MINERALS
MINERALS
Outcomes:
At the end of this unit, the learner should be able to:
·
List
the major minerals and describe their physiologic functions in the body.
·
List
the dietary sources of each of the minerals.
·
Outline
the factors affecting the absorption and utilization of the minerals.
·
Identify
the factors that influence the bioavailability of the minerals from food.
·
Describe
the relationship between minerals and other nutrients in the body.
·
Describe
the health problems caused by mineral deficiencies, their signs and symptoms
and there prevention.
In the previous topic we discussed one of the major
micronutrients, vitamins. In this topic we will review the remaining
micronutrient, minerals. Like vitamins, they perform important functions in the
body. They account for 4% of body weight. They are found in all body fluids and
tissues. Unlike the energy nutrients and vitamins, minerals are inorganic
elements that originate from the earth’s crust, not from plants and animals.
Minerals do not undergo digestion nor are they broken down or rearranged during
metabolism. Although they combine with other elements to form salts (e.g.
sodium chloride) or with organic compounds (e.g. iron in haemoglobin) they
always retain their chemical identities. Unlike vitamins, minerals are not
destroyed by light, air, heat or acid during food preparation. In fact when
food is completely burned, minerals are the ash that remains. Minerals are lost
only when foods are lost in water.
Definition of
terms
· Major
minerals:
These are essential mineral nutrients
required in the adult diet in amounts greater than 100 milligrams per day.
· Trace
minerals:
Essential mineral nutrients required in
the adult diet in amounts less than 100 milligrams per day.
· Fluid
and electrolyte balance:
Maintenance of the necessary amounts
and types of fluid and minerals in each compartment of the body fluids.
· Acids: compounds that release hydrogen ions
in solution.
· Bases: compounds that accept hydrogen ions in
solution.
· Acid-
base balance:
The balance maintained between acid and bases concentrations in the blood and
body fluids.
· pH: The concentration of hydrogen ions.
The lower the pH, the stronger the acid. Thus pH 2 is a strong acid while pH 6
is a weak acid; Ph 7 is neutral; and a pH above 7 is alkaline.
· Buffers: Compounds that can reversibly combine
with hydrogen ions to help keep solutions acidity constant.
· Ossification: The process of bone formation.
· Osteomalacia:
A weakening of the bones due to a calcium, phosphorus, and/or Vitamin D
deficiency.
· Osteoporosis:
An abnormal porousness of bone as the result of a calcium, phosphorus, and/or
vitamin D deficiency.
Classification
Minerals are classified into two groups based in the amount
found in the body. Some occur in large quantities and are required in
quantities above 100mgs per day. These are referred to as macro-minerals and
they include calcium, phosphorus, potassium, sodium, chloride, magnesium and
sulfur.
Others occur in small amounts and are required in only a few
milligrams (15mgs/day or less). They are therefore considered as trace elements
or micro-minerals and they include iron, copper, cobalt, zinc, manganese,
iodine, molybdenum, fluorine, selenium and chromium.
6.5General Functions of Minerals
include:
Minerals perform several general functions because of their
interrelationship. They provide structure to the body tissues and regulate body
processes such as fluid balance, acid-balance, nerve transmission, muscle
contraction, vitamin, enzyme and hormonal activities.
a. Structure
Calcium, phosphorus, magnesium and fluoride provide
structure to bones and teeth. Soft tissues gain structural support from
phosphorus, potassium, iron and sulphur. Sulphur is also a fundamental constituent
of skin, hair and nails.
b.
Maintenance
of acid-based balance
This
term refers to the maintenance of the body’s concentration of hydrogen ions
(pH). Biological reactions in the cell occur when the pH is right. Anything
that changes the pH of the cell environment may change the way cellular enzymes
act. Inactivation of enzymes results in
starvation and death of cells. Acid forming minerals such as, chlorine, sulphur
and phosphorus are found in the extra cellular fluid. They predominate in foods rich in protein
such as meat, fish, poultry, eggs and cereal products (acid forming foods).
Basic or alkaline minerals such as calcium, sodium, potassium and magnesium
tend to predominate in fruits, vegetables and nuts. Milk contains internal balance of base
forming calcium and acid forming phosphorus, and does not influence acid-base
balance
c. Catalysts
for biological reactions
Minerals serve as co-factors for many biological reactions.
