Download Module 1 - Carbohydrates part A [1] Today, we want to talk about

Module 1 – Carbohydrates part A
[1]
Today, we want to talk about carbohydrates, part A. Now this is going to be a little more technical
information. Carbohydrates are the fourth most important nutrient. Primarily we classify it as the fourth
most important nutrient because it does not harm us and it's a fairly inexpensive energy source; whereas
fat actually have important function that we will discuss as the next topic, but they also are a serious risk
factor for coronary heart disease. This is why we place them last, but let's not get ahead of ourselves.
We are going to talk today about carbohydrates, part A. Again, this is going to be more technical
information: what they are, where they come from, and how do we define them or categorize them. You
really need to pay attention here because a lot of good questions can come from this topic,
carbohydrates, part A.
[2]
Before we get into the technical issues, let's ask ourselves: where do calories come from in the American
diet? Presently, as this slide shows, 48% of our calories come from carbohydrates, 40% comes from fat,
and 12% comes from protein. These numbers are probably slightly off; the fat contribution is probably
high and should be below 40%. It is not that large of a reduction so we will leave it where it is. So
presently our contribution of calories in the American diet is about 48% from carbohydrates, 40% from fat
and about 12% from protein.
[3]
Ideally, based on dietary recommendations, we would like 60% of our calories coming from
carbohydrates, 30% of our calories coming from fat, and 10% coming from protein. So again, ideally
according to dietary recommendations, about 60% of our calories should come from carbohydrates, 30%
of our calories should come from fat, and 10% should come from protein. From our present contribution
to calories, on the previous slide, you can see we have a long way to go. We are still fairly high in
fat-oriented diets. However, as I said, those numbers are probably a little higher than what they are
supposed to be today. They are probably down. I think the last time I heard, it was somewhere around
38% of our calories were from fat, so we are getting the message. Yet even with this encouraging
statistic, greater than 50% of the population is considered overweight or above what is considered their
ideal body weight, and that is not a good thing. So there are a number of factors and trends that drive
consumers to eat one or the other type of macronutrients. Over the last 20 years, we have cycled back
and forth as to which macronutrient, carbohydrate or fat, is contributing to this obesity issue. Ideally, we
should probably watch out for both of these nutrients when it comes to watching our weight.
[4]
Let's begin to define carbohydrates. By name, they mean hydrates of carbon. Carbohydrates, hydrates
of carbon. They are carbon surrounded by water. The simplest of carbohydrates is C6H12O6. C is for
carbon, H is for hydrogen and O is for oxygen in those proportions: 6C, 12H, and 6O. Since, they are
carbons surrounded by water, we can explain the 12Hs and 6Os as part of the water for each of the
6Carbons. They are the most widely distributed compounds on earth. What does that mean? Why are
they considered the most widely distributed compounds on earth? Although plants are major dietary
carbohydrates source, we use a lot of plants for many other important applications, such as every time
you read or write, you are using carbohydrates. Paper is a carbohydrate; that is why they are the most
widely distributed compounds on earth. Another example is insects: their cuticle or shell is carbohydrate.
Between paper and insects, that is why it is the most widely distributed compound on earth. If you
consider the billions of different insects out there and the tonnage of paper that we recycle or discard, it is
amazing how much carbohydrates are found on earth. For us, the major dietary food sources of
carbohydrates are plants. Plants are where we get the bulk of our carbohydrates in the diet.
