Lecture 20 Flashcards
What are the major minerals?
Ca, P, K, S, Na, Cl, Mg
What are the minor minerals?
Fe, Zn, Mn, I, Se
What are the functions of iron?
- Hb
- Myoglobin
- Cytochromes
- Ribonucleatide reductase
What is haemoglobin?
A metalloprotein with heme, an iron porphyrin, attached to the protein moiety. Iron combines with oxygen in the lungs, where the concentration is high, and releases the oxygen at tissues where it is needed.
Where are RBC’s formed chiefly?
Bone marrow
What is haem synthesised from?
Glycine and iron in the presence of pyridoxine and combined with globin
How are old RBC’s removed? What happens to the iron?
Removed from circulation by cells of the reticuloendothelium. Iron is released from the porphyrin, taken up by transferrin and either returned to the bone marrow for the production of new cells or to the liver or spleen for storage.
How much iron is recycled daily?
20-25 mg
How much iron is required to be absorbed daily to replace losses?
1-2 mg
How is iron lost from the body?
- Gut enterocytes store the iron. When body doesn’t require it, these enterocytes get sloughed off.
- Bleeding
What is bioavailability dependent on?
Physiological factirs
Dietary factors
What are the major proteins for transport and storage of iron?
Transferrin and ferritin
What is transferrin?
An abundant plasma protein that carries iron from the site of absorption or storage to cells an tissues.
What happens to transferrin and iron once inside cells?
The transferrin/transferrin receptor complex is internalised by the cell and iron is released into the cytoplasm so that it may be used for various cellular functions.
What modification do cells that have particularly high iron requirements have?
Express high concentrations of transferrin receptors on their surface
What happens to the iron which is not required in the cell?
Combines with apoferritin to form ferritin.
Why is the storage of excess iron important?
Both ferric and ferrous iron are able to participate in reactions which generate potentially toxic oxygen radical
What happens in times of iron deficiency?
In times of iron deficiency cells increase their uptake of iron to ensure that sufficient is available for the synthesis of iron-containing proteins. To do this they increase the expression of cell surface transferrin receptors, but repress their synthesis of intracellular ferritin since there is negligible excess iron in the cytoplasm.
What are the two forms of dietary iron?
- Haem iron in the form of Hb and myoglobin (meat, fish, eggs)
- Non-haem iron (vegetables, pulses and inorganic iron added to cereals, dietary supplements)
Describe the absorption of haem iron.
Heme iron is absorbed into the mucosal cell as the intact porphyrin complex
Compare the absorption of haem and non-haem iron
Haem: 25%
Non-haem: 5%
Describe the absorption of non-haem iron
- Iron must be present in the duodenum and upper jejunum in a soluble form.
- In the stomach non-heme iron is ionised by gastric juice as the chyme passes from the stomach to the duodenum, the pH increases to 7. Unless chelated, most ferric iron is precipitated. At the cell membrane of the brush border of the epithelial cell ferric iron is bound to a receptor protein called membrane iron binding protein (MIBP) which then transfers iron into the cell
What is absorption of haem iron affected by?
- Iron status of subjects
* Amount of heme iron present in meat
What is absorption of non-haem iron affected by?
• Iron status of subjects
• Amount of bioavailable non-heme iron (adjustment for fortification iron and
contamination iron)
• Balance between dietary factors enhancing and inhibiting iron absorption
What is transfer of iron from mucosal cells to the body affected by?
- Size of the body stores
- The quantity of iron in the diet.
What are the factors enhancing iron absorption?
- Vitamin C
- Meat factor protein
- Lactic acid from foods
- Citric acid from foods
- HCl from stomach
- Sugars
What factors inhibit iron absorption?
- Phytates
- Fibres
- Polyphenols
- Tannins
- Calcium
- Oxalates
- Phosphorus
What factors decrease absorption of iron?
Intestinal motility
Steatorrhoea
What are the dietary sources of iron?
- Liver, lean meat, poultry, shellfish, eggs
- Legumes, dried fruit
- Iron fortification of cereals, flours, bread
- Iron contamination from cooking vessels
- Iron supplements
What are iron requirements determined by?
Demands for tissue growth and Hb accretion and by replacement needs for iron lost in faeces, urine, asn sweat
Amongst whom is the iron balance precarious in?
- Infants
- Women and adolescent girls who become concerned about weight and restrict food intake
- Vegans
- Dedicated recreational athletes
- Elite female athletes
- Pregnant women
What are the three stages of iron deficiency? What are they indicated by?
1) Depleted storage iron: Serum ferritin less than 15 mcg/L
2) Deficient: low serum iron, high serum total iron-binding capacity, reduced transferrin saturation index, high free erythrocyte protophyrin
3) Iron deficiency anaemia: Low Hb, hypochromic, microcytic anaemia, reduced mean blood corpuscular volume
What does the blood film of iron deficiency anaemia look like?
Pale, small RBC’s, normal shape
Microcytic anaemia
What is iron vital for?
Myelination
Neuronal growth and differentiation
Neurotransmitter regulation
What is the RDI for pregnant women?
27 mg/day
What are the side effects of iron supplements?
Diarrhoea
Constipation
