either genetic or environmental, in either case, the symptoms are caused by
insufficient levels of lactase. The activity of lactase is high in newborns, but declines after weaning, such
that the production of lactase drops off with age. Northern Europeans are exceptional in that lactase
activity often remains high in adults. Temporary lack of lactase activity may also be due to such causes as
viral infections, in which the epithelial tissues normally secreting the enzyme do not produce enough to
digest incoming lactose.
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Specifically, the enzyme beta-galactosidase which is located on the epithelial surface lining the
duodenum is required to digest lactose. If the enzyme is absent, lactose is too large to be directly absorbed
through the wall of the small intestine into the bloodstream, so, if ingested in the absence of lactase, it
travels along intact into the colon.
Bacteria in the colon can metabolize lactose, and the resulting fermentation produces copious amounts
of gas (a mixture of hydrogen, carbon dioxide, and methane) that causes the various abdominal
symptoms. The unabsorbed sugars and fermentation products also raise the osmotic pressure of the colon,
causing an increased flow of water into the bowels. In most cases, it can cause symptoms such as
abdominal bloating, cramps, flatulence, diarrhea, nausea, borborygmi (rumbling stomach), or vomiting
after consuming significant amounts of lactose.
3. Sucrose is commonly known as table sugar, and is obtained from sugar cane or sugar beets. Fruits and
vegetables also naturally contain sucrose. When sucrose is consumed, the enzyme beta-fructosidase
separates sucrose into its individual sugar units of glucose and fructose. Both sugars are then taken up by
their specific transport mechanisms. The body responds to the glucose content of the meal in its usual
manner; however, fructose uptake occurs at the same time. The body will use glucose as its main energy
source and the excess energy from fructose, if not needed, will be poured into fat synthesis, which is
stimulated by the insulin released in response to glucose.
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Oligosaccharides
From Greek "oligos," meaning "a few" and Latin "sacchar," meaning "sugar") they are carbohydrates that
consist of 3 to 10 monosaccharides covalently linked together. This type of saccharide polymer is located
in amino acid side chains. Most common plants with large amounts are: onions, artichokes, chicory
root, legumes, asparagus, wheat and jicama.
They are characterized by their mildly sweet taste and unique ‘mouth feel’.
These carbohydrates are an important part of plasma membranes and play a role in the cell-cell
recognition within. An example is the ABO blood typing cell recognition is from glycolipids markers that
are oligosaccharides.
Nutritional Benefits
Some Nutritionists believe eating oligosaccharides are beneficial, since the undigested portion serves as
food for the intestinal microflora. Depending on the type of oligosaccharide, it can increase the number of
beneficial bacteria in the colon while simultaneously reducing the population of harmful bacteria. Others
view them as problematic as nearly 90% of foods containing them are not broken down in the small
intestine which may cause issues for the colon.
Basic Examples
Fructo-Oligosaccharides (FOS), found in many vegetables, short chains of fructose molecules.
Galacto-Oligosaccharides (GOS), also occur naturally, short chains of galactose molecules.
Many types but 2 important specific examples are:
1. Raffinose- indigestible, made from galactose-glucose-fructose.
2. Stachyrose- indigestible, made from galactose-galactose-glucose-fructose.
Role in Human Reproduction System
In a 2011 study it was found that receptor for an identified oligosaccharide completely cover the outer
coating of the human egg cell or ova (called the zona pellucida). It is implicated in binding of Sperm to
ova leading to fertilization. The discovery of the largest receptor on the human ovum may help improve
infertility research.
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Polysaccharides
Monosaccharides can undergo a series of condensation or dehydration synthesis reactions, adding one unit
after another to the chain until very large molecules (polysaccharides) are formed. This is called
condensation (dehydration synthesis) polymerization, and the building blocks are called monomers.
The properties of the resulting polysaccharide molecule depend on:




its length (though they are usually very long)
the extent of any branching (addition of units to the side of the chain rather than one of its ends)
any folding which results in a more compact molecule
whether the chain is 'straight' or 'coiled'
Starch
Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose and
amylopectin. Both are made from α-glucose. Amylose is an un-branched polymer of α-glucose. The
molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a
branched polymer of α-glucose. It is completely insoluble in water.
