perspectives in nutrition

1 34 The American Journal of Clinical Nutrition 31 : JANUARY 1978, pp. 134-141 . Printed in U.S.A.

Estimation of available dietary 2Elaine R. Ph . D. , 4 M.D. , Miguel is5 M.D.,D. Mark 6 Ph .D. , James D. 7 M.D. , Walter 8 M.D.,and ClementA. Finch,3 M.D.

ABSTRACT Dietary iron requirements are dependent on the amount and availability offood iron ingested. On the basis of recent studies of food iron absorption employing extrinsictag techniques, the availability of heme iron has been defined and estimates of the availabilityof nonheme iron based on the amounts of enhancing substances appear possible . A model hasbeen developed whereby the availability of iron in a given meal may be estimated. Calculationsare made on a meal basis of 1 ) the amount of heme iron and its availability, and 2) the amountOf nonheme iron and its availability as influenced by the meals content of enhancing factors.Examples of these calculations are provided. Am. J. Clin. Nutr. 31 : 134-141 , 1978.

It is recognized that frank and occult irondeficiency is a relatively common problemin the United States. Considerations of re-quirements have focused on those of theadult woman since she represents the popu-lation consuming a general diet who is atgreatest risk. The amount of iron a womanmust absorb varies greatly and is dependenton the amount of iron lost during menstrua-tion (1-3). It is estimated that some 1 .8 mgof iron must be absorbed to meet the needsof 80 to 90% of women (4). The currentRecommended Dietary Allowance for foodiron intake was thus established at 18 mg/day, assuming an absorption of 10% (5).However, this amount can rarely beachieved with the ordinary foods available.Furthermore, these estimates were made ata time when there was limited informationconcerning the availability of food iron.

Accumulating evidence demonstrates thatthe amount of iron potentially availablefrom foods depends not only upon theamount of iron supplied but the nature ofthat iron and the composition of the mealwith which it is consumed (6-8). The totaliron content of the diet is thus a relativelypoor indicator of the adequacy of the dietwith regards to iron. Although numerous

questions remain to be answered about ironavailability and iron needs, sufficient infor-mation is now at hand so that better esti-mates of iron need in relation to diet can bemade, and such information should be usedin the development of diets and in makingdietary recommendations.

Food iron absorption

The availability of food iron is best dis-cussed under the headings of heme andnonheme iron , since considerable experi-

1 Supported in part by a grant from the UnitedStates Department of Agriculture.

2 Address reprint requests to: Dr. Elaine R. Mon-sen, University of Washington, Raitt Hall DL-10,Seattle, Washington, 98195.

3 Division of Human Nutrition, Dietetics andFoods; and Division of Hematology, Department ofMedicine , University of Washington , Seattle , Washing-ton 98195. Department of Medicine, Universityof Goteborg, Goteborg, Sweden. Department ofExperimental Medicine, Instituto Venezolano de In-vestigaciones Cientificas (I .V .1 .C .), Caracas, Venezu-ela. Department of Nutrition, Harvard School ofPublic Health, Boston, Massachusetts 021 15 . Di-vision of Hematology , Department of Medicine , Uni-versity of Kansas Medical Center, Kansas City, Kansas66103. Vitamin and Mineral Nutrition Labora-tory, U. S. Department of Agriculture, Beltsville,Maryland.

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FIG. 1 . The percent absorption of nonheme ironby individuals with no body iron stores, 250, 500, and1000 mg body iron stores is shown as influenced bythe availability of nonheme iron in a given meal.

mental data indicates that each of theseforms a different pool of iron (6 , 9) . Hemeiron which comes from hemoglobin andmyoglobin, is absorbed directly as the intactiron porphyrin complex , and its iron is freedin the intestinal mucosal cell (10-13). Theproportion of heme iron which humans usu-ally absorb from their diet is high in compar-

ESTIMATION OF AVAILABLE DIETARY IRON 135

TABLE 1Factors for estimating percent absorption of dietary iron at increased levels ofiron status (indicated by the quantity of iron stores)

