To start with the basics, carbohydrates are types of molecules in foods and drinks. They are one of the four main classes of macro (or major) nutrients – the others are proteins, fats (lipids) and alcohol. In typical human diets, carbohydrates or “saccharides” are the most abundant of these.
The word carbohydrate comes from chemistry and means “watered carbon” (carbon with water molecules). For example, the formula for glucose is C6H12O6 (six carbon atoms and six water molecules—H2O = water). Sometimes you will see it shortened to CHO (especially in scientific papers), which stands for carbon, hydrogen, and oxygen.
In food and nutrition we mostly think of carbohydrates as being sugars and starches, but fibres are carbohydrates too. So are oligosaccharides, a slightly sweet to flavourless category of carbohydrates sandwiched between sugars and starches.
Chemically, the sweet-tasting carbohydrates we call sugars are either monosaccharides or disaccharides.
Monosaccharides (mono means one) are single-sugar molecules and they are smallest carbohydrate molecules. Three common dietary monosaccharides (in alphabetical order) are:
- Fructose, which is found in fruits, honey and agave sap.
- Galactose, which is found in milk, yoghurt, and whey.
- Glucose, which is found in fruits, grains, vegetables, and honey.
What is tagatose?
It is very similar to fructose in its chemical structure. It is found in minute quantities in fruits, vegetables, and some dairy products (milk, yogurt, cheese) and in the gum of the tropical karaya gum tree (Sterculia setigera). It is around 90 percent as sweet as regular sugar, has about two-thirds the calories, and virtually no effect on blood glucose levels because it is poorly absorbed (the body basically treats it as a dietary fibre).
Disaccharides (di means two) are two single-sugar molecules joined (or bonded) together. Three common dietary disaccharides (in alphabetical order) are:
- Lactose, which is glucose plus galactose molecules, is found in milk. All mammal milk contains lactose, but human milk, which is very sweet, is the richest source of lactose by far.
- Maltose, which is glucose plus glucose molecules, is found in grains such as barley and in malt and malted foods and beverages.
- Sucrose, which is glucose plus fructose molecules, is found in tubers such as potatoes and sugar beet, sugarcane (a grass), the sap of maple and birch trees, and fruits.
What is trehalose?
It is a disaccharide consisting of two glucose molecules, and it is the main blood glucose in insects; you could say it’s the sugar that gives bees their buzz. Commercially produced from starch by an enzymatic process, it is used in foods and beverages as a multipurpose ingredient – sweetener, stabilizer, thickener, and flavour enhancer.
Oligosaccharides (oligo means a few) are chains of three to nine single-sugar molecules. They are slightly sweet. The two main classes of oligosaccharides used in food processing for sweetening are:
- Fructo-oligosaccharides (FOS), which are short chains of fructose molecules, are found in fruits and vegetables such as asparagus, bananas, chicory root, garlic, Jerusalem artichokes, leeks, legumes, onions, wheat, and yacon; and in whole-grain foods, especially rye. These can act as prebiotics, the non-digestible components of plant foods that help promote good gut health by stimulating the growth or activity of friendly bacteria in the large intestine. People on a low-FODMAP diet are advised to avoid (or minimise consuming) foods containing fructo-oligosaccharides.
- Maltodextrins (modified food starches), are man-made. They are produced enzymatically by the partial hydrolysis of starch (corn, potato, rice, wheat, or tapioca). They are not sugars but are commonly used as food additives to provide bulk and texture, because they are only moderately sweet or even flavourless. Maltodextrin is listed in the FDA’s Code of Federal Regulations (CFR) as a GRAS additive, meaning it is generally recognized as safe.
What are galacto-oligosaccharides (GOS)?
They are short chains of galactose molecules produced through the enzymatic conversion of lactose in milk. Like fructo-oligosaccharides, they are prebiotics, and are a preferred fuel of health-promoting bacteria in our large intestine. Mother’s milk contains these oligosaccharides naturally, which remain undigested, but discourage pathogens in the small intestine and facilitate the growth of friendly bacteria (e.g., Bifidobacteria and Lactobacilli) in a baby’s large intestine.
The carbohydrates we call starches are part of the large group of polysaccharides (poly means “many”) – long chains or polymers (some branching, some straight) of single-sugar molecules. There are two sorts of starches – amylose and amylopectin.
- Amylose is like a string of glucose molecules that tend to line up in rows and form tight, compact clumps that are harder to gelatinise and therefore digest.
- Amylopectin is a string of glucose molecules with lots of branching points, such as you see in some types of seaweed. Amylopectin molecules are larger and more open and the starch tends to be easier to gelatinise and digest.
What is resistant starch?
