The document discusses nutrient bioavailability, defining it as the fraction of ingested nutrients that is useful for physiological functions and storage. It covers factors influencing bioavailability, including digestion, absorption, and the impact of food processing, as well as the roles of macronutrients and dietary fiber. Additionally, it highlights issues related to malabsorption and the consequences for health, emphasizing the complex interplay of nutrition, physiology, and genetics.

1. Nutrient
Bioavailability
(What we get from what we have taken in)
Dereje Tsegaye(MPH,Assiss.Prof.PhD fellow)
1

2. Topics
•Definition
•What makes it up
•Critical phase
•How its measured
2

3. First basic law of nutrition:
No nutrient is absorbed and
utilized to the full extent that
it is fed
Steven Blezinger
3

4. Define Bioavailability
•Bioavailability is the fraction of an ingested nutrient
that is available for utilization in normal physiologic
functions and for storage.
–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 4

5. Biological Availability
(Bioavailability)
• 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
5

6. • Bioavailability is a post-absorption assessment of how
much of a nutrient that has been absorbed becomes
functional to the system
Nutritional Definition
• Bioavailability is a assessment of how much of a
nutrient is retained in the food product after
processing for the consumer
Food Science Definition
6

7. • Assimilation may be a major part of a mineral’s
bioavailability and needs to be assessed separately e.g.
75Se as selenite
75Se as selenomethionine
Absorption %
92
96
Retention %a
<50
>80
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
Why Not Absorption Alone as an index of
Bioavailability?
7

8. Different terms used in bioavailability
• Bioavailability is the fraction of an ingested nutrient
that is available for utilization in normal physiologic
functions and for storage.
• Bioconversion is the fraction of a bioavailable nutrient
(e.g., absorbed provitamin A carotenoids) that is
converted to the active form of a nutrient (retinol).
8

9. Terms…
• Bioefficacy is the fraction of an ingested nutrient (e.g.,
dietary provitamin A carotenoids) that is absorbed and
converted to the active form of the nutrient (retinol) in
the body.
• Functional bioefficacy is the fraction of an ingested
nutrient that performs a certain metabolic function, such
as the ability of ingested provitamin A carotenoids to
reverse or prevent abnormal dark adaptation. 9

10. • With respect to provitamin A carotenoids, the term
bioefficacy is defined as the product of the fraction of the
ingested amount that is absorbed (bioavailability) and
the fraction of that which is converted to retinol in the
body (bioconversion).
* Bioefficay = bioavailability X bioconversion
10

11. Components of Bioavailability
•Digestion
•Absorption
•Liver surveillance
•Transport
•Trans membrane
movement
•Intracellular movement
•Target binding
Assimilation
Phase
Absorptive
Phase
11

12. Bioavailability in toto
Raw Food Product (100%)
Processed Food
Digestion
Absorption
Cellular uptake
Functional Nutrient
Total (proximate analysis)
Chemically available
Biologically available
12

13. The fraction of the total amount absorbed that
performs a function
Digestion
Absorption
Functional Site
Blood Transport
Membrane transport
Losses
along
the way
Which is the
most critical
phase for
Nutrients ?
Intracellular movement
Liver and kidney excretion
13

14. Note that
A nutrient is considered outside the
body until it passes through the
intestinal barrier
14

15. The amount that gets absorbed depends on:
• Digestibility of the food source
• Solubility of the nutrient
• Elements in the food source that hinder or facilitate
absorption (enhancers and inhibitors)
With a focus on the organism, bioavailability
depends on:
Extrinsic Factors
• Age
• Health
• Nutritional state
• Physiological state
• Genetic predisposition
• Gender
• Developmental stage
• Species
Intrinsic
Factors
15

16. Bioavailability of
Macronutrients
16

17. Absorption & Transport
End-products of digestion:
1. CHO >>> Monosaccharides
2. Fats >>> Glycerol + fatty acids
3. Proteins >>> Amino acids
4. Vitamins, minerals & water – no
digestion 17

18. Absorption
18

19. Absorption & Transport
• Absorption occurs in the small intestine
– Wall of small intestine covered with 100s of folds
– Each fold covered with 1000s of villi
– Each villi contains 100s of microvilli
19

