Absorption and endogenous faecal excretion of calcium by low birthweight infants on feeds with varying contents of calcium and phosphate.

Low birthweight infants aged 4-41 days were given from birth one of three experimental milk formulae varying widely in content of calcium and phosphate. Ca and P in feed, urine, and faeces were measured between carmine markers corresponding to a metabolic period of 48 hours. Calcium enriched in 46Ca to provide a marker for the dietary Ca was added to one feed and 46Ca measured in urine and faeces. True absorption of Ca and endogenous excretion into the bowel could then be inferred. True absorption of Ca was proportional to Ca intake and independent of P intake. Endogenous faecal excretion seemed to be independent of both Ca P intakes, and varied widely between different infants in the range 4-150 mg/day. Urine Ca was low and retention was essentially the difference between true absorption and endogenous faecal excretion. Retention of Ca tended to be much greater on a high Ca intake, but the variability in retention between infants on a given intake was large, paralleling the variability in endogenous faecal excretion. The variability in plasma Ca concentrations in newborn infants may in part be a consequence of wide individual variability in endogenous faecal excretion. The 46Ca marker technique provides a means of investigating the factors determining this variability.     

Urinary and faecal excretion of marker calcium (46Ca) by low birthweight infants

Low birthweight infants were given calcium enriched in 46Ca in a single feed. The specific activity of successive urine samples showed that the absorption of the marker was largely complete in about 4 hours. The rate of decrease of urinary specific activity after 3 1/2 hours was exponential and very similar in 8 sets of observations in 6 infants. Its extrapolation backwards to 1 hour may indicate the size of the exchangeable calcium pool, usually about 200 mg/kg body weight. Completeness of faecal collection for estimation of 46Ca is essential for accurate determination of true absorption and endogenous faecal excretion of natural Ca. In 5 infants examined, nor marker was detectable in faeces excreted later than 48 hours after the first stool containing marker. Nevertheless, reasons are given why a collection period limited to 48 hours may sometimes involve error.     

Urinary and faecal excretion of marker calcium (46Ca) by low birthweight infants.

Low birthweight infants were given calcium enriched in 46Ca in a single feed. The specific activity of successive urine samples showed that the absorption of the marker was largely complete in about 4 hours. The rate of decrease of urinary specific activity after 3 1/2 hours was exponential and very similar in 8 sets of observations in 6 infants. Its extrapolation backwards to 1 hour may indicate the size of the exchangeable calcium pool, usually about 200 mg/kg body weight. Completeness of faecal collection for estimation of 46Ca is essential for accurate determination of true absorption and endogenous faecal excretion of natural Ca. In 5 infants examined, nor marker was detectable in faeces excreted later than 48 hours after the first stool containing marker. Nevertheless, reasons are given why a collection period limited to 48 hours may sometimes involve error.     

Dual tracer stable isotopic assessment of calcium absorption and endogenous fecal excretion in low birth weight infants

Using a dual tracer (44Ca orally and 46Ca i.v.) stable isotope technique, true dietary Ca absorption, endogenous fecal Ca excretion, and net Ca retention were measured in 12 low birth weight (1426 +/- 260 g) infants fed a high Ca-containing formula. Endogenous fecal Ca excretion averaged 7.2 +/- 4.1% of intake, and exceeded 10% of intake in three infants. Net Ca retention, 103 +/- 38 mg/kg/d, was consistent with previous studies of Ca retention obtained using mass balance techniques and correlated closely (r = 0.98, p less than 0.001) with true Ca absorption but not with endogenous fecal excretion (r = -0.40, p = 0.19). Although endogenous fecal excretion may represent a significant source of Ca loss for some low birth weight infants, these data suggest that net Ca retention in low birth weight infants fed a high Ca-containing formula is primarily determined by the total dietary Ca absorbed.     