They are not part of initial compounds or end products but they serve as vital
links between enzymes and the substance which they act on e.g. zinc in known to
catalyze about 100 different reactions. They catalyse many of the separate
steps involved in the metabolism of carbohydrates, fats and proteins. For
example, the synthesis of haemoglobin depends on several mineral elements that
are not part of haemoglobin such as iron. Similarly, although calcium is a
catalyst for blood clotting, it is not part of the clot. The absorption of
nutrients from the gastrointestinal tract and the uptake of nutrients by the
cells often depend on minerals (e.g. calcium facilitates the absorption of B12,
sodium and magnesium facilitates absorption of carbohydrates).
d. Components
of essential body compounds
Many hormones, enzymes and other vital compounds synthesized
in the body contain minerals as essential parts of their structure. As such, if
the required mineral is absent, the body will be unable to produce adequate
amounts of these substances. For example, thyroxin, a hormone which controls
energy metabolism requires iodine. Haemoglobin
which transports oxygen to the cells and carbon dioxide from them contains
iron; chlorine is essential for hydrochloric acid production in the
stomach. Mineral containing enzymes are
sometimes called metalloenzymes. If the mineral is unavailable, the enzymes
will not be synthesized or will be ineffective.
Molybdenum is part of an enzyme needed to release liver stores of iron.
Zinc is part of a protein splitting enzyme carboxypeptidases secreted into the
pancreatic juice.
e. Maintenance
of fluid balance
Water comprise of 60% total body weight or 70% of fat free
weight. It is present in 3 components, separated by semi-permeable membrane
that allows free exchange of fluids and electrolytes (ions). Intravascular (in
blood) compartment includes fluid in the arteries, veins and capillaries. Intercellular
(between cells) bathes individual cells and provides nourishment, this act as a
buffer area. Its volume will adjust to prevent changes in the volume of the
intravascular or extra cellular chambers. Intracellular fluid is found within
the cells. Transcellular compartment includes the fluid which lubricates the
joints and fluid which in the eyeball. This represents a very small portion and
it is not involved in fluid shifts.
The movement of fluid from one compartment to another is
controlled by the concentration of minerals on either side of the membranes.
Mineral elements in body fluids occur as salts that separate into component
ions with positive and negative charges. Charged ions are known as electrolytes
and maintain osmotic pressure of body fluids. As the concentration of
electrolytes increase, osmotic pressure increases, this draws fluid from the
compartment with low electrolyte concentration until the concentration (or
pressure) is equalized. Under normal circumstances, the body is able to prevent
a shift in electrolyte concentration between compartments. Shifts however do
occur in abnormal situations.
f. Transmission
of nerve impulses
Minerals play a vital role in conducting nerve impulses
along a nerve fibre. The exchange of sodium and potassium across nerve causes
the transmission of nerve impulses. The permeability of membrane changes
allowing potassium to leave the cell. At the same time sodium enters each cell,
but is quickly pumped out, this creates a temporary change in the electrical charge.
In this way the message is passed along the fibre. The transmission of a nerve
impulse from one nerve cell to another is also dependent on neurotransmitter
acetylcholine (ACH) at the junction of the two fibres. The release of ACH is
regulated by calcium.
g.
Regulation of muscle contraction:
The muscles of the body are bathed in the intercellular
fluid. For muscles to function normally, the composition of this fluid should
have a balance between mineral elements such as calcium which stimulates
muscular contraction and sodium, potassium, and magnesium which exerts relaxing
effect. This balance does not depend on dietary sources and can only be
disturbed by hormonal changes.
h. Growth
of body tissue
Some minerals such as calcium and phosphorus occur in large concentration
in bones and teeth and are the building material of body tissue. Absence of
these will lead to stunted growth or poorly developed bones and teeth. Other
minerals serve as catalysts to reactions which lead to synthesis of body
compounds or to release energy needed for growth.
Macro
Minerals
Most abundant mineral present in the body, amounting to
almost 40% of the total mineral mass.
Majority is present in bone, where together with
phosphorous, it plays an essential part in hardening the skeleton and teeth.
In addition, calcium is a reserve of the mineral for its
role in body fluids as ionic calcium, which is essential for nerve impulse
transmission, muscle contraction and blood clotting.
a. Absorption
of Calcium
Calcium salts are generally not highly soluble, which makes
their absorption from the diet problematic.
Several factors can enhance or inhibit absorption of Calcium.
Factors enhancing calcium absorption
1.
Vitamin
D, which causes the synthesis of a calcium-binding protein in the intestinal
cells that, transports calcium into the plasma.
2.
Lactose
(present in milk) also enhances calcium absorption by keeping it in a soluble
form. The presence of lactose and the
large amounts of calcium found in milk are make this an excellent source of the
mineral.
3.
Other
sugars and protein also enhance calcium absorption.
4.
The
acidic environment of the upper digestive tract also facilitates the solubility
of calcium.
5.
Body
Need - This is the major factor governing the amount of calcium that will be
absorbed during periods of growth, low calcium levels in the body fluids, and
when body stores are depleted, more calcium is absorbed.
6.
Other
factors that enhance absorption are;
-
The
acidic environment in the upper gastro intestinal tract
-
The
normal gastro intestinal motility
Factors inhibiting calcium absorption
· Calcium absorption is reduced by phytic
acid present in whole cereals, owing to the formation of insoluble calcium
phytate.
