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According to FAO 1997, Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their degree of polymerization and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides. (FAO/WHO. Carbohydrate in Human Nutrition Report of joint FAO/WHO expert consultation 14- 18 April 1997 Rome FAO Food and Nutrition paper No.66.Rome)
Major dietary carbohydrates and food sources
Dietary carbohydrates play a major role in human diets and contribute about 40-75% of energy. Carbohydrates of great importance to the diet are monosaccharides, disaccharides, oligosaccharides and sugar alcohols.
Table : Classification of carbohydrates
Class of carbohydrates
Type of saccharide
Readily absorbed and utilized
Small amounts in fruit and vegetable and largely in honey. Most of the body glucose is from digestion.
Well absorbed at normal levels of intake(~ 10g/day), but can be cause osmotic diarrhea especially in children where it can be directly absorbed without conversion to glucose
Fruit, vegetable and honey. Fructose used in manufacture of some products like jams
Absorbed by active transport and rapidly converted to glucose.
Not found in its free state but as component of lactose milk sugar.
Broken down by enzyme sucrase to glucose and fructose and are rapidly absorbed.
Table sugar. Naturally sweet food like fruit and young vegetables.
Broken down by lactase to glucose and Galactose. Some people loose ability to produce lactase beyond childhood resulting into lactose intolerance
Milk and milk products
Converted by maltase to glucose
Malted wheat, barley. Malt extracts.
Converted by trehalase enzyme to glucose
Mushrooms and edible fungi
Comprise of Galactose, glucose and fructose units. Human lack enzymes to digest them but can undergo colon fermentation.
FOS and GOS added to some functional foods as prebiotics and FOS is added to some commercial enteral feeds
Comprised of amylase (linear molecules of glucose units) and amylopectine (branched chain molecules of glucose units). Broken down by pancreatic amylase. Heat and moisture release starch from its storage granules.
Cereals and potatoes. Small amounts in vegetables and ripe fruits.
Similar to amylopectine. It is the storage form of glucose in living animals but no present on food derived from them.
A mixture of cellulose and non-cellulosic polysaccharides containing a variety of hexoses, pentoses, uronic acids and other components in their structure.
Vegetables, fruits, wholegrain cereals, cereal bran and pulses
Sugar alcohols (polyols)
These are only partially absorbed therefore they provide less energy per gram than other available carbohydrates. Large amounts of polyols cause osmotic diarrhea.
Small amounts in fruits and significant amounts in manufactured foods where they are used as substitutes for sucrose.
Source: Manual of dietetic practice
physiological and metabolic effects of dietary simple and complex carbohydrates- Energy source, blood glucose, fermentation and effects
Complex carbohydrates physiology and metabolism
Complex carbohydrates can be defined as large carbohydrate polymers. Nutritionally there are two classes of complex carbohydrates; starch and non-starch polysaccharides (NSP). Starch is the soluble carbohydrate or the digestible carbohydrate while NSP is the insoluble starch also referred to as the indigestible starch is fiber. The indigestible carbohydrates are fermented, in the large bowel by the action of various bacteria. The fermentation products are mainly short chain fatty acids (SCFAs) or volatile fatty acids (VFAs), methane (CH4), hydrogen (H2) and carbon dioxide (CO2). The SCFAs produced from the fermentation are absorbed at site of production and transported to the liver via entero-hepatic circulation.
Digestion of starch occurs in the ileum with glucose absorbed as the simple sugar. The NSPs are resistant to human digestive enzymes in the small intestines and are therefore undergo fermentation by anaerobic bacteria. First of all the large polymers are broken down to smaller units followed by fermentation via the glycolytic pathway to pyruvate and finally to SCFAs such as acetate, propionate and butyrate. The volatile fatty acids ratios produced on type carbohydrates consumed. Absorption of SCFAs is via ionic exchange or passive diffusion.
Metabolism of SCFAs in humans
The short chain fatty acids absorbed is used in order of priority for maintenance, growth and lipogenesis. Some of the volatile fatty acids are directly absorbed at the site of production into or by the liver via the hepatic portal vein. On absorption of SCFA, activation Co-enzymes are formed for different acids e.g Acetyl-CoA, Propionyl Co-A and butyryl-CoA. Acetate, propionate and butyrate give end different products of metabolism due to differences in activity of the respective acyl-CoA in the liver. Acetate majorly undergoes lipogenesis, while propionate can be used for glyconeogenesis and energy production in the TCA cycle. Butyrate is metabolized by both gut epithelium and liver. Butyrate is used for lipogenesis or energy.
