Exam 4 Review:  Chapter 24:  Nutrients and Vitamins

non-essential amino acids - Those building blocks, i.e., monomers (amino acids), of proteins, which may be synthesized within the body and, therefore, are not mandatory within the diet; they include: alanine, arginine, aspartate, glutamate, glycine, proline, and serine.  [Note:  As cysteine and tyrosine require the presence of methionine and phenylalanine respectively, they may be considered as non-essential amino acids only if their precursors are available within the diet.]

minerals - Various inorganic substances, usually individual elements in the form of ions, which constitute about 4% of the total body weight; they are concentrated most densely in the skeleton; they can be divided into two categories: (1) those present in relatively large amounts, e.g., sodium, potassium, calcium, magnesium, chlorine, phosphorous, and sulphur, and (2) trace elements - those present at a concentration of less than 50mg/kg, e.g., selenium; some are also considered "essential," i.e., those essential to growth or health, e.g. sodium and selenium; they may interact with each other on a variety of levels: (1) as co-factors in enzymes, they modulate the substrate/product relationship of biochemical pathways, (2) as ions across membranes, they contribute to: (a) the state of electrical excitability, e.g., the action potential, (b) the distribution of fluid volumes between fluid compartments, (c) competitive interactions, e.g., of zinc and copper for intestinal absorption, and (d) as common targets for regulatory compounds, e.g., vitamin D's action on calcium, phosphate and magnesium.

vitamins - Any of various fat-soluble or water-soluble organic substances essential in minute amounts for normal growth and activity of the body and obtained naturally from plant and animal foods; most vitamins, or their immediate metabolic derivatives, serve as co-enzymes or co-factors in a variety of enzyme catalyzed metabolic reactions; since they are recycled many times in these reactions before becoming useless, only small quantities are required by the body to replace those lost in each period of time.

oil soluble vitamins (A, D, E, K) - See the specific definitions below.   aka - fat-soluble vitamins.

water soluble vitamins (B complex, C) - See the specific definitions below.

vitamin A - One of the four oil-soluble [aliphatic alcohol] vitamins, occurring in nature in two forms: retinol and dehydroretinol; it is found in fish liver oils, liver, butter, egg yolk, cheese and most yellow and orange fruits and vegetables, in most of which it exists in its precursor form, carotene; in the developing world, most vitamin A comes from vegetable sources whereas in the developed world, animal sources are the main dietary source; Vitamin A is formed from carotene (and other provitamins) in the small intestine; bile salts and fat are required for its absorption; after absorption, vitamin A is transported in chylomicrons to the liver, where it is either stored as an ester, or re-exported to critical tissue sites in combination with retinol binding protein; vitamin A is necessary for the normal growth of epithelial tissue, aids in the synthesis of polysaccharides, is essential for formation of the visual pigment, rhodopsin, and it may have a role in protection against degenerative and neoplastic tissue changes.

B complex vitamins - A range of water-soluble vitamins with little structural similarity; they are grouped together because (1) they all act as coenzymes which participate in various metabolic reactions, and (2) they all tend to assimilate together in similar foods, e.g., milk and cereals; the group includes: vitamin B1 - thiamine, vitamin B2 - riboflavin, vitamin B6 - pyridoxine, vitamin B12 - cyanocobalamin, niacin, pantothenic acid, folate, and biotin.

vitamin C - A small, water soluble organic compound, ascorbic acid, which promotes many metabolic reactions, particularly the laying down of collagen during the formation of connective tissue; it does this by facilitating the activity of enzymes which hydroxylate lysine, proline and procollagen; hence, it promotes wound healing; it also has vital roles in maintaining folate as tetrahydrofolate and in the absorption of iron; sources of vitamin C include citrus fruits, tomatoes, green vegetables, kidney, liver and milk; cooking of these source reduces the vitamin C content by virtue of oxidation and absorption into the cooking water; absorption takes place within the ileum by secondary active transport along with sodium ions; vitamin C deficiency produces classical scurvy, a condition characterized by general weakness, anemia, gum disease (gingivitis), and skin hemorrhages, scurvy is now most frequently seen in older, malnourished adults and is very rare in developed countries; Vitamin C status is determined from an assay on white blood cells.  aka - ascorbic acid.  See examples of lesions of scurvy below:

 

Above, destruction to gingivae = gums (1-3) and small bleeding lesions and corkscrew hair on skin (4).	Below, larger skin hemorrhages and skin lesions on the legs of individuals with scurvy.

vitamin D - One of the four oil-soluble vitamins; a collective term for a range of oil- or fat-soluble compounds (e.g., vitamin D2 = ergocalciferol, pro-vitamin D3 = 7-dehydrocholesterol, vitamin D3 = cholecalciferol, 25-hydroxycholecalciferol = 25-HCC = calcidiol, 1,25-dihydroxycholecalciferol = 1,25-DHCC = calcitriol) with similar activity on calcium and phosphate physiology:  (1) the enhancement of gastrointestinal absorption and (2) an increase in mineral deposition in bone, in nature, cholecalciferol is supplied principally through the action of ultraviolet light on skin; there are very few natural foods which contain substantial amounts of vitamin D in either the D2 or D3 form - they include fish-liver oils, egg yolk and fortified milk, margarines and breakfast cereals; ergosterol within plant matter can be converted to vitamin D; the consequences of vitamin D deficiency are (1) lowered blood levels of calcium and phosphate producing a secondary hyperparathyroidism in response, and (2) a further reduction in phosphate levels and increased plasma levels of bone-derived alkaline phosphatase; clinically, these produce poor bone mineralisation - rickets in children, osteomalacia in adults; depending on the cause, there may be a rapid response to administration of small doses of calciferol.  See examples of rickets below.