[5]
Since plants are major sources of carbohydrates, let's talk a little bit about where they come from. It
starts in the leaves with photosynthesis. You have carbon dioxide from air, you have water coming in
and stored, and you have energy from the sun. The carbon dioxide provides the carbon, the water is
there for the hydration, and then the leaves, by photosynthesis, produce the first carbohydrate, C6H12O6,
called glucose. A glucose produced in the chlorophyll of the leaves is transported all over the place. If
the plant is fruit bearing, the glucose is converted to fructose, or glucose and fructose, combined to form
sucrose, and now you have the reason why fruits are so sweet--because you have glucose, fructose and
sucrose in the fruit, and that is why upon ripening, fruits are sweet. The sucrose in certain plants end up
in the stems and the sappy material, which is? Well hopefully, you said maple syrup, as we use that
syrup that we like on our waffles and pancakes, etc. Glucose, if it is found in the roots and seeds and
tubers of plants, forms with itself to form long polymers of carbohydrates called starch. So glucose is
stored as plant starch and as an energy source for plants. So when you eat various parts of the plant,
you do get various types of carbohydrates, depending on the plant source and what you are consuming.
[6]
Since we can get different carbohydrates when we eat different parts of the plant, I want to now focus on
the bottom of page 37, and more importantly, the top of page 38, which begin to define the various
carbohydrates. Focus on the little table as we begin to talk about the three main types of carbohydrates
and then give roughly 3 to 4 examples of important carbohydrates that fit into these main categories. So
the main carbohydrates can be found on page 38, and it's a great table to remember, memorize, or star,
because I can ask a number of questions from it. The first category we are going to talk about is called
monosaccharides. Mono meaning one, saccharide meaning sugar, these are one sugar compounds.
Most of them, are six sided rings, however, you can have a five sided ring is well. If the ring is six sided
in shape is called a hexose. You can see there are six bonds which make the six sided ring. For
fructose it is a five sided ring but it is still a six carbon sugar if you pay close attention to the numbers. A
six sided ring structure that you see here representing glucose, is also called a furanose structure. A five
sided ring structure representing fructose is called a pyranose structure. Both of these structures are
how carbohydrates exist in nature.
[7]
There are three types of monosaccharides we are going to talk about from this category. The first one is
glucose. It is also called dextrose or blood sugar. It is the sugar that they measure when measuring
your blood for carbohydrates. Glucose is called the hub of carbohydrate biology. It is the beginning and
the end in terms of carbohydrates in nutrition. It is the first carbohydrate made by the plants from
chlorophyll and photosynthesis and is the major carbohydrate required by the cell in the body when
carbohydrate is digested. That is why we call it the hub of carbohydrate biology because it is the first to
form and the last required. Here's what it looks like: it is a pyranose structure, a six member ring
structure with 5 of the carbons as part of the ring and the sixth carbon is bound to carbon 5. Also the
hydroxyl groups are either above or below the plane of the ring and the importance of this will be
explained in a few slides.
[8]
The second type of carbohydrate in this category is called fructose. Fructose is also called levulose and
fruit sugar. Fructose is twice as sweet as table sugar, which is partially why fruits are sweeter when they
ripen. Frutose is a furanose type of ring structure. It is a five membered ring. It also has six carbons
but the ring is made up 4 carbons with 2 carbons bound at carbons 2 and 5. Again the hydroxyls above
and below the plane of the ring help to identify its structure.
[9]
The last one we are going to talk about is called galactose. This one is found primarily in animal tissues
as part of the mammary gland. It is a part of the disaccharide lactose which we’re going to get into in the
next category. It is not found free in the diet. It is also a pyranose ring, a 6 carbon ring similar to
glucose but it is structurally different. Pay attention to the hydroxyl at position 4, we will discuss this next.
There are other monosaccharides we can talk about and we may mention them briefly as we go through
other lectures but these are the three most important types of monosaccharides found in the diet.
[10]
We now have to come back to the question what is the difference between glucose and galactose? They
are both pyranose ring structure. They are both six carbons sugars. They both have the same formula
C6H12O6, so what makes glucose different from galactose? Primarily as you see here in the pinkish
highlights with the red bond, it is the position of the hydroxyl group at carbon 4 on the ring, that makes a
glucose or galactose. That is what I want you to remember, what makes monosaccharides different,
especially ones that are six membered rings and those that are five membered rings, what makes them
different from their six member partners and their five member partners, is the position of the hydroxyl
groups on the ring. The position of the hydroxyl group at carbon 2, 3, and 4 determines a glucose
molecule from a galactose molecule. You can see the hydroxyl for glucose is below the plane of the ring
while the hydroxyl for galactose is above the plane in the ring. If asked the question “what is the
difference between glucose and galactose?” you should say the position of the hydroxyl groups on the
ring designates ones sugar from the other. I don't care to remember which carbon or ones below and
ones above; that is not important, what you need to remember is that the position of the hydroxyl groups
are what differentiates a glucose from a galactose.
