Janet R. Hunt

Bioavailability of iron, zinc, and other trace minerals from vegetarian diets.

Iron and zinc are currently the trace minerals of greatest concern when considering the nutritional value of vegetarian diets. With elimination of meat and increased intake of phytate-containing legumes and whole grains, the absorption of both iron and zinc is lower with vegetarian than with nonvegetarian, diets. The health consequences of lower iron and zinc bioavailability are not clear, especially in industrialized countries with abundant, varied food supplies, where nutrition and health research has generally supported recommendations to reduce meat and increase legume and whole-grain consumption. Although it is clear that vegetarians have lower iron stores, adverse health effects from lower iron and zinc absorption have not been demonstrated with varied vegetarian diets in developed countries, and moderately lower iron stores have even been hypothesized to reduce the risk of chronic diseases. Premenopausal women cannot easily achieve recommended iron intakes, as modified for vegetarians, with foods alone; however, the benefit of routine iron supplementation has not been demonstrated. It may be prudent to monitor the hemoglobin of vegetarian children and women of childbearing age. Improved assessment methods are required to determine whether vegetarians are at risk of zinc deficiency. In contrast with iron and zinc, elements such as copper appear to be adequately provided by vegetarian diets. Although the iron and zinc deficiencies commonly associated with plant-based diets in impoverished nations are not associated with vegetarian diets in wealthier countries, these nutrients warrant attention as nutritional assessment methods become more sensitive and plant-based diets receive greater emphasis.

Effect of dietary boron on mineral, estrogen, and testosterone metabolism in postmenopausal women.

A study was done to examine the effects of aluminum, magnesium, and boron on major mineral metabolism in postmenopausal women. This communication describes some of the effects of dietary boron on 12 women between the ages of 48 and 82 housed in a metabolic unit. A boron supplement of 3 mg/day markedly affected several indices of mineral metabolism of seven women consuming a low‐magnesium diet and five women consuming a diet adequate in magnesium; the women had consumed a conventional diet supplying about 0.25 mg boron/day for 119 days. Boron supplementation markedly reduced the urinary excretion of calcium and magnesium; the depression seemed more marked when dietary magnesium was low. Boron supplementation depressed the urinary excretion of phosphorus by the low‐magnesium, but not by the adequate‐magnesium, women. Boron supplementation markedly elevated the serum concentrations of 17β‐estradiol and testosterone; the elevation seemed more marked when dietary magnesium was low. Neither high dietary aluminum (1000 mg/day) nor an interaction between boron and aluminum affected the variables presented. The findings suggest that supplementation of a low‐boron diet with an amount of boron commonly found in diets high in fruits and vegetables induces changes in postmenopausal women consistent with the prevention of calcium loss and bone demineralization.— Nielsen, F. H.; Hunt, C. D.; Mullen, L. M.; Hunt, J. R. Effect of dietary boron on mineral, estrogen, and testosterone metabolism in postmenopausal women. FASEB J. 1: 394‐397; 1987.

The usefulness of in vitro models to predict the bioavailability of iron and zinc: A consensus statement from the HarvestPlus expert consultation

A combination of dietary and host-related factors determines iron and zinc absorption, and several in vitro methods have been developed as preliminary screening tools for assessing bioavailability. An expert committee has reviewed evidence for their usefulness and reached a consensus. Dialyzability (with and without simulated digestion) gives some useful information but cannot predict the correct magnitude of response and may sometimes predict the wrong direction of response. Caco-2 cell systems (with and without simulated digestion) have been developed for iron availability, but the magnitude of different effects does not always agree with results obtained in human volunteers, and the data for zinc are too limited to draw conclusions about the validity of the method. Caco-2 methodologies vary significantly between laboratories and require experienced technicians and good quality cell culture facilities to obtain reproducible results. Algorithms can provide semi-quantitative information enabling diets to be classified as high, moderate, or low bioavailability. While in vitro methods can be used to generate ideas and develop hypotheses, they cannot be used alone for important decisions concerning food fortification policy, selection of varieties for plant breeding programs, or for new product development in the food industry. Ultimately human studies are required for such determinations.

Dietary zinc reduces osteoclast resorption activities and increases markers of osteoblast differentiation, matrix maturation, and mineralization in the long bones of growing rats.

