BioChemistry, Nutrition

Understanding Carbohydrates

Carbohydrates are the body’s primary source of energy. Carbohydrates provide the fuel for all bodily processes, such as proper functioning of the immune system, cardiorespiratory system, nervous system, growth, and blood clotting. In addition, carbohydrates maintain a structural component in cells (cellulose in plants) and tissues (cartilage in animals), and they are also involved in the storage and transportation of energy throughout the entire body. Certain carbohydrates, such as fiber, play an important role in promoting the health and function of the gastrointestinal tract by supporting digestion, absorption and waste elimination.

Characterization of Carbohydrates

Carbohydrates are made up of three neutral molecules: carbon, hydrogen, and oxygen. Chemically, they are organic compounds known as aldehydes or ketones and are more commonly grouped into three categories: sugars, starches and cellulose. When categorized by their size or function, they are recognized as simple carbohydrates or complex carbohydrates.

The basic carbohydrate structures are: monosaccharides, such as glucose, galactose, and fructose. These are the smaller compounds that are composed of one sugar unit.  The two sugar units are: disaccharides, such as sucrose, maltose, and lactose. The monosaccharides and disaccharides are referred to as simple sugars because of their small size and the rapid rate in which they are digested.  These sugars are the reason certain foods taste sweet. 

The complex carbohydrate structures are polysaccharides, such as starch, fiber, glycogen, cellulose and dextrin. These are large carbohydrate structures that are made up of 10 or more monosaccharides linked together. 

The sugar units between polysaccharides and monosaccharides, in size, are oligosaccharides (fructo-oligosaccharides), galacto-oligosaccharides and mannan-oligosaccharides. Oligosaccharides are carbohydrates that are composed of three to ten monosaccharides. Oligosaccharides can be either simple or complex carbohydrates depending on their size and complexity, however, most are categorized as simple sugars because they are quickly digested and rapidly converted into usable energy.  

Sugar Groups

Monosaccharides – (one sugar molecule)

Glucose, fructose, galactose, mannose, sorbitol, and manitol

Disaccharides – (two sugar molecules)

Sucrose, maltose, and lactose

Polysaccharides – (three or more sugar molecules)

Starch, fiber, glycogen, cellulose and dextrin

Oligosaccharides – (three to ten sugar molecules)

Fructo-oligosaccharides, galacto-oligosaccharides and mannan-oligosaccharides

Simple Carbohydrates


The two principal forms of simple carbohydrates are monosaccharides and disaccharides.  Oligosaccharides are another, less common, form of simple carbohydrate. Simple carbohydrates provide a fast source of usable energy. In fact, because they are quickly absorbed into the bloodstream, they are often used as energy boosters during exercise.


Monosaccharides are found in most foods. They are most abundant in foods such as ripe fruit, honey and sugar cane. They are an important source of energy, however, when ingested in large quantities they will cause an extreme increase in the blood sugar level, followed by an abrupt drop. As a result, a rapid loss of energy soon follows, a condition known as hypoglycemia. Monosaccharides quickly affect the blood sugar level because of their basic molecular structure. They do not need to be broken down during digestion and are, therefore, quickly absorbed into the bloodstream. They provide immediate energy that is not sustainable over a long period of time.

Over time, frequent incidents of large sugar intake can lead to a dysregulation of the body’s blood sugar count, which in turn can lead to hypoglycemia or diabetes mellitus. It is, therefore, recommended that individuals at risk of diabetes, or hypoglycemia, limit their intake of processed foods, because they often have high amounts of hidden monosaccharides to enhance sweetness.  Soda and other beverages with added sugar should also be avoided or at least consumed in limited amounts.

Glucose – is the primary sugar that contributes to the body’s source of fuel. The word glucose is derived from the Greek word glykys, meaning “sweet” in addition to the suffix “ose” which denotes a sugar.  The glucose molecule is the predominant sugar that initiates metabolism and is also very easily converted into energy. The primary function of glucose, however, is to help sustain growth, development and functioning of all cells. Whether a simple carbohydrate or complex carbohydrate is consumed, it will eventually be broken down into a one-unit sugar. Carbohydrates that are not immediately used are stored as glycogen in muscle cells and in the liver.

Once glucose passes across the stomach lining, it enters the bloodstream, which stimulates the release of insulin. When a high level of glucose enters the bloodstream, it stimulates a higher release of insulin. It is this elevated level of insulin that leads to an immediate uptake of any available glucose, which is commonly referred to as a “sugar high”. The result is a quick boost of energy followed by a shaky, tired or run down feeling.

Fructose – is another major monosaccharide the body uses for fuel. This sugar is often referred to as “fruit sugar”. Even though fructose is a single sugar structure, it is less digestible than glucose because it has a more complex chemical make up, and therefore, tends to absorb into the bloodstream at a slower rate. Fructose does not lead to a rapid influx of insulin, which eliminates the blood sugar fluctuations observed by glucose. In addition, fructose will tend to store as glycogen in the liver rather than in muscle. This is useful because it has been found that brain fuel is derived primarily from the liver, which is how this process helps increase mental alertness.