20. The Small Intestine Villa
20

21. Absorption & Transport
• Absorbed nutrients enter either the:
1. Vascular system – water-soluble nutrients
(monosaccharides, amino acids, water- soluble vitamins,
minerals, water, short chain fatty acids) enter the blood
via the portal vein for transport to the liver
2. Lymph system – fat-soluble nutrients (lipids, fat-soluble
vitamins) enter here, eventually entering the blood near
the heart
21

22. Absorption & Transport
3. Transport of lipids and lipid soluble vitamins – since fats
are insoluble in water, they must be packaged for
transport as lipoproteins (triglyceride, phospholipid,
protein, cholesterol)
22

23. Absorption of Carbohydrates
a)Starch and other discharides
23

24. a)Starch & other disaccharides …
24

25. 25
They get fermented in the
colon by anaerobic bacteria
Oligosaccharides(eg. Raffinose, Stachyose)
and non-starch polysaccharides resistant
starch
Escape digestion in the
upper gut (small intestine
Increased faecal Biomass
resulting in increased
peristalsis
Production of
short chain fatty
acids (SCFA)
 Acetate
 Propionate
 Butyrate
Production of
gases likes co2,
methane and
hydrogen
sulphide
b)Digestion of oligosaccharides, resistant starch and non-
starch polysaccharides (Dietary Fiber)

26. 26
Dietary fiber
• Fiber refers to certain types of carbohydrates that our
body cannot digest (oligosaccharides and non-starch
polysaccharides).
• These carbohydrates pass through the intestinal tract
intact and help to move waste out of the body.
• Diets that are low in fiber have been shown to cause
problems such as constipation and hemorrhoids and to
increase the risk for certain types of cancers such as colon
cancer.

27. 27
Cont…
• Diets high in fiber; however, have been shown to
decrease risks for heart disease, obesity, and they
help lower cholesterol.
• Foods high in fiber include fruits, vegetables, and
whole grain products.

28. 28
How does fiber prevent different health
problems?
Cancer (Colonic, breast..)
• Prevents secondary bile acid circulation
• Decrease intestinal transit time
• Decrease contact of carcinogens with intestinal
cells
• Fermentation product butyrate has apoptotic
effect
• Decreases absorption fats and sugars

29. 29
Cont…
Dietary Fiber prevents Constipation, Hemorrhoids &
Diversticulosis by:
• Increasing perystalsis making stool bulk
• Decreasing straining to pass stool

30. Carbohydrate malabsorption
Lactose intolerance
• Most mammals normally cease to produce lactase,
becoming lactose intolerant, after weaning
• It is estimated that 75% of adults worldwide show some
decrease in lactase activity during adulthood
• The frequency of decreased lactase activity ranges from
5% in northern Europe through 71% for Sicily to more than
90% in some African and Asian countries
• This distribution is now thought to have been caused by
recent natural selection favoring lactase-persistent
individuals in cultures in which dairy products are available
as a food source.
30

31. Mechanism of Lactose-Induced Diarrhea
and Flatus
Lactase-sufficient
people absorb
>80% of lactose
Lactase-deficient
people absorb
<50% of lactose
6-20 grams malabsorbed
lactose = flatus
(1 g = 44 ml H2)
>20 grams malabsorbed
lactose = flatus+diarrhea
Small
bowel
Colon
Lactose
Glucose
Galactose
Lactose
CO2+H2
SCFA
lactose
glucose
galactose
FLATUS OSMOTIC DIARRHEA
31

32. Glycaemic Index
• The glycemic index or glycaemic index (GI) is a measure
of how quickly blood glucose levels (i.e., blood sugar)
rise after eating a particular type of food.
• Glucose (the defining standard) has a glycemic index of
100
32

33. Classification
Glycaemic
index range
Examples of foods
Low GI 55 or less
beans (white, black, pink, kidney, lentil, soy,
almond, peanut, walnut, chickpea); small seeds
(sunflower, flax, pumpkin, poppy, sesame); most
whole intact grains (durum/spelt/kamut wheat,
millet, oat, rye, rice, barley); most vegetables, most
sweet fruits (peaches, strawberries, mangos);
tagatose; fructose
Medium GI 56–69
not intact whole wheat or enriched wheat, pita
bread, basmati rice, unpeeled boiled potato, grape
juice, raisins, prunes, pumpernickel bread,
cranberry juice, regular ice cream, sucrose, banana
High GI 70 and above
white bread (only wheat endosperm), most white
rice (only rice endosperm), corn flakes, extruded
breakfast cereals, glucose, maltose, maltodextrins,
potato, pretzels, parsnip, bagels
Glycaemic index...
33

