T E Fox

Bioavailability of selenium from fish, yeast and selenate:a comparative study in humans using stable isotopes

Objective: To measure the bioavailability of selenium from cooked and raw fish in humans by estimating and comparing apparent absorption and retention of selenium in biosynthetically labelled fish with labelled selenate and biosynthetically labelled selenium in brewers yeast.Design: The intervention study was a parallel, randomised, reference substance controlled design carried out at two different centres in Europe.Setting: The human study was carried out at the Institute of Food Research, Norwich, UK and at TNO Nutrition and Food Research, Zeist, The Netherlands.Subjects: In all, 35 male volunteers aged 18–50 y were recruited; 17 subjects were studied in Norwich (UK) and 18 in Zeist (Netherlands). All of the recruited subjects completed the study.Interventions: Biosynthetically labelled trout fish (processed by two different methods), biosynthetically labelled brewers yeast and isotopically labelled selenate were used to estimate selenium apparent absorption and retention by quantitative analysis of stable isotope labels recovered in faeces and urine. Subjects consumed the labelled foods in four meals over two consecutive days and absorption was measured by the luminal disappearance method over 10 days. Urinary clearance of isotopic labels was measured over 7 days to enable retention to be calculated.Results: Apparent absorption of selenium from fish was similar to selenate and there was no difference between the two processing methods used. However, retention of fish selenium was significantly higher than selenate (P<0.001). Apparent absorption and retention of yeast selenium was significantly different (P<0.001) from both fish selenium and selenate.Conclusions: Fish selenium is a highly bioavailable source of dietary selenium. Cooking did not affect selenium apparent absorption or retention from fish. Selenium from yeast is less bioavailable.

The bioavailability of iron in different weaning foods and the enhancing effect of a fruit drink containing ascorbic acid

ABSTRACT: There is limited information on the bioavailability of Fe in infant weaning foods, mainly because of the difficulties of measuring Fe utilization directly in infants. The aim of this study was to develop a safe and relatively noninvasive method for studying Fe bioavailability (measured as percent Fe incorporation into red blood cells) in infants using 54Fe, 57Fe, and 58Fe stable isotopes. Four commonly used weaning foods were selected for study, labeled extrinsically with 57Fe- or 58Fe-enriched ferrous sulfate, and fed to five female and five male 9-mo-old fasting infants, using a multiple-dosing technique. Each food was given three times, labeled with one isotope, with a fruit juice drink containing 50 mg of ascorbic acid, and three times, labeled with a different isotope, with an ascorbic acid-free drink. Fourteen days after the last test meal, a blood sample was obtained from a heel-prick, spiked with a known amount of 54Fe, digested, and purified by ion exchange; isotopic enrichment and total Fe content were measured by quadrupole thermal ionization mass spectrometry. The proportion of administered dose of isotope circulating in the blood was calculated from an estimate of blood volume. The geometric mean bioavailability (range) was 3.0% (1.2–9.5%) in a proprietary dehydrated vegetable product, 3.0% (1.1–21.2%) in Weetabix whole-wheat breakfast cereal, 3.1% (1.2–15.4%) in wholemeal bread and 4.3% (1.7–10.3%) in baked beans. When taken with the drink containing ascorbic acid, there was a 2-fold increase in bioavailability in all foods except the vegetable meal, presumably because this was already fortified with ascorbic acid. Thus, drinks containing 50 mg of ascorbic acid, taken with a meal, can significantly improve Fe bioavailability to infants from weaning foods low in ascorbic acid.

Factors affecting iron stores in infants 4–18 months of age

Objectives: To determine the effects of dietary, physiological or environmental factors on body iron levels in infants aged 4–18 months. Design: The daily iron intake of the infants was measured from a diet history obtained by interview using a standardised question sheet, previously validated against weighed intake (minimum 3 days) in an independent sample of 8 and 18 month old infants. Capillary blood samples were analyzed for haemoglobin, mean cell volume, haematocrit, zinc protoporphyrin and plasma ferritin concentration. Ferritin values were log-transformed prior to analysis to give a better approximation to the normal distribution and forward stepwise multiple linear regression was carried out using SPSS. Setting: The city of Norwich, UK and some of its suburbs. Subjects: One hundred and eighty-one healthy infants in age groups 4, 8, 12 and 18 months. Results: Main determinants of iron stores in the 4 month old infants were birth weight (+ve (P<0.001)) and body weight (−ve (P<0.005)). In the 8 month old infants intake of cow’s milk (−ve (P<0.05)), belonging to a smoking household (−ve (P<0.05)) and quantity of commercial babyfood consumed (+ve (P<0.05)) were significant. In this age group there was a gender effect (girls>boys (P<0.01)) and the gender effect remained at 12 months (girls>boys (P<0.05)), but at 18 months only non-haem iron intake was a significant factor (−ve (P<0.05)). Conclusions: At 4 months of age birth weight and body weight exert the greatest influence on iron stores, whereas by 8 months components of the weaning diet have an effect (commercial babyfood (+ve), cow’s milk (−ve)); there is also a gender effect (girls>boys), possibly reflecting the different growth rate between boys and girls. At 12 and 18 months the only significant factors are gender (girls>boys) and non-haem iron intake (−ve) respectively. Sponsorship: Ministry of Agriculture, Fisheries and Food and the Biotechnology and Biological Sciences Research Council.

