Lecture 9: Intake vs. Amount Available to the Body Flashcards
What is bioavailability?
The proportion of a nutrient in a food that is digested, absorbed and utilised
What is the absorption of non-haem iron?
approx. 5-15%
How is iron in the body?
Through epithelial cells and fluids
How is iron lost in epithelial cells?
- Skin (0.2mg)
- Intestinal mucosal cells (0.1mg)
- Urinary tract cells (0.1mg)
How is iron lost in fluids?
- Blood (gut 0.4mg)
- Menstruation
- Bile (0.2mg)
What are the dietary requirements of iron?
Men: 8mg
Women: 18mg
What are the average iron losses per day?
Men: 1mg
Women: 2mg
What is bioavailability influenced by?
- Chemical form
- Composition of food and meal
- Nutrient status
- Physiological status
- Amount consumed
What happens when someone has low iron stores?
Increased Fe absorption
What happens to iron absorption when someone is pregnant?
It increases
Why do we measure bioavailability?
- NRV’s are set using %absorption
- Identify & quantify new enhancers and inhibitors
- Compare efficiency of new supplements
- Develop algorithms to estimate absorption
What are isotopes?
Atoms with the same number of protons but different number of neutrons - similar chemistry different mass
What are the three components of an atom?
- Protons (+)
- Neutrons (0)
- Electrons (-)
What are isotopes of Fe?
Fe-55, Fe-59, Fe-54, Fe-56, Fe-57, Fe-58
What are radioisotopes?
Atoms with an unstable nucleus that emit radioactive decay
What are the three types of radioactive decay (ionising radiation)?
- Alpha particles
- Beta particles
- Gamma rays
How does ionising radiation work?
- Remove electrons from atoms and products damage tissues
- Risk depends on route, dose, and ionisation energy
What are the characteristics of alpha particles?
- Low penetrating power
- Cause high ionisation power
What are the characteristics of beta particles?
- More penetrating than alpha
- Not as ionising as alpha
What are the characteristics of gamma particles?
- High penetrating power
- Causes least ionisation
What types of studies use radioisotopes?
- Balance studies
- Whole body counting
- Plasma appearance
- Hb incorporation
What are the advantages of using radioisotopes?
- True tracers
- Often cheaper than stable isotopes
- Minimal sample preparation needed
What are the disadvantages of using radioisotopes?
- Radiation dose
- Cannot be used with pregnant or lactating women, infants or children
- Ethical approval
What are stable isotopes?
- Stable nucleus so do not emit radiation
- Naturally occuring
What type of studies are stable isotopes used in?
- Balance studies
- Plasma appearance
- Hb incorporation for Fe
What are the advantages of using stable isotopes?
- No known health effects
- Can be used in any population
- Multiple isotopes for Fe
What are the disadvantages of using stable isotopes?
- Expensive
- More sample preparation
- Need larger doses
- Only 1 stable isotope for iodine so can’t study it
What is the chemical balance method?
Apparent Fe absorption = Fe intake - Faecal Fe
What are the advantages of using chemical balance?
- Whole diet not single meals
- No radiation
- Several nutrients at once
What are the disadvantages of using chemical balance?
- Large margin of error
- Nutrient retention may depend on status
- No correction for endogenous excretion
What are algorithms?
- Mathematical equations
- Use intakes of selected enhancers and inhibitors to estimate % of nutrient absorbed
- Applied to whole diet
- Assume a specific nutrient status
What are the advantages of using algorithms?
- Relatively quick
- Relatively inexpensive
- Can provide useful comparative data
What are the disadvantages of using algorithms?
- Underestimate absorption
- Need food comp data
- Don’t account for interactions between modifiers
- Effects based on single meal not whole diet studies
