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BIO AVAIL ABILITY. (what we get from what we have taken in). BIOAVAILABILITY. Definition What makes it up Critical phase Hormonal factors in How its measured Membrane transporters Special cases for redox metals. First basic law of nutrition :

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(what we get from what we have taken in)



What makes it up

Critical phase

Hormonal factors in

How its measured

Membrane transporters

Special cases for redox metals

First basic law of nutrition:

No nutrient is absorbed and utilized to the full extent that it is fed

Steven Blezinger

Define bioavailability
Define Bioavailability

  • That which becomes bioavailable

  • The fraction (or percentage) of nutrient absorbed that is useful to the body

  • The degree to which an absorbed nutrient is available to the system

Nutritional Definition

Bioavailability is a post-absorption assessment of how much of a nutrient that has been absorbed becomes functional to the system

Food Science Definition

Bioavailability is a assessment of how much of a nutrient is retained in the food product after processing for the consumer

Bioavailability in toto

Raw Food Product (100%)

Total (proximate analysis)

Processed Food

Chemically available



Biologically available

Cellular uptake

Functional mineral

Basic Scheme of Proximate Analysis

Air dried Sample (oven)

Water free sample

Extract with ether


Boil in acid







Nitrogen free extract

Boil in alkali

Residue (crude fiber)




(Total N content)

Biological Availability


Definition can be based on either the percentage of a nutrient ingested or the percentage of a nutrient absorbed that becomes useful to the organism

The percentage ingested is preferred by some because the percentage absorbed is difficult to determine and relies on an indirect analysis

The percentage absorbed is, nonetheless, a more accurate appraisal of bioavailability

The fraction of the total amount absorbed that performs a function



Blood Transport

Which is the most critical phase for minerals?

Liver and kidney excretion

Losses along the way

Membrane transport

Intracellular movement

Functional Site

Recall function

A nutrient is considered outside the body until it passes thru the intestinal barrier

The amount that gets absorbed depends on: function

Digestibility of the food source

Solubility of the mineral

Extrinsic Factors

Elements in the food source that hinder or facilitate absorption

With a focus on the organism, bioavailability

depends on:



Nutritional state

Physiological state

Genetic predisposition


Developmental stage


Intrinsic Factors

Components of bioavailability
Components of Bioavailability function

  • Digestion

  • Absorption

  • Liver surveillance

  • Transport

  • Transmembrane movement

  • Intracellular movement

  • Target binding

Absorptive Phase

Assimilation Phase

Why Not Absorption Alone as an index of Bioavailability function

As pointed out by O’Dell, assimilation may be a major part of a mineral’s bioavailability and needs to be assessed separately

Absorption %

Retention %a

75Se as selenite



75Se as selenomethionine



aArbitrary units

2-picolinic acid enhances zinc absorption in rats by nearly 60%. But, also increases zinc excretion so there is no net effect on retention and hence no increase in bioavailability

Assessing bioavailability by the slope ratio method function

Assessing Phytate (in soy flour) as a factor in Zinc bioavailability

Zn carbonate

Zn Beef

Beef/carbonate = 0.97

Body wt of 3 wk old chicks

Soy/carbonate = 0.31

Zn Soy flour

4 6 8 10 14

Zn in the diet (mg/kg)

Quantifying Bioavailability function

% Bioavailable = % absorbed x % assimilated x 10-2

If a diet contained 5 mg of a metal ion and 42% of the 10% absorbed

was retained in the system, the % bioavailable would be ___________

which means ________ mg was rendered functional








Solution function

5 mg

4.5 mg (90%)

stays in lumen


0.5 mg absorbed (10%)

% BA = %Abs x %Fun x 10-2

= 10 x 42 x 10-2

Functional site

= 4.2 or 0.21 mg of the 5 mg taken in the diet

0.21 mg (42%) of absorbed becomes functional

0.29 (58%) of absorbed is non-functional

Exam 1
Exam 1 function

An Overall Assessment of Minerals

  • What they are (chemistry)

  • What they do (biochemistry)

  • How they get in our body (absorption)

  • How they get into cells (transport, assimilation)

  • How efficient are they (bioavailability)

  • How are they regulated

Chemistry function

  • Chemical properties relative to function

    • Ionization

    • Solubility

    • Valence

    • Electronic configuration


  • Biochemical properties relative to function

    • Macro vs microminerals

    • Enzyme cofactors

    • Pathway components

    • Crystallization

    • Binding proteins


  • Solubility

    • Mucins, ligands, pH

  • Valence

  • Transport proteins (intestinal sites)

  • Cytosolic transport and storage

  • Export factors

Transport and Assimilation function

  • Transport proteins in plasma

  • Membrane receptors and channels

  • Membrane transport

    • Simple diffusion

    • Mediated diffusion

    • Active transport

    • Receptor mediated endocytosis

  • Cytosolic transport

    • Vesicles

    • Metallochaperones


  • Regulation of iron absorprtion

    • Iron control of ferritin and transferrin receptor (IRPs)

    • Hemochomatosis factors hepcidin and HFE

  • Regulation of calcium absorption

    • Control of calbindin and CaT1 by dihydroxy-vitamin D3


1. Ways to measure mineral bioavailability