Effect of complex carbohydrates on blood glucose
Non starch polysaccharides lower the rate of diffusion of various carbohydrates in small intestines. High viscosity is generated by the soluble SNP therefore this lowers the rate of absorption of products of digestion. The viscous NSP reduce the convective currents in the GIT generated by intestinal muscles there by reducing interaction of enzymes with the corresponding nutrients hence lowering the rate of hydrolysis of polysaccharides. This further lowers absorption rates contributing to lowered blood glucose levels. Studies in rats have shown NSP to alter enzymatic activity, for example guar feeding in rats reduces sucrase activity.
Complex carbohydrates associated disease
Diseases like diabetes mellitus, constipation, appendicitis, colorectal cancer, cardiovascular diseases have a correlation with complex carbohydrates. Occurrence of cardio vascular diseases is associated with cholesterol increase, reduction of low density lipoprotein (LDL) and increase in high density lipoprotein (HDL). LDL receptor activities are affected by dietary NSP and some dietary fats hence reducing the blood cholesterol levels. The mechanisms by which NSP reduce cholesterol include but not limited to increased digest viscosity, enhanced bile acid excretion and altered digestion and absorption of lipids. Production of SCFAs inhibits HMG-CoA reductase activity that is responsible for cholesterol production. Cholesterol reduction in the body is further done through enhancement fecal bile excretion. Soluble NSP such as guar gum in rats increase the activity of enzyme cholesterol 7alpha-monoxygenase during conversion of cholesterol to bile acids in the liver is increased and at the same time there is increased fecal bile acid excretion. Further still NSP increase viscosity GIT mass slowing down movement and consequently interfere with digestion and absorption of lipids.
3.0 role carbohydrates and disease
3.1 Glycemic index and load
Glycemic index (GI) can defined as the incremental area under the blood glucose response curve after consumption of 50 grams of available carbohydrates from a test food, divided by the area under the curve after consumption of 50 grams of carbohydrates from a reference food (e.g. glucose or white bread). The response of glucose rise is measured after intake of a certain carbohydrate meal. Carbohydrate foods are ranked on a scale of 0-100 according to rate of releasing blood sugar levels after a meal. High GI foods release those that are rapidly digested and, absorbed and cause up-down effects to blood sugar levels. Another point of view at low GI foods, they cause slow digestion and absorption, produce gradual rises in blood sugar and insulin levels, and have proven benefits for health.
Physiologic and therapeutic effects of low Glycemic Index foods
Meals containing low GI foods reduce both blood glucose and insulin responses. Animal studies suggest that incorporating slowly digested starch into the diet delays the onset of insulin resistance. Some epidemiologic studies suggest that a low GI diet is associated with reduced risk of developing non-insulin diabetes in men and women. Clinical trials in normal, diabetic and hyperlipidemic subjects show that low GI diets reduce mean blood glucose concentrations, reduce insulin secretion and reduce serum triglycerides in individuals with hypertriglyceridemia . In addition, the digestibility of the carbohydrate in low GI foods is generally less than that of high GI foods. Thus, low GI foods increase the amount of carbohydrate entering the colon and increase colonic fermentation and short chain fatty acid production. This has implications for systemic nitrogen and lipid metabolism, and for local events within the colon.
Calculation of glycemic index of meals
GI of entire meal can calculated from amount of carbohydrates in each food and the GI of each food in the meal. The index ranks carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. The requirements for this calculation are nutrition labels and GI charts.
An example would be to calculate GI of a meal containing milk, shredded wheat and toast. The caluculation would be arranged in the table as illustrated below.
Grams of Glycemic carbohydrate
Portion of Glycemic carbohydrate
Food Glycemic index
Meal Glycemic index
Use the nutritional label for your food to identify the carbohydrates in serving for that particular food. The milk contains 23g carbohydrates, the shredded wheat 43g carbohydrates and the toast 28g carbohydrates.
Calculate the total amount of carbohydrates in your meal. The milk has 23g, the shredded wheat 43g and the toast 28g. The total carbohydrate load in the meal is 23 + 43 + 28=94g
Calculate the portion each food in the meal that is contributed to the total carbohydrate. Milk 23/94 = 0.245, Wheat 43/94 = 0.457, Toast 28/94 = 0.298
Multiply each portion (fraction) by it GI. You can get the GI values for various foods from the GI charts. Milk GI: 32, so its contribution to the GI of the meal is 32 x 0.245 = 7.8. Shredded GI 63, GI contribution; 63 x 0.457 = 28.8. Toast GI 70,GI contribution; 70 x 0.298 = 20.9
Finally compute the total GI by adding up the different contributions.