34. Absorption of Lipids
• Abnormal to find more than 6 or 7% of ingested lipids still
intact in the feces( Digestibility = 93-94%).
• Most fat absorption takes place in the duodenum or
jejunum – micelles carry monoglycerides and free fatty
acids to the brush border where they diffuse into
enterocytes.
• Cholesterol and other sterols are poorly absorbed.
• Overall, about 50% of dietary cholesterol is absorbed.
• Fiber (especially soluble fiber) decrease cholesterol
absorption 34

35. ABSORPTION…
• Bile salts are absorbed in the ileum (enterohepatic
circulation)
• Short-chain fatty acids(< 12 carbon) and glycerol
are absorbed directly into bloodstream.
• They do not enter the lymph system
• Once in the enterocytes, Larger chain fatty
acids(>=12 carbon), monoglycerides and free fatty
acids are reformed into triglycerides
35

36. ABSORPTION…
• The triglycerides, and cholesterol will be esterfied in to
cholestrol ester and these will be coated with
phospholipids, and protein to form a Lipoprotein called
Chylomicron
• CHYLOMICRONS enter the lymphatic system through the
lacteals
• ChilomicronsVLDLLDLHDL
Lipoprotein lipase lyses contents!
36

37. triglycerides
Absorption of Lipids…
37

38. 38

39. Absorption & Transport
4 basic types of lipoprotein:
1. Chylomicrons – very, very low density (85%
triglyceride); absorbed from small intestine into
lymph & circulated to cells where some of lipid
material is picked off & remnants return to liver
2. VLDL – very low density lipoprotein (50%
triglyceride); made by liver & travels to cells
39

40. Absorption & Transport
3. LDL – low density lipoprotein (50% cholesterol);
remains of VLDL; high levels increase risk of
heart attack
4. HDL – high density lipoprotein (50% protein);
removes cholesterol from blood for return to
liver; high levels decrease risk of heart attack
40

41. Malabsorption due to
Luminal Maldigestion of Fat (Steatorrhea)
:Common causes
Pancreatic insufficiency: Chronic pancreatitis
Bile salt deficiency: Loss of terminal ileum:
loss of bile salts in stool
insufficient bile salts
Bacterial overgrowth: Deconjugation and loss
of bile acids
Gastric hypersecretion: Acid inactivation of
pancreatic enzymes
41

42. 0
20
40
60
80
100
0 20 40 60 80 100
Relationship between Pancreatic
Function and Steatorrhea
Fecal
Fat
(g/day)
Pancreatic Function (%)
This leads to deficiency of
Lipid soluble vitamins
42

43. ABSORPTION OF PROTEINS
• Most protein absorption takes place in the duodenum
and jejunum
• Most amino acids are absorbed into the bloodstream,
but some remain in the enterocytes and are used to
synthesize enzymes and new cells
• >99% of protein enters the bloodstream as amino acids
• Absorption of whole protein can cause a severe allergic
reaction
43

44. ABSORPTION OF PROTEINS…
• In the enterocyte, other peptidases immediately digest
everything into single amino acids which are absorbed
into the bloodstream
• Some amino acids share the same transport system, so
if you take in a large amount of one particular amino
acid, you may be inhibiting the absorption of others
44

45. Free Amino Acid Absorption
• Free amino acids
– Carrier systems
• Basic AA
• Neutral AA
• Acidic AA
• Imino acids
– Entrance of some AA is
via active transport
• Requires energy
Na+ Na+
45

46. Peptide Absorption
• Form in which the majority
of protein is absorbed
• More rapid than
absorption of free amino
acids
• Active transport
– Energy required
• Metabolized into free
amino acids in enterocyte
• Only free amino acids
absorbed into blood
46

47. Absorption of Intact Proteins
• Newborns
– First 24 hours after birth
– Immuno globulins
• Passive immunity
• Adults
– Paracellular routes
• Tight junctions between cells
– Intracellular routes
• Endocytosis
• Pinocytosis
• Of little nutritional significance...
– Affects health (allergies and passive immunity)
47

