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Potential of the rat as a model for predicting iron bioavailability for humans
NUTRITION RESEARCH, Vol. 4, pp. 913-922, 1984 0271-5317/84 $3.00 + .00 Printed in the USA. Copyright (c) 1984 Pergamon Press Ltd. All rights reserved.
POTENTIAL OF THE RAT AS A MODEL FOR PREDICTING IRON BIOAVAILABILITY FOR HUMANS
Arthur W. Mahoney and Deloy G. Hendricks
Department of Nutrition and Food Sciences - 87 Utah State University Logan, Utah 84322
ABSTRACT The absorption of iron by iron deficient and normal rats and humans was compared. Qualitatively, rats and humans respond similarly to many dietary and physiological factors known to affect iron u t i l i z a t i o n . I t was found that iron absorption by rats was highly correlated (r=0.94) with that of humans. The average human response was 68 percent that by the rats. KEY WORDS: Iron b i o a v a i l a b i l i t y , rat, man INTRODUCTION Bioavailability of a nutrient or compound implies attention to that fraction of the amount consumed that is metabolizable. Determining b i o a v a i l a b i l i t y of iron thus requires quantitating the total amount of metabolizable iron present in the source. Because the amount of iron absorbed by people and animals varies d i r e c t l y with their iron status, b i o a v a i l a b i l i t y values are confounded to an unknown extent by the characteristics of the food and the physiological characteristics of the model subject. In animal models this confusion can be minimized by using an iron deficient subject given quantities of iron below its known capacity for absorption. The amount of iron absorbed is thus considered to reflect that which is metabolizable in the source consumed. Traditionally, rats have been used to study the b i o a v a i l a b i l i t y of iron supplements (I-6) and the effects of processing procedures on b i o a v a i l a b i l i t y (7,8). Most of this research has been directed towards evaluating the b i o a v a i l a b i l i t y of unknown sources relative to that of a reference iron source (1,2,6). On the other hand, in human studies the interest has been to quantitate the bioavailable iron in the diet or individual meals (9-11). Hallberg ( I I ) recently reviewed much of the methodology of and data from human studies of iron b i o a v a i l a b i l i t y . Bing (12) and Mahoney and Hendricks (13) have reviewed rat assays for quantitating bioavailable iron. Here, we present evidence summarized from published data that human iron absorption is highly correlated with that of the rat when the tested representatives of both species are of similar iron status. 913
914
A.W. MAHONEYand D.G. HENDRICKS METHODS
The iron b i o a v a i l a b i l i t y l i t e r a t u r e was scanned for iron u t i l i z a t i o n data that could be used to compare rat and human responses in two ways. In the f i r s t comparison, representative l i t e r a t u r e were selected showing the q u a l i t a t i v e responses of rats and man to various dietary and physiological factors affecting t h e i r iron u t i l i z a t i o n (Table I ) . In the second comparison, l i t e r a t u r e were selected which could be used to q u a n t i t a t i v e l y compare iron u t i l i z a t i o n by rats and humans. For l i t e r a t u r e to be accepted into this part of the study, two c r i t e r i a were chosen. The iron sources had to be s i m i l a r , and data r e l a t i v e to a p a r t i c u l a r iron source had to be available from at least two published reports in which rats and/or humans of similar iron status were used. Data for three iron sources met these c r i t e r i a (Table 2). The values for rat and human data were averaged by species. The averages were subjected to linear regression analysis (14). RESULTS AND DISCUSSION Rats and humans respond q u a l i t a t i v e l y in a similar way to many dietary and physiologic factors known to affect iron b i o a v a i l a b i l i t y (Table i ) . The responses of these species to ascorbic acid and heme iron are of p a r t i c u l a r interest. Even though the rat does not require a dietary source of ascorbic acid, i t responds to dietary ascorbic acid by increasing nonheme iron u t i l i z a t i o n as does the human being. The absorption of heme iron is not affected by ascorbic acid in either species. Chelators of inorganic iron do not affect heme iron absorption by rats nor man, but they do decrease inorganic iron absorption by both species. Heme and inorganic iron absorption are increased in iron deficient subjects of both species. We believe that these q u a l i t a t i v e s i m i l a r i t i e s between the iron u t i l i z a t i o n responses of rats and man are good evidence that the rat may have important potential for predicting human iron b i o a v a i l a b i l i t y . Clearly, both rats and people increase t h e i r iron u t i l i z a t i o n from both ferrous sulfate and hemoglobin when they are iron deficient (Table 2). The s i m i l a r i t y of the averages of iron u t i l i z a t i o n by the people and rats is remarkable when considering that the data r e f l e c t many differences in methodology for determining percent iron u t i l i z a t i o n . Accepting these differences in methodology, i t is s t r i k i n g that such a high correlation ( r = 0.94) was found for percent iron u t i l i z a t i o n between the animal and human studies. I f the meat data, for which rat data from only one laboratory are presented, are not included in the analysis, the correlation (r : 0.99) is very high between the human and rat responses. When all data are included in the analysis, the human response is 68 percent that of the rat. When the meat data are excluded, the slope is 80 percent that of the rat. These correlations are similar to that (r : 0.98) found by Bressani (68) between nitrogen balance index in children and nitrogen growth index in rats. Although these p a r t i c u l a r prediction equations are based on too few data to have practical value, the data do strongly support the potential for using the rat model to estimate the b i o a v a i l a b i l i t y of iron in human dietaries. A concerted research e f f o r t w i l l be required to evaluate this p o s s i b i l i t y and calibrate the model.