The nutritional influence of zinc on markers of bone extracellular matrix resorption and mineralization was investigated in growing rats. Thirty male weanling rats were randomly assigned to consume AIN-93G based diets containing 2.5, 5, 7.5, 15 or 30 microg Zn/g diet for 24 days. Femur zinc increased substantially as zinc increased from 5 to 15 microg/g diet and modestly between 15 and 30 microg/g (P<.05). By morphological assessment, trabecular bone increased steadily as dietary zinc increased to 30 microg/g. Increasing dietary zinc tended to decrease Zip2 expression nonsignificantly and elevated the relative expression of metallothionen-I at 15 but not 30 microg Zn/g diet. Femur osteoclastic resorption potential, indicated by matrix metalloproteinases (MMP-2 and MMP-9) and carbonic anhydrase-2 activities decreased with increasing dietary zinc. In contrast to indicators of extracellular matrix resorption, femur tartrate-resistant acid and alkaline phosphatase activities increased fourfold as dietary zinc increased from 2.5 to 30 microg Zn/g. Likewise, 15 or 30 microg Zn/g diet resulted in maximum relative expression of osteocalcin, without influencing expression of core-binding factor alpha-1, collagen Type 1 alpha-1, or nuclear factor of activated T cells c1. In conclusion, increased trabecular bone with additional zinc suggests that previous requirement estimates of 15 microg Zn/g diet may not meet nutritional needs for optimal bone development. Overall, the up-regulation of extracellular matrix modeling indexes and concomitant decrease in resorption activities as dietary zinc increased from 2.5 to 30 microg/g provide evidence of one or more physiological roles for zinc in modulating the balance between bone formation and resorption.

Inhibitory effects of dietary calcium on the initial uptake and subsequent retention of heme and nonheme iron in humans: comparisons using an intestinal lavage method

BACKGROUND Calcium is the only reported dietary inhibitor of both heme- and nonheme-iron absorption. It has been proposed that the 2 forms of iron enter a common pool in the enterocyte and that calcium inhibits the serosal transfer of iron into blood. OBJECTIVES We aimed to ascertain whether the inhibitory effect of calcium occurs during initial mucosal uptake or during serosal transfer and to compare the serosal transfer of heme and nonheme iron, which should not differ if the 2 forms have entered a common mucosal iron pool. DESIGN Whole-gut lavage and whole-body counting were used to measure the initial uptake (8 h) and retention (2 wk) of heme and nonheme iron with and without a calcium supplement (450 mg). Two experiments tested basal meals with low iron bioavailability and 360 mg Ca (n = 15) or with high iron bioavailability and 60 mg Ca (n = 12). RESULTS Added calcium reduced the initial uptake of heme iron by 20%, from 49% to approximately 40% from both meals (P = 0.02), and reduced the total iron absorbed from the low- and high-bioavailability meals by approximately 25% [from 0.033 to 0.025 mg (P = 0.06) and from 0.55 to 0.40 mg (P < 0.01), respectively]. Calcium did not affect the serosal transfer of either form of iron. CONCLUSIONS Calcium supplementation reduced heme and total iron without significantly affecting nonheme-iron absorption, regardless of meal bioavailability. Calcium inhibited the initial mucosal uptake rather than the serosal transfer of heme iron. Differences in serosal transfer indicate that heme and nonheme iron did not enter a common absorptive pool within 8 h after a meal.

Dietary and physiological factors that affect the absorption and bioavailability of iron.

Iron deficiency, a global health problem, impairs reproductive performance, cognitive development, and work capacity. One proposed strategy to address this problem is the improvement of dietary iron bioavailability. Knowledge of the molecular mechanisms of iron absorption is growing rapidly, with identification of mucosal iron transport and regulatory proteins. Both body iron status and dietary characteristics substantially influence iron absorption, with minimal interaction between these two factors. Iron availability can be regarded mainly as a characteristic of the diet, but comparisons between human studies of iron availability for absorption require normalization for the iron status of the subjects. The dietary characteristics that enhance or inhibit iron absorption from foods have been sensitively and quantitatively determined in human studies employing iron isotopes. People with low iron status can substantially increase their iron absorption from diets with moderate to high availability. But while iron supplementation and fortification trials can effectively increase blood indices of iron status, improvements in dietary availability alone have had minimal influence on such indices within several weeks or months. Plentiful, varied diets are the ultimate resolution to iron deficiency. Without these, more modest food-based approaches to human iron deficiency likely will need to be augmented by dietary iron fortification.

EFFECTS OF ASCORBIC ACID SUPPLEMENTS AND A DIET MARGINAL IN COPPER ON INDICES OF COPPER NUTRITURE IN WOMEN

Biochemical indicators of copper status were studied in young women housed in a metabolic unit for 135 days. The women were fed a diet low in Cu (0.67 mg/d), that was supplemented with 1.5 g of ascorbic acid and variable amounts of copper. Copper balance was not significantly affected by diet. Plasma copper and monoamine oxidase, and erythrocyte superoxide dismutase were not affected by dietary copper or by ascorbic acid. Enzymatic (CpENZ), but not immunoreactive (CpRID), ceruloplasmin was lower at the end of low copper intake and high ascorbic acid intake periods. The specific activity of ceruloplasmin (CpENZ/CpRID) was significantly lower at the end of the ascorbic acid period (1.26) than at either the end of control (1.81) or repletion (1.80) periods, and slightly lower than after only low copper intake (1.44). Cytochrome-c oxidase activities in platelets and leukocytes were significantly affected by copper intake, but not ascorbic acid intake. These findings indicate that the specific activity of ceruloplasmin and cytochrome-c oxidase activity of platelets and white cells may be sensitive indicators of copper status, and that ascorbic acid supplements do not markedly affect commonly measured indices of copper metabolism, except for the specific activity of ceruloplasmin in young women.

