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 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.

    We have an innate liking for sweetness, which simple sugars provide. Historically, simple sugars were available in more modest quantities than they are today, and eating too many calories from sugar can become a problem. Unhealthy diets, including those with too many calories from simple sugars, are associated with an increased risk for diseases like obesity, diabetes mellitus, and cardiovascular disease, but the good news is that a healthy diet can reduce that risk as well.

    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 likely 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 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, including 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 — it’s 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 quite 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.

    There’s also dietary fibers, which are carbohydrates that intestinal enzymes can’t break down, and so the body cannot digest them. Dietary 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 keep blood glucose levels relatively stable. Fiber is also good for heart health and increases stool weight which helps prevent constipation.

    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 — a water molecule, which goes away. In the case of maltose, that leaves an alpha 1-4-glycosidic bond — 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 another, like this. Lactose has a beta 1-4 glycosidic bond, meaning that carbon 1 of glucose and carbon 4 of galactose are bonded, but this time the molecules are stacked with one higher than the other, like this. Finally, sucrose has an alpha 1-2-glycosidic bond, meaning that carbon 1 of glucose and carbon 2 of fructose are bonded, like this.

    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. 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. And poof! The 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 the body’s needs. Now that doesn’t mean that the type of food that carbohydrates can be found in doesn’t matter, in fact, it matters quite a bit. One way to quantify this concept is the glycemic index. The glycemic index is a measure of how blood glucose levels rise two hours after eating 50 grams of available carbohydrates that the body can absorb — so that excludes fiber. The glycemic index value is expressed as the percentage of the rise in blood glucose that you would experience from eating 50 grams of a given food compared to 50 grams of glucose.

    So, generally speaking, foods that have a high glycemic index are foods with the types of carbohydrates that are broken down and absorbed quickly, cause a quicker rise in blood glucose, and therefore have a higher glycemic index. Although, foods with high sucrose or fructose content would not have a high glycemic index score because fructose has a very low glycemic index value. Foods that have more fiber, fat and protein generally result in a slower rise in blood glucose and thus have a lower glycemic index.

    The glycemic index of a particular food can vary — things like ripeness of a fruit, whether pasta is overcooked or served al dente, the type of food processing, and what the food is eaten with can all affect glycemic index values. Your age and activity level can also impact your glycemic response as well. Of course, people don’t usually eat exactly 50 gram portions of available carbohydrate from a single food, so portion size matters. Another value — the glycemic load — accounts for both the carbohydrate quality of the food — its glycemic index, as well as the quantity of food consumed. For example, watermelon has a high glycemic index of 72, but a low glycemic load of 4, because there’s only 5 grams of available carbohydrate per 100 grams of watermelon.

    The glycemic load and glycemic index are tools mainly used in research settings to identify which foods may have a greater impact on people with impaired glucose metabolism, and their relationships with health outcomes are still being investigated. In general, the idea is that choosing lower glycemic foods may lead to fewer spikes in blood glucose and insulin, which may be helpful for managing blood glucose in people with diabetes mellitus.

    It’s recommended that a healthy diet contain between 45-65% 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 an active 40 year old woman, who’s 5 foot 7 inches, 130 pounds, requires a 2000 calorie diet, and let’s say she wants to aim for the 65% end of the carbohydrate range — so that’s 65% of 2000 or 1300 calories from carbohydrates. Now the type of carbohydrate matters as well.

    First, there’s fiber. The general recommendation is to get about 28 grams of fiber for a 2000 calorie diet. With roughly zero to 2 calories per gram of fiber, that’s about 56 calories, or about 4% of her total carbohydrates. Next there’s added sugar. Both the World Health Organization and the US Dietary Guidelines recommend fewer than 10% of total calories as added sugars, so that’s a maximum of 200 calories or 50 grams, or about 15% of her total carbohydrates. Next there’s natural sugars from whole foods, and there are few formal recommendations here. The Canadian food label lists a daily value of 100 grams or 400 calories from total sugars, which can be a combination of natural and added. We already included 200 calories or 50 grams of added sugars, so we can assume the remainder — 200 calories or 50 grams — would come from natural sugars, which is another 15% of her total carbohydrates. So that means that the remainder — 844 calories or 211 grams come from starch, that’s about 66% of her total carbohydrates.

    Now, it turns out that a 40 year old man, who’s 5 foot 10 inches, 165 pounds, and pretty sedentary also requires a 2000 calorie diet. Let’s say he wants to eat fewer calories from carbohydrates and aims toward the 45% end of the range — 45% of 2000 is 900 calories. Working through the same math, there’s still the same recommendation to get about 28 grams of fiber — so that’s 56 calories or about 6% of his total carbohydrates. At minimum, he should aim to get the same proportion of carbohydrates from fruits, vegetables, dairy and grains — so let’s assume that 15% of the total carbohydrates come from naturally-occurring sugars. That amounts to 135 calories or about 34 grams of naturally-occurring sugars. Similarly, let’s assume that 66% of the total carbohydrates come from starches. That amounts to 594 calories or 149 grams of starch. So that leaves him with room for 13% of the total carbohydrates from added sugars which is about 115 calories or 29 grams. So his overall number of calories is the same 2000, but because he’s aiming for fewer total carbohydrates, his maximum for added sugars would be 29 grams, whereas it would be 50 grams for our moderately active woman.

    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.

Video credit: Osmosis (