Simple carbohydrates are sugars whose bonds are easily broken down by digestion. Sugars are classified as mono- (one) or di- (two) saccharides.

Monosaccharides are glucose, fructose and galactose. The body can only absorb the monosaccharides into the bloodstream, but only glucose affects blood sugar levels, and therefore all other carbohydrates that are consumed are converted to this usable form in the body. Glucose can be obtained through digestion or through the process of gluconeogenesis. Fructose, also known as fruit sugar, has the same chemical composition of glucose, but has a different shape. Upon absorption into the bloodstream, it is transported to the liver, where it is metabolized to form glucose. Galactose does not exist freely in nature but can be found combined with glucose in milk. The 5-carbon monosaccharide ribose is an important driver of the coenzymes involved in energy production, including adenosine triphosphate (ATP), flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD). It is also an integral part of the genetic molecules of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Disaccharides are two monosaccharides bonded together and include maltose (two glucose molecules), sucrose (fructose and glucose) and lactose (galactose and glucose). Maltose is found in beer, cereals and seeds, whereas sucrose, commonly known as table sugar, is found in cane sugar, honey and maple syrup. Lactose is the only sugar not found in plants; it is found in milk.

Complex carbohydrates are defined as polysaccharides and are formed from many (three to thousands) monosaccharides bonded together. They are either linear or branched in structure. Starch is an example of a polysaccharide from plants. It exists in two forms, as amylose or amylopectin. Amylose is a linear, long, straight chain of glucose molecules helically twisted, while amylopectin is a highly branched group of bonded monosaccharides. Fiber is a non-starch structural polysaccharide found in plants, which is for the most part resistant to digestion. Fibers remain intact until they reach the large intestine. There are two types of fibers: soluble or insoluble. Soluble fibers can be dissolved in water; these fibers absorb water and thus move through the digestive system slowly. Fruit is an example of soluble fiber. Insoluble fibers do not affect the speed of digestion, and include cellulose from vegetables. The last form of polysaccharide that is the most important for the body is glycogen, which is hundreds of glucose molecules linked together and is stored in the muscles or liver.

The storage of glucose as glycogen is called glycogenesis. During glycogenesis, one molecule of glucose at a time is added to an existing glycogen molecule. The amount of stored glycogen in the body is determined by the dietary intake of carbohydrates; a person on a low-carbohydrate diet will have less glycogen than a person on a high-carbohydrate diet. To use stored glycogen, it must be broken down and separated into the individual glucose molecules through a process called glycogenolysis, which means breakdown of sugar.

Carbohydrate digestion begins in the mouth with the enzyme amylase, which is present in the saliva. This enzyme begins to break down long chains of starch molecules into shorter chains of glucose and the disaccharides maltose or maltodextrin. In the stomach, this process is halted when the gastric acids of the stomach destroy the amylase; however, the breakdown continues upon reaching the small intestine, where secreted pancreatic amylase continues breaking down the short-branched glucose chains into individual glucose molecules. Absorption through the intestinal walls begins, but is dependent on the type of carbohydrate that is digested. Large complex carbohydrates take longer to break down and to be absorbed than the simple carbohydrates. The speed of digestion is determined by a variety of factors including which other nutrients are consumed with the carbohydrate, how the food is prepared, individual differences in metabolism and the chemistry of the carbohydrate. Once separated, glucose molecules are transported across the intestinal wall and into the bloodstream. After digestion and absorption, the body will utilize or store the glucose. It can be burned by the powerhouse of the cells, the mitochondria, immediately releasing carbon dioxide, water and energy in the form of ATP. If the glucose is not needed immediately, it is converted by the liver or the muscles into glycogen. Muscle glycogen provides energy only to the muscles, whereas liver glycogen can be used for any other part of the body. If the body takes in too much glucose, this excess is converted to fat by the liver and stored in adipose or fat tissue throughout the body.

No matter which type of carbohydrate is ingested, the end result is a rise in blood sugar levels, stimulating the pancreas to release the anabolic shuttling hormone insulin. Insulin is an important regulator of the glucose transporters at the membrane surface of the muscle or adipose tissue. Essentially, upon release of insulin, it binds and shuttles glucose and other important nutrients in the bloodstream to the glucose transporters at the membrane surface of the muscle or adipose tissue for uptake. At the membrane surface, insulin binds to an insulin receptor on muscle tissue causing the GLUT4 receptors to translocate from the interior space to the cell membrane surface, where they become exposed and available for glucose uptake. In the absence of glucose, these receptors actually remain dormant within the cell. The GLUT4 transporter moves glucose into the cells by facilitative diffusion down concentration gradients, in contrast to energy-dependent uptake of glucose in the gut or kidney. On termination of the insulin stimulus, the glucose transporters are recycled back to the interior of the cell, where they remain for the next insulin release. Once in the cells, glucose is either used to generate energy as ATP or stored as glycogen for later use. In the liver, glucose can be used to meet the liver�s energy needs, stored as glycogen or converted to triglycerides to be stored as fat. Some of these newly formed triglycerides will be stored in the liver, but most are packaged with proteins called lipoproteins and secreted into the blood. Lipoproteins that contain more fat than protein are called very low-density lipoproteins (VLDL). These VLDL are then transported through the blood to be stored in adipose tissue as triglycerides, also known as fat.

