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.