3-Carbohydrates and exercise

Nutrition and Fitness

Dr. Siham Gritly

Dr. Siham Gritly

Objectives of this lecture;

• At the end of the lecture student should be able to understand;

• The role of carbohydrates as main energy fueling macromolecules

• the role of carbohydrates in exercises.

• The general outlines of the pentose phosphate pathway

Dr. Siham Gritly

Carbohydrates

• Introduction;-• Carbohydrates are organic compound consist of

carbon, hydrogen, and oxygen atoms. ratio of hydrogen to oxygen is 2:1. Carbohydrates range from simple monosaccharides (glucose, fructose, galactose) to complex polysaccharides (starch).

• The most important monosacchrides are; glucose, fructose and galactose (hexoses sugar ) the main dietary sources of metabolic energy or body fuels.

Dr. Siham Gritly

Classification of carbohydrates

• a- Monosaccharides or simple sugar which contain one sugar unit such as Trioses (3carbon), tetro, ses (4 carbon molecules), pentoses (5 carbon), hexoses (6carbon), heptoses (7 carbon) .

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b-disaccharides, are formed by the condensation of two mono-sacchrides with the elimination of one molecule of water. the disaccharides of nutritional importance are

• 1-Sucrose; cane sugar, beat sugar, sucroses are manufactured on large scale from cane sugar. sucrose is present in certain fruits and vegetables. Sucrose consists of –glucose +fructose

Dr. Siham Gritly

• 2-Maltose; present in malt, cereal grains. it is formed when starch that present in the food is digested by the salivary and pancreatic enzyme Amylase. maltose consists of 2 glucose molecules; maltose—glucose + glucose

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• 3-Lactose or called milk sugar is a disaccharide occur in mammals milk, synthesis in mammary gland by the glucose that supplied through the food. in small intestine lactose is hydrolyzed to glucose and galactose by the action of lactase enzyme present in intestinal juice. Lactose—glucose + galactose.

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• C-polysaccharides; consist of many mono-saccharides. Polysaccharides of nutritional important are;

• a-starch; occur mainly in plant kingdom. Important sources are cereals, millets, roots, tubers formed in nature in large amounts.

• Starch hydrolyzed by amylase enzyme present in saliva and in pancreatic juice to form maltose (disaccharide).

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• during hydrolysis starch formed intermediate product called dextrin. complete digestion of starch formed glucose;

• starch--------dextrin----------maltose------------glucose

• b-glycogen; polysaccharides, it is the storage form of carbohydrates in the human body. the site of storage is in the liver and muscles.

• It is the reserve of carbohydrates in body. the liver contain enzyme which convert glycogen to glucose through the process known as glycogenlysis.

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• c- Dietary fiber is a carbohydrates (or a polysaccharide) that is incompletely absorbed in humans and in some animals.

• *Dietary fiber consists mainly of cellulose, a large carbohydrate polymer that is indigestible because humans do not have the required enzymes to disassemble it. There are two subcategories: soluble and insoluble fiber. Whole grains, fruits (especially plums, and figs) and vegetables are good sources of dietary fiber.

Dr. Siham Gritly

Digestion and absorption of carbohydrates

• Digestion is the process of breakdown large particles into smaller one. the digestion of carbohydrates start in the mouth and is continue in the stomach and intestine. Carbohydrates absorbed through the intestinal mucosa as monosaccharides (glucose).

• Glucose enters the metabolic pathways to release energy, which then is used for the formation of the chemical compound ATP.

Dr. Siham Gritly

exercise;Discus the role of enzymes in carbohydrate digestion

Dr. Siham Gritly

Major function of carbohydrate in human body

• *CHO is the main fuel for certain tissues during rest such as brain, central nervous system (CNS) and red blood cell (RBC)

• *Supply energy; glucose oxidized producing water, energy, carbon dioxide and energy (ATP)

• *monosaccharide can form other carbohydrate molecules such as trioses and pentoses, these substances combined with other substances to form essential compounds (glycolipids and glycoprotein)

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• Glycoprotein are very important components of cell membrane

• Ribose sugar (pentose) are important in formation of RNA (ribonucleic acid of the cell)

• excess of carbohydrates stored in the form of glycogen in liver (glycogenesis).

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Carbohydrate Needs

• The carbohydrate contribution is mainly from;- *glycogen stores in skeletal muscle and that stored in the liver,

• the diet for athletes and active people is that it should include more carbohydrate-containing foods that recommended by the health professionals.