48. Groff & Gropper, 2000
*Whole proteins are nutritionally insignificant...
Basolateral Membrane
• Transport of
free amino acids
only*
– Peptides are
hydrolyzed
within the
enterocyte
• Transport mainly by
diffusion and
Na-independent
carriers
48

49. Protein Malabsorption
Celiac Sprue I
• Immune-mediated destruction of enterocytes in response
to ingestion of the protein gluten found in wheat and
certain other grains.
• A fraction termed gliadin contains the immunogenic
material (gluten induced eteropathy)
• Small intestinal villi are damaged or destroyed - "flat gut"
appearance.
• Mature digesting and transporting enterocytes are
virtually absent.
49

50. Celiac Sprue - II
• Patchy disease - usually affects proximal intestine more
than distal intestine .
• Mucosal digestion and absorption are both severely
impaired.
• Characteristic antibodies used in diagnosis: IgA
antibodies to tissue transglutaminase or gliadin.
50

51. Clinical Manifestations of Sprue
• Weight loss, often with increased appetite
• Bulky, oily stools – steatorrhea - fat malabsorption
• Flatus/frothy stools – carbohydrate malabsorption
• Anemia – deficiencies of iron, folate
• Osteopenic bone disease – Vitamin D and calcium
malabsorption
• Edema/hypoproteinemia – protein deficiency and
malnutrition
• Cheilosis and glossitis – B vitamin deficiencies
• The only known effective treatment is a lifelong Gluten
Free Diet 51

52. Absorption of
Minerals
(Unifying principles that apply to all minerals)
52

53. Digestion
• Preparing for absorption
• Liberating minerals from a bound state to an aqueous
phase
• Digestive enzymes
• Bile acids and salts that work with digestive enzymes
(e.g., lipases)
53

54. Purpose of digestion to mineral nutrition
• Minerals in a food source are locked within a matrix
composed primarily of proteins, complex
carbohydrates and fats
• The purpose of digestion is to render large composite
molecules into smaller manageable units…minerals are
liberated during this process
• Digestive processes consists mainly of hydrolytic
enzymes that break chemical bonds between modular
units without total destruction (metabolism) of the
liberated components
54

55. Digestive Enzymes (hydrolases)
Enzyme Location Target Action
Pepsin gastric juice proteins breaks peptide bonds
Trypsin and
chymotrypsin
duodenum proteins breaks peptide bonds
Amylases saliva and
duodenum
starch and
glycogen
breaks glycosidic bonds
Lipases duodenum complex
lipids
breaks ester bonds
Glycosidases microvilli di- and tri-
saccharides
breaks glycosidic bonds
Peptidases microvilli small
peptides
breaks peptide bonds
I
II
Phase I is primarily salivary and pancreatic secretions
Phase II involves enzymes on the surface of absorbing cells
55

56. Critical factors in Mineral Absorption
• Absorption tends to be selective for the mineral (makes
finding a unified mechanism more difficult)
• A deficiency increases the fraction of that mineral
absorbed (absorption is tuned to internal bodily needs)
• Certain food chemicals (e.g., phytate, oxalate) lower
absorption by tying up the mineral
• There is competition for absorption machinery
56

57. Critical factors…
• Metal ions antagonism (Cu-Zn; Zn-Fe; etc.) occurs at ion
channels during the transmural passage phase of
absorption
• Vitamin dependency is seen with Vitamin D and C that
regulate body load of Ca+2 and Fe2+respectively
• Absorptive cells excrete factors that aid in the solubility
of metal ions
• Some transport proteins are in vesicles that fuse with the
membrane and move vectorially within the cell
57

58. Steps in mineral absorption
1. Transport through the luminal (apical) cell membrane,
i.e., start of transcellular
2. Handling within the enterocyte, i.e., mediate
transcellular
3. Transport through the antiluminal basolateral membrane
into the circulation, i.e., end of transcellular.
4. Transport between the cells, i.e., paracellular
*Only metals in an aqueous phase can be transported into
the enterocyte
58