It is the starch that resists digestion and absorption in the small intestine and passes through to the large intestine, where it acts like dietary fibre to improve bowel health. Sources of resistant starch are unprocessed cereals and whole grains, firm (unripe) bananas, legumes, and starchy foods that have been cooked and then cooled, such as cold potatoes or old-fashioned oatmeal. (If you cook a pot of old-fashioned oatmeal and reheat individual portions of it each day, the heated-cooled-and-heated-again oatmeal is higher in resistant starch.) Resistant starch is also added to some refined-grain-based products, including breads and breakfast cereals, to increase their fibre content. Like other starches, it is not sweet at all.
What is glycogen?
Our bodies can store a certain amount of excess carbohydrates (about 1500 to 1900 calories worth) in the liver and muscles as a kind of starch known as glycogen. One way to think of glycogen is as an energy reservoir we can draw on when going without food for a long time or exercising intensely. That’s when our bodies convert it back to glucose to provide energy for our bodies and brains. Athletes restore glycogen by consuming carbohydrates on a regular basis because their supply becomes depleted as the intensity and duration of their exercise increases. In high-intensity sports, it is generally the availability of carbohydrate stores that limits performance.
How and why is starch converted back to glucose?
Just as enzymes in our bodies break down starchy foods (such as bread, pasta, or oatmeal) into glucose in our digestive tract, starch processors use enzymes to convert starch into a range of glucose products for the food and beverage industry. It is much cheaper and less sweet than sucrose and has a number of practical attributes, such as adding bulk and reducing crystallization. Enzymatic conversions also provide us with a range of sweetener products including glucose (dextrose) powder, tablets, or syrup; and glucose and maltose syrups such as barley malt syrup, corn syrup, oat syrup, rice syrup, tapioca syrup, wheat syrup, etc.
Dietary fibres are large carbohydrate molecules containing many different sorts of monosaccharides. Unlike starches and sugars, they are not broken down during digestion when we consume them. They come mostly (but not exclusively) from plants, and they are the poorly digested portions that pass through into the large intestine (bowel) and provide much of the bulk in our stool (along with water and bacteria, among a few other things).
There are a number of ways of classifying the different types of fibre. One of the most popular systems is whether they are soluble in water or not:
- Water-soluble fibres include gums (e.g., agar), fructans (e.g., inulin), mucilages (e.g., psyllium), and pectins. They are found in a range of foods, including fruits, vegetables, legumes (beans, peas, and lentils), and some grains (oats and barley). Soluble dietary fibre may help reduce blood cholesterol levels and modulate blood glucose levels – but whether it does so or not depends in part on the degree of food processing and, of course, on how much of it you eat.
- Water-insoluble fibres include cellulose, hemicellulose, and lignin. They are mostly found in vegetables, wheat and other whole grains, nuts, and seeds. Insoluble dietary fibre primarily helps with laxation, which in turn may decrease the risk of constipation, hemorrhoids, and colorectal (bowel) cancer.
Glucose, the simplest form of carbohydrate, is a universal fuel for most organs and tissues in our body; the only fuel source for our brain, red blood cells and a growing fetus; and the main source of energy for our muscles during strenuous exercise.
Carbohydrate foods bring more than energy to the dining table – they are good sources of vitamins, minerals, phytonutrients and fibre.
Regardless of their source, most of the carbohydrates in foods that we eat are digested in the stomach and small intestine, absorbed into the bloodstream, and, one way or another, converted to glucose. This includes all starches, maltodextrins, and most sugars (but not dietary fibre). Here’s how the body does this.
For most carbohydrates the process is relatively simple. Starches and maltodextrins are simply chains of glucose. Small proteins (known as enzymes) called amylases found in saliva and intestinal digestive juices (secreted from the pancreas) snip each bond between the glucose molecules, so that by the end of the digestive process you wind up with pure glucose in the small intestine, which is then actively transported through the intestinal wall into the bloodstream.
Sugars are a little more complicated. In order to be absorbed into the bloodstream, the common disaccharides maltose, lactose, and sucrose need to be broken down into their constituent monosaccharides (single sugars). Like starches and maltodextrins, this is done by specific enzymes in our small intestine:
- Maltose is very quickly broken down by the enzyme maltase into two glucose molecules (single units of glucose), which are transported straight into the bloodstream.
- Sucrose is broken down into its constituent glucose and fructose molecules (single units of fructose) by the enzyme sucrase
- Lactose is broken down to glucose and galactose molecules (single units of galactose) by the enzyme lactase (except in those who are lactose intolerant).
The glucose, fructose, and galactose molecules are then absorbed into the bloodstream.