IRON BIOAVAILABILITY
Table I.
915
COMPARISONOF EFFECTS OF VARIOUS DIETARY AND PHYSIOLOGICAL FACTORS ON IRON ABSORPTIONBY RATS AND HUMANS
Factor
Human
Rat
Vitamin C on nonheme Fe Increase Increase Increase Increase Increase
Vitamin C on Heme Fe
Cysteine Cysteine Cysteine Histidine Histidine Histidine
No change No change No change
Increase
Martinez-Torres & Layrisse (23) Layrisse et al. (24) Van Campen (25)
Increase Increase No change Increase Increase
Calcium & phosphate Calcium & phosphate Calcium or phosphate
Decrease
Increase
Blood donations Phlebotomy Phlebotomy
Increase Increase
Nonheme Fe absorption is inverse of Fe stores
Yes Yes Yes Yes
Yes Yes Yes
Bjorn-Rasmussen & Hallberg (27) Cook & Monsen (28) Shah et al. (29) Monsen & Cook (30) Chapman & Campbell (31) Monsen & Cook (30)
Decrease No change Decrease Decrease
Mahoney & Hendricks (32) Chapman & Campbell (31) Barton et al. (33) Hegsted et al. (34)
Increase
Magnusson, et al. (36) Olsson et al. (37) Whittaker (35) Pirzio-Beroli & Finch (38) Magnusson et al. (36) Kuhn et al. (39) Turnbull et al. (21)
Yes Yes Heme Fe absorption is inverse of Fe stores
Van Campen (25) Van Campen & Gross (26) Layrisse et al. (24)
Decrease No change
Phosphates Phosphate Calcium
Hallberg et al. (15) Sayers et al. (16) Morris (17) Miller & McNeal (12) Monsen & Page (19) Callender et al. (20) Turnbull et al (21) Bannerman (22)
Increase Increase
Meat Meat Meat
Reference
Bannerman et al. (40) Whittaker (35) Turnbull et al. (21) Conrad et al. (41) Gabbe et al. (42)
916 Table 1.
A.W. MAHONEYand D.G. HENDRICKS (continued)
Rat
Reference
Heme Fe absorption is inverse of Fe stores (continued)
Yes Yes Yes
Wheby et al. (43) Weintraub et al. ( 4 4 ) Bannerman (22)
Chelators on Nonheme Fe Decreased
Decreased
Well known
No change No change No change
Turnbull et al. (45) Hallberg & Solvell (43) Wheby et al. (46) Bonnerman & Malpas (46)
Factor
Chelators on Heme Fe
Inorganic Fe absorption Decreases with age
Human
No change
True False (However, decreased erxthropoi esis is suggested)
Jacobs & Owen (47) Marx (48)
True (However Yeh et al. (49) erythropoiesis is normal)
Percent Fe absorption decreases with Fe dose
True
True
Well known
Decrease
Cook et al. (50) Morck et al. (51) Welch & Van Campen (52)
Soy products on nonheme Decrease Decrease Fe absorption Tea
Decrease Decrease Decrease
Coffee
Decrease No change
Pectin
Decrease No change
Monnier et al. (55) Cook et al. (56) No change
Reduced gastric acidity
Rossander et al. (53) Morck et al. (54) Zhang, Hendricks, Mahoney (unpublished data) Morck et al (54) Zhang, Hendricks, Mahoney (unpublished data)
Decrease Decrease
Baig et al. (57) Cook et al. (58) Skikne et al. (59)
Decrease Decrease
Murray & Stein (60) Mahoney & Hendricks (32)
aNo attempt has been made to provide an "exhaustive" literature search but to provide "i I I ustrati ve" references.