Amylose
about 40% of the starch
straight chain polymer of D-glucose units
α-1->4 glycosidic linkage
Amylopectin
about 60% of the starch
branched chain polymer of D-glucose units
α-1->4 glycosidic linkage and α-1->6 glycosidic linkage
at branch points
α and β amylases can hydrolyze amylose
α and β amylases hydrolyze α-1->4 glycosidic linkage but
not at α-1->6 glycosidic linkage points of amylopectin
more soluble in water
less soluble in water
Soluble in hot water without swelling
Soluble in hot water with swelling
Soluble in hot water; does not form starch gel Soluble in hot water; forms starch gel or paste
or paste
Examples of Food that contain significant Starch:
Grains, like wheat, rice, barley, oats, corn, potatoes and beans
Grains are made into bread, cereal and pasta, as well as crackers, biscuits, cookies, cakes, pie crust,
and anything else made with flour (amylose). This amylose is broken down quite slowly. The higher the
amount of amylose in a starch, the more slowly it is digested.
Different types of rice have differing percentages of amylose, and therefore different glycemic indices!
Long grain rice, which tend to stay more separate, are higher in amylose, thus lower on the
glycemic index.
Short grain rice, which tend to produce creamier and stickier rice are low in amylose and are
more glycemic.
New potatoes (sometimes described as "waxy") have a starch that is closer to amylose in structure than
more mature potatoes, and they are somewhat less glycemic.
Most of the starch in beans has a structure which is only slowly broken down into sugars.
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Cooking time and thickness of the pasta greatly affects how it is digested and consequently affects the
glycemic index. Pasta is a processed food however, when the pasta is cooked "al dente" (slightly firm)m it
is digested more slowly, also due to the starch molecules being so tightly packed that only about half is
rapidly digested.
Some cooked starches, such as potatoes and rice, when cooked and cooled, a small percentage of the
starch takes longer to digest.
Glycogen
The primary role of glycogen in animals is to act as a storage molecule for glucose. Glycogen is
polymer of glucose and is an amylopectin with very short distances between the branching side-chains.
When we eat foods that contain starch from plants, it is hydrolyzed in the digestive system to liberate
glucose. When new sources of glucose arrive in the body, a fairly limited amount of it is stored as
glycogen, predominantly in the liver. This creates a fairly fast way to liberate glucose in times of
hypoglycemia. Glycogen is highly branched compared to starch.
Glucose passes into the cell and is used in metabolism. Inside the cell, glucose can be polymerized to
make glycogen which acts as a carbohydrate energy store.
Glucose easily passes into cells and is used in metabolism. Any spare glucose is grabbed by the liver
which has a limited capacity to store it as Glycogen. Inside the hepatocytes of liver, glucose can be
polymerized to make glycogen which acts as a carbohydrate energy store.
Glycogen is stored in three places in the body:
1) the Liver; 2) Skeletal Muscle; and 3) the Uterus
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Cellulose
Cellulose is a third polymer made from glucose. But this time it's made from β-glucose molecules and the
polymer molecules are 'straight'. There are many other element involved in creating the varieties of
structures, but the polymer below is the most common,
Figure 7. Section of a cellulose molecule. Notice the ‘every other glucose is upside down’ pattern.
Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in
plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of
glucose units in the polymer chain and the hydrogen-bonding between neighboring chains. Cellulose is
not hydrolyzed easily and, therefore, cannot be digested so it is not a source of energy for humans. The
stomachs of Herbivores contain a specific enzyme called cellulase which enables them to digest cellulose.
Starch and cellulose are two very similar polymers, both made from the monomer, glucose. In starch, all
the glucose repeat units are oriented in the same direction. But in cellulose, each successive glucose unit
is rotated 180 degrees around the axis of the polymer backbone chain, relative to the last repeat unit.
