Factors influencing the absorption of iron from soya-bean protein products

The role of soy in vegetarian diets

Soyfoods have long been prized among vegetarians for both their high protein content and versatility. Soybeans differ markedly in macronutrient content from other legumes, being much higher in fat and protein, and lower in carbohydrate. In recent years however, soyfoods and specific soybean constituents, especially isoflavones, have been the subject of an impressive amount of research. Nearly 2,000 soy-related papers are published annually. This research has focused primarily on the benefits that soyfoods may provide independent of their nutrient content. There is particular interest in the role that soyfoods have in reducing risk of heart disease, osteoporosis and certain forms of cancer. However, the estrogen-like effects of isoflavones observed in animal studies have also raised concerns about potential harmful effects of soyfood consumption. This review addresses questions related to soy and chronic disease risk, provides recommendations for optimal intakes, and discusses potential contraindications. As reviewed, the evidence indicates that, with the exception of those individuals allergic to soy protein, soyfoods can play a beneficial role in the diets of vegetarians. Concerns about adverse effects are not supported by the clinical or epidemiologic literature. Based on the soy intake associated with health benefits in the epidemiologic studies and the benefits noted in clinical trials, optimal adult soy intake would appear to be between two and four servings per day.

Iron nutrition in the UK: getting the balance right

Fe homeostasis is considered in the context of the UK diet, using information on Fe intake and status from the National Diet and Nutrition Surveys. The importance of assessing Fe availability rather than total Fe intake is discussed. Dietary and host-related factors that determine Fe bioavailability (Fe utilised for Hb production) are reviewed using information from single-meal studies. When adaptive responses are taken into consideration, foods associated with higher Fe status include meat (haem-Fe and the ‘meat factor’) and fruits and fruit juice (vitamin C). Foods that may have a negative impact include dairy products (Ca), high-fibre foods (phytate) and tea and coffee (polyphenols), but the effects are more apparent in groups with marginal Fe deficiency, such as women of childbearing age. Analysis of dietary intake data on a meal-by-meal basis is needed to predict the influence of changing dietary patterns on Fe nutrition in the UK. Current information suggests that in the UK Fe deficiency is a greater problem than Fe overload.

Effect of beef and soy proteins on the absorption of non-heme iron and inorganic zinc in children

BACKGROUND

Iron and zinc deficiency remain substantial problems in small children in both developed and developing nations. Optimizing mineral absorption is an important strategy in minimizing this problem.

OBJECTIVES

To assess the effects of beef and soy proteins on the bioavailability of non-heme iron and zinc in children.

METHODS

We measured iron (n = 26) and zinc (n = 36) absorption in 4-8 y old children from meals differing only in protein source (beef or a low-phytate soy protein concentrate). Iron and zinc absorption were measured using multi-tracer stable isotope techniques. Iron absorption was calculated from the red blood cell iron incorporation measured after 14 days and zinc absorption from the ratio of the oral and intravenous excretion of the zinc tracers 48 hours after dosing.

RESULTS

Iron absorption from the beef meal was significantly greater (geometric mean, 7.6%) than from the soy meal (3.5%, p = 0.0015). Zinc absorption from the beef meal was greater (mean +/- SD, 13.7 +/- 6.0%) than from the soy meal (10.1 +/- 4.1%, p = 0.047).

CONCLUSION

These findings indicate that beef protein increases both non-heme iron and zinc absorption compared to soy protein. The effect of protein source on non-heme iron and inorganic zinc absorption should be one of the factors taken into account when designing diets for children. The inhibitory effect of the soy based meal on iron and zinc absorption could be overcome by fortifying the soy protein with these minerals during the production process.

