‘Nutrition, digestion, and energy balance’ [47%, 2017]


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‘Nutrition, digestion, and energy balance’

{ESSAY CONTENTS: Introduction; Carbohydrates – structure and functions; Table 1 – carbohydrate sources; Fibre – structure and functions; Table 2 – fibre sources; Proteins – structure and functions; Table 3 – protein sources; Table 4 – ‘essential’, ‘conditionally essential’, and ‘non-essential’ amino acids; Fats – structure and functions; Table 5 – fat sources; Table 6 – cholesterol sources and saturated fat content; Fat-soluble vitamins; Water-soluble vitamins; Table 7 – vitamin sources, functions, and symptoms of deficiency; Minerals – structure and classification; Table 8 – mineral classifications, functions, and sources; Water – importance, functions, and requirements; Digestion: oral, oesophageal; Digestion: stomach; Digestion: small intestine; Digestion: nutrient absorption, usage, and storage; Governmental dietary recommendations; Essay summary; Bibliography.}

This essay will give an account of the importance of the seven essential nutrients in a healthy balanced diet. Following this examination of the importance of carbohydrates, fibres, proteins, fats, vitamins, minerals, and water, the process of digestion and energy storage will be discussed. Finally, the energy density of various nutrients will be evaluated with respect to maintenance of a healthy weight. [^]

Carbohydrates consist of carbon, hydrogen, and oxygen atoms. They can be classified as simple sugars (‘monosaccharides’ or ‘disaccharides’), starches (‘polysaccharides’), or fibre (‘non-starch polysaccharides’) depending on their chemical structure and digestibility. Examples of simple sugars are the monosaccharides glucose, fructose, and galactose. These monosaccharides can be joined together via a condensation reaction, whereby a molecule of water is lost in the creation of a compound molecule; for example, the dehydration of a water molecule from two glucose molecules creates the disaccharide maltose. Polysaccharides are made up of long chains of monosaccharides. The addition of water splits a compound molecule into its constituent parts; this is known as a hydrolysis reaction (New Health Advisor, 2014). An adequate carbohydrate supply is important for several reasons: it provides energy and heat; it ensures that protein is used for its primary purpose, rather than in gluconeogenesis; and it forms a bodily energy store, both as glycogen in the liver and skeletal muscles as a short-term energy store, and as fat stored in adipose tissue. [^]

(Tabulated list based on information provided by NHS Choices, 2015, and 2016a)
Bread [preferably seeded, wholemeal, or granary]
Cereals [preferably wholegrain – porridge; whole oats] [also includes barley, corn, couscous, quinoa, and tapioca]
Fruit and vegetables
Fruit juices
Sweets, chocolates, cakes, biscuits, soft drinks [^]

Non-starch polysaccharide, commonly referred to as dietary fibre, is the indigestible part of the diet, composed of bran, cellulose and other polysaccharides found in fruit, vegetables, and cereals; fibre is only found in food that comes from plants (Grant and Waugh, 2014). There are two types of fibre – soluble and insoluble. Soluble fibre dissolves in the water of the digestive system, helping to lower blood cholesterol as well as softening stools and making defaecation easier (NHS Choices, 2017a). Insoluble fibre does not dissolve in the water of the digestive system, passing instead through the gut without being broken down, helping to move other foods through the digestive system more easily. [^]

(Tabulated list based on information provided by NHS Choices, 2017a)
Oats, barley, rye Wholemeal bread
Fruit Bran
Root vegetables Cereals
Golden linseeds Nuts and seeds (except golden linseeds) [^]

Proteins are composed of long chains of amino acids. These amino acids are themselves made up of carbon, hydrogen, oxygen and nitrogen atoms, with some amino acids also containing minerals such as iron, copper, zinc, iodine, sulphur and phosphate (Grant and Waugh, 2014). Many (‘non-essential’) amino acids can be made within the body, while some (‘essential’) amino acids must be included in the diet. Histidine is an example of a ‘conditionally essential’ amino acid, since infants cannot synthesise it while adults can. The amino group of excess amino acids is excreted in urine, with the rest of the molecule converted to either glucose or a ketone body. Amino acids serve several purposes: growing and repairing body cells and tissues; producing enzymes, plasma proteins, antibodies, and some hormones; and providing energy – for example, during prolonged exercise (over two hours) when glycogen reserves have been exhausted and fat stores are depleted. [^]