IRON BIOAVAILABILITY TABLE 2.
Percent absorption of iron by normal and iron d e f i c i e n t ~eople and r a t s given ferrous s u l f a t e , meat or hemoglobin i r o n .
Normal FeS04
Iron d e f i c i e n t Hb
..............
FeS04
Meat
References
46,48,52
. . . . . .
Hoglund & Reizenstein (61)
67
. . . . . .
Marx (48)
20,26
---
---
14.4
25
I0
---
22.1
4.1
8.8
55.1
4.0
6.9
---
4.3
. . . . . .
20.3
Gabbe et al (42)
22
Callender e t al (20)
35.1
Conrad et al (41)
---
15.6
Turnbull et al (21)
54.6
---
21.3
Hallberg & S o l v e l l (45)
46.7
---
15.9
Hallberg & S o l v e l l (45)
30
21
B e u t l e r (62)
32.82
---
Marinez-Torres & Layrisse (63)
3T.4
~
Averages Cy)
58
---
. . . . . .
12
---
. . . . . . . . . 13.8
H__b_b
People . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .
---
917
~
T.6
..............
Rats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .
52
. . . . . .
65,58
---
. . . . . .
58
. . . . . .
Miller
4.8
83.9
---
38
Bannerman(22)
19
2.0
57
---
20
Wheby et al (43)
11.2
2.1
30.0
---
---
2.5
---
3.0
6.5
~
34,53,61,46,49
Mahoney & Hendricks (13)
19,20,25
Park et al (64) (65)
3.0
Weintraub et al (44)
. . . . . .
5.8
R a f f i n et al (66)
. . . . . .
22.7
Amine & Hegsted (67)
~
Averages (x)
T.7
~
IThese data have been taken in part from Park et al (64). The r e l a t i o n s h i p between the human (y) and r a t (x) responses may be presented as: y= 0.68 x+ 7.27; r= 0.94. 2Calculated from t h e i r (23) t a b l e I f o r subjects having hemoglobin concentrations less than 10 g/100 ml.
918
A.W. F~AHONEYand D.G. HENDRICKS ACKNOWLEDGEMENTS
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Accepted for publication
July 16, 1984
Effect of dietary pectin J. Nutr. 113:2389, 1983.
POTENTIAL OF THE RAT AS A MODEL FOR PREDICTING IRON BIOAVAILABILITY FOR HUMANS
Arthur W. Mahoney and Deloy G. Hendricks
Department of Nutrition and Food Sciences - 87 Utah State University Logan, Utah 84322
ABSTRACT The absorption of iron by iron deficient and normal rats and humans was compared. Qualitatively, rats and humans respond similarly to many dietary and physiological factors known to affect iron u t i l i z a t i o n . I t was found that iron absorption by rats was highly correlated (r=0.94) with that of humans. The average human response was 68 percent that by the rats. KEY WORDS: Iron b i o a v a i l a b i l i t y , rat, man INTRODUCTION Bioavailability of a nutrient or compound implies attention to that fraction of the amount consumed that is metabolizable. Determining b i o a v a i l a b i l i t y of iron thus requires quantitating the total amount of metabolizable iron present in the source. Because the amount of iron absorbed by people and animals varies d i r e c t l y with their iron status, b i o a v a i l a b i l i t y values are confounded to an unknown extent by the characteristics of the food and the physiological characteristics of the model subject. In animal models this confusion can be minimized by using an iron deficient subject given quantities of iron below its known capacity for absorption. The amount of iron absorbed is thus considered to reflect that which is metabolizable in the source consumed. Traditionally, rats have been used to study the b i o a v a i l a b i l i t y of iron supplements (I-6) and the effects of processing procedures on b i o a v a i l a b i l i t y (7,8). Most of this research has been directed towards evaluating the b i o a v a i l a b i l i t y of unknown sources relative to that of a reference iron source (1,2,6). On the other hand, in human studies the interest has been to quantitate the bioavailable iron in the diet or individual meals (9-11). Hallberg ( I I ) recently reviewed much of the methodology of and data from human studies of iron b i o a v a i l a b i l i t y . Bing (12) and Mahoney and Hendricks (13) have reviewed rat assays for quantitating bioavailable iron. Here, we present evidence summarized from published data that human iron absorption is highly correlated with that of the rat when the tested representatives of both species are of similar iron status. 913
914
A.W. MAHONEYand D.G. HENDRICKS METHODS