Effect of calcium supplementation on calcium and phosphorus balance and renal net acid excretion in preterm infants fed a standard formula

In 19 preterm infants fed a standard formula for prematures (calcium (Ca) 13.5 mmol/l; phosphorus (P) 12.9 mmol/l), biochemical parameters of blood, serum and urine were determined before and during supplementation with Ca-L-lactate (final Ca concentration 20 mmol/l). In 8 preterm boys Ca and P balance were evaluated in addition. During Ca supplementation, the serum Ca levels, urine pH (without supplement 6.31, with supplement 6.73), and calciuria (46 mumol/kg/d vs. 98 mumol/kg/d) were increased, and urinary P (1.05 mmol/kg/d vs. 0.65 mmol/kg/d) and net acid excretion (1.70 mEq/kg/d vs. 0.89 mEq/kg/d) were decreased. Balance studies showed increased net intestinal Ca absorption during supplementation (37% vs. 56%) as well as improved Ca (0.8 mmol/kg/d vs. 1.85 mmol/kg/d) and P retention (0.97 mmol/kg/d vs. 1.45 mmol/kg/d). These data show that increased Ca intake given to optimize the Ca:P ratio improves mineral retention in preterm infants fed a standard formula. Ca and P intake should be thoroughly balanced to avoid side-effects like hypercalciuria or high renal net acid excretion.     

Effect of Calcium Supplementation on Calcium and Phosphorus Balance and Renal Net Acid Excretion in Preterm Infants Fed a Standard Formula

ABSTRACT. In 19 preterm infants fed a standard formula for prematures (calcium (Ca) 13.5 mmol/l; phosphorus (P) 12.9 mmol/l), biochemical parameters of blood, serum and urine were determined before and during supplementation with Ca-L-lactate (final Ca concentration 20 mmol/l). In 8 preterm boys Ca and P balance were evaluated in addition. During Ca supplementation, the serum Ca levels, urine pH (without supplement 6.31, with supplement 6.73), and calciuria (46 μmol/kg/d vs. 98 μmol/kg/d) were increased, and urinary P (1.05 mmol/kg/d vs. 0.65 mmol/kg/d) and net acid excretion (1.70 mEq/kg/d vs. 0.89 mEq/kg/d) were decreased. Balance studies showed increased net intestinal Ca absorption during supplementation (37 % vs. 56 %) as well as improved Ca (0.8 mmol/kg/d vs. 1.85 mmol/kg/d) and P retention (0.97 mmol/kg/d vs. 1.45 mmol/kg/d). These data show that increased Ca intake given to optimize the Ca:P ratio improves mineral retention in preterm infants fed a standard formula. Ca and P intake should be thoroughly balanced to avoid side-effects like hypercalciuria or high renal net acid excretion.     

Calcium and phosphorus retention in extremely preterm infants supplemented individually

The aim of this study was to investigate the correlation between the retention of calcium (Ca) and phosphorus (P) and weight gain and intake of Ca and P when using the concept of individualized Ca and P supplementation in extremely low birthweight infants. Three-day Ca and P balances were performed in 20 infants with a mean gestational age of 26.6wk (between 24.1 and 28.7 wk) and a birthweight of 744 g (450-990), when the infant was able to tolerate at least 100 ml/kg/d of milk. The daily supplementation with Ca and P was individually adjusted to achieve a simultaneous excretion of > or = 1.2 mmol/L Ca and > or = 0.4 mmol/L P in the urine. In 16 of the 20 infants, the urinary concentrations of both Ca and P exceeded the lower limits. The retention of Ca (mean 3.8 mmol/kg/d, minimum 0.9; maximum 8.1; 57% of intake, 34-80) and P (2.4,1.1-4.2; 76%, 52-96) was significantly correlated with both the daily weight gain (16 g, 3-28; Ca r2 = 0.22, p = 0.02; P r2 = 0.21, p = 0.03) and the intake of Ca (6.5 mmol/kg/d, 2.4-10.2; r2 = 0.67, p < 0.001) and P (3.1, 1.9-5.3; r2 = 0.85, p < 0.0001). The molar ratio of the Ca and P intake was 2.2 (1.3-4.0). CONCLUSION: It was found that Ca and P retention was a function of growth and intake.     

