Nutrition Lecture 5: Zinc Flashcards
What are rich food sources of zinc?
Meat, poultry, shellfish, fish, liver, kidney, peas, beans, legumes
What factors influence zinc bioavailability?
- Host related factors
- Enhancers and inhibitors
What happens to zinc absorption during pregnanacy?
Markedly increases
What are zinc absorption enhancers?
Animal protein - flesh protein, milk, cheese
What are inhibitors of zinc absorption?
- High doses of iron
- Phytate
What is phytate?
The molecule that plants use to store phosphate groups
- Anything that can germinate contains phytate
(Nuts, seeds, legumes)
What is the structure of phytate?
6 carbon ring structure - each carbon has a phosphate group on it
What are inositol phosphates?
a group of 6 naturally occurring organic compounds derived from inositol
What inositol phosphate is not inhibitory of zinc absorption?
IP4
What inositol phosphate is inhibitory of zinc absorption?
IP6
How can we make IP6 not inhibitory?
get rid of the phosphates to make IP4
How do we remove phosphates from IP6?
- Germinate the thing (bean, seed etc.) so it uses the phosphate groups
- Fermentation (bread is a fermentation process)
- Canning will also reduces the amount of phytate
How does phytate inhibit absorption?
Zinc is a divalent positively charged ion
- It binds nicely to oxygens on the phytate
- Once bound it is stuck there and cant be absorbed in the gut
Where does zinc absorption happen?
In proximal small intestine
What are the two mechanisms that zinc enters enterocytes through the brush border?
- Carrier-mediated transport (ZIP4)
- Diffusion (at high doses e.g., supplements)
What is ZIP4?
ZIP4 is responsible for uptake of zinc from the intestine. It facilitates the transport of zinc across the cell membrane into enterocytes (intestinal cells)
What happens to zinc inside cells (2 options)?
- Used (enzymes etc.)
- Stored
- Transported out into bloodstream
What is zinc stored as inside of cells?
Part of metallothionein, or in vesicles, or trans-golgi network
How is zinc transported out of cells across basolateral membrane?
By ZnT1
What is ZnT1?
A protein that is responsible for transporting zinc out of cells
How is zinc transported in the blood?
By binding to proteins in the blood such as albumin and transferrin
What is zincs main function?
Over 200 enzymes are zinc-dependent
How does zinc influence enzyme function?
- Stabilises protein structure by binding to amino acid residues
- Participates in reaction at active site
How is zinc involved in our genetic material?
“zinc fingers” play a significant role in gene expression by binding to DNA, RNA, or proteins
How is zinc involved in cell function?
- Cell growth
- Cell membrane integrity
- Cell replication
How is zinc involved in our immune function?
Skin integrity, cell-mediated and humoral immunity
How many proteins use zinc fingers?
Used by 1,000’s of proteins
What is the structure of zinc fingers?
2 Cys and 2 His close together in a protein chain + zinc - protein folds tightly around it
What is metallothionein?
a small, cysteine-rich protein that binds heavy metals
How many zinc ions can metallothionein bind at a time?
Up to 7 zinc ions
What are the main functions of metallothionein? (4)
- Store zinc
- Zinc transporter
- Cell signalling
- Protect against oxidative stress
When were the first cases of zinc deficiency reported?
1960s
What were the clinical features of the first zinc deficiency cases?
- Stunting
- Little or no secondary sexual development
What is acrodermatitis enteropathica?
a rare genetic disorder that causes defective zinc absorption in small intestine
What mutation causes acrodermatitis enteropathica?
Mutation of SLC39A4 gene which encodes the ZIP4 transporter on enterocytes
What are the symptoms of acrodermatitis enteropathica?
Poor immune function, wound healing, alopecia, diarrhoea and dermatitis around mouth
What are the causes of zinc deficiency?
- Inadequate dietary intake
- High physiological requirement
- Excessive losses
What is the relationship between zinc and diarrhoea?
The longer you have diarrhoea, the lower your serum zinc will be
What are the impacts of zinc deficiency?
- Impaired growth
- Impaired immune competence
- Poor taste acuity
What groups are at risk for zinc deficiency?
- Infants/children
- Pregnancy
- Elderly
- Vegetarians/vegans
- Low income
- Prolonged diarrhoea
What does zinc supplementation do for diarrhoea?
Reduces the incidence of it
What is the upper limit of zinc intake?
> 40mg/day
What happens when someone consumes extremely high doses of zinc (>1g/day)?
- Metallic taste
- Nausea
- GI stress
- May be fatal
Zinc absorption upregulates the production of what?
Metallothionein
What does Metallothionein preferentially bind?
binds copper over zinc
What is the EAR for zinc?
Men = 12mg/day
Women = 6.5mg/day
What is the RDI for zinc?
Men = 14mg/day
Women = 8mg/day