Dephytinisation of soyabean protein isolate with low native phytic acid content has limited impact on mineral and trace element absorption in healthy infants

Infant formulas based on soyabean protein isolate are often used as an alternative to cows'-based formulas. However, the presence of phytic acid in soya formulas has raised concern about the absorption of trace elements and minerals from these products. The aim of the present study was to evaluate mineral and trace element absorption from regular and dephytinised soya formula in healthy infants. Soyabean protein isolate with a relatively low native content of phytic acid was used for production of a regular soya formula (300 mg phytic acid/kg liquid formula) and an experimental formula was based on dephytinised soya protein isolate (<6 mg phytic acid/kg liquid formula). Using a crossover study design, apparent mineral and trace element absorptions were measured by a stable isotope technique based on 72 h faecal excretion of non-absorbed stable isotopes (Zn, Fe, Cu and Ca) and by the chemical balance technique (Mn, Zn, Cu and Ca) in nine infants (69-191 d old). Fe absorption was also measured by erythrocyte incorporation 14 d after intake. The results from the present study demonstrated that Zn absorption, measured by a stable isotope technique, was significantly greater after dephytinisation (mean value 16.7 v. 22.6 %; P=0.03). No other statistically significant differences between the two formulas were observed. The nutritional benefit of dephytinisation was marginal in the present study. Based on these results, the use of soyabean protein isolate with low native content of phytic acid should be promoted for production of soya formulas and adequate addition of ascorbic acid to enhance Fe absorption should be ensured in the products.

Iron and ascorbic Acid: proposed fortification levels and recommended iron compounds

An adequate supply of dietary iron during the 1st 24 mo of life is essential for preventing iron deficiency with its attendant negative effects on mental, motor and emotional development as well as later cognitive performance. Iron reserves and the small amount of highly bioavailable iron in human milk are adequate to satisfy the iron requirements of breast-fed infants of adequate birth weight for the 1st 6 mo of life. Thereafter, complementary foods, iron supplements or both are needed to meet this requirement. Complementary foods should not displace the consumption of human milk. The quantities eaten, particularly by younger infants, may therefore be quite small. As a consequence it is essential that the iron be supplied in a highly bioavailable form. This can be achieved by fortifying complementary foods with ferrous sulfate and ascorbic acid provided that the ascorbic acid is not lost during storage or meal preparation. Suggested fortification levels for ferrous sulfate and ascorbic acid for some types of complementary foods are given. The use of ferrous fumarate or an elemental iron powder instead of ferrous sulfate has not been evaluated adequately. There is a need to develop alternative strategies for improving iron bioavailability in complementary foods because it may not be possible to preserve ascorbic acid activity in many of them.

Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet

BACKGROUND

Ascorbic acid has a pronounced enhancing effect on the absorption of dietary nonheme iron when assessed by feeding single meals to fasting subjects. This contrasts with the negligible effect on iron balance of long-term supplementation with vitamin C.

OBJECTIVE

Our goal was to examine the effect of vitamin C on nonheme-iron absorption from a complete diet rather than from single meals.

DESIGN

Iron absorption from a complete diet was measured during 3 separate dietary periods in 12 subjects by having the subjects ingest a labeled wheat roll with every meal for 5 d. The diet was freely chosen for the first dietary period and was then altered to maximally decrease or increase the dietary intake of vitamin C during the second and third periods.

RESULTS

There was no significant difference in mean iron absorption among the 3 dietary periods despite a range of mean daily intakes of dietary vitamin C of 51-247 mg/d. When absorption values were adjusted for differences in iron status and the 3 absorption periods were pooled, multiple regression analysis indicated that iron absorption correlated negatively with dietary phosphate (P = 0.0005) and positively with ascorbic acid (P = 0.0069) and animal tissue (P = 0.0285).

CONCLUSIONS

The facilitating effect of vitamin C on iron absorption from a complete diet is far less pronounced than that from single meals. These findings may explain why several prior studies did not show a significant effect on iron status of prolonged supplementation with vitamin C.

