Use of the stable isotope, 58Fe, for determining availability of nonheme iron in meals

Iron uptake and ferrokinetics in healthy male subjects of an iron-based oral phosphate binder (SBR759) labeled with the stable isotope (58)Fe

SBR759 is a novel polynuclear iron(III) oxide-hydroxide starch·sucrose·carbonate complex being developed for oral use in chronic kidney disease (CKD) patients with hyperphosphatemia on hemodialysis. SBR759 binds inorganic phosphate released by food uptake and digestion in the gastro-intestinal tract increasing the fecal excretion of phosphate with concomitant reduction of serum phosphate concentrations. Considering the high content of ∼20% w/w covalently bound iron in SBR759 and expected chronic administration to patients, absorption of small amounts of iron released from the drug substance could result in potential iron overload and toxicity. In a mechanistic iron uptake study, 12 healthy male subjects (receiving comparable low phosphorus-containing meal typical for CKD patients: ≤1000 mg phosphate per day) were treated with 12 g (divided in 3 × 4 g) of stable (58)Fe isotope-labeled SBR759. The ferrokinetics of [(58)Fe]SBR759-related total iron was followed in blood (over 3 weeks) and in plasma (over 26 hours) by analyzing with high precision the isotope ratios of the natural iron isotopes (58)Fe, (57)Fe, (56)Fe and (54)Fe by multi-collector inductively coupled mass spectrometry (MC-ICP-MS). Three weeks following dosing, the subjects cumulatively absorbed on average 7.8 ± 3.2 mg (3.8-13.9 mg) iron corresponding to 0.30 ± 0.12% (0.15-0.54%) SBR759-related iron which amounts to approx. 5-fold the basal daily iron absorption of 1-2 mg in humans. SBR759 was well-tolerated and there was no serious adverse event and no clinically significant changes in the iron indices hemoglobin, hematocrit, ferritin concentration and transferrin saturation.

High resolution inductively coupled plasma mass spectrometry allows rapid assessment of iron absorption in infants and children

Stable isotope absorption studies of iron have been limited by the high cost and limited availability of isotope ratio analysis using thermal ionization MS (TIMS). The development of high-resolution double focusing inductively coupled plasma MS (ICP-MS) may permit more cost-efficient sample analysis due to its high throughput, lower cost, easy sample pretreatment, and greater availability. Our objective was to develop an ICP-MS methodology for the measurement of iron isotope ratios using very small blood volumes. We developed a technique using multiple iron-nickel mixing standard solutions to adjust for nickel interference calibration. RBC samples from human subjects previously given 58Fe and 57Fe were analyzed for iron isotope ratios and compared with our current methodology (TIMS). Reproducibility of iron isotope ratios provided external relative SD < 0.5 and 0.7% (1 SD) for 57Fe/54Fe and 58Fe/54Fe, respectively. Iron isotope ratios from ICP-MS analysis did not differ from those from TIMS based on statistical analyses, nor did the calculated iron absorption values. The mean and SD of iron absorption did not differ when measured by TIMS or ICP-MS. A 2-microL RBC sample was sufficient for ICP-MS iron isotope ratio analysis with an internal relative SD < 0.5% and analytical time < 5 min. This technique may assist groups in increasing their use of stable isotope methods to assess iron absorption in infants and children.

Inevitable iron loss by human adolescents, with calculations of the requirement for absorbed iron

In growing individuals, the requirement for absorbed iron consists of iron needed for growth and iron needed to replace inevitable iron loss. We were able to estimate inevitable iron loss by adolescents because total body iron of the adolescents had been enriched with the stable isotope, (58)Fe, as the result of earlier studies of iron absorption. During an interval beginning at least 1.56 y after isotope administration (a time sufficient for complete mixing of the isotope with total body iron) and extending for no less than 3.29 y, we determined the isotopic enrichment of circulating iron. On the basis of several assumptions, we calculated total body (58)Fe and total body iron at the beginning and end of the interval. Because of complete mixing of the isotope with total body iron, fractional total (58)Fe loss was the same as fractional loss of total iron. In males, the fractional loss of iron was 9.70%/y and the quantitative loss was 256 mg/y or 0.70 mg/d. In females, the fractional loss of iron was 14.60%/y and the quantitative loss was 306 mg/y or 0.84 mg/d. Using several assumptions, we then calculated that the iron requirement for growth during this interval was 0.76 mg/d for males and 0.31 mg/d for females. Adding the iron loss to the iron requirement for growth, the requirement for absorbed iron was estimated to be 1.46 mg/d for males and 1.15 mg/d for females.

