Merck Manual

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Overview of Carbohydrates and Sugars
Overview of Carbohydrates and Sugars
Overview of Carbohydrates and Sugars

    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 rings link up together to make long chains. Carbohydrates provide us with calories, or energy, and simple sugars in particular play a lot of 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 sugarcane or honey, it's still considered an added sugar. In fact, a variety of ingredients listed on food labels might be sources of added sugars, some of which you're probably familiar with.

    Sugar actually refers to a family of molecules called saccharides: monosaccharides ("mono" means 1, so 1 sugar molecule; disaccharides ("di" means 2, so 2 sugar molecules linked together); oligosaccharides ("oligo" means a few, so it's 3 to 9 sugar molecules linked together); and polysaccharides ("poly" means many, so it's 10 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 sources 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 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 milk as well as 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 2 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, and 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 the taste buds in the same way. And there are also dietary fibers, which are carbohydrates that intestinal enzymes can't break down. And so the body can't 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. Fibers are critical because they 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, which helps prevent constipation. Fibers like beta-glucan are also good for heart health.

    Monosaccharides link together through glycosidic bonding, which is when an OH group from the carbon on one monosaccharide binds with the hydrogen from the carbon of another monosaccharide. Together this 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 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 the disaccharides, oligosaccharides, and polysaccharides into monosaccharides so they can be absorbed. Different enzymes help to break down 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 this helps move glucose into all cells for energy. 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. It basically flips the orientation of the OH group on the 4th carbon. Just like that, galactose can become just another glucose molecule in the liver. Fructose, though, is handled a bit differently by the liver. Fructose has a total of 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 3 monosaccharides are metabolized through glycolysis, then the citric acid cycle and oxidative phosphorylation. Ultimately all digestible carbohydrates, regardless of whether they come from simple sugars and honey or from starches and 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 contained 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 and 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 0 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 naturally found 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 2000 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 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.

    Alright, 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.

Video credit: Osmosis from Elsevier (https://osmosis.org/)