(Tabulated list based on information provided by Jan Annigan, 2015)
Meat [an example of a ‘complete protein’ (containing all essential amino acids), as with all sources of protein derived from animal sources]
Fish [same as meat, above]
Eggs and milk products (fluid milk, whey supplements, yoghurt, cheese)
Plants [the majority of plant proteins are ‘incomplete’, as they do not contain all the essential amino acids; complementary plant proteins can be combined to form a complete protein – for example: black beans on a corn tortilla; baked beans on toast; kidney beans and rice; baked beans and sweetcorn as a jacket potato topping] [^]
(Tabulated list based on information provided by Nutrients Review, 2016, and Grant and Waugh, 2014)
Isoleucine Histidine Alanine
Leucine Arginine Asparagine
Lysine Cysteine Aspartic acid
Methionine Glutamine Glutamic acid
Phenylalanine Glycine Selenocysteine
Threonine Proline
Tryptophan Serine
Valine Tyrosine [^]

Like carbohydrates, fats are made up of carbon, hydrogen, and oxygen atoms, but unlike carbohydrates the hydrogen and oxygen elements are not in the same proportions as in water (Grant and Waugh, 2014). The most commonly referred to ‘fat’ is known as a triglyceride, a molecule that is comprised of three fatty acids linked to a glycerol molecule. Triglycerides can be classified according to whether the carbon atoms are fully saturated with hydrogen atoms (saturated), or if one (monounsaturated) or more (polyunsaturated) double bonds have formed between carbon atoms for molecular stability. Fats serve several functions in the body: as a source of chemical heat and energy; as organ support; in the movement and storage of fat-soluble vitamins; in energy storage, as fat in adipose tissue; insulation; and helping to maintain a feeling of ‘fullness’ after eating, since high fat food takes longer to be emptied from the stomach. Cholesterol is another type of fat that is needed to produce steroid hormones and form cell membranes. Cholesterol bonds with proteins and travels in the blood as lipoproteins, in the form of either low-density lipoprotein that carries cholesterol from the liver to the body cells, or as high-density lipoprotein that carries cholesterol back from the body cells to the liver. [^]

(Table based on information provided by Harvard School of Public Health, 2017)
Monounsaturated – Olive, peanut, and canola oils
– Avocados
– Almonds, hazelnuts, pecans
– Pumpkin and sesame seeds
Improve blood cholesterol levels, ease inflammation, stabilize heart rhythms.
Polyunsaturated – Sunflower, corn, soybean, flaxseed oils
– Walnuts
– Flax seeds
– Fish
These foods are great sources of omega-3 fats. Omega-3 fats cannot be synthesized by the body, so they must come from the diet. Replacing saturated fats with polyunsaturated fats can lower the risk of heart disease.
Saturated – Pizza
– Cheese
– Milk, butter, dairy desserts
– Meat products
– Cookies
– Mexican fast food dishes
Saturated fats should be kept to a minimum in the diet. Replacing saturated fats with polyunsaturated fats lowers the ‘bad’ low-density lipoprotein cholesterol and can help prevent insulin resistance.
‘Trans fats’
[Trans Fatty Acids]
– Partially hydrogenated vegetable oil
– Beef fat, dairy fat (small amounts)
Created by heating liquid vegetable oils in the presence of hydrogen gas and a catalyst. Raises ‘bad’ low-density lipoprotein cholesterol and lowers ‘good’ high-density lipoprotein cholesterol. Creates inflammation, contributes to insulin resistance, and increases the risk of coronary heart disease.
(Table based on information provided by Heart UK, 2017)
Full cream dairy: milk; cheese; yoghurt; cream. High
Animal fats: butter; ghee; animal fat-derived margarines and spreads; lard; suet; dripping. High
Fatty meat and processed meat products. High
Lean meat: liver; kidney; sweetbreads; heart; tripe. Low
Much seafood: prawns; crab; lobster; squid; octopus; cuttlefish. Low
Egg yolk Low [^]

