Carbohydrates & sugars – biochemistry


Carbohydrates include both simple sugars which
are little ring-shaped molecules made of carbon, hydrogen and oxygen – either alone or in pairs,
as well as more complex carbohydrates, which are formed when these the rings link up together
to make long chains. Carbohydrates provide us with calories or
energy, and simple sugars in particular play many roles in our diet – they sweeten lemonade,
balance out an acidic miso soup, fuel yeast in rising dough and alcohol, and help preserve
jams and jellies. Now, Sugars are found naturally in plants
like fruits, vegetables, and grains, as well as animal products like milk and cheese. Added sugars are the sugars that get added
to foods like cereals, ketchup, energy bars, and even salad dressings. To be clear, even if the sugar being added
comes from a natural source like sugar cane or honey, it’s still considered an added
sugar. In fact, a variety of ingredients listed on
food labels may be sources of added sugars, some of which you’re probably familiar with. Sugar actually refers to a family of molecules
called saccharides – monosaccharides where “mono” means one, so one sugar molecule,
disaccharides where “di” means two, so two sugar molecules linked together, oligosaccharides
where “oligo” means a few, so it’s three to nine sugar molecules linked together, and
polysaccharides where “poly” means many, so it’s ten or more sugar molecules linked
together. Glucose is the most important member of the
sugar family and it’s a monosaccharide. It’s one of the main source of calories
for the body, and is able to cross the blood brain barrier and nourish the brain. Another monosaccharide is fructose which is
commonly found in honey, fruits, and root vegetables. Finally, there’s the monosaccharide, galactose,
known as milk sugar. It’s known as milk sugar because it’s
only found in nature when it links with glucose to form lactose, a disaccharide found in the
milk of mammals, which includes cow and human breast milk. Sucrose, or table sugar, is another disaccharide
and it’s formed when fructose links up with glucose. Sucrose is found in various fruits and vegetables,
with sugarcane and sugar beets having the highest quantities. Maltose is another disaccharide -and this
one is two glucose molecules linked together, and it’s found in molasses which can be
used as a substrate to ferment beer. Sugars, like fructose for example, are most
always found in combination with other sugars, and the combinations can be pretty different
– even in seemingly similar foods. For example, in honey 50% of the sugar is
fructose, 44% is glucose, 4% is galactose, 2% is maltose; whereas in maple syrup less
than 1% of the sugar is fructose, 3% is glucose, and 96% is sucrose. So simple sugars, whether they’re natural
or added, are mixtures of monosaccharides or disaccharides. Next there are the complex carbohydrates. There are oligosaccharides like galacto-oligosaccharides
which are short chains of galactose molecules like those found in soybeans. Then there are polysaccharides which are even
larger chains with branches, and are the most abundant type of carbohydrates found in food. Starches are polysaccharides with molecular
bonds between sugar molecules that human intestinal enzymes can break down. Starches are an important source of calories,
and can be found in foods like rice, potatoes, wheat, and maize. Starches don’t taste sweet like simple sugars
because they don’t activate taste buds in the same way. And there also dietary fibers, which are carbohydrates
that intestinal enzymes can’t break down, and so the body cannot digest them. Now there are many different types of Dietary
fibers, and they’re not all the same when it comes to their structure or impact on health. Fibers have molecular bonds that are resistant
to human intestinal enzymes, so they pass through the small intestine undigested, get
broken down a bit by bacteria in the large intestine, and ultimately end up as bulk matter
in the stool. Fiber is critical because it can slow down
the rate of absorption of simple sugars like glucose in the small intestine which can help
maintain healthy blood glucose levels. They also increase stool weight stool weight
which helps prevent constipation, and fibers like beta-glucan are also good for heart health. Monosaccharides link together through glycosidic
bonding, which is when an “OH” from the carbon on one monosaccharide bonds with an
“H” from the carbon of another monosaccharide. Together that forms an H2O or a water molecule,
which goes away. In the case of maltose, that leaves an alpha
1-4-glycosidic bond which is a bond between carbon number 1 of one glucose monosaccharide
and carbon number 4 of the other glucose monosaccharide. And alpha refers to the fact that the molecules
are lined up next to one each other. Lactose on the other hand has a beta 1-4 glycosidic
bond, meaning that carbon 1 of galactose and carbon 4 of glucose are bonded, but this time
the molecules are stacked with one higher than the other. Finally, sucrose has an alpha 1-2-glycosidic