The role of taurine in infant nutrition

The importance of taurine in the diet of pre-term and term infants has not always been clearly understood and is a topic of interest to students of infant nutrition. Recent evidence indicates that it should be considered one of the "conditionally essential" amino acids in infant nutrition. Plasma values for taurine will fall if infants are fed a taurine-free formula or do not have taurine provided in the TPN solution. Urine taurine values also fall, which is indicative of an attempt by the kidney to conserve taurine. The very-low-birth-weight infant, for a variety of reasons involving the maturation of tubular transport function, cannot maximally conserve taurine by enhancing renal reabsorption and, hence, is potentially at greater risk for taurine depletion than larger pre-term or term infants, and certainly more than older children who have taurine in their diet. Taurine has an important role in fat absorption in pre-term and possibly term infants and in children with cystic fibrosis. Because taurine-conjugated bile acids are better emulsifiers of fat than glycine-conjugated bile acids, the dietary (or TPN) intake has a direct influence on absorption of lipids. Taurine supplementation of formulas or TPN solutions could potentially serve to minimize the brain phospholipid fatty acid composition differences between formula-fed and human milk-fed infants. Taurine appears to have a role in infants, children, and even adults receiving most (> 75%) of their calories from TPN solutions in the prevention of granulation of the retina and electroencephalographic changes. Taurine has also been reported to improve maturation of auditory-evoked responses in pre-term infants, although this point is not fully established. Clearly, taurine is an important osmolyte in the brain and the renal medulla. At these locations, it is a primary factor in the cell volume regulatory process, in which brain or renal cells swell or shrink in response to osmolar changes, but return to their previous volume according to the uptake or release of taurine. While there is a dearth of clinical studies in man concerning this volume regulatory response, studies in cats, rats, and dog kidney cells indicate the protective role of taurine in hyperosmolar stress. The infant depleted of taurine may not be able to respond to hyper- or hyponatremic stress without massive changes in neuronal volume, which has obvious clinical significance. The fact that the brain content of taurine is very high at birth and falls with maturation may be a protective feature, or compensation for renal immaturity Defining an amino acid as "conditionally essential" requires that deficiency result in a clinical consequence or consequences which can be reversed by supplementation. In pre-term and term infants, taurine insufficiency results in impaired fat absorption, bile acid secretion, retinal function, and hepatic function, all of which can be reversed by taurine supplementation. Therefore, this small beta-amino acid, taurine, is indeed conditionally essential.

Measurement of iron bioavailability by means of stable 54Fe and mass spectrometry

Stable isotopes represent a useful tool for the assessment of biokinetic parameters in in vivo studies on humans. A procedure is described to evaluate the bioavailability of iron in pharmaceutical preparations by means of 54Fe as a tracer and mass spectrometry for the determination of time dependent changes in the isotope ratio of 54Fe/56Fe in red blood cells. Iron tablets with an increased portion of 54Fe were administered to iron deficient subjects and red cell iron utilization was used as a measure of iron bioavailability. Iron utilization was derived from changes in the 54Fe/56Fe ratio as evaluated by means of fast atom bombardment-mass spectrometry (FAB-MS) on processed blood samples. A good intraindividual reproducibility was observed for blood samples drawn at various times after application of the trial drug. Figures for bioavailability and its interindividual variations were in the range expected from comparable studies on similar iron preparations using radioiron as tracers. The results obtained show that quantitative data of bioavailability from pharmaceutical iron preparations may be obtained without radiation exposure of the subjects investigated.