Fat-soluble vitamins require bile for absorption via the small intestine. There are four fat soluble vitamins – A, D, E, and K. Vitamin A serves many functions in the body: strengthening eyesight; aiding cell growth and differentiation; improving immunity; and promoting growth. Vitamin D can be produced in the human body by the action of sunlight on 7-dehydrocholesterol in the skin. It increases calcium and phosphate absorption and retention, promoting the calcification of bones and teeth. Vitamin E protects components of the body against the oxidative reactions caused by free radicals, and has been shown to improve heart health. Vitamin K is essential for the liver to produce prothrombin and factors VII, IX, and X, all of which are necessary for blood clotting. It can be produced by bacteria in the large intestine. Owing to the bacteria-free sterility of their colons, premature babies may be given vitamin K orally or via injection in order to guard against haemorrhagic disease of the newborn (Grant and Waugh, 2014). [^]

Water-soluble vitamins are excreted in urine, meaning body stores are limited. There is a group of B vitamins that stimulates enzymatic catabolism of nutrients to release energy – vitamins B1, B2, B3, B6, and B12. Folic acid is produced by bacteria in the large intestine, and plays an essential role in DNA synthesis and cell division. Pantothenic acid aids carbohydrate metabolism, while biotin is produced by intestinal microbes and is involved in the metabolism of carbohydrates, fats, and some amino acids. Vitamin C is involved in protein metabolism; deficiency (‘scurvy’) becomes apparent after 4-6 months. The sources, functions, and symptoms of deficiency relating to the aforementioned vitamins are encapsulated in the following table. [^]