WomenMen

Iron stores

mg

I. Hemeiron 0 250 500 1,000II. Nonhemeiron 35% 28 23 15

A. Low availability meal 5 4 3 21 . Meat, poultry, or fish

75 mgor

3. Meat, poultry, or fish30-90 gplus ascorbic acid 25-75 mg

a The factors for 500 mg iron stores are suggested for most dietary calculations. b These factors are

approximate values based on a semi-logarithmic relationship between iron stores (the linear function) and hemeiron absorption (the logarithmic function).

low medimAVAILABILITY OF NON HEME IRON

ison with nonheme iron (14, 15). An mdi-vidual who has no iron stores may be ex-pected to absorb approximately 35% ofheme iron when ingested as meat, while anindividual with an adequate iron store of500 mg would be expected to absorb ap-proximately 25% . Analysis of a variety offoods has shown that approximately 30 to40% of the iron in pork, liver, and fish and50 to 60% of the iron in beef, lamb, and

high chicken is in the form of heme (16).The nonheme iron pool consists of iron

from other foods such as vegetables, grains,fruits, eggs, and dietary products as well asfrom the nonheme iron of meats, poultry,and fish and from soluble iron supplements.Although numerous isotopic studies haveshown virtually complete mixing of the non-heme iron from all foods in a complex meal(9 , 1 7 , 1 8) , certain forms of dietary ironmay not be so completely available . Only aportion of the iron of ferritin and hemosid-em appears to enter this pool, the remain-der being unavailable (19). The availability

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Food Wt Total.iron

Hemefactor

Hemeiron

Nonhemeiron

Ascorbic acid(as served)

g mg ,pig

85 2.7 0.478 0.420 0.115 0.120 0.235 0.310 0

128 0.463 1.0

5.2

% absorbable ironAbsorbable iron (mg)

total absorbable iron (mg)

120 0.929 0.6 0.480 1.514 0.265 1.440 0.6

5.2

% absorbable ironAbsorbable iron (mg)Total absorbable iron

95 2.698 0.578 0.914 055 0.1

8 0.58 0.4

226 0.15.1

% absorbable ironAbsorbable iron (mg)

total absorbable iron

23% 8%0.25 0.33

0.58mg

0.9 Trace0.2 0.4 0

1.5 100.2 11.4 00.6 Trace

0.2 5.0 11

23% 5%0.05 0.24

0.29mg

2.6 00.5 00.9 10 00.1 10.5 00.4 Trace0.1 25.1 4

3%0.15

0.15 mg

136 MONSEN ET AL.

of iron in contaminating dirt may vary de-pending on the chemical properties of thedirt iron . Insufficient information is avail-able to make any quantitative predictionsof the absorption of such iron which seemsto form only a minor part of the dietary

TABLE 2Examples of method to calculate absorbable iron from single meals

I. Meat, poultry, fish-containing-high availability(26 g protein; 650 kcalories)

Beef-vegetable stewBeef, lean, raw, 3 ouncePotatoes, V2 cupCarrots, 2 tablespoonOnions, 2 tablespoon

Green pepper, raw, 2 slicesBreadstick, 2 mediumMargarine , 2 teaspoonPeaches, canned, V2 cupGingerbreadTotals

Ascorbic acid 46 mgMeat, fish, poultry 3 ounces, raw

II. Meat, poultry, fish-containing- medium availa-bility (27 g protein; 700 kcal)

Macaroni , tuna fish , and cheese casserole/2 cup macaroni and cheese1 ounce tuna, drained

Green peas, V2 cupCucumber, 3 large slicesMolasses cookies, 2Blueberry muffin, 1Totals

Ascorbic acid 1 1 mgMeat, fish , poultry 1 ounce , raw

III. Nonmeat, -poultry, or -fish containing-lowavailability (22 g protein; 730 kcal)

Beans, navy, cooked V2 cupRice, brown, cooked i/2 cupCornbread, 1 pieceMargarine, 1 tablespoonApple slices, /2 cupWalnuts, black, raw 1 tablespoonAlmonds, raw, 1 tablespoonYogurt, skim milk, 1 cupTotals

Ascorbic acid 4 mgMeat, fish, poultry, 0 ounces, raw

iron intake with usual American meals. Cer-tam forms of fortification iron are also oflimited availability, especially pyrophos-phate, orthophosphate, and some prepara-tions of ferrum reductum (20, 21).