Zinc absorption, distribution, excretion, and retention by healthy preterm infants

Zinc (Zn) is an essential nutrient for growth, but little is known about Zn absorption, distribution, excretion, and retention in preterm infants. Nine infants with gestational age 32+/-1 wk (mean+/-SE), birth weight 1.44+/-0.08 kg, postnatal age 14+/-3 d, on Zn intake of 23+/-3 micromol/kg per d via enteral feeding of preterm formula were studied. A stable Zn isotope (70Zn) was administered orally or i.v., and plasma, red blood cells, urine, and feces were sampled for up to 30 d. Samples were analyzed for Zn by inductively coupled plasma atomic emission spectrometry and for isotope enrichment by inductively coupled plasma mass spectrometry. Data were analyzed by compartmental analysis using the Simulation Analysis and Modeling program, and absorption, distribution, excretion, and retention were calculated. Absorption was 36+/-5% or 7+/-1 micromol/kg per d; distribution in plasma was 15+/-1 micromol Zn/L and in RBC was 41+/-4 micromol Zn/L; excretion in urine was 0.55+/-0.03 micromol Zn/kg per d and in feces was 17+/-3 micromol Zn/kg per d and retention was 5+/-1 microl/kg per d. Results show that healthy preterm infants with Zn intake of 23 micromol/kg per d and expected growth rates (> 15 g/kg per d) absorb and retain Zn at rates comparable to in utero accretion. The values for absorption, distribution, and excretion by this population of healthy preterm infants provide a normal range for future studies, although further studies are required to determine endogenous excretion rates in healthy preterm infants. We speculate that these values can be used to determine whether Zn kinetics are abnormal in sick infants or in infants with slow growth.     

Absorption of calcium in premature infants as measured with a stable isotope 46Ca extrinsic tag

Absorption of dietary calcium was evaluated with the extrinsic tag approach and stable isotope methodology in growing premature infants. Fractional absorption of a bolus dose of 46Ca was determined on 16 occasions in 13 premature infants (birth weight 1135 +/- 40 g, gestational age 29.5 +/- 0.4 wk, mean +/- SE) and was found to be 84.4 +/- 2.2%. Fractional absorption of 46Ca ranged between 65 and 97%, and did not appear to be influenced by postnatal age, postconceptual age, body weight, or intake of preterm human milk, fortified preterm human milk, or premature formula. Therefore, if absorption of the 46Ca dose reflects that of dietary calcium, about 80% of dietary calcium is absorbed.     

Low renal net acid excretion, high calciuria and biochemical signs of sodium deficiency in low-birth-weight infants fed a new low-phosphorus formula

In 11 infants (birth weight greater than 1800 g) fed a new type of humanized formula with a low phosphorus (P) content (calcium (Ca) 11 mmol/l, P 7.2 mmol/l, sodium (Na) 8.3 mmol/l) biochemical parameters of blood, serum and urine were determined. In nine boys Ca and P balances were evaluated also. Renal net acid excretion was low (0.85 mmol/kg/day). Mean concentrations of P and Ca in urine were 0.34 mmol/kg/day (10.5 mg/kg/day) and 0.1 mmol/kg/day (4 mg/kg/day), respectively. In four infants, Ca concentration in urine was, however, greater than 0.15 mmol/kg/day) (6 mg/kg/day). In infants with birth weights greater than 1800 g fed the new, low-P formula, the low renal net acid excretion, the normal P and the high Ca concentrations in urine were comparable to term infants fed human milk. The high calciuria in several infants may be normal physiologic values. However, it remains to be established that the urinary solubility product of infants fed the new, low-P formula is in the same range as those for infants fed human milk. Unexpectedly, low urinary Na excretion (0.26 mmol/kg/day) and increased urinary excretion of aldosterone-18-glucuronide indicated biochemical evidence of Na deficiency secondary to low Na intake and a high weight gain. If the new, low-P formula is to be fed to infants with a birth weight as low as 1800 g. Na content should be higher than in mature human milk because of the often relatively higher weight gain.     

Relation between changes in plasma calcium in first week of life and renal function.