[11]
Let us move into category 2. Disaccharides, di meaning two sugars, two sugar units or two
monosaccharides linked together. There are three examples of disaccharides that we will talk about.
These are mainly found in the diet. Repeating: a disaccharide is two monosaccharides or two sugar
units bound together. The first one we're going to talk about is sucrose. Sucrose is also called table
sugar, refined sugar and white sugar. They all mean the same. They are called sucrose. It is made up
of one molecule of glucose and one molecule of fructose, they are linked together by eliminating water so
you have a linkage between the monosaccharides and that forms the disaccharide sucrose. One
molecule is glucose and the other molecule is fructose. That is why the table on page 38 is important to
remember, because this is a good question to ask. A question like: sucrose is made up of one molecule
of glucose and? You would say fructose because glucose and fructose make up sucrose. So, again,
you want to go over the table on page 38 so that you know this information.
[12]
The next disaccharide in the category is called maltose. It is a product of grain starches. It is also the
carbohydrate used for fermentation. Some people call it beer sugar. If you look at the picture as well as
the bullet, the final bullet states maltose is made up of two glucose molecules linked together. Again as
a product of grain starches, glucose is bound to glucose, and it is the molecule form when starch breaks
down. It forms maltose, which can be used in the fermentation process to make alcoholic type
beverages, and that is why we label it beer sugar.
[13]
The last example in the disaccharide category is lactose. Again, lactose is primarily found in animal
tissues as a part of milk sugar. It is called milk sugar because it consists of one molecule of glucose and
one molecule of galactose, both found in the body and mammary gland. These make up the
disaccharide lactose. So this is an example of a carbohydrate mainly from animal sources and from our
diet primarily found in dairy foods.
[14]
The last category we are going to talk about are called polysaccharides. Polysaccharides are many
sugar units or polymers of monosaccharides, specifically glucose. Two classic examples found here are
starch and fiber. Starch is digestible; fiber is not digestible by humans. This is one reason they are
different. Since many of the polysaccharides are polymers of glucose, how are they different? Here is
an example: starches have glucose bound together while fiber also has glucose bound together. So we
come back to that question: if they are both polymers of glucose how are they different? So think about
that, we won't answer this right now but we will answer it a few slides from now.
[15]
Within starches there are three examples. Natural starches are made of two types of molecules; they
are called amylose and amylopectin. As you see here, the amylose is a straight chained starch, it may
have some curving in it but it is straight. Amylopectin is a branched chained starch, and all starches
have combinations or compositions of both of these compounds. The difference in potato starch from
tapioca starch is the different proportions of amylose to amylopectin and this makes them have different
properties. Dextrin is a smaller starch, it is a predigested starch; some people have problems digesting
starches, so dextrin is a smaller starch molecule that has been predigested to help digestion issues. As I
said, the starches here can be branched or straight chained and you'll find both in various plants, just in
different proportions. A more linear or straight chained starch has a tendency to be used in food systems
as a thickening agent, good for making gravy. Along with a smaller proportion of amylopectin, it thickens
up to use like cornstarch in your drippings, or the flour you put in to thicken gravy up, because you just
don't want runny gravy; you like it to stay there but you don't want it to go too far. When you pour it on
you don't want it to go all over the place, it stays with your mashed potatoes. However, you do not want
it to go to the other extreme where it solidifies into a jelly type of product and just wiggles on top of your
mashed potatoes. When you have starches that have more amylopectin, they have a tendency to gel.