59. Two categories of ingested metal Ions
1. Solubility not dependent on pH
2. Solubility pH dependent
Examples: Na+, K+, Mg2+, Ca2+
Examples: Cu2+, Fe2+, Mn2+, Zn2+
Category 2 metal ions are soluble in acid, but form insoluble
hydroxy-polymers at neutral or alkaline pH.
Category 1 metal ions are soluble throughout
the gastrointestinal pH range (1-8)
Solubility and Metal Ion Absorption
59

60. Secretions of Digestion
60

61. Mucosal Side
Serosal Side
Basolateral
Surface
(antiluminal
surface)
Apical
surface
Microvilli
Ca Ca
Ca
Enterocyte
To access the serosal
side, the mineral must
pass either through the
enterocyte (transcellular
99%) or the junction
between enterocytes
(paracellular <1%))
Fe Fe
Fe
A large fraction of the
iron can be trapped
(sequestered) within
the cytosol of the
enterocyte)
61

62. Role of Vesicles in the Regulation of
Mineral Absorption
Vesicles are internal membrane compartments that move between the cytosol
and membranes. This movement is regulated by external factors
Vesicles contain the transport proteins that absorb the mineral into the lumen of
the vesicle and bring it into the cell
Vesicles that have fused with the membrane are positioned to absorb minerals.
Absorption thus depends on the number of vesicles that fused
with the membrane.
Resting Cell Absorbing Cell
62

63. MACROMINERALS
• Monovalent cations, Na+, K+
• Monovalent anions, Cl-
• Divalent cations, Ca2+, Mg2+
• Complexes, HPO4
=, HCO3
-
63

64. Rule 1: Macrominerals in general enter intestinal cells
through transport portals designated for the mineral
(major) or between cells (minor).
Rule 2: The energy for entry is provided by a concentration
gradient across the membrane or by hydrolysis of ATP
(active transport)
Rule 3: Electro-neutrality is sought in the operation of
membrane co-transporters
Rules that apply to the absorption of Macrominerals
64

65. Macrominerals
Na+, K+, Cl-, HPO4
-, Mg2+, Ca2+
• The macro-minerals for the most part rely on diffusion
controlled mechanisms combined with specific channel
proteins to pass into the system.
• Gradients across the membrane can be driven by
unidirectional and bidirectional ATPase enzymes
Example
Na+/K+ ATPase
Ca2+/H+ ATPase
65

66. Properties of Macro-minerals Relative to
Absorption
1. Monovalent ions exist mostly as free ions
2. Monovalent ions are unable to form stable complexes
3. Divalent ions exist partially as free ions
4. Divalent ions are more apt to form complexes with
proteins and organics
5. Complexes exist mainly as free ions
66

67. Absorption of Sodium and Chloride
Na+
Na+
Glucose
Amino acids
H+
Cl-
HCO3
-
Apical (lumen) side
Glucose cotransporter
Amino acid transporter
Na+/H+ antitporter
CO2 CO2
H2CO3
H+
H+ + HCO3
-
H2O
2K+
3Na+
Anion antiporter
Blood
Intestinal Enterocyte
Carbonic anhydrase
ATP
ase
Na+/K+ ATPase
67

68. Calcium and Magnesium
68

69. 50
100
0
0
100 200
Dietary Calcium
Calcium
Absorbed
50
100
0
0
100 200
Dietary Calcium
No Vit D Vit D ( 1,25-(OH)2-D3)
Saturable
Non-Saturable
Duodenum Vitamin D deficient rats
69

70. 0
0
100 200
50
100
0
0
100 200
Saturable
0
0
100 200
Saturable
Non-saturable
Duodenum Jejunum Ileum
Calcium Instilled, mM
Uptake in ileum is by diffusion only; it is, therefore, not regulated
by vitamin D. Thus, most of the Ca2+ is absorbed in the
duodenum.
70

71. • CaT1, a Ca channel protein in the brush border of human
enterocyte, is regulated by 1,25-dihydroxyvitamin D.
• The vitamin appears to mediate changes in CaT1-mRNA
levels. CaT1, therefore, could be the primary gatekeeper
regulating homeostatic modulation of intestinal calcium
absorption efficiency.
• Our best understanding is that calcium enters the
duodenal cell through calcium channels which may
contain a vitamin D responsive Ca-binding component.
Entry is down an electrochemical gradient( Bonner, 1999)
71