Similarly to insulin, muscle contraction can also stimulate and increase the rate of glucose uptake by the GLUT4 glucose transporters into the muscles. Exercise and insulin utilize different signalling pathways, both of which lead to the activation and translocation of GLUT4 to cell membrane surface for uptake of glucose. Muscle contraction causes calcium release, which increases calcium concentrations in the cell, stimulating the GLUT4 to translocate to the cell surface, allowing for glucose uptake in absence of insulin.

The body’s most important energy-producing molecule is ATP, which is generated in the powerhouse of the cells, the mitochondria. Every cell in the body stores and uses energy through ATP synthesis and production, which fuels protein synthesis, muscle contraction, metabolism and transport of nutrients. The primary fuel source for ATP production is glucose, which is broken down into pyruvate within the cell to generate two molecules of ATP through a hydrolysis reaction. ATP is made up of the five-carbon sugar ribose, adenine and three phosphates. The string of three phosphates lies on the other side of the ribose molecule. ATP saturates the long thin fibers composed of a protein called myosin, which forms the basis of our muscle cells. Because ATP is so important to energy production and is used for a wide range of energy requirements, the body has several different ways to facilitate its production. There are three different energy systems that can be used to generate ATP: the phosphagen system, the glycogen-lactic acid system and the aerobic system. The aerobic system is engaged during periods of exercise that can be sustained for more than two minutes, and it is the primary means by which the muscles will be supplied with ATP, primarily from carbohydrates. Protein will be used for energy under conditions of depletion such as dieting. ATP is produced at the slowest rate through aerobic respiration, in which glucose is broken down into carbon dioxide and water. This system produces enough energy to sustain an athlete for several hours. Glycogen is obtained for aerobic respiration from numerous sources, as discussed previously: via uptake of glucose by working muscles, from glycogen stored in the muscles or via breakdown of the liver’s glycogen into glucose. The phosphagen system generates ATP through a high-energy phosphate molecule called creatine phosphate. This is used to restore ATP levels under very short-term, high-intensity conditions, such as during a high-intensity set of weight training or during a short, fast sprint. The enzyme creatine kinase moves a phosphate group from creatine phosphate and transfers it to ADP to form ATP. The creatine phosphate levels begin to decline over a ten-second period, and energy subsides. The glycogen-lactic acid system is supplied at a slower rate than with the phosphagen system, producing enough ATP for about 90 seconds of high-intensity activity. Lactic acid is formed from glucose in the muscle cell as a result of anaerobic metabolism. A good example of the type of activity performed under these conditions would be the 400-meter sprint. Muscle soreness prevents the athlete from continuing due to high lactic acid concentrations in the muscle.

Skeletal muscle, the nervous system, the brain and all other cells of the body use glucose for their energy requirements. When glucose needs are not met through dietary consumption, glycogen is used. The muscles have the ability to store between 250 to 400 grams of glycogen, and the liver has the ability to store up to 100 grams of glycogen. Once glycogen storages are depleted, the body must find a way to get more glucose. We usually do this through calories consumed; however, on low-carbohydrate and low-calorie diets, glycogen replenishment may not meet the needs of the body. As a result, the gluconeogenesis process can lead to the formation of new glucose from non-glucose sources such as amino acids, glycerol, lactate or pyruvate. In fact, in the absence of glucose, the body will automatically begin to catabolize or break down muscle protein to supply amino acids for this process. Therefore, it is important for athletes to eat a sufficient amount of dietary carbohydrates. Dietary guidelines recommend a high proportion of calories consumed be in the form of carbohydrates - in fact, as much as 40 to 50 percent. Ultimately, the amount of carbohydrates a bodybuilder or athlete needs can be determined based on the following factors:

• Type of training - endurance versus weight training, aerobic versus anaerobic
• Intensity of training, duration and frequency
• Overall caloric needs based on basal metabolic rate (BMR)
• Performance goals
• Body-composition goals and current conditioning
• Insulin sensitivity or resistance

The glycemic index (GI) is a ranking system that determines how much a carbohydrate food raises blood glucose levels after consumption. In scientific terms, the glycemic index of a food can be defined by the area under the two-hour blood glucose response curve (termed AUC for short) following the ingestion of a fixed portion of carbohydrate, which is usually 50 grams. The AUC of the test food is compared to the reference standard, glucose or white bread, by dividing the AUC of the test food by the standard and multiplying by 100. The values determined through this process make up the GI numerical ranking system of carbohydrate foods, where the value of the reference standard is 100. In general, it has been found that carbohydrates that break down quickly during digestion have the highest glycemic indices, while carbohydrate foods that break down slowly and release glucose gradually into the bloodstream have a lower glycemic index value.

