Nutrition Lecture 4: Selenium Flashcards
What is selenium?
An essential trace element
How does selenium intake range globally?
Large geographical intakes from deficiency to toxicity - China’s intake is extremely high
What is the RDI for selenium?
Women = 60ug/day
Men = 70ug/day
What are the different species of selenium?
Organic and Inorganic
What are the two organic forms of selenium?
SeCys and SeMet
What form of selenium is found in animal products?
SeCys
What form of selenium is found in plant products?
SeMet
What are the two inorganic forms of selenium?
Selenate (SeO2- 4)
Selenide (SeO2- 3)
What is the active part of selenium?
SeCys - responsible for 99% of the metabolic actions
What are inorganic forms of selenium not attached to?
Not attached to amino acids
How does Se enter the food chain?
Through plants
What is the Se content of plants affected by?
Availability of Se in soil
What matter in soil absorbs Se?
Organic matter
What binds Se?
Clay minerals
What form of Se do alkaline soils favour?
Selenate - more bioavailable
What form of Se fo acidic-neutral soils favour?
Selenide - less bioavailable
What are the three classifications of Se accumulation in plants?
- Hyperaccumulators
- Secondary-accumulators
- Non-accumulators
What are hyperaccumulators?
Accumulate >1000mg/Se/kg
What is an example of a hyperaccumulator plant?
Brazil nuts
What are secondary-accumulators?
Accumulate up to 100-1000mg/Se/kg
What are examples of secondary-accumulators?
Brassica (rapeseed, broccoli, cabbage) Allium (garlic, onions, leeks)
What are non accumulators?
Accumulate <100mg/Se/kg
What are examples of non-accumulators?
Wheat crops (wheat, oats, barley, rye) Rice
Is NZ considered a deficient, low or rich region of Se?
Low region - we have moderate to low intakes
What are good food sources of Se?
Brazil nuts, Seafood, kidney, livers and brains
What food sources do NZ’ers get their Se from?
Bread, seafood, chicken
What are the regional differences in Se intake in NZ?
Intake is significantly higher in the North Island
Why are animal sources of Se more reliable than plant sources?
Plant sources are affected by soil variability, whereas animals have an absolute requirement for Se - If the soil is deficient in selenium, the animals must be fed it from somewhere else
What is the absorption of Se?
High - 80% from food sources
Where is selenium processed?
In enterocytes
What is organic selenium converted into?
Into H2Se and then packaged into lipoproteins
After selenium is packaged into lipoproteins where does it go?
Taken into the liver and converted into a functional protein - selenoprotein
What does organic Se need to be converted into before it can be used in the body?
Selenoprotein
What are inhibitors of Se absorption?
None are known
How many classes of selenoproteins are there?
25 known classes
What are the biological processes of selenoproteins?
- Antioxidant defence
- Inflammation
- Muscle development
- Reproduction
- Thyroid hormone metabolism
- Immune function
What is selenocysteine?
The active part of a selenoprotein - an essential amino acid component in selenoproteins
Is selenoprotein synthesis regulated?
Highly regulated - there is a maximum amount of selenoproteins that we can and will synthesise
How do differences arise in selenoprotein function and usage between people?
Within the 3UTR you can sometimes get differences in polymorphisms which affects the function of the protein
What are glutathione peroxidases (GPx)
selenium-containing antioxidant enzymes
What are the different forms of GPx?
GPx1
GPx3
GPx4
Where is GPx1 found?
in the cytosol of cells
Where is GPx3 found?
is extracellular in the blood and plasma, and ECF
Where is GPx4 found?
in phospholipids
What is the main function of GPx?
Antioxidant defences
What 2 main antioxidant defences does GPx carry out?
- Reduces hydrogen peroxide and other organic hydroperoxides
- Protects tissues from inflammatory and oxidative damage
What are thioredoxin reductases?
Selenium containing enzymes
What is the function of Thioredoxin reductases?
involved in vital cell processes, found everywhere
What does liver disease reduce?
reduces selenoproteins
What does kidney disease reduce?
GPx3
What type of stress upregulates selenoprotein synthesis?
Oxidative stress
What does inflammation reduce?
Plasma Se is reduced during acute phase
What are selenium-containing proteins?
Differ from selenoproteins, proteins in the body that incorperate seleno-methionine
What does SeMet stand for?
Seleno-methionine
What happens when Se availability is limited?
Incorporation of Se into individual selenoproteins is prioritised
What selenoprotein is at the top of the hierarchy (activities maintained when deficient)?
GPx4
What selenoprotein is at the bottom of the hierarchy (activities drop when deficient)?
GPx1
What body tissues does Se prioritise when deficient?
Brain, endocrinological, reproductive tissues
What body tissues does Se disregard when deficient?
Heart, liver, kidney, lung tissues
What body parts can Se concentration be measured in? (long term intake)
Urine, toenails, hair
How is Se commonly measured? (short-medium term intake)
Plasma/serum Se concentration (SelP and GPx3)
What plasma selenoprotein reflects liver concentration?
SelP
What plasma selenoprotein reflects kidney concentration?
GPx3
What is the relationship between plasma Se and dietary intake?
Plasma Se concentrations increase with increasing Se intake
When do we see a sharper increase in plasma Se?
When Se intakes are less than 70ug/day
When do plasma Se concentrations decrease?
During acute phase (inflammation/sickness)
When does plasma GPx activity reach maximum? (plateus)
at Se intakes of 40-60ug/day (plasma Se of 85ug/L)
When does plasma SelP concentration reach maximum? (plateus)
at Se intakes of 95ug/day (plasma Se of 120ug/day
What is the relationship between selenium and all-cause mortality?
Risk of all cause mortality is highest risk at low and high concentrations - want to sit at recommended (U shape relationship)