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• Their diets should be about 60% of their daily energy intake obtained from carbohydrates, 30 % or less from fat and 10 to 15 % from proteins

• Adequate carbohydrate intake also helps prevent

protein from being used as energy

• It also fueling the central nervous system (CNS) and brain

Dr. Siham Gritly

• During heavy training kcal must be increased specially from carbohydrates (CHO), to meet the energy demands. Failure to do so may result in:

• Chronic muscular fatigue.

• - Weight and muscle mass loss

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Carbohydrates utilized by the body in different ways;

• *for the immediate energy needs through oxidation to carbon dioxide and water via the processes of glycolysis and tri-carboxylic acid cycle or Kerb cycle

• *stored as glycogen in the liver and muscles tissue through the process glycogenesis.

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• *converted to fatty acids and stored as triglyceride.

• *convert to other necessary CHO such as ribose, fructose and deoxyribose which is necessary for the formation of genetic material DNA.

• *some become the carbon skeletons for production of essential amino acids

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Carbohydrates and exercises

• Carbohydrates come in two sources during exercises ;

1-Simple sugar glucose

2-Storage form of glucose –glycogen; Glycogen is stored in Muscles and Liver

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• During athletic endeavors (or activities) blood glucose, liver glycogen and muscle glycogen is the main source of energy

• - Carbohydrate are the main nutrient fueling exercise of a moderate to high intensity, while fat providing energy during exercise that occurs at a lower intensity (supply about 40-80%of the total calories in human diet).

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• During moderate exercise muscle glycogen and liver glycogen contribute equally to CHO oxidation

• - As exercise intensity increases, carbohydrate metabolism efficiency drops off and anaerobic metabolism takes over.

• - This is because the body can not take in and distribute oxygen quickly enough to use either fat or carbohydrate metabolism easily.

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• - The liver is the only tissue that can release stored glycogen in the form of glucose

• Muscle glycogen store is the first source of

glucose for the exercising muscle

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• when no glycogen;- the process of glycogenolysis started and follow by gluconeogenesis in the liver to make glucose available (main function of the liver is to release glucose through glycogenolysis and gluconeogenesis )

• if no enough glucose after 3 hours of heavy exercise athletes enter to a condition known as hoypglycemia (low glucose level in the blood)

Dr. Siham Gritly

Carbohydrates loading

• CHO Loading, or glycogen loading and glycogen super-compensation;- is dietary technique designed to promote a significant increase in glycogen content in both the liver and muscles to delay fatigue among athletes.

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• It is generally used 3-7 days in preparation for athletic competition

• It is used by endurance athletes with continuous energy expenditure for prolonged periods (long distance runners, swimmers) to optimize physical performance during prolonged endurance events.

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Pentose Phosphate Pathway, (hexose-mono-phosphate shunt)

• The Pentose Phosphate Pathway, generate important intermediates not produced in other pathways. it acts in the conversion of hexoses sugar into pentose sugar (ribose) and in regeneration of NADPH .

• when glucose increase in the blood it converted to glycogen through the process of glycogenesis, or when it needed as energy it enter into glycolytic pathway for energy formation but;

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• some of glucose may enter alternative route in the cytoplasm, this route is known as Pentose phosphate pathway.

• in the presence of oxygen, hexoses sugar (glucose ) converted to pentose mainly Ribose and deoxy-ribose that enter in the formation of DNA and RNA, the neuclic acid of the cells (Genetic material of the cell).

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• when excess pentose present can also converted to hexoses (glucose) and thus enter the glycolytic pathway for energy production.

• *in this process no carbon dioxide released, the hydrogen molecules combined with NAD to become NADPO and enter in the lipid synthesis

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• When carbohydrate intake is low, oxidation of fatty acids accelerate to provide energy through the production of acetyl CoA (TCA substrate)

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References

• Sareen Gropper, Jack Smith and James Groff, Advanced Nutrition and Human Metabolism, fifth ed. WADSWORTH

• Melvin H Williams 2010; Nutrition for Health, Fitness and Sport. 9th

ed, McGraw Hill• • Heymsfield, SB.; Baumgartner N.; Richard and Sheau-Fang P. 1999.

Modern Nutrition in Health and Disease; Shils E Maurice, Olson A. James, Shike Moshe and Ross A. Catharine eds. 9th edition

• Guyton, C. Arthur. 1985. Textbook of Medical Physiology. 6th edition, W.B. Company