(Table based on information provided by Health Supplements Nutritional Guide, 2017)
A (retinol) Animal liver; fish liver oil; eggs; milk; green and yellow vegetables;
reddish-yellow fruits.
Strengthens eyesight; ensures normal functioning of the reproductive system;
bone and teeth development; antioxidant; improves immunity; builds healthy skin,
mucous linings, hair, and nails; anti-ageing.
Night blindness; teeth and bone issues; xerophthalmia, leading eventually to blindness; skin problems; increased susceptibility to colds, viruses, sinusitis and urinary tract infections; abscesses in the ears; rapid weight loss; loss of smell, taste, appetite.
D Fish liver oils; mackerel; salmon; sardine; shrimp; alfalfa; animal liver;
egg yolk; fortified milk; oatmeal; cod; halibut; tuna; sweet potatoes.
Promote correct absorption and usage of calcium and phosphate, ensuring correct skeletal, bone, and teeth development; helps regulate blood sugar levels by stimulating the production of insulin; protects against high blood pressure,
heart disease, and cancer; regulates inflammatory response; guards against arthritis, Type One diabetes, psoriasis, and irritable bowel syndrome; decreases risk of multiple sclerosis; improves mood regulation.
Impaired function of insulin-producing cells; increased likelihood of Type One diabetes; bone, teeth, and muscular issues; increased risk of high blood pressure and low blood calcium level; impaired mood regulation.
E (tocopherol) Cold-pressed vegetable oils; wheat germ oil; dark green leafy vegetables;
avocadoes; blueberries; brown rice; dried beans; egg yolk; kiwifruit; legumes;
almonds; sunflower seeds; macadamia nuts; hazelnuts; peanuts; oatmeal; liver;
olives; papaya; soybeans; sweet potatoes; asparagus; bell peppers; broccoli;
Brussels sprouts; collard greens; kale; parsley; tomato; wheat germ; whole grains and whole grain products.
Antioxidant; anti-ageing; boosts immune system; works with vitamin A to protect lungs from pollution; protects skin from harmful ultraviolet rays;
helps vision; associated with beneficial neurological effects; lowers blood pressure; protects against atherosclerosis and heart disease; necessary for red blood cell formation, as well as fertility.
Loss of appetite; nausea; anaemia; impaired eyesight; chest pains; muscular weakness; nerve damage; decreased muscular coordination; digestive tract issues,
leading to poor absorption of food; decreased fertility.
K Soybeans; dark green leafy vegetables; alfalfa; animal liver; carrots;
egg yolk; garbanzo beans; green beans and peas; cereals (oats, oatmeal, rye,
Regulates blood clotting; antioxidant; helps form cartilage, bone,
and dentine; helps to regulate calcium balance and bone mass.
Bruise or bleed easily; higher chance of vomiting/excreting blood;
slower blood clotting; weak bones.
B1 (thiamin) Mushrooms; sunflower seeds; tuna; asparagus; broccoli; Brussels sprouts;
romaine lettuce; spinach; tomatoes; animal liver; brewer’s yeast; egg yolk;
fish; lean meat; nuts; chickpeas; dhal; lentils; soybeans; raw rice bran;
beans; beets; cauliflower; eggplant; green peas; wheat germ; whole grains and cereals.
Improves digestive health by supporting hydrochloric acid production;
carbohydrate metabolism; optimizes brain function and mood; eye health;
nervous system health; involved in blood cell formation; coordinates interaction of nerves and muscles; required for normal muscle tone of heart, stomach, and intestines.
Beriberi – a condition characterized by: excessive fatigue;
muscular weakness; loss of sensation in hands and legs; nerve damage, leading to leg paralysis; heart abnormalities; breathing difficulties; gastrointestinal disorders; confusion; irritability; depression; brain damage in extreme cases (Wernicke-Korsakoff Syndrome).
B2 (riboflavin) Animal liver; mushrooms; spinach; beans; chicken eggs; cheese; milk;
yogurt; fish; lean meat; asparagus; broccoli; Brussels sprouts; chard; collard greens; mustard greens; romaine lettuce; turnip greens; venison.
Eyesight; energy production; body tissue growth and repair; maintaining the mucous membrane of the digestive tract; migraine prevention; antioxidant;
required for antibody and red blood cell production; essential for iron and vitamin B6 absorption; helps maintain levels of B vitamins in the body; helps to treat carpal tunnel syndrome; helps to ensure correct foetal development during pregnancy; helps to moderate the risk of cardiovascular disease.
Dermatitis; swollen or sore throat; inflamed tongue; loss of sense of taste;
loss of appetite; foetal damage; anaemia; nerve damage; eyesight issues.
B3 (niacin) Mushrooms; raw rice bran; tuna; beef liver; brewer’s yeast; chicken breast;
corn flour; dairy products like cheese and milk; eggs; halibut; peanuts;
lean pork; salmon; shrimp and scallops; sea vegetables; asparagus; broccoli;
carrots; collard greens; green peas; mustard greens; romaine lettuce;
tomatoes; turkey; venison; whole wheat.
Food metabolism and digestive health; necessary for intra-cellular genetic DNA formation; nervous system health; skin health; eyesight; can reduce LDL cholesterol and increase HDL cholesterol; cardiovascular health; helps to protect against mental decline. Dermatitis; weariness; muscular weakness; gastrointestinal disturbances;
poor mood regulation; dementia.
B6 (pyridoxine) Bananas; bell peppers; chick peas; potatoes (baked with skin); prune juice;
raw rice bran; turnip greens; spinach; beans; brewer’s yeast; eggs; chicken;
garlic; cod; halibut; snapper; trout; tuna; lean meat; nuts; sunflower seeds;
turmeric powder; asparagus; broccoli; Brussels sprouts; cabbage; carrots;