72. Ca2+
Ca2+
Calbindin
ATP
ase
ATP
ase
Ca2+
CAT1
Mg2+
(Na+)
Ca2+
Calcium ATPase
Enterocyte
Paracellular
Ca2+
Ca bound to
fiber, phytate,
oxalate, fatty
acids
Calcium ATPase
antiporter
Lumen Blood
Ca2+
Calcium Absorption
Albumin
Vitamin D
responsive
CAT1 is a Ca2+ channel protein located in the brush border of mucosal cells
Calbindin is a small (9 kD) protein in the cytosol of mucosal cells
72

73. Magnesium
1. Absorption depends on concentration
2. Absorption is saturable and non-saturable (7-10%)
3. Fully saturable in ileum but not jejunum (contrast with calcium)
4. Absorption in the colon significant
Human Study
Fed Fractional Absorption
7 mg 65-75%
36 mg 11-14%
73

74. Magnesium
Enterocyte
Mg2+
TRPM6
Distal jejunum and ileum
Cation channel protein
(transient receptor
protein TRP)
Mg2+
Since TRPM6 operates by
diffusion without co-
transporters, Mg2+
absorption efficiency
depends on the amount
of Mg2+ in the diet and
within the cell
ATP
ase
ATP
ADP
Mg2+
Mg2+ -bound to
phytate, fiber,
fatty acids
74

75. Microminerals
Because of their very low cellular concentrations, the
micronutrients rely on specific high affinity transporters
and binding proteins for movement. Some collect in
vesicles and use the vesicle as the transport factor.
Fe2+, Cu2+, Mn2+,
Zn2+
Redox-sensitive metals (Fe2+/Fe3+, Cu+/Cu2+) rely on
valence state changes to be sequestered or transported
from the cell.
75

76. Metals such as Fe3+ and Zn2+ are more soluble in
acid solutions due to a shift in the equilibrium
towards the free ion
Fe(OH)3(s) Fe3+(aq) + 3OH-(aq)
H+
Zn(OH)2(s) Zn2+(aq) + 2OH-(aq)
Pulls equilibria
pH
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Solubility Fe(OH)3 solubility
Zn(OH)2 solubility
76

77. Elements of Micromineral Absorption
• Insolubility or iron and zinc is partially overcome by
mucins secreted from the cells
• Fe3+ and Cu+ can engage their respective transporters
• Cytosolic sequestering and regulatory factors have the
potential to lock the mineral within the cell and block its
release
• Internal movement of Zn2+, Cu+ and Fe3+ is primarily via
vesicles
• Basolateral surface release is redox sensitive for Fe and Cu
77

78. Iron regulation
78

79. Iron metabolism
79

80. • Mucins are complex polysaccharides secreted into
the lumen that assist in stabilizing the solubility of
metal ions
• Mucins prevent alkaline-induced polymerization of
category 2 metal ions and make the metal ion available
to transporters on the enterocyte surface
Mucins
80

81. Mineral Bioavailability
81

82. Methods of
Assessing
Bioavailability
82

83. 1. Depletion-Repletion Techniques
• In the “Sheffield experiment” conducted in the 1940s, in
which male participants were conscientious objectors to
military service, 16 (2 women and 14 men) subjects
consumed a vitamin A–deficient diet and 7 control
subjects (1 woman and 6 men) consumed the same diet
but with additional supplements of retinol or B-carotene
from various sources for periods ranging from 8.5 to 25
mo.
83

84. Depletion-Repletion…
• Dark adaptation was used as a functional indicator of
bioefficacy.
• In the control group, either 750 µg retinol/d (n = 2) or
3000 µg B-carotene/d in oil (n = 4) or in margarine (n =
2) prevented impaired dark adaptation.
• In the 16 subjects fed the deficient diet, only 3 men,
who also had the lowest plasma retinol concentrations,
developed impaired dark adaptation (depletion phase
of the study).
84

85. Depletion-repletion…
• Either 390 µg retinol/d (n = 1) or 1500µg B-carotene/d
in oil (n = 2) was required to reverse this impaired dark
adaptation (repletion phase).
• Other depletion-repletion studies conducted by Booher
et al (15; n = 5) and by Wagner (16; n = 10) shortly before
World War II reached similar conclusions.
• Whereas in the 1970s Sauberlich et al (17; n = 8)
concluded that, on a weight basis, only twice as much B-
carotene as retinol was required to reverse impaired
dark adaptation.
85