3.2 High GI and metabolic syndrome (waist circumference, impaired fasting glucose, high blood pressure, dyslipidemia and low HDL-CHOO) and Cancer
An individualâ€™s exposure to cardiovascular risk factors such as Type 2 diabetes (acquired), increased blood fat (triglycerides), increased cholesterol (LDL or decreased HDL), increased blood pressure, obesity (intrabdominal type) can referred to as metabolic syndrome. High intake of sugar triggers more insulin production by the body. This will lead to an unexpected/sudden energy drop down; again the body craves sugar to boost the energy demand resulting into fluctuating effects. Abnormally high glucose levels will keep the insulin levels high and the following effects will occur; formation of atherosclerotic plaque, making it more likely that you will develop artery blocking clots, retention of water and sodium elevating blood pressure and, high levels of triglycerides in blood. At low levels of blood sugar, the pituitary glands secrete adrencocorticotropic hormone which triggers adrenal glands to secrete cortisol hormone which leads to stimulation of the glycogen release from the liver. GI of foods helps to look at those that will require less insulin for digestive breakdown into energy. Diets with low GI show improvements in insulin sensitivity in patients at risk cardiovascular disease (metabolic syndrome). Among the observed responses are improved fat profile, that is elevated HDL, weight loss because they control appetite and delay hunger, improved response in type 1 and type 2 diabetes. Commonly accepted ranges of GI of foods are; 70 and above is high, Medium at 56-69 and Low at or below 55.
3.3 Role of dietary fiber and disease prevention- Cancer, CHD, Diabetes
A high intake of dietary fiber has been thought to reduce the risk of colorectal cancer. Dietary fibre reduces insulin secretion by slowing rate of nutrient absorption, protects against hypertension, hyperlipedemia and CVD. Dietary fibre increases stool bulk and bind carcinogens that cause colon cancer causing carcinogens there by reducing risks of colon cancer. However the detail of the mechanism and components responsible for colon cancer reduction in humans are not clear. Therefore a healthful diet of vegetables, fruits and grains (especially whole grains) may be helpful, combined with other healthful lifestyle practices. Diabetes can be controlled through carefully timed, high-dose supplementation with specific types of concentrated
fiber sources. In the early days of research on dietary fiber and diabetes, 40g of fiber were recommended for daily intake by diabetic patients. Recent research shows that amount required for significant control of diabetes almost triples the previously recommended and specific for certain fiber sources that are more viscous like the guar gums. Coronary heart disease prevention is associated with high fiber diets. This may be by its effects on obesity, blood cholesterol and glucose metabolism or effect on atherosclerosis. Blood cholesterol levels are only lowered by specific fiber foods that contain significant amount of soluble fiber. These food and supplements include guar gum, oat meat, oat bran and legumes. Wheat bran and cellulose do not reduce cholesterol levels and for their effect to be significant relatively large doses are required and taken consistently daily. It is essential to consume lots of fluid during high fiber intake periods otherwise the patient will be prone to constipation.
3.4 Role of Phytochemicals in health and disease
Phytochemicals are compounds occurring naturally in plants responsible for color and organoleptic sensory properties for example smell in garlic, deep red color in tomatoes. They play a role of antioxidants in the body by deactivating cancer causing substances and binding free radicals in the body. They work with the defense system and play a role of enhancing cellular growth and repair by incorporation of their components in various cellular and metabolism processes. Phytochemicals include compounds like antioxidants, bioflavonoid, sulfides (allium), indoles, phytosterols, protease inhibitors, phenols, tannins and terpenes. Beta-carotene is responsible for the deep orange color, and bright yellow in fruits. In the body beta-carotene are converted to vitamin and are responsible for growth and repair of body tissues, formation of bones and teeth, resistance to infection, and development of healthy eye tissues.
Green color in peas and green leafy vegetables contain lutein an antioxidant that is responsible reduce the risk of cataracts and macular degeneration. Indoles an antioxidant in cruciferous vegetables; broccoli, cauliflower and cabbage protect against breast cancer in women and prostate cancer in men. Deep red and bright pinks are source of lycopene and carotenoids and are fat soluble. Tomatoes have the highest concentration
Hi Watermelons, pink grapefruits, and tomatoes
are all good sources of lycopene, one of the 600
carotenoids. Tomato-based products, such as tomato
sauce, tomato soup, and tomato juice have the most
concentrated source of lycopene.15 Cooked tomato
sauces are associated with greater health benefits,
compared to uncooked, because the heating
process makes lycopene more easily absorbed by
the body. Lycopene is fat soluble, so it must be
eaten with at least a small amount of fat.
3.5 Dietary recommendations
3.6 Advances in human Nutrition.