cauliflower; celery; chard; collard greens; kale; mustard greens; peas;
walnuts; wheat germ; whole grains; whole wheat.
Immune system health; carbohydrate and fat catabolism; blood glucose regulation; protein metabolism; new cell formation and growth; skin health and mucous membranes; haemoglobin and red blood cell formation; normal nerve and brain function; helps the body convert tryptophan to vitamin B3; promotes cardiovascular health; reduces symptoms of pre-menstrual syndrome; helps to treat carpal tunnel syndrome. Eczema/dermatitis; inflammation of oral mucous membrane; convulsions and seizures; cramps or numbness; poor mood regulation; nausea; migraines; anaemia; chronic fatigue; muscle weakness; asthma; decreased immunity against infections.
B12 (cobalamin) Animal liver; cod; halibut; herring; trout; mackerel; salmon; snapper;
dairy products; egg yolk; kidney; meat; clams; scallops; shrimps.
Helps cells metabolize fats, proteins, and carbohydrates; red blood cell formation; needed to synthesize DNA; healthy nerve function. Pernicious anaemia; low blood pressure; loss of appetite;
nerve damage; heart palpitations; poor mood regulation; dandruff; eyesight problems; associated with breast cancer; retarded growth, movement disorders,
and anaemia in infants; increased susceptibility to cardiovascular disease.
Folic acid (folate) Organ meat like animal liver or kidney; legumes; dark-green leafy vegetables; avocadoes; bananas; cantaloupe; oranges and lemons; dates; egg;
nuts; papaya; poultry and pork; root vegetables; salmon; shellfish; tuna; wheat bran; wheat germ; whole grains and whole wheat; yeast; cabbage; green peas;
leeks; red bell peppers; squash; string beans; tomatoes.
Lowers risk of cardiovascular disease and dementia; involved in the production of neurotransmitters that help to regulate appetite, mood, and sleep;
energy production; amino acid metabolism and protein synthesis; DNA replication and healthy cell growth and function; essential for correct foetal development;
red blood cell formation; anaemia prevention; healthy skin and nails; protects oral and intestinal lining; may be used to treat leg ulcers.
Megaloblastic macrocytic anaemia; chronic muscular fatigue;
affects production of neurotransmitters, leading to poor mood regulation and nervous system-related problems of the hands and legs; skin disorders; gastrointestinal tract disorders; glossitis; cervical dysplasia; premature grey hair; increased risk of heart disease; slowed growth rate in infants and children; incorrect foetal development.
Pantothenic acid (‘Vitamin B5‘) Animal liver; cauliflower; mushrooms; raw rice bran; beans / legumes; beef;
brewer’s yeast; chicken; eggs; broccoli; chard; collard greens; tomato; turnip greens; winter squash; yellow corn; grapefruit; pork; salt-water fish; shellfish; strawberries; sunflower seeds; whole grains; yogurt.
Involved in many chemical processes in the body as a constituent of coenzyme A (CoA); important role in adrenal hormone, antibody, and haemoglobin production; involved in nerve impulse transmission between nerve cells; food metabolism; helps synthesize fatty acids and cholesterol; digestive health; enables body to use other vitamins and minerals. Numbness/burning/shooting pains in the feet; chronic fatigue; depression/listlessness/irritability; arthritis; headaches; nausea; insomnia; abdominal cramps; decreased immune system function.
Biotin Organ meats such as liver; carrots; romaine lettuce; swiss chard; tomatoes;
almonds; raspberries and strawberries; cooked egg yolk; milk; cheese; yogurt;
oats; poultry; halibut; soybeans; walnuts; cabbage; cucumber; cauliflower;
onions; whole grains; yeast.
Promotes healthy skin, hair, and nails; slows hair loss; needed for DNA replication and cell division and growth; healthy sweat glands, nerve tissue,
and bone marrow; production of fatty acids; metabolism of food for energy;
regulates blood glucose levels, benefiting people with Type Two diabetes;
decreases triglyceride levels; helps prevent intestinal disorders; helps the body correctly use the other B vitamins.
Poor nail and hair health; cradle cap in infants, seborrhoeic dermatitis in adults; depression; insomnia; intestinal tract issues; fatigue owing to poor energy production; muscle pain and cramps; movements characterized by poor muscle tone and coordination.
Vitamin C (‘ascorbic acid’) Guava; lychees; papayas; strawberries; broccoli; Brussels sprouts; cabbage; cauliflower; collards; kale; mustard greens; bell peppers; turnip greens; spinach; blueberries; cranberries; raspberries; blackcurrants; grapefruit;
lemons; limes; oranges; tangerines; cantaloupe; kiwifruit; mangos; melons;
persimmons; pineapple; onions; fennel; parsley; peppermint; rose hips; radishes,
turnips, potatoes and sweet potatoes (baked with skin); asparagus; beet greens;
chard; garden cress; green peas; lettuce; tomatoes; winter squash; zucchini.
Immune health; antioxidant; needed for the growth and repair of body tissues; skin, teeth, gum, and bone health; wound healing; helps the body absorb the iron necessary for red blood cell formation; reduces LDL cholesterol and raises HDL cholesterol; reduces susceptibility to allergens; helps to expel toxins from the body. Scurvy, whereby the body ceases to produce collagen and begins to fall apart – cartilage and tendons break down, blood vessels rupture, gums ulcerate, teeth fall out, the immune system deteriorates and death ensues; easy bruising; small bleeds under the skin; painful joints; slow wound healing; nosebleeds; bleeding gums and loose teeth; decayed teeth; brittle hair; skin problems; anaemia; fatigue; lowered immunity; loss of appetite. [^]