 

Rickets

vitamin E - One of the four oil-soluble vitamins, an oil- or fat-soluble dietary vitamin necessary for normal reproduction, muscular development, resistance of erythrocytes to hemolysis, and various other biochemical functions in humans and animals; also, it can act as an antioxidant; a collective term for a range of eight naturally occurring oil- or fat-soluble compounds (the tocopherols and tocotrienols, of which, d-alpha-tocopherol is the most widely available and the most biologically active); sources of vitamin E include fresh nuts, wheat germ seed oils, and green leafy vegetables;  animal products are generally poor sources; absorption depends on the digestion and absorption of fat; free tocopherols passively enter the lymphatic circulation, whereas metabolites and a small amount of vitamin E enter the portal circulation; when circulating in the blood, it is principally bound to low-density and high-density lipoprotein (LDL and HDL); it is stored in the liver, adipose tissue, muscle, pituitary gland, testes and adrenals.

vitamin K - One of the four oil-soluble vitamins; a collective term for a range of oil- or fat-soluble organic compounds: (1) vitamin K1 = the phylloquinones, present in green vegetables and fruit, and (2) vitamin K2 = the menaquinones, synthesized by bacteria within the gut; vitamin K plays an essential role in blood clotting by acting as a cofactor for the post-translational carboxylation of glutamate residues in clotting factors II (prothrombin), VII, IX and X by the liver; bile salts and fat are required for its absorption, and triglyceride-rich lipoproteins (VLDLs) seem to be the major carriers of vitamin K in the plasma; it is stored in the liver and spleen; dietary sources include leafy vegetables such as spinach, cabbage and cauliflower, certain legumes, and some vegetable oils such as rapeseed and soyabean oils.

Describe:

3.  the relationship between deamination of amino acids and the formation of urea in the liver. 

The liver has the major responsibility for maintaining the appropriate levels of the approximately twenty amino acids required by the body for protein synthesis and other functions.  When a particular amino acid is present in the liver in excess of body needs, it may be transformed into some other needed amino acid by the process of transamination or it may be catabolized for energy by the process of deamination.  It is necessary to remove all the -NH2 amino groups from the amino acid so that the remaining carbon "skeletons" can by transferred to one or another of the central carbohydrate catabolic pathways, glycolysis or the Kreb's Cycle = TCA Cycle = Citric Acid Cycle.  The amino groups cannot be further broken down for any useful chemical energy, and if left as is, would spontaneously combine with water to for a dangerous strong base, ammonium hydroxide NH4OH.  To prevent that outcome, enzymes in the liver immediately combine any amino groups generated from protein or nucleic acid catabolism or any ammonium ions absorbed from the large intestine which were by-products of normal flora bacterial metabolism with carbon dioxide molecules to produce a safe, highly water-soluble non-toxic nitrogenous waste compound, urea, which can be transported safely in the blood plasma to the kidneys for excretion.  This reaction is 2CO2 + NH3 Û urea.

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Here are some reasonable cautions about vitamin and mineral supplementation:

Vitamin A: Excess amounts accumulate and can be toxic. Too much A can blur vision, cause headaches and vomiting, and also lead to liver, bone and central nervous system problems, among others. Beta Carotene: The body converts this into vitamin A. One study found that high levels of beta carotene in the blood were linked to three times the risk of aggressive prostate cancer. Vitamin C: There's no conclusive evidence that it prevents colds, heart disease, cataracts or cancer. Vitamin E: Large doses can thin the blood and may increase the risk of hemorrhagic stroke in those with uncontrolled blood pressure. Has not been proven to protect the heart or prevent cancer. Selenium: Most Americans get enough of this trace mineral in their diet. One study suggests that adding more via a pill may increase the risk of developing type 2 diabetes. Folic Acid: It's a must during pregnancy to help prevent birth defects, but recent studies show no real effect for the rest of us against heart disease, cancer or depression. The connection between folate and reduced risk of Alzheimer's is not yet conclusive either. Niacin: This B vitamin can be used to treat high cholesterol, but only under a doctor's supervision due to the risk of potentially serious side effects, including liver damage. Lycopene: Two studies, one by the FDA, recently concluded that consuming lycopene as a supplement or in rich food sources, such as tomatoes, does not offer strong cancer-fighting protection, as was previously promoted. Iron: Only women who are pregnant or have heavy periods, as well as people with diagnosed deficiency disorders such as anemia, need extra amounts of this mineral. Iron supplements can interact with meds, other dietary supplements and food, and can worsen conditions like ulcers. Zinc: High doses can interfere with how the body metabolizes copper and iron, may weaken the immune system and may also reduce levels of HDL (good) cholesterol. Studies are mixed about its effect on the common cold. Zinc supplements can also interact with certain drugs, including some antibiotics, blood pressure medications and NSAIDs.

For the majority of individuals, a balanced diet provides most necessary vitamins and minerals and one ordinary multi-vitamin/mineral per day is the only supplementation that is required for those who are concerned that their diet is not "balanced."