· Oxalates (present in spinach, rhubarb,
beetroot, chocolate, tea infusions, wheat bran, peanuts and strawberries) may
also inhibit calcium absorption owing to the insoluble nature of the calcium
oxalate salt
· Non-starch polysaccharides may trap
some calcium making it unabsorbable in the small intestine. However fermentation of the soluble NSP in
the large intestine may release the calcium for absorption here.
· Unabsorbed fats will combine with
calcium to form soaps, removing calcium from the body. This is a particular problem in steatorrhea,
in which loss of vitamin D, a fat-soluble vitamin, may aggravate the problem of
calcium absorption.
The skeleton is an active reservoir of calcium.
Role of calcium in the body
In bones – Calcium is principally located in the
bones. The mineral is continually laid
down by osteoblasts and resorbed by osteoclasts. Bone growth happens in childhood and
adolescence and maintained in adults.
Changes in levels of some hormones such as cortisol or oestrogens will
cause bone loss. The amount of bone
declines with age. It happens earlier
and rapidly in women at the time of menopause. When bone breakdown exceeds
repair the bones become fragile and can easily break. This condition is known as osteoporosis.
In blood and body fluids – Necessary for homeostasis where levels
are maintained at 2.2 – 2.6 mmol/L. When
plasma calcium is low (or phosphate levels are high) parathyroid hormone is
secreted. This increases calcium level
by promoting synthesis of active vitamin D and thus, increasing calcium
absorption from the gut, reducing calcium excretion at the kidney and
stimulating calcium release from the bone.
Conversely, high calcium levels cause the release of calcitonin from the
thyroid gland. This inhibits bone
mobilization and promotes calcium uptake into bone.
Calcium is essential for blood clotting where insoluble prothrombin is
converted to thrombin by the action of fibrin and other clotting factors. If calcium levels are insufficient, blood
will not clot.
Muscle contraction and nerve impulse
contraction
at nerve endings both involve the movement of calcium across the cell membrane,
increasing intracellular levels and triggering contraction or depolarization.
Calcium is required for the normal
transmission of nerve impulses. The calcium ions operate at the tips of nerve branches and
help to excite the muscle fibre.
Calcium activates a number of enzymes, including pancreatic lipase (digest
fat) and adenosinetriphosphate (involved in the release of energy for muscular
activity). Calcium also aids in the absorption and utilization of vitamin B12.
Permeability of the cell membrane - Calcium regulates the passage of
substances across cell membranes
Calcium excretion
Calcium is lost from the body via urine and faeces, with
very small amounts through sweat.
Urinary calcium represents the final adjustment of calcium
levels, with the majority (up to 97%) of the calcium filtered being reabsorbed
by the renal tubules.
Levels of urinary calcium are increased:
· On a high calcium intake
· By a high protein diet
· By high sodium intake
· In women at the menopause
· In old age
· By a high potassium intake
· By a high magnesium intake
· By a high phosphorous intake
Factors that help to develop peak bone mass
· Body weight – Excessively thin females
with BMI below 20 may be amenorrhoeic.
The absence of normal menstrual cycles and lack of oestrogen will
prevent normal bone accretion. This may
be a problem in girls with eating disorders.
Body size is a good indicator of bone mass. Muscular development increases bone
development to support movement.
· Alcohol and smoking. These both reduce bone accretion and are
therefore detrimental to bone health
· Vitamin D. Adequate exposure to sunlight is important
for vitamin D synthesis which is necessary for calcium absorption.
· Vitamin K is important to facilitate
bone development (Osteocalcin and matrix glaprotein are dependent on Vitamin
K).
· Vitamin C. This is an essential factor in the synthesis
of collagen that forms part of the structural framework of bones.
Maintenance of Serum Calcium
Levels
Serum calcium levels and the ratio of serum-calcium to
phosphorous are held relatively constant at the expense of bones if dietary
intake of calcium is not adequate. To maintain blood calcium concentration
within normal limits, several hormones play an interactive role, whereby they
control the amount of calcium absorbed and excreted, and also bone metabolism.
The hormones include active vitamin D, PTH, calcitonin, estrogen, testosterone and
other. They respond to changes in the serum calcium levels in three ways.
Active Vitamin D increases calcium absorption, mobilizes
calcium from the bones and increased renal absorption to some extent
PTH also increases calcium absorption, mobilizes calcium
from the bones, and increases renal re-absorption.
Calcification helps to correct high levels by preventing
further bone re-absorption.
Note:
Altered serum calcium levels are usually due to hormonal
abnormalities or may occur secondary to other clinical conditions such as
inadequate secretion of PTH due to hypoparathyrodism which may be surgically
induced or otherwise (e.g. in thyrodectomy, or neck dissection)
Hypercalcemia can cause nausea, vomiting anorexia, abdominal
pain, constipation, polydipsia, mental changes, calcium renal stones and
excessive calcification of bones and soft tissues. Coma and death may result if
not treated.