Minerals are not man made, have never been alive, and are not derived from plant or animal sources. Every mineral is a solid, is made up of a particular mix of chemical elements, and has an ordered atomic arrangement (Oxford University Museum of Natural History, 2006). Minerals can be classified as either macrominerals or trace minerals according to how much of the mineral is necessary in order to ensure normal cellular functioning. The macrominerals consist of calcium, magnesium, phosphorous, sodium, potassium, chloride, and sulphur. A lesser amount of the following trace minerals is also essential for good health: iron, manganese, copper, iodine, zinc, cobalt, fluoride, and selenium (MedlinePlus, 2017). The following table lists the classification, functions and sources of each of the aforementioned minerals. [^]

(Table based on information provided by SFGATE, 2017, The New York Times Health Guide, 2017, The Healthy Home Economist, 2017, NHS Choices, 2017b, 2017c, and Centers for Disease Control and Prevention, 2017)
Calcium Macromineral Strong bones and teeth; nerve transmission; muscle function; hormone secretion. Dairy products – milk, yoghurt.
Vegetables – kale, broccoli, cabbage.
Magnesium Macromineral Supports more than 300 biochemical reactions; muscle and nerve function;
heart health; strong bones; boosts immunity.
Beans, nuts, whole grains, green vegetables.
Phosphorous Macromineral Strong bones and teeth; involved in protein production and cell repair. Dairy products, meat, whole grains.
Sodium Macromineral Stimulate nerve and muscle function; maintain cellular fluid balance;
absorption of other nutrients, such as chloride, amino acids, and glucose.
Breads and rolls, cold cuts and cured meats, pizza, poultry, soups,
sandwiches, cheese, pasta dishes, meat dishes, snacks.
Potassium Macromineral Controls electrical activity of the heart; required to build proteins;
catabolize carbohydrates; maintain blood pH balance; support normal growth.
Beef, fish, chicken, cantaloupe, potatoes, tomatoes, lima beans.
Chloride Macromineral Balances the fluids in the body; essential for the production of digestive juices in the stomach. Table salt, seaweed, rye, tomatoes, lettuce, celery, olives.
Sulphur Macromineral Vital for effective glucose metabolism. Eggs, onions, garlic, cabbage.
Iron Trace mineral Produce haemoglobin and myoglobin – proteins that transport oxygen in the blood. Liver, meat, beans, nuts, dried fruits, whole grains, fortified breakfast cereals, soybean flour, dark green leafy vegetables.
Manganese Trace mineral Regulates blood sugar; enhances calcium absorption; aids the production of connective tissues and bones. Tea, bread, nuts, cereals, peas, runner beans.
Copper Trace mineral Works with iron to help form red blood cells; keeps blood vessels, nerves,
immune system, and bones healthy; aids iron absorption.
Oysters and other shellfish, whole grains, beans, nuts, potatoes,
kidneys, liver, dark leafy vegetables, dried fruits.
Iodine Trace mineral Involved in the production of thyroid hormones that regulate nearly every cell in the body. Table salt, seafood, kelp, dairy products.
Zinc Trace mineral Correct immune system function; cell division; cell growth; wound healing;
catabolizes carbohydrates.
Animal proteins, nuts, whole grains, legumes, yeast.
Cobalt Trace mineral Makes up part of vitamin B12 – helps cells metabolize fats,
proteins, and carbohydrates; red blood cell formation; needed to synthesize DNA;
healthy nerve function.
Fish, nuts, broccoli, spinach, oat cereals.
Fluoride Trace mineral Strong, healthy teeth. Fresh fruits and vegetables; cups of tea.
Selenium Trace mineral Producing antioxidant enzymes. Vegetables, brazil nuts, fish, red meat, grains, eggs, chicken,
liver, garlic. [^]

Water is the most essential nutrient of a healthy balanced diet. An individual should imbibe around 35 millilitres of water for every kilogram of body weight (MyNetDiary, 2017). Since the average UK adult female weighs 70.6 kilograms, while the average UK adult male weighs 83.9 kilograms (On Average, 2017), the daily water requirements for these groups are 2,471 millilitres and 2936.5 millilitres respectively. The human body consists of approximately 60% water; an individual will either die or be very close to death after three days without it (Binns, 2012). As the most profuse component of the human body, water serves a number of functions. Water is required by all living cells for metabolic reactions, such as converting glucose to carbon dioxide and water during respiration (The Royal Society of Chemistry, 2004). In the form of cooling sweat, water is involved in the regulation of body temperature. It dilutes and provides the means of excretion for bodily waste products. As the main component of blood and tissue fluid, water facilitates the movement of substances round the body and paves the way for exchange between the blood, tissue fluid, and body cells (Grant and Waugh, 2014). The main constituent of saliva, water lubricates dry food, helping to transform it into a bolus ready for transport down the oesophagus into the stomach. [^]