Unique to nonheme iron is the fact that

1.1 1.6 00.4 130.1 10.1 20.2 260.3 Trace0 00.4 41.0 Trace

1.1 4.1 46

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ESTIMATION OF AVAILABLE DIETARY IRON 137

TABLE 2-continued

Food W Total Hemet iron factor Hemeiron

Nonhemeiron

Ascorbic acid(as served)

a mg ngIV. Nonmeat, -poultry, or -fish containing- high

availability (23 g protein; 650 kcal)Red kidney beans, #{189}cup 93 2.2 2.2Tomato sauce, 2 tablespoon 30 0.2 0.2 4Broccoli,2/3cup 120 0.9 0.9 70French bread, 1 slice 35 0.8 0.8 TraceMargarine, 1 tablespoon 14 0 0 0Cottage cheese, I4 cup 55 0.2 0.2 0Pineapple, canned, 2 large slices 210 0.6 0.6 14Banana, sliced, 1/4 cup 37 0.3 0.3 4Totals 5.2 5.2 92Ascorbic acid 92 mgMeat, fish, poultry, 0 ounces, raw

% absorbable ironAbsorbable iron (mg)Total absorbable iron 0.4

8%0.42

2 mg

the amount absorbed can be modified mark-edly by components of foods ingested con-comitantly (22) . Dietary factors which dna-matically increase the absorption of non-heme iron, as much as four fold, are ascor-bic acid (23-25) and a meat factor pres-ent in meat, poultry , and fish (1 6). As thequantities of these substances in a complexor composite meal increase , iron absorptionincreases. Neither protein nor animal pro-tein per se enhance iron absorption . Whilebeef, lamb, pork, chicken, liver, and fishsubstantially raise the rate of nonheme ironabsorption, milk, cheese, and eggs do notincrease and may decrease iron availability(1 6) . These enhancing factors play a veryimportant role, for if they are not presentin a meal, the absorption of nonheme ironis very low. It is also apparent that a groupof other less well defined substances act todecrease nonheme iron availability . Tannicacid in tea (26), phosvitin of egg yolk (27),phytates (28), calcium and phosphate salts(29), EDTA (30), and antacids clearly sup-press absorption when present in sufficientamounts. There may be other substances aswell, and further studies are needed toclarify their role and particularly to deter-mine the effect of the interaction of blockingand facilitating substances.

The absorption of nonheme iron dependsnot only on the general composition of themeal but along with heme iron is furtheraffected by the iron status of the individual.Absorption of nonheme iron in the iron

deficient individual may be as much as 20%when enhancers are abundant (to be con-trasted with about 35% heme iron absonp-lion) . A much greater difference is observedin the iron replete subject with a meallacking enhancers and/or with high levels ofblocking substances, where nonheme ab-sorption is about 2% as contrasted to about1 5 % heme iron absorption . Even thoughthe percent absorption of nonheme iron isconsiderably below that of heme iron, thequantity of nonheme iron in the diet ismany fold above that of heme iron; thus, inmost meals, the major contribution of avail-able iron is made by nonheme iron.

Calculation of available food ironIn calculating the amount of iron ab-

sorbed from different meals, the amount ofheme and nonheme iron ingested must beconsidered separately . Present evidence in-dicates that the nature of the meal has littleor no influence on the amount of heme ironabsorbed, with the possible exception ofheme iron when used for fortification. Per-centage absorption of heme iron will, how-ever, be influenced by iron status in aninverse logarithmic function; in subjectswith 0, 250, 500, and 1000 mg of ironstores, absorption from heme iron is esti-mated to be 35, 28, 23, and 15%, respec-tively, from the amounts of heme iron pres-ent in meat, poultry, and fish in usual Amer-ican meals (Table 1). The proportion ofheme iron contained in various animal tis-

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