Fructose is found in honey, tree fruits, berries, melons; and some root vegetables, such as beets, sweet potatoes, parsnips, and onions.  Fructose can also be manufactured in the body from the digestion of sucrose (table sugar).


Disaccharides are two-sugar molecules, and they are found in sugars such as lactose (milk sugar), sucrose (table sugar), and maltose (formed form the breakdown of starch). Like monosaccharides, disaccharides are simple sugars and provide a sweet flavor and quick energy. Disaccharides are found abundantly in processed foods, and like monosaccharides, can lead to dysregulation of the blood sugar count if consumed on a frequent basis.

Disaccharides must be broken down into single-unit sugars before they are fully digested, and each different sugar has its own unique digestive enzyme. For example, sucrase is the enzyme required to break down sucrose. These specific enzymes are secreted into the intestines soon after consumption and are immediately digested.

In many people, the lactase enzyme is not available; therefore they are unable to breakdown lactose, present in milk products. This condition is known as “lactose intolerant”. Lactose intolerance is not as common in children and teens as it is in adults.  Many individuals, as they age, cease producing the lactase enzyme. Because undigested lactose is not absorbed, it can lead to bacterial growth in the intestinal tract, which produces gas in the small intestine. This can cause discomfort, acid burn and/or nausea. It can also lead to a degradation of the intestinal lining, which limits the body’s ability to produce digestive enzymes overall, perpetuating a cycle of maldigestion.


Oligosaccharides are simple sugars that are composed of 3–10 monosaccharides. There is not a significant proportion of natural occurring oligosaccharides. Most oligosaccharides are linked to lipids (glycolipids) or to proteins (glycoproteins); many of which are hormones or enzymes.  

Complex carbohydrates

Complex carbohydrates are glucose polymers that are identified, by their structure, as the polysaccharide group of sugars. Polysaccharides are long chains of glucose molecules linked together by hydrogen bonds, and can exist in numbers ranging from a hundred to several thousand. Like simple carbohydrates, complex carbohydrates are also suppliers of energy. However, due to their size and complex nature as polymers, they are not readily absorbed into the bloodstream like monosaccharides. Because the body only absorbs glucose, these long complex chains of sugars must first be broken down to single-unit glucose molecules before they can enter the system, thus slowing the digestive process. Glucose, from simple sugars, can enter the bloodstream from one-half hour to two hours after being ingested. Unlike simple sugars, glucose from complex sugars will tend to be released at a much steadier and slower rate, which prevents any dramatic fluctuations of blood sugar levels and gives way to sustained energy for longer durations of time.

The primary polysaccharides, utilized by the body for energy, are starches and glycogen. Fiber is also an important polysaccharide because it promotes a healthy digestive system and plays an important role in lowering the risks of intestinal related diseases/conditions.

Starch – is a glucose polymer that consists of chains of several hundred glucose molecules. It is found in rice, potatoes, wheat and corn.  There are two main classes of starches that exist in foods: amylose and amylopectin. The two are similar in the number of glucose molecules, but differ in their linkage. Amylopectin is more readily digested than amylose, which makes foods that contain amylose a preferred choice amongst many diabetics.


The rate in which starches are digested depends on how the starch is “packed” within the food. Starches in their pure form have a more complex packing than processed starches, and therefore, tend to be digested at a slower rate.  In whole foods, such as potatoes or rice, starch is folded together as a complex macromolecule encased in a protein or fiber, which must also be broken down before digestion occurs. This ‘casing’, coupled with the complex folding, slows the digestive process even further.  Once the casing is broken down, the unraveling can begin and the ‘breaking down’ into individual sugar units follows. Because there are several hundred glucose molecules linked together, this is a long process, taking up to three hours. For this reason, starches can provide a good source of sustained energy over long periods of time.

Glycogen – is a polysaccharide made up of a long chain of glucose molecules. Ingested glycogen (from meat) can only be stored minimally; therefore the body must manufacture its own glycogen. Because the glycogen storage capacity is limited, carbohydrates must be consumed throughout the day in order to keep the glycogen stores replenished and to help avoid energy loss. Carbohydrates that are not immediately metabolized and released into the bloodstream are converted to glycogen and stored in the fat cells, muscle tissue and liver.

Glycogen, stored in muscle, is important for individuals involved in sports. When fast-twitch muscle fibers are activated (as a need for quick bursts of energy), the body will use the glycogen stores as its primary source of fuel.

The primary function of liver glycogen, during exercise, is in the regulation of blood glucose levels and nutrient metabolism. Liver glycogen, in addition to the glandular roles it plays during exercise, is also important in brain function, which helps increase the concentration level in athletes.