Hypercalcemia may be cause by cancer excessive combined
intakes of calcium and Vitamin D, hyperparathyroidism (increase secretion of
parathyroid hormone) thyroid disorders and hypophosphatemia.
Re-absorptive Hypercalciuria is a condition that occurs
during periods of prolonged immobility, when an excessive amount of calcium is
withdrawn from the bones and excreted in urine, thus increasing the risk of
calcium renal stone formation.
Deficiency
Calcium deficiency may be related to an inadequate intake or
may occur secondary to mal-absorption syndromes, vitamin D deficiency or
endocrine disorders. A long standing deficiency will lead to:
· Rickets and badly formed teething
children
· Hypertension
· Osteoporosis is adults due to
withdrawal of calcium from the bones to raise the serum calcium levels, leading
(bone anthrop) and resulting in shortening of stature. The bones are also
porous and brittle and fracture very easily.
Note: Exercises appear to reduce loss of calcium from bones; also
there is clear evidence that providing the female hormone estrogen to women
after menopause reduces bone loss and osteoporosis.
· Stunted growth in children
· Tetany – a condition characterized by
uncontrolled muscular contractions (spasms) and failure to relax thus causing
the body to stiffen with pain, due to low serum calcium levels or too much
phosphorous, which causes a decrease in calcium absorption.
· Failure of blood clotting.
Food sources
All the calcium in the human body except that inherited from
the mother, comes from food and water consumed. It is necessary to have
adequate quantities of calcium she draws extra supplies from her bones. An
entirely breastfed infant will obtain adequate calcium from breast milk as long
as the volume of milk is sufficient.
A lactating mother secreting 1 litre will thus lose 300mg of
calcium per day.
Cow’s milk is a very rich source of calcium (richer than
human milk). A litre of human milk contains 300mg of calcium than a child needs
hence it is meant for the calf.
Other rich sources
Excellent Sources: milk, hard cheese, yoghurt, dark green
leafy vegetables and sardines.
Good Sources: Softer cheese, ice cream and dried legumes
Fair Sources: oranges, nuts and eggs
Poor sources: most of other fruits, vegetables and grains
and meats.
Calcium Content of Various Milks
Sources of Milk Calcium Content (mg/10ml)
Human 32
Cow 119
Camel 120
Goat 134
Water Buffalo 169
Sheep 193
The calcium content of drinking water varies from place to
place. Hard water usually contains high levels of calcium.
Recommended dietary requirements
It is not easy to state categorically the human requirements
for calcium, because there are several factors influencing absorption and
considerable variations in calcium losses among individuals. Need for calcium
is increase during pregnancy and children require more calcium because of
growth. Those on high protein diets require more calcium in the diet. The
following are recommended levels of daily calcium intake:
Children ages 1 to 10 years 800mgs/day
For men and women aged 18 to 24 years 1200mgs/day
Adults aged 25 years and above 800mgs/day
Pregnant and lactating mothers’ 1200mgs/day
After calcium, the most abundant mineral in the body is
phosphorus. It constitutes 22% of the minerals in the adult human body or 1% of
the body weight. It is a major constituent of bones and teeth or 85-90% of
total body’s phosphorus. The remainder is distributed in body tissues with half
in muscles. It is part of the nucleic acids; DNA and RNA which are found in the
nucleus of every living cell. As a phospholipid, it is a structural component
of the cell wall and participates in all biological reactions.
Functions of phosphorus
1.
Regulation
of energy release from oxidation of carbohydrates, fats and proteins: Energy is
stored as ATP (adenosine triphosphate) and when energy in needed, ATP is
changed to ADP (adenosine diphosphate).
2.
Absorption
and transport of nutrients: Phosphorus in the form of phosphate makes it
possible for nutrients to cross cell membranes or be carried in the blood. This
process is known as phosphorylation and occurs in several areas: absorption in
the intestines, release of nutrients from the blood stream to the intercellular
fluids, uptake into the cells, and uptake by organelles of the cell. When
glycogen is released from the liver or muscles for energy, it appears in the
blood combined with phosphorus.
3.
Part
of essential body compounds: Some vitamins and enzymes can function only when
they are phosphorilated e.g. thiamin pyrophosphate.
4.
Essential
part of DNA and RNA which are essential for cell reproduction.
5.
Calcification
of bones and teeth.
6.
Regulation
of acids-base balance: Serves as buffers since they can take additional
hydrogen ions.
Absorption and metabolism of
phosphorus
It is released from food by enzymes known as phosphatases.
It is absorbed with the help of vitamin D. The level in blood is regulated by
the parathyroid gland.
Dietary sources of phosphorus
Phosphorus is present in most foods especially those rich in
protein e.g. meat, poultry and fish, eggs dairy products and cereal products.
Deficiency of phosphorus
Phosphorus deficiency is unknown in humans but may affect
people who consume large amounts of antacids especially those with aluminium
hydroxide which interfere with phosphorus absorption. Premature infants may
need more phosphorus than what is provided by breast milk.