Supplemental sodium phytate and microbial phytase influence iron availability in growing rats

Five groups of individually housed albino rats (n = 7 each, initial average weight = 42 g) were fed diets based on corn starch and casein over a 4-week period. All diets were supplemented with 35 mg/kg of iron from FeSO4 x 7 H2O. Group I (control) was fed the basal diet free of phytic acid (PA) and phytase. By replacing corn starch by 7.5 g (groups II and IV) and 15 g phytic acid (groups III and V) from sodium phytate per kg diet, molar PA/iron ratios of 18 and 36 were obtained. In groups IV and V, 1000 U phytase from Aspergillus niger per kg diet were added. Food conversion efficiency ratio and growth rate as well as iron in plasma and spleen, hemoglobin, red blood cell count and erythrocyte zinc protoporphyrin were not influenced by the different dietary treatments. Dietary phytate reduced apparent iron absorption in groups II and III. Furthermore hematocrit, transferrin saturation and iron concentration in liver and femur were lowered in rats fed diets with PA, while total and latent iron-binding capacity of plasma increased. Microbial phytase supplementation (groups IV and V) partly counteracted the antinutritive effects of phytic acid on iron availability.

Interaction of iron with other nutrients

Iron is essential for oxygen transport, oxidative metabolism, and cellular growth. Interactions between iron and other dietary factors play a significant role in determining the adequacy of iron nutrition and have important implications for food fortification in developing countries. Vitamin A and vitamin C deficiency states may affect iron transport, metabolism, and storage within the body.

Implications of antinutritional components in soybean foods

There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

Soy protein, phytate, and iron absorption in humans

The effect of reducing the phytate in soy-protein isolates on nonheme-iron absorption was examined in 32 human subjects. Iron absorption was measured by using an extrinsic radioiron label in liquid-formula meals containing hydrolyzed corn starch, corn oil, and either egg white or one of a series of soy-protein isolates with different phytate contents. Iron absorption increased four- to fivefold when phytic acid was reduced from its native amount of 4.9-8.4 to less than 0.01 mg/g of isolate. Even relatively small quantities of residual phytate were strongly inhibitory and phytic acid had to be reduced to less than 0.3 mg/g of isolate (corresponding to less than 10 mg phytic acid/meal) before a meaningful increase in iron absorption was observed. However, even after removal of virtually all the phytic acid, iron absorption from the soy-protein meal was still only half that of the egg white control. It is concluded that phytic acid is a major inhibitory factor of iron absorption in soy-protein isolates but that other factors contribute to the poor bioavailability of iron from these products.

Soybean protein isolate and soybean lectin inhibit iron absorption in rats

Inhibitory effects of soybean protein isolate (SPI) and soybean lectin on the intestinal absorption of nonheme iron were investigated by in vivo studies in rats. Rats fed the SPI-based diet absorbed significantly less iron than did control rats fed the casein-based diet. Supplementing the SPI diets with 8% D-galactose significantly increased the incorporation of iron into liver ferritin, although D-galactose did not significantly increase iron absorption. Heat treatment of SPI significantly increased iron absorption. Ascorbate did not enhance iron absorption in rats fed the SPI-based diet. The presence of lectin in an aqueous extract of SPI was suggested by hemagglutination activity as well as by immunoreactivity with soybean lectin antibody. Soybean lectin introduced into ligated segments of the upper small intestine of rats inhibited ferrous iron absorption. This inhibitory effect, especially in the mucosal uptake, was significantly improved by addition of N-acetyl-D-galactosamine to soybean lectin. Soybean lectin had no effect on ferric iron absorption. Our results suggest that a portion of the reduction in iron absorption in rats fed SPI may be due to lectins.