Dietary calcium does not exacerbate phytate inhibition of zinc absorption by women from conventional diets

BACKGROUND Although calcium inhibits zinc bioavailability in rats, especially from high-phytate diets, the effect of calcium on zinc absorption by humans from practical diets remains unclear. OBJECTIVE The objective was to test the inhibitory effect of dietary calcium, in Western diets with high and low phytate content, on zinc absorption. DESIGN Using a 2 x 2 factorial design, zinc absorption was determined in 10 healthy women from 1-d diets with moderate and high calcium contents of approximately 700 and 1800 mg/d and low and high phytate contents of approximately 440 and 1800 mg/d. Absorption was measured by using extrinsically added (65)Zn and subsequent whole-body scintillation counting. RESULTS Mean (+/-SE) fractional zinc absorption was 32.8 +/- 2.3% from the moderate-calcium, low-phytate diet; 26.9 +/- 2.4% from the moderate-calcium, high-phytate diet; 39.4 +/- 2.4% from the high-calcium, low-phytate diet; and 26.2 +/- 2.3% from the high-calcium, high-phytate diet. The respective values for absolute zinc absorption were 3.8 +/- 0.3, 3.0 +/- 0.3, 4.5 +/- 0.3, and 3.2 +/- 0.3 mg/d. Phytate significantly reduced fractional zinc absorption by approximately 10 percentage points and reduced absolute zinc absorption by 25%, or approximately 1 mg/d. Differences in dietary calcium did not affect zinc absorption, regardless of a high or low dietary phytate content. CONCLUSIONS In healthy women consuming 1-d menus of ordinary foods (some fortified with calcium), dietary phytate reduces zinc absorption, but calcium does not impair zinc absorption, regardless of whether dietary phytate is low or high.

Zinc retention from an experimental diet based on the U.S.F.D.A. total diet study

Abstract To investigate retention of Zn from U.S. diets, two research diets were developed using 200 foods which represent average intakes of 25–30 year-old U.S. men and women, respectively (FDA Total Diet Study). The womens diet contained 7.8 mg Zn (analyzed) and 1570 kcal (calculated) daily; the mens diet contained 14.0 mg Zn and 2545 kcal. The diets were consumed by 14 women and 14 men for nine weeks. Although the food lists were intended to represent typical intakes in the U.S., they did not provide sufficient energy for weight maintenance. Despite energy adjustments with sucrose, followed by a 10% increase in all foods, the women lost (mean±SD) 4.4±1.4 and the men, 4.0±1.7 kg. The entire 3-d menu was extrinsically labelled with Zn-65 during week 3. Zn retention was monitored by whole body scintillation counting. Oral contraceptives did not affect Zn retention. Women absorbed a larger proportion of Zn from their diet (29±8 vs 22±4%), but less total Zn (2.3±0.6 vs 3.1 ±0.5 mg), and retained the Zn with a shorter biological half-life (147±32 vs 182±32 d) than men. Men and women absorbed similar amounts of Zn per kg. Although the difference in body weight between men and women was less than the difference in dietary Zn, the women apparently compensated by absorbing Zn more efficiently. Approximately a quarter of the Zn in a representative U.S. diet was absorbed.

Moderate and high amounts of protein from casein enhance human absorption of zinc from whole-wheat or white rolls

Abstract The enhancement of zinc absorption by protein was investigated using a broad range of ingested protein in combination with either whole-wheat or white rolls. Twenty or 40 g of protein from casein were added to rools containing 10 g protein; zinc content was adjusted to 4 mg. The rolls were extrinsically labelled with 65 Zn and fed to 20 healthy participants. Zinc absorption was measured by whole body scintillation counting. Average (±SD) zinc absorption ranged from 8±4% for the whole-wheat rolls with the least protein to 26±7% for the white rolls with the most protein. Less zinc was absorbed from whole-wheat than from white rolls containing the same amount of zinc. The protein and type of flour did not interact to affect zinc absorption. As protein increased from 10 to 50 g, zinc absorption was enhanced with no indication that maximum enhancement had been achieved.