The concentration of magnesium in the body is roughly 30mg;
60% of which is found in the bone. About a third of this is closely bound to
phosphate and the remainder is on the surface of the bone from which it is
readily mobilized to maintain normal blood and tissue levels. The blood
contains about 1% of total body magnesium, a third of this is bound to the
protein albumin, a third is free and a third is part of other compounds. It is
absorbed in the small intestines with the help of a carrier. About half of the
dietary intake is absorbed. Absorption is reduced by presence of calcium and
alcohol phosphate, phytates and fats. Absorption is increased by dietary
vitamin D and lactose. Excretion is regulated by the kidneys and reabsorption
is facilitated.
Functions of magnesium
a.
Within
the cell, magnesium acts as a catalyst to many biological reactions especially
those taking place in the mitochondrion of the cell. Magnesium is involved in
cell reactions involving expenditure of energy from carbohydrate, fat and
protein; and nucleic acid metabolism in those reactions related to the
synthesis, degradation and stability of the genetic material including DNA.
b.
It
is necessary for conduction of nerve impulses and for normal muscular
contraction. In these roles, magnesium and calcium play antagonistic roles.
Calcium stimulates whereas magnesium relaxes. A very high intake of magnesium
could lead to coma and heart failure.
c.
Magnesium
is also involved in bone formation together with phosphorus and calcium.
Dietary sources of magnesium
Dark green vegetables are the best sources of magnesium
followed by legumes, sea food, nuts, cereals and dairy products. Water contains
good amounts especially soft water; sea water has very high levels.
Deficiency of magnesium
Magnesium deficiency is associated with starvation,
persistent vomiting and trauma of surgery or diarrhoea. A deficiency leads to weakness, confusion,
growth failure in children. Severe deficiency leads to tetany (uncontrolled
neuromuscular tremors which progress to convulsive seizures). Deficiency also
leads to vasodilation (increase in diameter of blood vessels) and skin changes.
Sulphur represents only 0.25% of total body weight. It is
concentrated in cytoplasm of the cell. High concentrations are found in the
hair, skin and nails, hence the smell of sulphur dioxide when they are burned.
The sulphur containing amino acids; cycteine and methionine are characteristic
of the protein found in the tissues. Sulphur gets to the body through these sulphur-containing
amino acids.
Functions of magnesium
1.
Sulphur
in combination with hydrogen plays an important role in metabolism.
2.
Involved
in formation of blood clots.
3.
Involved
in the transfer of energy.
4.
It
is part of at least 3 vitamins; thiamin, pantothenic acid and biotin. These are
part of coenzymes which activate several enzymes.
5.
Compounds
containing sulphur act as detoxifying substances by combining toxic substances
and converting them into harmless compounds that are excreted by the kidney.
6.
Sulphur
is also necessary for collagen synthesis and the formation of many
mucopolyssacharides.
Fluorine is found mainly in the teeth and skeleton. Traces
of fluorine in the teeth help to protect against decay. When consumed in early
adulthood, it becomes part of the enamel, making it more resistant to the
organic acids formed from foods that get stuck in between the teeth. Fluorine
is also known to harden and strengthen the bone especially later in life and
thus slow or inhibit the development of osteoporosis.
Mode of action
Fluorine forms crystals of flourapatite which replaces
crystals of hydroxypatite normally deposited during tooth formation.
Flourapatite in the tooth enamel is more resistant to cavity forming acids.
Absorption is known to occur in the mouth through the tooth surface. Intakes of
8ppm in adult life protects against osteoporosis (a condition where there is a
reduction of the bone density due to erosion of calcium).
Dietary sources of fluorine
· The main source is water. If it
contains 1ppm (part per million), this will be adequate.
· Other sources include small fish when
consumed whole.
· Tea has a high content.
· Foods grown in high fluoride water will
have more than fresh foods.
Toxicity
Too much fluorine intake causes browning of teeth enamel and
excessive amounts may cause dental flourosis in which the teeth become mottled.
Micro elements
They are required in small amount by the body compared to
the macro elements.
Iron is found in every living cell. The total body content
is about 5grams; varying slightly with age, sex, body size, nutritional status
and general health. It is the only nutrient for which adult women have greater
requirements than adult men.
Absorption and distribution of
iron
The body uses several mechanisms to regulate iron absorption
because the body cannot easily eliminate excess iron once it is absorbed.5-10%
of iron in food is absorbed in healthy people.10-20% of iron in food is
absorbed in by people with iron deficiency. The extent of iron absorption
depends on a variety of factors, the most important being the amount of current
iron stores in the body.Iron in food occurs in several forms which differ in
their absorption by the body. Iron that is part of haemoglobin and myoglobin
molecules in animal flesh is about 40% of total iron present called heme
iron and is readily absorbed more than twice as efficiently as simple
elemental iron known as non-heme iron. Non-heme iron is provided from
plant sources and elemental iron components of animal tissues. It is less efficiently absorbed than heme
iron. The difference in absorption of heme and non heme iron make animal flesh
a rich source of dietary iron, considering both its iron content and the
increased efficiency of absorption of the heme iron present.Consuming heme iron
and non heme iron together increases non-heme iron absorption e.g. eating meat
with vegetables and grain products.
