CarbohydratesBiol 135 Lecture: IV. Structure, Function and Source of Macronutrients.

Levels of Organization in Human NutritionOne 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.

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-, H2PO3

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3. 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.

Non-Polar: equal sharing of electrons. e.g. C=C

Polar: unequal sharing of electrons. e.g. H2O

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A. CarbohydratesCarbohydrates (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.

MonosaccharidesMonosaccharides 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-1,6-diphosphate 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.

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

Benedict's & Water: Negative Control.

Benedict's & Unknown: Trace (No sig reducing sugars)

Benedict's & Unknown: Positive (Some reducing sugars).

Benedict's & Glucose: Positive Control.

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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. Glucose2. Galactose3. Fructose

1. GlucoseGlucose 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.

glucokinaseGlucose Glucose-6-Phosphate

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2. GalactoseGalactose 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 Sweetnessfructose 173%sucrose 100%glucose 74%maltose 33%galactose 33%lactose 16%

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High-Fructose Corn SyrupHigh-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 HFCSInvented 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 high-fructose 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 SugarThere 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. High Blood Pressure 2. Abdominal Fat 3. High Blood Triglyceride levels 4. High Uric Acid levels5. Insulin Resistance6. 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 Disaccharides

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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 BenefitsSome 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 ExamplesFructo-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 SystemIn 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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PolysaccharidesMonosaccharides 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'

StarchStarch 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 Amylopectinabout 40% of the starch about 60% of the starchstraight chain polymer of D-glucose units branched chain polymer of D-glucose unitsα-1->4 glycosidic linkage α-1->4 glycosidic linkage and α-1->6 glycosidic linkage

at branch pointsα 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 amylopectinmore soluble in water less soluble in waterSoluble in hot water without swelling Soluble in hot water with swellingSoluble in hot water; does not form starch gel or paste

Soluble in hot water; forms starch gel 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.

GlycogenThe 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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CelluloseCellulose 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 FiberThe 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 fibere.g., in skin of fruit

Pectin - soluble fiber e.g., in flesh of fruit

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Viscous FiberSome 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 FiberAn 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 StarchResistant 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 HighlightingSeveral 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.