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Vitamins
• Groups of organic compounds that are required in the diet in small amounts (µg or mg / day) for the
maintenance of normal health and metabolic integrity.
• Thus they are differentiated from the essential minerals and trace elements (inorganic) and from essential
amino acids and fatty acids, which are required in larger amounts
• Shown to be a dietary essential & its elimination from the diet must result in a deficiency disease and
restoration must cure or prevent that deficiency disease because they cannot be synthesized by the body.
• Compound that has pharmacological actions, and cures a disease, does not classify as a vitamins, even if it is a
naturally occurring compound (found in foods).
According to their solubility:
:
Classification
1) lipid-soluble vitamins: some kinds can be synthesized inside the body
• Non polar (hydrophobic) compounds that can be absorbed efficiently when there is normal fat
absorption.
• They are transported in the blood, like any other non-polar lipid, in lipoproteins or attached to specific
binding proteins.
2) 2- Water-soluble vitamins: can be synthesized inside the body
• Comprise the B complex and vitamin C and function as enzyme cofactors.
•
Estimated Average Requirement (EAR):
The average daily intake estimated to meet the requirement of half of the healthy individuals in a particular
life stage and gender group.
•
Recommended Daily Allowance (RDA):
The average daily intake sufficient to meet the nutrient requirement of nearly all 98% healthy individuals in a
particular life stage and gender group.
•
Lower Reference Nutrient Intake (LNRI)
The lower end of daily intake, below this intake a deficiency may occur.
•
Adequate Intake (AI):
Recommended daily intake based on experimentally determined intake by a group (or groups) of healthy
people that are assumed to be adequate [used when an RDA cannot be determined].
•
Tolerable Upper Intake Level (UL):
The highest average daily intake that doesn't cause risk of adverse effects to all individuals in a particular life
stage and gender group, as intake increases above the UL the potential risk of adverse effects may occur.
General aspects of vitamins nutrition
1) Individual response to nutrients varies:
This variation is a result of the genetic variation, because genes influence structures and functions associated
with the digestion and absorption of nutrients, metabolism or excretion, so that the optimal nutritional
guidelines should be population-specific, rather than general.
2) Many vitamins are absorbed by active transport; this is a saturable process and, therefore, the percentage that
is absorbed will decrease as the intake increases.
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3) Fat-soluble vitamins (A, D, E, and K)
absorbed, dissolved in lipid micelles, so that, absorption will be impaired when the meal is low in fat, also
gastrointestinal diseases that results in impaired fat absorption and steattorhea will impair the absorption
of fat-soluble vit., because they remain dissolved in the unabsorbed lipid in the intestinal lumen
Lipase inhibitors [Xenical] used for the treatment of obesity will impair the absorption of fat-soluble
vitamins.
4) Many of the water-soluble vitamins are present in foods bound to proteins, and their release may require either
the action of :
gastric acid (as for vitamin B12)
specific enzymatic hydrolysis ex:
• Conjugase to hydrolyze folate conjugates.
• The hydrolysis of biocytin to release biotin.
5) The state of body reserves of the vitamins may affect :
A. The extent to which it's absorbed by affecting the synthesis of binding and transport proteins. Absorption
[HOW]
B. The extent of metabolism to which it's metabolized: ex after uptake into the intestinal mucosa the
oxidative cleavage of carotene to retinal is regulated by vitamin A status.
6) Compounds naturally present in foods may have antivitamin activity: ex : many foods contain thiaminase that
catalyze enzymatic cleavage of thiamin to biologically inactive products.
7) Drugs and naturally compounds present in foods compete with vitamins for absorption ;
Drugs :
Ex: Chlorpromazine, tricyclic antidepressants, and some antimalarial drugs inhibit the intestinal
riboflavin;
transport and metabolism of
natural compounds ex :
• Carotenoids lacking vitamin A activity compete with β-carotene for intestinal absorption and
metabolism.
• Alcohol inhibits the active transport of thiamin across the intestinal mucosa.
8) Some vitamins are present in foods in chemical forms that are not susceptible to enzymatic hydrolysis during
digestion, although they are released during the preparation of foods for analysis.
9) Protein binding of a vitamin on foods increases its absorption and hence it's biological availability, e.g. folate
from milk is better absorbed than that from either mixed food folates or free folic acid, because folate bound
to a specific binding protein in milk is absorbed in the ileum, whereas free folate is absorbed in the (smaller)
jejunum.
Vitamin Requirements
• Requirement: The lowest intake that will maintain a defined level in an individual.
• Basal requirement: The level of intake required to prevent pathological and clinical signs of deficiency.
• Normative requirement: The level of intake to maintain a desirable body reserve of the nutrient.
For any nutrient, there is a range of intakes between that inadequate, leading to clinical deficiency
disease, and that so much in excess of the body’s metabolic capacity may lead to signs of toxicity. Between
these two levels are a level of intake that is adequate for normal health and the maintenance of metabolic
integrity,
Notes:
Supplement dose: when vitamin body reserves are less than normal but deficiency doesn’t occur.
Treatment dose: when vitamin body reserves are very little below & deficiency occur.