Factors affecting iron absorption
These include the following;
a. Body
needs
A normal person with normal haemoglobin levels absorbs only
2-10% of dietary non-heme iron and 23% of heme iron. Iron deficient individuals
may absorb as much as a third of dietary iron from heme and a fifth from
non-heme. Need for iron is increased with exercise because the production of
red blood cells increases.
b. Form
of iron
Ferrous iron is absorbed better than the ferric iron.
Organic acids such as vitamin C moderately increase non-heme absorption by
adding an electron to ferric iron (Fe3+) yielding ferrous iron (Fe2+).
Vitamin C then forms a complex called chelate with Fe 2+ thereby
enhancing absorption. As such iron supplements should be taken together with
vitamin C rich drinks such as fresh orange juice.
c. Composition
of meal
When meat is eaten with a vegetable and grain products, the
absorption of non heme iron present is improved. A protein factor in meat, fish
and poultry(MFP factor) facilitate non heme absorption. This factor appears to
contain amino acids such as cysteine that bind iron to enhance absorption.
d. Acid in the stomach
Acid also plays an important role in iron absorption by
promoting the conversion of Fe3+ to Fe2+ and by
solubilizing non heme iron. The decreased production of stomach acids
experienced by many elderly people can lower their iron absorption and
ultimately their body stores of iron.
Dietary
factors
Several dietary factors interfere
with the body’s ability to absorb iron and these include;
Phytic acid and oxalic acid in vegetables which bind iron thus
reducing its absorption.
Dietary fibre intakes above 35g/day bind iron and other trace elements.
Polyphenols such as tannins found in tea and related substances
found in coffee also reduce iron absorption. People trying to rebuild iron
stores are advised to reduce coffee and tea consumption, particularly meal
times.
Calcium supplements in amounts greater than 300 mg/d can
reduce iron absorption. Calcium supplements should be taken several hours
before or after an iron rich meal.
Function of iron
1.
The
major role of iron is to permit the transfer
of oxygen and carbon dioxide from one tissue to another. Iron does this as
part of haemoglobin in the blood and myoglobin in the tissues.
2.
Blood formation: Haemoglobin is an essential component
of the red blood cells which are formed in the bone marrow when stimulated by
erythropoietin, a hormone synthesized by the kidney. Erythropoietin is released
in response to a decrease in oxygen occlusion in the blood, blood loss or
binding of carbon dioxide to red blood cells. It targets the bone marrow to
increase production of red blood cells.
3.
Other
functions of iron: A small amounts of iron are incorporated into tissue
enzymes. In a dietary deficiency, the level of these enzymes drops before the
haemoglobin level drops.
Iron is also
important in many other functions such as;
· Catalyzing the conversion of beta
carotene into vitamin A.
· The synthesis of purines which form
integral part of nucleic acid
· The removal of lipids from the blood.
· Collagen synthesis.
· Antibody production needed for proper
immune formation.
· Detoxification of drugs in the liver.
Iron needs
Iron needs differ for different age groups and sex. Pregnant
women and adolescent girls need twice as much as iron compared to adult men.
Iron needs are determined by need to replace losses through sweat, hair, nails,
dead skin cells, masturbation or other forms of bleeding. Growth increases iron
needs due to increased blood volume and tissue mass. The adult recommendations
for iron are 10mg/d for men and 15mg/d for women to account for menstruation
losses.
Pregnant women require 30mg/d daily. This takes care of the
increased placenta blood volume and the foetal needs. New borns are born with
enough iron stores to last for the first 4-6months until supplementary feeding
is introduced. Human milk is a poor source of iron 15mg/d. Infants and
children: Reserve of iron last 4–6months after delivery hence complementary
foods introduced at 6 months need to be rich in iron.
Dietary sources of iron
Liver is the richest source, lean meat, chicken, eggs, lean
pork, dark green vegetables, legumes, nuts, molasses and cereals.
Iron deficiency
Continued iron deficiency leads to nutritional anaemia
caused by lack of dietary essentials involved in haemoglobin formation or poor
absorption of these dietary essentials. Some types of anaemia have been
reported to be caused by a lack of either dietary iron or high quality protein;
lack of pyridoxine (vitamin B6), which catalyses the heme portion of
the haemoglobin molecule, by a lack of vitamin C which influences the rate of
iron absorption and the release of iron from transferring to the tissues; and
by lack of vitamin E, which affects the stability of the red blood cell
membrane. Copper is not part of the haemoglobin but aids in its synthesis by
influencing the absorption of iron, its release from the liver, or its incorporation
into haemoglobin.