This is referred to as 1-4 alpha linkages in starch and 1-4 beta linkages in cellulose. This seemingly small
structural difference in the arrangement of the glucose molecules in the chain makes a large difference in
how humans metabolize these molecules. We can digest starch, but can't digest cellulose. Our body’s
contains enzymes that break starch down into glucose to fuel your body, but we don't have enzymes that
can break down cellulose. Animals like termites and cattle (who eat wood and grass) break down
cellulose in their special multi chambered stomachs.
Fiber – the Full Story and How the Different Types Affect You
Many people have a very basic understanding of fiber and tend to lump it all into one category. However,
the truth is that not all fiber is created equal. From gut bacteria to weight loss, fiber it is often
considered a fundamental part of a healthy diet. Some types are highly beneficial, while others can cause
digestive problems in some people. Let us briefly explore the important things you need to know about
the different types of fiber.
What is Fiber?
Fibers or ‘roughage’ refers to a diverse group of indigestible carbohydrates found naturally in plant
foods, including vegetables, fruits, legumes, whole grains, nuts and seeds. This is a category of
carbohydrates that humans cannot digest.
The recommended daily intake is 38 g/day for men and 25 g/day for women. However, most people are
only eating around half of that, or 15 g/day.
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Advantages of Consuming Fiber are: the production of healthful compounds during the fermentation
of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk,
soften stool, and shorten transit time through the intestinal tract. A disadvantage of a diet high in fiber is
the potential for significant intestinal gas production and bloating.
Dietary fibers include cellulose, lignin, and pectin, which are predominantly polymers of glucose.
For example, starch and cellulose are two very similar polymers, both made from the monomer, glucose.
In starch, all the glucose repeat units are oriented in the same direction. But in cellulose, each successive
glucose unit is rotated 180 degrees around the axis of the polymer backbone chain, relative to the last
repeat unit. This is referred to as 1-4 alpha linkages in starch and 1-4 beta linkages in cellulose.
When looking at structural representations of the molecules (see Fig 1 below), every other glucose
molecule in cellulose appears ‘upside down’ and this arrangement requires another type of digestive
enzyme to break that bond (cellulase) – one which humans do not possess! Therefore, we cannot digest or
metabolize cellulose, so they pass through most of the digestive system unchanged
Figure 8. Structural differences between starch and cellulose
Figure 9. Comparison of the branching of polymers of glucose found in starch, glycogen and cellulose.
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How are Fibers Classified?
The term "fiber" is somewhat of a misnomer, since many types of dietary fiber are not actually fibrous;
there is actually a huge variety of different fibers found in foods. One issue is that that they can be
classified in different ways, which can be highly confusing. In 2001, fiber was formally classified into
two main types:

Dietary fiber: Fiber found naturally in foods.

Functional fiber: Fiber that is extracted and isolated from whole foods, then added to processed
foods.
This is nice, but this classification gives no indication of their health effects on humans. A much more
effective alternative method is to classify fiber based on its solubility (soluble vs. insoluble), viscosity
(viscous vs. non-viscous) and fermentability (fermentable vs. non-fermentable). There is yet another
class of nutrients called resistant starches, which are often classified as dietary fibers, but we can go into
that later.
Soluble vs. Insoluble Fiber
The solubility of fiber refers to its ability to dissolve in water. Based on this, fiber has often been
categorized as either soluble or insoluble: Soluble fiber has various benefits for metabolic health, while
insoluble fiber functions mostly as a bulking agent. Different plant foods have varying proportions of
soluble and insoluble fibers.
Soluble fiber blends with water in the gut, forming a gel-like substance. It can reduce blood sugar spikes,
and has various metabolic health benefits.
Soluble fibers include gums, pectins, psyllium, beta-glucans and others.
Insoluble fiber does not blend with the water and passes through the digestive system virtually intact. It
functions mostly as a “bulking” agent, and may help speed the passage of food and waste through your
gut.
Insoluble fibers include lignin and cellulose.
Cellulose - insoluble fiber
e.g., in skin of fruit
Pectin - soluble fiber
e.g., in flesh of fruit
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Viscous Fiber
Some types of soluble fibers form a thick gel when they blend with water. These are known as viscous
fibers. Put simply, the viscosity of a fluid refers to its “thickness.” For example, honey is more viscous
than water. When you eat viscous fiber, it forms a gel-like substance that “sits” in the gut. This slows
down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced
appetite.