Fiber – is a unique polysaccharide, in that the body cannot break it down into a simple digestible form of sugar. Thus, it moves through the intestinal tract intact, which helps in the removal of waste and toxins, as well as, increasing the effectiveness of the entire digestive process. Fiber is important for a healthy digestive tract and helps in decreasing the risks of certain diseases, such as colon cancer and breast cancer.  In addition, fiber promotes healthy intestinal tract bacteria (“friendly bacteria”), which adds to the body’s overall health. The large intestine is home to a multitude of beneficial bacteria called symbiotic microbes that support health and immune function. Microbes require fiber as fuel and produce short-chain fatty acids (SCFA) as a metabolic by-product, which has been associated with lowering cancerous cells in the colon, promoting healthy intestinal cells, maintaining good blood sugar levels and reducing serum cholesterol. Fiber is found in whole grains, legumes, vegetables, fruit skin and potato skin. 

Carbohydrate Requirements


There is no minimum daily requirement for carbohydrates. The amount of carbohydrates a person should eat depends on the number of daily calories consumed and his/her weight and fitness goals.  

Carbohydrates and Athletic Competition

For athletic performance, athletes should consume large amounts of complex carbohydrates in the range of two to three hours prior to exercise, to help sustain energy levels and delay fatigue. It is recommended that 65% to 70% of athlete’s pre-game calories come from complex carbohydrates. To improve athletic performance and sustain high energy levels, many athletes practice a technique known as ‘carb-loading’. This is a strategy athletes use to maximize their storage of glycogen in the muscles, by consuming large amounts of carbohydrates, pre-event. Athletes can avoid the “hitting the wall” phenomenon by implementing carb-loading into their regimen and effectively train their bodies to increase their glycogen storage capacity, helping utilize glycogen stores more efficiently.

Athletes should consume only moderate amounts of “pre-event” glucose, but are typically advised to drink fluids containing glucose throughout the event, to help preserve glycogen stores and sustain energy levels. In fact, when large amounts of simple carbohydrates are consumed pre-event, an elevation of insulin levels can occur, which promotes a high volume of glucose uptake from the bloodstream. This is counterproductive, as it leads to fatigue and decreased performance.  


Carbohydrates and Weight Loss

Understanding carbohydrate properties can benefit individuals concerned with weight loss. Like athletic performance, knowing how to manipulate macronutrient combinations can help achieve weight loss and fitness goals. A general daily intake baseline to follow is a 45:35:20 macronutrient ratio (carbohydrate, protein and fat, respectively). Nutrient dense foods are recommended over large quantities of simple carbohydrates, because they will maintain constant levels of energy and suppress appetite for longer periods of time. The consumption of large portions of carbohydrates should be avoided three hours prior to bedtime, because unused carbohydrates will tend to store in fat cells during rest.

When seeking weight loss, it is also important to consider the glycogen-to-water ratio as it relates to storage within the body. For every 1-ounce of glycogen stored, 3 ounces of water are also stored.  This means that for every 1-ounce of glycogen burned; 3 ounces of water will be removed. This reduction in water weight is often mistaken for fat loss. In addition, many dieters will often experience several pounds of gastrointestinal bulk lost due to the change in diet. This biological process, coupled with water loss, could be a possible explanation as to why many dieters re-gain their weight. In conclusion, dieting alone is not the solution to a long-standing weight loss goal.

Glycemic index

The Glycemic Index (GI) is a rating system that compares how carbohydrates affect blood glucose levels within two hours of being consumed. Carbohydrates that are high on the GI respond very similarly to glucose, in terms of how fast blood sugar levels rise upon ingestion. Carbohydrates that are low on the GI, tend to be digested at a slower rate, therefore releasing smaller, more constant, amounts of glucose. Foods that are low on the GI are recommended for sustained energy and are a better choice for weight loss than foods ranked high on the Glycemic Index.



    Ketosis is a metabolic state in which there is a build-up of ketone bodies (a liver by-product prevalent during low carbohydrate conditions). When the body is deprived of carbohydrates, insulin levels drop, activating the secretion of glucagon (a pancreatic hormone that assists in the breakdown of liver glycogen). Eventually, glycogen is fully depleted, and the body will begin to use free fatty acids as its primary source of fuel. This promotes two schools of thought on the usage and importance of ketosis. Some agree that ketosis is a beneficial tool in helping burn fat; while others believe ketosis is harmful to the body because it promotes a high level of uric acid. High levels of uric acid have been linked to health risks such as gout and kidney stones. Bodybuilders, on the other hand, have found ketosis to be an effective means of utilizing body fat for energy. When the body is in a ketogenic state, it is argued, fat is being metabolized as a primary source of energy. During this state, it is also believed, the body prefers ketones to glucose, and while in ketosis the body does not metabolize proteins, thus preserving muscle mass during exercise.

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