The measurement of exchangeable pools of zinc using the stable isotope 70Zn

The present study was designed to assess the feasibility of using small doses of a stable isotope of Zn to follow plasma kinetics over a 10 d period and, hence, make deductions about Zn turnover and body pool sizes. At the beginning of the 10 d metabolic balance, two adults, consuming their habitual diet, were given an intravenous injection of 70Zn. There was a fourfold difference in the administered dose between the two subjects (0.445 and 2.078 mg). Blood samples were taken at regular intervals and plasma enrichment with 70Zn measured by thermal ionization mass spectrometry. Urine and faeces were collected and analysed for Zn and 70Zn. Kinetic analysis of the plasma 70Zn decay by several different methods was undertaken. It was apparent from both deconvolution analysis of the short-term (0-90 min) decay data and four-compartment modelling of the longer-term (0-24 h) data that isotopic Zn very rapidly equilibrates with the plasma Zn and with a rapidly exchanging non-plasma pool, probably located within the liver. This latter pool appears to contain less than 10 mg Zn and the peak of isotope enrichment occurs at about 20 min post injection. The later decay of plasma Zn enrichment appears to be dictated by exchange with a much larger pool of approximate size 350 mg.

True fractional calcium absorption in Chinese children measured with stable isotopes (42Ca and 44Ca)

True fractional Ca absorption (TFCA) was compared in children with different habitual Ca intakes using a double-label stable-isotope technique. Chinese children aged 7 years from Hongkong (n22) and Jiangmen (n12) participated in the study. An oral administration of 8 mg 44Ca in 100 g chocolate milk was given shortly after an intravenous injection of 0.75 mg 42Ca. Ca isotopic ratios were determined in urine samples collected 24 h later using thermal-ionization mass spectrometry. There was no significant difference in TFCA between Jiangmen and Hongkong children (P = 0.16). TFCA of a lower-Ca-intake group (Ca < or = 500 mg/d, n19) with mean Ca intake 359 mg/d was 63.1 (SD 10.7)% and that of a higher-Ca-intake group (Ca > 500 mg/d, n15) with mean Ca intake 862 mg/d was 54.8 (SD 7.3)%; the difference in TFCA was significant (P = 0.016). Serum levels of 25-hydroxycholecalciferol of the children were adequate (33.7 (SD 7.7) ng/ml). The present study indicates that growing children accustomed to a low-Ca diet appear to be able to enhance their absorptive capacity. If it is assumed that dietary Ca absorption by Chinese children resembles their TFCA from a single meal of chocolate milk, then the recommended dietary allowance (RDA) for Ca for Chinese children would be lower than the US RDA (800 mg/d), which is based on an estimated 40% Ca absorption as reported for Caucasian children. A comparative absorption study is necessary to determine whether there is any difference in TFCA between Caucasian and Chinese children.

Apparent zinc absorption by rats from foods labelled intrinsically and extrinsically with 67Zn

A variety of foods (peas (Pisum sativum), chicken meat, eggs, goats milk, human milk) enriched with the stable isotope 67Zn were prepared by means of intrinsic- and extrinsic-labelling procedures. They were fed to rats and apparent absorption of 67Zn determined from faecal excretion measurements using thermal ionization mass spectrometry. There were significant differences in the absorption of the extrinsic and intrinsic label which differed in magnitude between the foods tested. The extrinsic 67Zn was less well absorbed in peas, chicken meat, eggs, and human milk than intrinsic 67Zn, but in goats milk the extrinsic 67Zn was better absorbed than the intrinsic label. These results demonstrate that extrinsically-added stable Zn isotopes do not fully exchange with endogenous Zn in many foods, and illustrate the need for caution when using extrinsic labels for Zn bioavailability studies.