As well as water, saliva is composed of mineral salts, mucous, antimicrobial substances, and salivary amylase. Salivary amylase begins the chemical digestion of polysaccharides, breaking them down to the disaccharide maltose. This chemical digestion is accompanied by the mechanical action of chewing, facilitated by the tongue, cheek muscles, molars, and premolars after food has been bitten by the incisors and canine teeth. The result of this chemical and mechanical digestion is the formation of a bolus, a soft mass that is ready for swallowing. Voluntary muscles of the tongue and cheeks transport the bolus backwards into the pharynx, before involuntary contraction of the pharyngeal muscles moves the bolus down into the oesophagus. Peristaltic waves are triggered in the oesophagus once swallowing begins, and the cardiac sphincter relaxes to facilitate the propulsion of the bolus into the stomach (Grant and Waugh, 2014). [^]

The process of mechanical digestion is continued in the stomach, with the three layers of smooth muscle fibres effecting a churning motion. Churning helps to break down the bolus and mix it with the gastric juice. Approximately two litres of gastric juice is secreted daily, helping to facilitate the process of chemical digestion in the stomach – primarily protein digestion. Hydrochloric acid secreted by parietal cells in the gastric glands mixes with pepsinogens secreted by chief cells to form pepsins, enzymes that begin to break down proteins into smaller chains of amino acids (Laparoscopic.md, 2015). Once gastric contents are sufficiently condensed they are forced through the pyloric sphincter – via peristalsis – into the duodenum as chyme. Carbohydrate meals leave the stomach faster than protein meals, which in turn are emptied faster than fatty meals (Grant and Waugh, 2014). [^]

Chyme is strongly acidic upon leaving the stomach, but its pH is raised to between 6 and 8 once it has been mixed with pancreatic juice, bile, and intestinal juice in the small intestine. Pancreatic juice contains amylase, lipase, trypsinogen, and chymotrypsinogen. All digestible polysaccharides are converted to disaccharides by pancreatic amylase. Pancreatic lipase changes fats to fatty acids and glycerol, with the help of bile – produced by the liver, stored in and secreted by the gall bladder – that emulsifies fats. Trypsinogen and chymotrypsinogen are activated by enterokinase in the microvilli, converting them to trypsin and chymotrypsin and triggering the breakdown of proteins to tripeptides, dipeptides, and amino acids. Direct contact between these nutrients and the enterocytes covering the intestinal villi complete the chemical digestion of food that takes place in the small intestine: sucrase, maltase, and lactase convert disaccharides to monosaccharides; pancreatic lipase converts emulsified fats to fatty acids and glycerol; and trypsin and chymotrypsin breaks down protein into its constituent amino acids (Vivo Pathophysiology, 2006). [^]

Enterocytes facilitate the absorption of nutrients in the small intestine. Monosaccharides pass into the blood capillaries in the villi, and are transported by the portal circulation to the liver. Glucose serves several functions in the body: it is used for the metabolic activity of the liver itself; some remains in the blood to maintain normal blood glucose levels; some is converted to glycogen, stored in the liver and skeletal muscles, that can be broken down by glucagon to fuel muscular and liver activity when needed; and glucose that is surplus to the above requirements is converted to fat and stored in the fat depots. Like monosaccharides, amino acids pass into the blood capillaries in the villi and are transported in the portal circulation to the liver and to all the cells and tissues of the body. Body cells choose the particular amino acids they require for producing antibodies, enzymes, or hormones, and for growth or repair. Amino acids surplus to the body’s requirements are broken down by the liver; the nitrogenous part is excreted in urine after conversion to urea, with the remaining part used to provide energy or stored as fat. Fatty acids and glycerol are absorbed into the lacteals and are transported via the lymphatic system to the bloodstream, arriving this way at the liver. Fats serve several functions in the human body: cells use circulating fatty acids and glycerol for energy and secretion synthesis; energy and heat is provided by fatty acids and glycerol in the liver; and fat is stored in the form of triglycerides (Grant and Waugh, 2014). [^]