86. 2. Balance studies
• In oral-fecal balance techniques, the difference between
the amount of -carotene in food consumed and that
excreted in feces is assumed to represent the amount of
nutrient absorbed(eg.B-cartotene).
• With these techniques, gastric or bacterial degradation
of unabsorbed carotenoids may contribute to
overestimation of absorption.
• Rao and Rao found indications that such bacterial
degradation occurs .
86

87. Balance study…
• On the other hand, endogenously secreted betacarotene
might be excreted in feces, thus leading to an
underestimate of bioavailability.
• Thus, data on the bioavailability of carotenoids obtained
with balance techniques should be interpreted with care.
87

88. 3. Radioisotopes
• Nearly all of the early work characterizing body iron pools
and iron bioavailability was undertaken using iron
radioisotopes specifically 55Fe and 59Fe
• Radioactive isotopes have some advantages in human
studies in that they
– require a very small, essentially ‘mass free’,
– amount of iron label and the instrumentation needed
for measurement of iron radioisotopes in biological
samples is found in many research facilities.
– An additional advantage of the gamma emitting 59Fe
radioactive isotope is that whole body counting can be
used to measure the fraction of iron isotope that is
retained in the body after ingestion. 88

89. Radioisotopes…
• Disadvantages to the use of radioisotopes include
the fact that they have finite half-lives.
– This may introduce time limitations for shipping isotopes to
isolated settings, storing isotopes before the study is
implemented
– and collecting and analyzing the biological samples post-dosing
• Typical radiation exposures from radioactive iron
isotopes
89

90. 4. Stable Isotopes
• The potential risk and increased reluctance to use
radioisotopes has stimulated the use of stable isotopes.
• Thus, in recent years, stable isotopes and compounds
labeled with stable isotopes have become increasingly
available.
• Basically, the methodology to assess iron bioavailability
involves the administration of single or multiple oral
stable iron isotope(s).
• Iron absorption is then estimated using one of three
approaches
90

91. • A metabolic balance study can be undertaken to recover
the amount of oral stable iron isotope(s) excreted in
faeces (faecal recovery method).
• (b)A plasma sample can be obtained several hours post-
dosing to assess plasma appearance kinetics (plasma
appearance method).
• (c)A blood sample can be collected 2 weeks post-dosing
to assess the amount of stable iron isotope(s)
incorporated into RBCs (erythrocyte iron incorporation
method). 91

92. • Administration of stable iron isotopes does not have any
known adverse risks at the doses typically used in human
studies.
• All three minor abundance stable iron isotopes can be
given to the same subject to compare bioavailability of
various foods or to allow for simultaneous administration
of oral and intravenous isotopes.
• Multiple stable isotopes of different minerals can also be
given to the same individual to examine interactions of
iron with other nutrients such as calcium or zinc
92

93. • Stable-isotope-tracer techniques were successfully
developed for studying the metabolism of minerals, such
as iron, magnesium, and zinc, and of vitamins and their
precursors, such as folate and provitamin A carotenoids.
• Because of the limited availability of organic nutrients
labeled with stable isotopes and the difficulty in their
quantification, few studies have been carried out so far.
93

94. 5. Algorithm Method
• The algorithm may be used to translate data from dietary
surveys into amounts of a nutrient expected to be
absorbed.
• The main requirement for such calculations is that
detailed information is available about the meal
composition and its variation over a representative and
sufficiently long period of time.
• A 7-d record, for example, may not represent the iron
absorption from the habitual diet.
94

95. • An important use of the algorithm would be to translate
physiologic requirements into dietary requirements
under different dietary conditions known to prevail in a
certain population.
• In the FAO recommendations, 3 levels of bioavailability
(5%, 10%, and 15%) were used arbitrarily for this
translation.
• The validity of choices of representative bioavailability
values can be tested by using the algorithm.
95

96. • The algorithm may be useful in the future search for
realistic recommendations to be used in food based
strategies to improve iron nutrition in developing
countries.
• Challenges
– Estimation of Phytate
– Estimation of vitamin C
96

97. 97

98. 98

99. Thank you!
99