Nutritional anaemia is characterised as both hypochromic and microcytic. Hypochromic suggests a lack of red pigment haemoglobin
and microcytic indicates the presence of small red blood cells. When red blood
cells synthesis is reduced, the amount of oxygen carried to the blood is
decreased. There are many types of anaemia but the major type found in the
world is iron deficiency anaemia. It is common in adolescent girls (growth and
menstrual demands), young infants and women of child bearing ages.
Iron toxicity
Excess intake may lead to hemosiderosis i.e. increased iron
reserves due to excessive supplements. Hemochromatosis: A genetic disorder of
iron metabolism characterized by increased absorption, saturation and
deposition of iron in the liver tissues.
Iodine is present in the body in minute amounts, 15-23 mg
and 75% of this is concentrated in the thyroid gland that uses it to synthesize
the hormone, thyroxin. The remainder is in other tissues particularly in the
salivary, mammary and gastric glands and in the kidney. It is present in food
as iodide and other non elemental forms. Iodine was linked to the presence of
an enlarged thyroid gland (goitre) during World War I.
Functions of iodine
1. Regulation of growth and development as
part of thyroxin hormone. Thyroxin can stimulate metabolism by as much as 30%.
2. Conversion of carotene to vitamin A.
3. Synthesis of protein.
4. When thyroxin levels are normal,
absorption of carbohydrates from the intestines is more efficient.
5. Synthesis of cholesterol.
Recommended daily allowances:
For adults 150mg/day
Dietary sources of iodine
These include: Salt water, fish, sea foods, iodized salt,
molasses, dark green leafy vegetables especially spinach leaves.
Deficiency of iodine
a. Goitre
This is a condition
characterised by swelling in the neck as a result of the enlargement of the
thyroid gland. The gland enlarges as it attempts to compensate for lack of
iodine essential to its major role in the synthesis of thyroxin. The pituitary
gland hormone that stimulates the thyroid hormone production, thyroid
stimulating hormone also stimulates the growth of the thyroid gland. Too much
growth of the gland causes difficulties in breathing.
Goitre is also
associated with the consumption of goitrogens (substance in foods such as the
cabbage family and water that interfere with thyroid gland metabolism and thus
may cause goitre if consumed in large amounts). Defects in the enzymes
necessary for the synthesis and release of thyroxin can also lead to goitre.
Goitre is common in areas where the soil is poor in iodine due to leaching.
Goitre incidences are six times more in females than males with adolescent
girls and pregnant women being more susceptible.
b.
Cretinism
If
an expectant mother consumes iodine deficient diets during the early pregnancy,
her infant may be born with a short stature (dwarfed) and with mental
retardation. Maternal iodine needs take precedence over foetal needs. The
retarded body growth is referred to as cretinism characterized by physical
drawfness, mental retardation, thick-dry skin and enlarged protruding stomach.
c.
Myxedema
This
is a condition seen in adults who have had symptoms of hypothyroidism
throughout their development and growth period. It is characterized by coarse
sparse hair, dry, yellowish hair, poor tolerance to cold and low husky voice.
d.
Hyperthyroidism
This
is where basal metabolic rate is accelerated by as much as 100% above normal
and is associated with over activity of the thyroid gland and experience
nervousness, weight loss, increased appetite, intolerance to heat, tremors and
protruding eyeballs.
Zinc
Zinc
is present in all body tissues and fluids .The total human body zinc content
has been estimates to be 30 mmol (2grams). Skeleton muscle accounts for
approximately 60% of the total body content and bone mass, with a zinc
concentration of 1.5-3.0mol/g (100-200g/g) for approximately 30%. The
concentration of zinc in lean body mass is approximately 0.46 mol/g (30(g/1).
Plasma zinc has a rapid turn over and represent only about 0.1% of total body
zinc content. This level appears to be under close homeostatic control. High
concentrations of zinc are found in the choroids of the eye (4.2 mol/g or 274
g/g) and in prostastic fluids (4.6-7.7 mmol/l or 300-500mg/l). Zinc is lost through faeces, urine and shed
tissues. High tissue losses are from the skin, the mucosal cells, menstrual
fluids and semen.
Absorption of zinc
Like
iron absorption, zinc absorption is influenced by the types of food ingested.
More is absorbed when animal protein is consumed. Fibre and phytates inhibit
zinc absorption.
Functions of zinc
Over
300 enzymes require zinc as a co-factor for optimal activity. Adequate zinc is
necessary to support many bodily functions such as;
a.
It is involved in
nucleic acid synthesis and function (some factors that control expression of
genes contains zinc-rich regions that bind DNA).
b.
It is involved in
protein metabolism, growth and wound healing.
c.
Immune function.
d.
Development of sexual
organs and bone.
e.
Involved in storage,
release and function of insulin.
f.
Cell membrane structure
and function.
Recommended daily requirements
for zinc
The
adult recommended daily requirement is 15 mg/day for men and 12 mg/day for
women.