The iron b i o a v a i l a b i l i t y l i t e r a t u r e was scanned for iron u t i l i z a t i o n data that could be used to compare rat and human responses in two ways. In the f i r s t comparison, representative l i t e r a t u r e were selected showing the q u a l i t a t i v e responses of rats and man to various dietary and physiological factors affecting t h e i r iron u t i l i z a t i o n (Table I ) . In the second comparison, l i t e r a t u r e were selected which could be used to q u a n t i t a t i v e l y compare iron u t i l i z a t i o n by rats and humans. For l i t e r a t u r e to be accepted into this part of the study, two c r i t e r i a were chosen. The iron sources had to be s i m i l a r , and data r e l a t i v e to a p a r t i c u l a r iron source had to be available from at least two published reports in which rats and/or humans of similar iron status were used. Data for three iron sources met these c r i t e r i a (Table 2). The values for rat and human data were averaged by species. The averages were subjected to linear regression analysis (14). RESULTS AND DISCUSSION Rats and humans respond q u a l i t a t i v e l y in a similar way to many dietary and physiologic factors known to affect iron b i o a v a i l a b i l i t y (Table i ) . The responses of these species to ascorbic acid and heme iron are of p a r t i c u l a r interest. Even though the rat does not require a dietary source of ascorbic acid, i t responds to dietary ascorbic acid by increasing nonheme iron u t i l i z a t i o n as does the human being. The absorption of heme iron is not affected by ascorbic acid in either species. Chelators of inorganic iron do not affect heme iron absorption by rats nor man, but they do decrease inorganic iron absorption by both species. Heme and inorganic iron absorption are increased in iron deficient subjects of both species. We believe that these q u a l i t a t i v e s i m i l a r i t i e s between the iron u t i l i z a t i o n responses of rats and man are good evidence that the rat may have important potential for predicting human iron b i o a v a i l a b i l i t y . Clearly, both rats and people increase t h e i r iron u t i l i z a t i o n from both ferrous sulfate and hemoglobin when they are iron deficient (Table 2). The s i m i l a r i t y of the averages of iron u t i l i z a t i o n by the people and rats is remarkable when considering that the data r e f l e c t many differences in methodology for determining percent iron u t i l i z a t i o n . Accepting these differences in methodology, i t is s t r i k i n g that such a high correlation ( r = 0.94) was found for percent iron u t i l i z a t i o n between the animal and human studies. I f the meat data, for which rat data from only one laboratory are presented, are not included in the analysis, the correlation (r : 0.99) is very high between the human and rat responses. When all data are included in the analysis, the human response is 68 percent that of the rat. When the meat data are excluded, the slope is 80 percent that of the rat. These correlations are similar to that (r : 0.98) found by Bressani (68) between nitrogen balance index in children and nitrogen growth index in rats. Although these p a r t i c u l a r prediction equations are based on too few data to have practical value, the data do strongly support the potential for using the rat model to estimate the b i o a v a i l a b i l i t y of iron in human dietaries. A concerted research e f f o r t w i l l be required to evaluate this p o s s i b i l i t y and calibrate the model.
IRON BIOAVAILABILITY
Table I.
915
COMPARISONOF EFFECTS OF VARIOUS DIETARY AND PHYSIOLOGICAL FACTORS ON IRON ABSORPTIONBY RATS AND HUMANS
Factor
Human
Rat
Vitamin C on nonheme Fe Increase Increase Increase Increase Increase
Vitamin C on Heme Fe
Cysteine Cysteine Cysteine Histidine Histidine Histidine
No change No change No change
Increase
Martinez-Torres & Layrisse (23) Layrisse et al. (24) Van Campen (25)
Increase Increase No change Increase Increase
Calcium & phosphate Calcium & phosphate Calcium or phosphate
Decrease
Increase
Blood donations Phlebotomy Phlebotomy
Increase Increase
Nonheme Fe absorption is inverse of Fe stores
Yes Yes Yes Yes
Yes Yes Yes
Bjorn-Rasmussen & Hallberg (27) Cook & Monsen (28) Shah et al. (29) Monsen & Cook (30) Chapman & Campbell (31) Monsen & Cook (30)
Decrease No change Decrease Decrease
Mahoney & Hendricks (32) Chapman & Campbell (31) Barton et al. (33) Hegsted et al. (34)
Increase
Magnusson, et al. (36) Olsson et al. (37) Whittaker (35) Pirzio-Beroli & Finch (38) Magnusson et al. (36) Kuhn et al. (39) Turnbull et al. (21)
Yes Yes Heme Fe absorption is inverse of Fe stores
Van Campen (25) Van Campen & Gross (26) Layrisse et al. (24)
Decrease No change
Phosphates Phosphate Calcium
Hallberg et al. (15) Sayers et al. (16) Morris (17) Miller & McNeal (12) Monsen & Page (19) Callender et al. (20) Turnbull et al (21) Bannerman (22)
Increase Increase
Meat Meat Meat
Reference
Bannerman et al. (40) Whittaker (35) Turnbull et al. (21) Conrad et al. (41) Gabbe et al. (42)