Effect of traditional oriental soy products on iron absorption

The effect of a variety of traditional oriental unfermented and fermented soy products on iron absorption was evaluated in 242 Indian women. When compared with a soy-flour meal containing an equal amount of protein, iron absorption was found to be significantly improved with silken tofu, natto, tempeh, rice miso, barley miso, and soybean miso. This improvement could not be adequately explained except with reference to changes in the protein composition of the products. The protein fraction profiles of the soy products were obtained by size-exclusion high-performance liquid chromatography. An inverse relationship between food iron absorption and the high-molecular-weight fraction of the soy products was demonstrated (r = 0.66, p = 0.01). However, anomalous results obtained with three products (sufu, tempeh, and fully hydrolyzed isolated soy protein) did not make it possible to reach firm conclusions concerning the effect of the protein fraction of soy on iron bioavailability.

[The effect of a supplement of citric acid on the bioavailability of zinc from corn germ]

The purpose of this 2 factorial designed study was to investigate the influence of citric acid on the availability of zinc from diets containing 140 g corn germs as a native phytate source (0.5% phytate in diet). Growing male rats with an average initial weight of 42 g were divided into 8 groups of 8 animals each. After a 7 d depletion period (2.4 micrograms Zn/g diet) the animals were fed ad libitum for 21 d a diet on the basis of egg white solid and corn germs. The diets were supplemented with zinc in order to obtain phytate:zinc molar ratios of 31, 20, 14, and 0 (control without corn germs, 11 micrograms Zn/g diet). Each diet was fed with and without a supplementation of 1% citric acid. A phytate:Zn molar ratio of 31:1 resulted in typical symptoms of zinc-deficiency like anorexia, alopecia and a significant depression of growth. These effects were apparently reduced by citric acid. The zinc concentration in serum and organs followed the graded levels of phytate:zinc molar ratios. Primary significant effects of the phytate:Zn molar ratio but also effects of citric acid and interactions between the 2 factors phytate:Zn and citric acid could be detected. Only total liver zinc but not liver zinc based on fresh matter was affected by the phytate:Zn molar ratio. In serum and tissues the activity of alkaline phosphatase showed a significant response to the phytate:zinc molar ratio. Furthermore the supplementation with citric acid increased the femur alkaline phosphatase and slightly reduced it in the liver. The concentrations of metallothionein in liver duodenum, jejunum and ileum were significantly affected by the phytate:Zn molar ratio.

Nutritional iron requirements and food iron absorption

To prevent nutritional iron deficiency, sufficient iron must be absorbed from the diet to meet the normal physiological requirements. Daily iron losses in males are about 1 mg (14 micrograms kg-1), while the average additional requirements incurred in women include menstruation (0.6 mg), pregnancy (2.7 mg) and lactation (less than 0.3 mg). Requirements during pregnancy are not evenly distributed and increase to between 5-6 mg in the last trimester of pregnancy, which is more than can be absorbed from even an optimal diet. While the amounts absorbed are affected by the iron content of the diet, the composition of the latter is even more relevant. About one-quarter of the iron in haem proteins is absorbed regardless of the other components in the diet, while non-haem iron absorption is subject to the interplay of promoting and inhibiting substances in the diet. Thus diets rich in enhancers of non-haem iron absorption, chiefly meat and/or ascorbic acid, have high iron bioavailability (about 3 mg d-1) while diets in which inhibitors, such as polyphenols and phytates, predominate are poor sources of iron (less than 1 mg d-1). Examination of the relative proportions of promoters and inhibitors of iron absorption in individual foodstuffs and the measured iron absorption from them may be useful in predicting the overall iron bioavailability from mixed diets.

Is solubility in vitro a reliable predictor of iron bioavailability?

In this paper we have described physiological factors that are likely to influence iron bioavailability, reviewed chemical properties of iron important to its absorption, and addressed the question posed in the title. Intestinal transit times, luminal pH, and diffusion barriers resulting from gastrointestinal mucus may affect absorption. The tendency of iron to form large-molecular-weight species and to bind to macromolecules may limit its absorption. Iron solubility following in vitro gastrointestinal digestion is a reliable predictor of ascorbic acid effects on bioavailability but not of protein effects. Solubility of low-molecular-weight iron is better than simple solubility for predicting iron bioavailability.