The glucose molecules circulate throughout the body and can be absorbed directly into the cells of most of the body’s tissues and organs, where it usually ends up as pyruvate and adenosine triphosphate (ATP), which is our body’s main energy currency. Normally, pyruvate is also converted to ATP, thus producing more energy.
Galactose and fructose molecules, however, go to the liver for further processing. Here it gets rather technical, but we have simplified as much as we can. In the liver, fructose is rapidly removed from the bloodstream, phosphor is added to the fructose, and the resulting phosphorylated fructose enters what is known scientifically as the glycolytic pathway, where, through a series of chemical reactions, it usually ends up as pyruvate and ATP. Similarly, galactose is extracted from the blood and converted to glucose in the liver, and again converted to pyruvate and ATP, just like glucose.
The release of these alternate fuels (e.g., glucose, pyruvate) into the bloodstream depends on your energy balance. For example, if your body’s energy stores are low after an overnight fast (which typically occurs when we are sleeping!), the liver will release glucose from its glycogen stores to keep the brain, nervous system, and other vital organs functioning. However, just after a meal, the liver will store these fuels (typically as glycogen) for use later. Also, all monosaccharides can be converted to fat (triglycerides) in the liver through a series of complex chemical reactions. The fat can either be stored for later use or released into the bloodstream, depending on the body’s requirements. Fructose is converted more readily than the other monosaccharides, and when consumed in large amounts (more than 50 grams of pure fructose in one “dose”), it will raise blood triglyceride levels. However, most people do not eat large amounts of pure fructose in one sitting!
When glucose enters the bloodstream, the pancreas releases the hormone insulin, which signals most of the body’s organs and tissues to absorb the glucose from the blood. People with diabetes produce either no insulin (type 1) or not enough insulin (type 2), and this is why their blood glucose levels rise too high after consuming a high-carbohydrate meal or drink.
What happens to the fructose? It depends. We use most of it for energy, and under normal circumstances, very little is stored as fat. A recent review of the scientific evidence found that our bodies:
- Use up 45 percent of pure fructose (that’s fructose consumed on its own, such as Fruisana—pure crystalline fructose) within 3 to 6 hours for energy.
- Use up 66 percent of fructose consumed with glucose (as it typically is in nature, such as sucrose, or table sugar) within 3 to 6 hours for energy.
- Convert roughly a third (29 percent) to a half (54 percent) of all fructose we consume to glucose.
- Seem to convert less than 1 percent of fructose directly to blood fat (triglycerides).
Where does the glycemix index (GI) fit into the digestive picture?
When we consume carbohydrate foods and beverages, our bodies convert the sugars and starches in them to glucose (blood glucose or blood sugar) to fuel our brains, cells, tissues, and muscles (particularly during strenuous exercise). But it converts them at very different rates.
The glycemic index or GI is simply a number that is a relative ranking that gives us a useful indication of how fast our body is going to digest, absorb, and metabolize foods and drinks containing carbohydrates. (This does not include fibre, which the body does not digest.) Think of it as a kind of “carb speedo” that tells you how fast and high your blood glucose level is likely to rise after you consume carbohydrates.
The ranking is based on testing each food in healthy people, not in test tubes. Pure glucose, which is digested and absorbed in a flash, is given a value of 100, and all other carbohydrates are ranked against this. Some foods break down quickly during digestion (“gushers,” with a glycemic index of 70 or above), and the glucose in our blood increases rapidly; others break down slowly during digestion, and the glucose is released gradually into the blood (“tricklers,” with a glycemic index of 55 or below). And, of course, there are the moderates in between.
What is glycemic load?
How high your blood glucose actually rises and how long it remains high after you eat carb foods depends on both the amount of carbohydrate in the food and the carbohydrates’ GI value. That’s where the glycemic load (GL) comes in.
One unit of glycemic load is equivalent to 1 gram of pure glucose. The higher the GL of a food or meal, the more insulin your pancreas needs to produce to drive the glucose into your cells. When you are young, your pancreas is able to produce enough insulin to cover the requirements of high-glycemic load foods and meals, but as you get older, it may no longer be able to cope with higher insulin requirements. This is when type 2 diabetes and other “lifestyle” diseases can start to develop.
The glycemic load is calculated by multiplying the glycemic index of a food by the available carbohydrate content (carbohydrates minus fibre) in the serving (expressed in grams), divided by 100 (because GI is a percent). (GL = GI/100 x available carbs per serving.) For example:
- A typical medium-size apple has a glycemic index of 38 and contains 15 grams of available carbohydrate. Therefore, its glycemic load is 38 × 15 ÷ 100 = 6.
- If you are hungry, and the apples are particularly crispy, juicy, and delicious, so you eat two, the overall glycemic load of this snack is 12.