A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused
weight loss. Viscous fibers include glucomannan, beta-glucans, pectins, guar gum and psyllium. Good
whole-food sources include legumes, asparagus, Brussels sprouts, oats and flax seeds.
Fermentable Fiber
An estimated 100 trillion live bacteria reside in the human gut, mainly in the large intestine. These
bacteria are actually crucial for optimal health in humans. They play various roles related to weight
management, blood sugar control, immunity, brain function and mental health. The friendly gut bacteria
(also called gut flora) are so important that they are often referred to as the “forgotten organ”. Because
humans can’t digest fiber, it ends up reaching the large intestine mostly unchanged.
This is where fermentable fiber comes into play. These are fibers that our good gut bacteria are able to
digest (ferment) and use as fuel. This increases the number and balance of friendly gut bacteria, which
also produce vitamin K and short-chain fatty acids with powerful health benefits. Most fermentable
fibers are soluble, but there are also some insoluble fibers that can function in this way.
Fermentable fibers include pectins, beta-glucans, guar gum, inulin and oligofructose. The best whole-food
sources of fermentable fibers are beans and legumes. A 1-cup serving often provides up to half of the
recommended daily intake of fiber.
All that being said, one of the by-products of fiber fermentation is gas. This is why foods high in
fermentable fiber can cause flatulence and stomach discomfort, especially if people are not used to eating
a lot of fiber.
Resistant Starch
Resistant starch is a type of starch that escapes digestion, hence the term ‘resistance starch’. It functions
like soluble, fermentable fiber, and has numerous health benefits. Starches are the main types of digestible
carbohydrates in the diet, but some can resist digestion and act more like fibers.
Resistant starch has numerous powerful health benefits. It improves digestive health, enhances insulin
sensitivity, lowers blood sugar levels and significantly reduces appetite. There are several good food
sources of resistant starch, including green bananas, various legumes, cashews and raw oats and raw
potato.
Unique Fibers That Are Worth Highlighting
Several fibers have specific health implications, and are worthy of highlighting. Fructans are fibers that
can cause adverse digestive symptoms in some people. Beta-glucans and glucomannan are soluble,
viscous fibers with potent health benefits.
Fructans - A fructan is the term used to describe a small chain of fructose molecules.
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Oligofructose and inulin are the two main fructan varieties in the diet. They can feed the friendly
bacteria in the gut, and have been shown to help treat certain types of diarrhea.
However, fructans are also classified as FODMAPs, types of carbohydrates known to cause digestive
issues in many people. The acronym FODMAP is from "Fermentable Oligo-, Di-, Mono-saccharides
And Polyols." These are carbohydrates commonly found in the modern western diet. The restriction of
these FODMAPs from the diet has been found to have a beneficial effect for sufferers of irritable bowel
syndrome and other functional gastrointestinal disorders (FGID). Fructans and other FODMAPs trigger
adverse symptoms in 3 out of 4 people with irritable bowel syndrome, a common digestive disorder.
The biggest source of fructans in the modern diet is wheat.
Beta-Glucan - The health benefits of beta-glucans have been extensively documented. These fibers have
a specific molecular structure that makes them highly viscous in the gut. Beta-glucans can improve insulin
sensitivity and lower blood sugar levels. They can also significantly reduce cholesterol levels and increase
feelings of fullness.
The main food sources of beta-glucans are oats and barley.
Glucomannan - is a viscous fiber that is commonly marketed as a weight loss supplement. Numerous
studies have shown that glucomannan can cause modest weight loss, fight constipation and improve risk
factors for heart disease.
In Summary: Fibers that are soluble, viscous and fermentable appear to be the healthiest. Resistant
starches are also incredibly healthy. Good sources of healthy fibers include vegetables, fruits, oats,
legumes, nuts, dark chocolate, avocados, chia seeds and various other foods.
Figure 10. A summary of the important molecular differences between glucose, starch and cellulose that has a
significant impact on how the human body utilizes these molecules.