bond, meaning that carbon 1 of glucose and carbon 2 of fructose are bonded. Now when you eat something like a piece of
onion bread, enzymes start breaking down disaccharides, oligosaccharides, and polysaccharides into
monosaccharides so they can be absorbed. Different enzymes help to break different
linkages – for example amylases break down large polysaccharides like starch into smaller
units, whereas lactase, sucrase, and maltase break down lactose, sucrose, and maltose into
their monosaccharides. The individual monosaccharides that result
from the digestion of larger carbohydrate molecules – glucose, fructose, and galactose
– cross the gut lining and get into the bloodstream to get used by the body. When glucose levels in the blood increase
after eating, the pancreas releases the hormone insulin and it helps move glucose into the
cells and into the liver. Insulin helps stimulate the liver to store
glucose as glycogen in a process called glycogenesis – which is when some of the glucose molecules
get linked together with alpha 1-4 and alpha 1-6 glycosidic bonds to form a polysaccharide
called glycogen. Insulin also promotes fat and protein synthesis. Now, metabolism of galactose has an initial
step where an enzyme in the liver converts galactose into glucose – basically flipping
the orientation of the OH group on the 4th carbon. Just like that galactose has become just one
more glucose molecule in the liver. Fructose is handled a bit differently by the
liver. Fructose has 6 carbons, and most of it is
broken down into two 3-carbon molecules and sent into glycolysis to help generate energy. When energy is needed – all three monosaccharides
are metabolized through glycolysis, the citric acid cycle, and oxidative phosphorylation. Ultimately, all digestible carbohydrates,
regardless of whether they come from simple sugars in honey or from starches in baked
potatoes, are broken down into their component monosaccharides for immediate energy use or
stored away for a rainy day, depending on what the body needs. The National Academies of Sciences, Engineering,
and Medicine recommended that a healthy diet contain 45 to 65% of its calories from carbohydrates. The number of calories you need to maintain
your weight depends on things like your age, sex, height, weight, and activity level. For example, let’s take this slightly active
40 year old woman, who’s 5 foot 9 inches, 160 pounds, with a BMI of 23.6 and requires
a 2000 calorie diet, and let’s say she wants to aim for 55% of her calories from carbohydrates
– so that’s 55% of 2000 or 1100 calories. And there are different types of carbohydrates. First, there’s fiber. The general recommendation is to get 28 grams
of fiber in a 2000 calorie diet. With roughly zero to 2 calories per gram of
fiber, that’s about 56 calories, or about 3% of her total calories. Next there’s sugars, both the kind that
are added to foods and those found naturally in whole foods. There are few formal recommendations for total
or natural sugar intake, however updated Canadian nutrition labels are based on a daily value
of 100 grams or 400 calories from total sugars, which is 20% of a 2,000 calorie diet. When it comes to added sugars, both the World
Health Organization and the US Dietary Guidelines recommend that they make up fewer than 10%
of total calories. Just like her goal for total carbohydrates,
she’s aiming for the mid-range of the added sugars recommendation, which would be 100
calories or 5% of her total calories. Using this approach, the remainder of her
total sugar calories – 300 calories or 75 grams would could come from the sugars found
in fruits, vegetables, dairy and grains. This would be 15% of her total calories and
that leaves 640 calories or 160 grams from starches, or 32% of her total calories, to
reach her carbohydrate intake goal. Now eating a healthy diet means choosing foods
that are as nutrient-rich as possible, and foods that contain fiber, starch, and natural
sugars like fruits and vegetables tend to be richer in nutrients than those with added
sugars. Having said that, processed and packaged foods
are a part of most people’s diets, so carefully reading nutrition labels can help you compare
foods and choose more nutrient-rich options. Generally speaking, picking foods and beverages
that are higher in nutrients like fiber and lower in added sugars is best. All right, as a quick recap – There are various
types of carbohydrates: simple sugars are monosaccharides and disaccharides that the
body can readily absorb, starches are polysaccharides that take longer to absorb, and fibers are
polysaccharides that the body can only partially absorb with the help of gut bacteria. Ultimately, a healthy diet includes all types
of carbohydrates coming from a variety of sources like fruits, vegetables, dairy and
grains. It can include added sugars too, with the
World Health Organization and the US Dietary Guidelines recommending that added sugars
make up less than 10% of your overall calories.

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