Dietary sources of zinc
In
general, protein-rich diets are also rich in zinc. Lean meats especially beef
and other red meats, eggs and shell fish are the best sources. Cereals and
legumes contain significant amounts of zinc but contain phytic acid and other
substances that can interfere with intestinal absorption of zinc.
Zinc deficiency
Zinc
deficiency is associated with;
·
Severe growth
retardation and arrested sexual maturation.
·
Altered taste acuity.
·
Impaired immune function.
·
Slow wound healing.
·
Impaired central
nervous system function.
Selenium
is deposited in all body tissues except fat. The highest concentrations occur
in the liver, kidney, heart and spleen. Serum levels are about 0.22 µg/dl. Most
selenium in foods is bound to derivatives of the amino acids; methionine and
cysteine. Because these substances are readily absorbed, bioavailability of
selenium is considerably higher than that of zinc and iron. About 50%-100 % of
dietary selenium intake is absorbed. In addition, since no physiological
mechanism appears to control selenium absorption, selenium has a definite
potential for toxicity. Most selenium is excreted via the urine and faeces.
Functions of selenium
a.
It functions as an
integral component of an anti oxidant enzyme; glutathione peroxidase that
protects cells and lipid membranes against oxidative damage.
b.
It also acts as a
structural component, incorporated into the protein matrix of the teeth.
Recommended daily allowances for selenium
The
recommended daily allowance estimated to be safe and adequate for an adult is
about 50-200 µg/day.
Dietary sources of selenium
Food
sources vary with the selenium soil content. Good sources include sea food,
legumes, whole grains, low fat meats and dairy products with smaller amounts in
vegetables.
Selenium deficiency
The
signs and symptoms of selenium deficiency include pain, muscle wasting and cardiomyopathy, a form of heart
disease.
Selenium toxicity
High
doses (a milligram or above daily is toxic. Selenium toxicity causes vomiting,
diarrhoea, loss of hair and nervous system.
Other minerals
In
addition to macro and micro elements discussed above, other mineral elements
are present in the body in variable amounts and are essential in human
metabolism. In the human body, they are almost always bound to organic
compounds rather than being free in organic elements. They are copper,
manganese, molybdenum, chromium, nickel, vanadium, silicon, tin and cobalt.
Copper is a component of oxidative enzymes; cytochrome c oxidase which is
active at terminal end of the mitochondrial electron transport chain. Molybdenum acts as part of several
metalloenzymes that are involved in oxidation-reduction reactions and may be
involved with detoxification process in the liver. It appears to be
particularly important in the early stages of brain development. Chromium participates in carbohydrate and
lipid metabolism. The only known function of cobalt is as part of vitamin B12.
Summary
·
Minerals are inorganic
substances that are widely distributed in nature. They build body tissues,
active, regulate and control metabolic processes, transmit neorologic messages.
·
They are classified as;
macro elements that are required in relatively large quantities, which make up
60-80% of all inorganic material in the body and the micro elements are
required in quantities as small as microgram, which make up less than 1% of the
body’s inorganic material.
·
Many minerals are vital
for sustaining life. For humans, animal products are typically the best sources
of most minerals.
·
Calcium forms a vital
part of the bone structure and is also very important cation (Ca2+)
in blood clotting, muscle contraction and nerve-impulse transmission. Calcium
absorption is enhanced by stomach acid and many other factors.Dairy products
are important calcium sources.
·
Phosphorus aids the
function of many enzymes and forms part of ATP molecules, numerous metabolites
and phospholipids in cell membranes. It is efficiently absorbed, deficiencies
are rare. Good food sources are dairy products, bakery products and meats.
·
Magnesium is the only
mineral found mostly in plants where it functions in chlorophyll. In humans, it
is important for nerve, lung and heart function, as a co-factor for many
co-enzymes. Good sources include dark green vegetables, legumes sea food, nuts,
cereals and dairy products. Sulphur is incorporated into certain vitamins and
amino acids.
·
Fluorine is found
mainly in teeth and skeleton and the main sources is water, small fish and some
amounts are also found in tea.
·
Iron is found in every
cell in the body. It is a critical component of haemoglobin, myoglobin and
cytochromes. It aids in the transfer of oxygen and carbon dioxide from one
tissue to another. It is also involved in blood formation, conversion of
ß-carotene to vitamin A, synthesis of purines, collagen formation, and removal
of lipids in blood, antibody production and detoxification of drugs in the
liver.
·
Iodine is concentrated
in the thyroid gland that uses it to synthesise thyroxin hormone that regulates
growth and development. A lack of dietary iodide results in development of
goitre and other metabolic problems including stunted growth and mental
retardation.
·
Zinc functions as a
co-factor for over 300 enzymes that are important for growth, development,
immune function, wound healing and taste sensation. The most understood role of
selenium is as a co-factor for glutathione peroxidase, whose action reduces
oxidative damage of cells.
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