Glycemic index can also be used as an indicator of insulin release, where high-GI foods tend to cause a large release of insulin and low-GI foods generally cause a low insulin release. In general, from a diet perspective, diets that contain a large volume of high-GI foods, such as white bread, pasta and white potatoes, will likely experience elevated insulin levels and high plasma triglycerides. Diets that contain a large volume of low-GI foods, such as veggies, dairy, nuts and berries, may have lower levels of blood glucose and insulin and experience positive health benefits, including lower triglycerides. Most bodybuilders follow a low-GI diet, staying away from high-sugar carbohydrates or at the very least eating them in moderation. It should be noted that the glycemic index comprises only a list of single food items and doesn’t take into account the combination of foods eaten together. Also, it has been found that when eaten in combination with fats or proteins, carbohydrate foods are digested much more slowly and result in a reduction in the GI of a food. Also, GI takes into account only the effect of glucose and doesn’t consider the effect of other sugars such as fructose. Another point of interest is the fact that the GI of a food can vary depending on its ripeness, its variety and how it was processed or prepared. The GI of a food can also vary from person to person, and can also change depending on the time of day and an individual’s blood glucose levels, insulin levels and predisposed genetic metabolic parameters. Despite these factors, GI research has taught us that eating low-GI foods will result in greater appetite suppression and fewer cravings between meals, can help stabilize blood sugar levels and reduce insulin response, and may even help reduce the risk of heart disease and metabolic diseases. And finally, low-GI foods eaten prior to exercise can help sustain longer energy levels and increase fat oxidation!

As discussed previously, carbohydrates come in simple and complex sources, and each has a specific need with respect to recovery and performance. Complex, low-glycemic carbohydrates provide long-term energy and are therefore best consumed throughout the day. During a workout, the body depletes the glycogen stores, and to get recovery into full swing, the body needs glucose. Simple, high-glycemic carbohydrates are therefore most beneficial. Low carbohydrate consumption will lead to a shortage of energy during exercise requiring glycogen due to a reduced availability. If you don’t provide your body with enough carbs, not enough will be stored and available during exercise. In fact, about all glycogen stores can be used up after about two hours of strenuous exercise. Therefore, it is important to make sure you get in an ample amount of a low-glycemic carbohydrate prior to exercise to sustain energy levels and extend time to exhaustion. During a workout, your body will need both high and low-glycemic index foods. Low-glycemic foods can help sustain aerobic activities, while high-glycemic foods can help drive energy systems needed for weight training such as the creatine phosphate system. Immediately following training, high-glycemic foods should be consumed to restore and replace lost glycogen in the muscle as well as to help stimulate the release of insulin, which can not only shuttle glucose into the cells to replace lost glycogen but also stimulate the uptake of important aminos needed to drive muscle anabolism and repair, such as those taken through supplementation, including creatine or leucine.

Low-carbohydrate diets can increase the amount of GLUT4 receptors. With increased receptors, more glucose can be pulled into the cells from the bloodstream; they are in fact actively searching for uptake of glucose to store and use as glycogen. Depriving your body of carbohydrates for long periods of time can result in the body switching to an energy system that utilizes muscle protein instead of glucose. Additionally, when reintroducing carbohydrates into the muscle cells, the result can be a supercompensation of glycogen to the muscle cells with more water than usual, giving your muscles a fuller and harder look. This method is termed the depletion phase of a pre-contest diet. Each gram of glycogen is stored with approximately 3 grams of water. As a result, the muscle cells become fuller when packed due to ensuing water absorption.

There are many options when it comes to carbohydrate fueling. After a workout, the carbohydrate of choice should be from simple sources to kick-start the muscle recovery and rebuilding process. Throughout the day and especially for breakfast, stick to complex carbohydrates that are high in fiber and provide lasting energy between meals. Fiber has many benefits and is an extremely important part of any serious bodybuilder’s nutrition plan. Fibrous foods take longer to be processed and digested than simple carbohydrates or foods that do not contain fiber. This results in longer sustained energy and blunting of the appetite (or a feeling of being full for longer). Some high-fiber, slow-burning foods are sweet potatoes, green vegetables, brown rice and oatmeal. Fiber also helps to keep the intestinal walls functioning at a high capacity, freeing up the walls of undigested food during processing. The result is enhanced absorption of nutrients to keep the body functioning at an optimal anabolic level. To top that off, fiber also has a positive impact on fat absorption. Fiber can bind to fat and help pull it through the digestive system, reducing the amount that is absorbed by the body. High-fiber foods have been shown to enhance the affinity for insulin receptors, improving insulin sensitivity. The muscles have receptors for the important anabolic hormone insulin. The greater the affinity for the receptor, the better the insulin can drive carbohydrates, nutrients and protein into the muscle cells to begin muscle synthesis and repair.