One gram of fat provides the body with 9 calories of energy, whereas one gram of both carbohydrate and protein supplies 4 calories (Youdim, 2017). The average adult male requires approximately 2,500 calories per day from a balanced diet to maintain a healthy weight, while the average adult female needs about 2,000 daily calories. There are several factors that affect calorie requirements: age; lifestyle; size; some hormones; some medications; and feeling unwell. In order to maintain a healthy weight, the energy absorbed from food must be balanced with energy expenditure (NHS Choices, 2016b). The NHS Eatwell Plate stipulates that a healthy balanced diet consists of the following food groups: fruit and vegetables (33%); bread, rice, potatoes, pasta, and other starchy foods (33%); milk and other dairy products (15%); meat, fish, eggs, beans, and other non-dairy sources of protein (12%); and foods and drinks high in fat or sugar (7%) (New Health Guide, 2017). [^]

The following table summarizes the conclusions drawn from this essay:

(Simplified table based on information discussed above)
– provide 4 calories/gram
– starchy foods should constitute 33% of the diet
– should constitute 33% of the diet – provide 4 calories/gram
– non-dairy sources of protein should constitute 12% of the diet
– provide 9 calories/gram
– foods and drinks high in fat or sugar should constitute 7% of the diet
– fat-soluble vitamins: A, D, E, K
– water-soluble vitamins: B1, B2, B3, B6, B12, folic acid, pantothenic acid, biotin, C
Macro minerals: calcium, magnesium, phosphorous, sodium, potassium, chloride, sulphur
trace minerals: iron, manganese, copper, iodine, zinc, cobalt, fluoride, selenium
– most vital nutrient
– optimal intake = 35mls/ kg bodyweight
– milk and other dairy products should constitute 15% of the diet
UK adult male requires approximately 2,500 calories/day
UK adult female requires approximately 2,000 calories/day
– begin digestive life as polysaccharides (long chains of monosaccharides)
– chemical digestion begun by salivary amylase
– begin digestive life as non-starch polysaccharides (indigestible chains of monosaccharides) – begin digestive life as long chains of amino acids – begin digestive life as triglycerides –Mechanical digestion started by chewing in the mouth–
soluble fibre dissolves in the water of the digestive system – chemical digestion begun by pepsin in the stomach –Mechanical digestion continued with churning in the stomach–
– chemical digestion continues with pancreatic amylase in the small intestine insoluble fibre does not dissolve in the water of the digestive system – chemical digestion continues with pancreatic protease in the small intestine – chemical digestion begun by pancreatic lipase and bile in the small intestine –Mechanical digestion continued with peristalsis in the small intestine–
monosaccharides pass into the blood capillaries in the villi, and are transported by the portal circulation to the liver amino acids pass into the blood capillaries in the villi and are transported in the portal circulation to the liver and to all the cells and tissues of the body – fatty acids and glycerol are absorbed into the lacteals and are transported via the lymphatic system to the bloodstream, arriving this way at the liver –Nutrient absorption–
– used for the metabolic activity of the liver itself; used for blood glucose maintenance; some is converted to glycogen, stored in the liver and skeletal muscles – can be broken down by glucagon to fuel muscular and liver activity when needed; and glucose that is surplus to the above requirements is converted to fat and stored in the fat depots – passes through the GI tract, softening stools, moderating blood glucose levels, and lowering blood cholesterol levels – body cells choose the particular amino acids they require for producing antibodies, enzymes, or hormones, or for growth and repair; surplus amino acids are broken down by the liver; the nitrogenous part is excreted in urine after conversion to urea, with the remaining part used to provide energy or stored as fat – cells use circulating fatty acids and glycerol for energy and secretion synthesis; energy and heat is provided by fatty acids and glycerol in the liver; and fat is stored in the form of triglycerides –Nutrient functions–


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Forty-year-old father of three wonderful children [William, Seth, and Alyssa]. Works as an Assistant Technical Officer in the Sterile Services Department of Treliske Hospital, Cornwall. Enjoys jogging, web design, learning programming languages, and supporting Arsenal FC. Obtained a BA degree in English from the University of Bolton in 2008, and has continued to gain qualifications in a diverse range of subjects thereafter.

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