Acute Effect of Poly-γ-Glutamic Acid on Calcium Absorption in Post-Menopausal Women

Objective: Poly-γ-glutamic acid (PGA) increases calcium (Ca) solubility in vitro and in vivo, and is associated with reduced bone loss in post-menopausal Japanese women. This study is the first to examine the effect of PGA on Ca absorption in humans. Methods: A single-blind, randomized, crossover study with a 3–4 week wash-out was performed to determine the effect of PGA (80.6% glutamic acids) on Ca absorption measured by the double stable isotope method. Twenty-four healthy, non-smoking, postmenopausal women (mean age: 56.4 ± SE 0.9) were given 200 g of orange juice containing 200 mg Ca as Ca-44 enriched CaCO3, with or without 60 mg of PGA, after an overnight fast. The two tests were separated by 3–4 weeks. An intravenous injection of Ca-42 (CaCl2 solution) was given 30 min after consuming the drink and a complete urine collection carried out from 24–48 h post-dosing. Ca absorption was calculated from the Ca isotope ratios measured by thermal ionization quadrupole mass spectrometry (TIQMS). Results: Mean Ca absorption with PGA was significantly higher (P < 0.01) than without PGA, 39.1 (SE 1.6) % and 34.6 (SE 1.9) %, respectively. The effect of PGA on increasing Ca absorption was more marked in a sub-group of subjects whose baseline Ca absorption (without PGA) was lower than the population mean value. Conclusion: Postmenopausal women who received a single dose of PGA increased their intestinal Ca absorption particularly those individuals with lower basal absorptive capacity.

Use of the stable isotope 106Cd for studying dietary cadmium absorption in humans

Hydroponically grown wheat was intrinsically labelled with the stable isotope 106cadmium (106Cd) and the flour made into a porridge. The abundance of the isotope in the porridge was approximately 30 times the natural abundance, but the total level of Cd in the porridge was 0.03 mg/kg fresh weight, which was the same as expected in a normal diet. Cadmium measurements were made using inductively coupled plasma-mass spectrometry (ICP-MS). The porridge was eaten at breakfast by adult and infant volunteers. Bulked faecal collections were analysed for unabsorbed Cd. Initial results suggest that the apparent absorption of Cd may be higher than 5% as commonly quoted, but longer faecal collection times may be necessary to confirm this.

Evaluation of iron bioavailability in infant weaning foods fortified with Haem concentrate

BACKGROUND Nutritional iron deficiency in infants over 4 months of age is one of the most common deficiency disorders. Dietary iron is comprised of non-haem and haem iron, the latter being absorbed by a separate pathway and more efficiently than non-haem iron. Fortification of infant weaning foods is one of the strategies adopted for preventing iron deficiency and the aim of this project was to examine the potential use of haem iron concentrate as a fortificant. METHODS Sixteen non-anaemic 6-month old infants were recruited and allocated to two groups of 8. Each infant consumed 2 meals/day of a commercial weaning food (100 g) for 7 consecutive days containing 40 mg ascorbic acid and 2.5 mg haem iron/100 g (Group 1) or the same quantity of iron as ferrous sulphate plus 40 mg ascorbic acid (Group 2). Bioavailability was assessed by chemical balance using carmine to mark the beginning and end of the faecal collection. The effect of haem iron concentrate (as a candidate for the factor in meat that enhances iron absorption) was examined by measuring its effect on 57Fe-labelled non-haem iron absorption. RESULTS There was no difference in iron balance between the two groups. Mean iron retention was 3.5 (SD 2.1) mg/day in Group 1 (haem iron) and 3.0 (SD 2.4) mg/day in Group 2 (ferrous sulphate). Haem concentrate did not enhance the absorption of 57Fe-labelled non-haem iron, Group 1: 1710 (SD 11.1)%, Group 2: 28.4 (SD 17.7)%. CONCLUSIONS Haem iron concentrate appears to be a highly bioavailable form of iron when added to infant weaning foods. This protein is not, however, responsible for the enhancing effect of animal protein on non-haem iron absorption.

Absorption of selenium from wheat, garlic, and cod intrinsically labeled with Se-77 and Se-82 stable isotopes.

There is limited information on the absorption of selenium from different foods in humans because of technical difficulties associated with isotopic labeling of dietary selenium. Wheat, garlic, and cod fish were intrinsically labeled with Se-77 or Se-82 stable isotopes. Labeled meals were fed in random order to 14 adults, with a minimum washout period of six weeks between each test meal. Apparent absorption was measured as luminal loss using a fecal monitoring technique over an 8-day period. Plasma appearance of the isotope was measured at 7, 24, and 48 hours post-ingestion. Selenium absorption (+/- SD) was significantly higher (p < 0.001) from wheat (81.0 +/- 3.0%) and garlic (78.4 +/- 13.7%) than from fish (56.1 +/- 4.3%). Lowest plasma concentration was observed after the fish meal at all three time points, with a peak at 24 hours, whereas wheat produced the highest plasma concentration at all three time points and peaked at 7 hours. Selenium absorption from wheat and garlic was higher than from fish, and inter-individual variation was low. Form of selenium and food constituents appear to be key determinants of post-absorptive metabolism.