They have a tendency to make a better pudding and things like that, where you're dealing with gel
formation. Gel entraps water, which is the result of their branching characteristic. They bind or entrap
water to make an excellent gel product where the amylose only thickens are gets more viscous.
[16]
The last one in this group is glycogen. Glycogen is animal starch; again, this is the carbohydrate found
in animal tissues. This is the main storage carbohydrate for animal products. We don't usually get
much glycogen in our diet, because upon slaughtering the animals it burns or breaks down the glycogen
as they get dressed for slaughtering. So we don't find a lot of animal starch in muscle and tissues that
normally store glycogen. Glycogen is again very similar to starch. It is a polymer of glucose but it has
more branching. It is digestible like plant starches. So it is highly branched and behaves very similar to
plant starches in terms of its digestibility.
[17]
Let us come back to our fundamental question of polysaccharides: why is cellulose and starch different, if
they are polymers of glucose? Cellulose, the non-digestible carbohydrate polysaccharide is called fiber;
we mention it because it is beneficial component of carbohydrates. One type is called cellulose, there
are other fibers such as pectin, hemicellulose, lignin, etc. You get a benefit in the body to have this
compound be non-digestible, and we will discuss this more under carbohydrates and health. Now let's
answer the question posed. That is, if all of these are polymers of glucose, why are starches and
glycogen digestible, while fiber is non-digestible? They are all made up of the same compound, so what
is the difference?
[18]
This shows why they are different. Starch (amylose) is made up of glucose molecules, just like cellulose
is made up of glucose molecules. Notice the red bonds or linkages between the glucose molecules. First
this bond is called a glycosidic bond and it is a lot different from starch to cellulose. In starch, it's an
alpha linkage, and thus, this molecule is digestible. In cellulose, this is called a beta linkage; thus this
linkage is not digestible. So what do you want to remember if a question is asked: what is the difference
between starch and cellulose? First, one’s an alpha linkage, one’s a beta linkage; second, one's
digestible, one’s not digestible, and that's what makes them different. The linkages being different
require enzymes to break down these, for amylose, the enzyme is amylase. For cellulose, it is beta
amylase, which we do not have. That is why if you get hungry, you cannot eat Uncle Marty's notebook.
If you were hungry, and you had the beta amylase, you could take your notebook, rip it up, and start
chewing on it, and it would provide you energy and some nourishment. However, we don't have this
enzyme, so eating Uncle Marty’s notebook won't do you any good. It will just pass right through you as
part of the non-digestible component –called fiber. There are some biological organisms where the
reverse occurs--they digest cellulose but can't digest starch. Do you know what those are? You
probably get your house treated before you build, or you get your house inspected for them because they
are devastating organisms--termites. Termites have enzymes that break the beta linkage but don't have
enzymes that break the alpha linkage, so they can eat wood or cellulose, they digest it fine, we can't.
They can eat paper or fiber, while we can‘t. I remember when I first went to Rutgers for my Masters
degree, there was a food scientist who was trying to take the termite enzyme and immobilize it on the
enamel of your teeth, in the hopes that if you got hungry you could go and eat a piece of tree or plant, that
normally you could not digest. It actually worked and would break down fiber but the physical force of
maceration in the mouth caused the enzyme to release from the enamel. I think it would last an hour or
two and then it would just come off. Because of this it had no practical application commercially. He got
a lot of grant money to look at this, initially, and to my knowledge, nothing more came out of his attempt to
allow people to eat nondigestible cellulose. This concludes carbohydrates, part A. Next time we will be
talking about carbohydrates, part B. We will continue our discussion about the fourth most important
nutrient. We will get into functions of carbohydrates, sources of carbohydrates, some additional
carbohydrates. Also, some commercial sugars and products will be discussed and we will talk about
alcohol, which results from the fermentation of carbohydrates. So next time, I hope things are going well.
Again, if you have any questions please do not hesitate to contact me by email and I will try to answer
those as quickly as possible. I will see you when we begin discussion of carbohydrates, part B.