916 Table 1.
A.W. MAHONEYand D.G. HENDRICKS (continued)
Rat
Reference
Heme Fe absorption is inverse of Fe stores (continued)
Yes Yes Yes
Wheby et al. (43) Weintraub et al. ( 4 4 ) Bannerman (22)
Chelators on Nonheme Fe Decreased
Decreased
Well known
No change No change No change
Turnbull et al. (45) Hallberg & Solvell (43) Wheby et al. (46) Bonnerman & Malpas (46)
Factor
Chelators on Heme Fe
Inorganic Fe absorption Decreases with age
Human
No change
True False (However, decreased erxthropoi esis is suggested)
Jacobs & Owen (47) Marx (48)
True (However Yeh et al. (49) erythropoiesis is normal)
Percent Fe absorption decreases with Fe dose
True
True
Well known
Decrease
Cook et al. (50) Morck et al. (51) Welch & Van Campen (52)
Soy products on nonheme Decrease Decrease Fe absorption Tea
Decrease Decrease Decrease
Coffee
Decrease No change
Pectin
Decrease No change
Monnier et al. (55) Cook et al. (56) No change
Reduced gastric acidity
Rossander et al. (53) Morck et al. (54) Zhang, Hendricks, Mahoney (unpublished data) Morck et al (54) Zhang, Hendricks, Mahoney (unpublished data)
Decrease Decrease
Baig et al. (57) Cook et al. (58) Skikne et al. (59)
Decrease Decrease
Murray & Stein (60) Mahoney & Hendricks (32)
aNo attempt has been made to provide an "exhaustive" literature search but to provide "i I I ustrati ve" references.
IRON BIOAVAILABILITY TABLE 2.
Percent absorption of iron by normal and iron d e f i c i e n t ~eople and r a t s given ferrous s u l f a t e , meat or hemoglobin i r o n .
Normal FeS04
Iron d e f i c i e n t Hb
..............
FeS04
Meat
References
46,48,52
. . . . . .
Hoglund & Reizenstein (61)
67
. . . . . .
Marx (48)
20,26
---
---
14.4
25
I0
---
22.1
4.1
8.8
55.1
4.0
6.9
---
4.3
. . . . . .
20.3
Gabbe et al (42)
22
Callender e t al (20)
35.1
Conrad et al (41)
---
15.6
Turnbull et al (21)
54.6
---
21.3
Hallberg & S o l v e l l (45)
46.7
---
15.9
Hallberg & S o l v e l l (45)
30
21
B e u t l e r (62)
32.82
---
Marinez-Torres & Layrisse (63)
3T.4
~
Averages Cy)
58
---
. . . . . .
12
---
. . . . . . . . . 13.8
H__b_b
People . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .
---
917
~
T.6
..............
Rats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .
52
. . . . . .
65,58
---
. . . . . .
58
. . . . . .
Miller
4.8
83.9
---
38
Bannerman(22)
19
2.0
57
---
20
Wheby et al (43)
11.2
2.1
30.0
---
---
2.5
---
3.0
6.5
~
34,53,61,46,49
Mahoney & Hendricks (13)
19,20,25
Park et al (64) (65)
3.0
Weintraub et al (44)
. . . . . .
5.8
R a f f i n et al (66)
. . . . . .
22.7
Amine & Hegsted (67)
~
Averages (x)
T.7
~
IThese data have been taken in part from Park et al (64). The r e l a t i o n s h i p between the human (y) and r a t (x) responses may be presented as: y= 0.68 x+ 7.27; r= 0.94. 2Calculated from t h e i r (23) t a b l e I f o r subjects having hemoglobin concentrations less than 10 g/100 ml.
918
A.W. F~AHONEYand D.G. HENDRICKS ACKNOWLEDGEMENTS
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Accepted for publication
July 16, 1984
Effect of dietary pectin J. Nutr. 113:2389, 1983.