(1) Of 71 infants fed on reconstituted dried or evaporated cow's milk, 31 showed a fall in plasma calcium between the 1st and 6th days of life, whereas in 35 breast-fed infants this occurred in only 5. (2) Those artificially-fed infants who had shown a rise in plasma calcium over this period had significantly lower plasma creatinine values and significantly higher excretion of creatinine than those infants who showed a fall in calcium levels. (3) Artificially-fed infants who had shown a rise in calcium had significantly lower plasma osmolality and significantly higher osmolar excretion in the urine than those infants who showed a fall in plasma calcium. (4) It is suggested that a delay in the normal increase in glomerular filtration rate during the first week of life in some infants leads to phosphate retention. This, together with a higher dietary intake of phosphate, leads to a decrease of the plasma calcium to hypocalcaemic levels.     

Influence of protein intake and liver function on acid base balance in premature infants

In 43 patients with late metabolic acidosis (LMA) the factors promoting LMA were investigated. Postnatal adaptation was distributed in all cases, in 35 patients acidosis developed after introduction of formula feeding. Whereas no differences were observed in renal function (urine volume and renal molar excretion) between acidotic and non-acidotic patients, there was a significantly higher concentration of bile acids in serum (26.1 +/- 9.6 vs 98.6 +/- 21.6 mumol/l), a significantly increased fractional volume of stools (8.2 +/- 1.3 vs 11.4 +/- 1.9% of intake, and higher faecal fat excretion (26.5 +/- 5.2 vs 39.1 +/- 6.6% of faecal weight) in LMA patients than non-acidotic formula-fed infants. It is suggested that impaired postnatal development of liver function caused by severe disturbances of postnatal adaptation (respiratory distress, persistent fetal circulation, sepsis) is one of the most important factors in the pathogenesis of LMA. Thus, liver function should be checked before a protein intake surpassing that of a breastfed infant is introduced. Concentration of the serum bile acid level seems a reliable marker of LMA.     

EXCRETION OF BILE ACIDS IN EXTRAHEPATIC BILIARY ATRESIA AND INTRAHEPATIC CHOLESTASIS OF INFANCY

This series included 24 infants, 16 boys and 8 girls, who were admitted to hospital with the diagnosis of obstructive jaundice. Five of the infants were subsequently found to have extra-hepatic biliary atresia (BA) and the other 19 infants intrahepatic cholestasis of infancy (IHC). The infants were investigated given special attention to: the quantitative urinary excretion of cholic and chenodeoxycholic acids, the isotope excretion after intramuscular injection of cholic acid-24–¹⁴C, the nature of labelled urinary bile acids, the half-life and the pool size of cholic acid. At the first examination of the infants after admission the urinary excretion of cholic and chenodeoxycholic acids varied greatly between the patients. However, on comparing the values obtained in the two groups, it was found that there was virtually no difference between the mean daily values of cholic and chenodeoxycholic acids in urine, and the ratio cholic to chenodeoxycholic acid between the BA group and the IHC group. After the injection of isotopic cholic acid most of the isotope was recovered in the urine in all cases. In the infants with BA the faecal excretion of the isotope was low, being less than 3 per cent of the injected isotope. Out of the 19 infants with IHC the recovery of the injected isotope in faeces was also less than 3% in 11 infants. In 8 infants with IHC the faecal isotope excretion was significantly high to exclude extrahepatic biliary atresia. The first 24 hour urine specimen contained small amounts of unconjugated labelled cholic acid in all cases whereas in no case did the patients excrete unconjugated labelled cholic acid 48 hours after the injection of the isotope. No transformation of cholic acid was observed. There was no difference between the BA group and IHC group with regard to the percentage labelled glycine conjugates of total excreted urinary conjugates. Neither was there any difference between the two groups with regard to half-life and pool size of cholic acid. There was no difference with respect to the bile acid metabolism between infants with congenital CMV infection, decreased serum concentrations of alfal-antitrypsin and the other patients.     

Tubular calcium,magnesium, and phosphate excretion during therapeutic hypothermia for neonatal hypoxic–ischemic encephalopathy: A prospective study

ObjectivesHypocalcemia, hypomagnesemia, and hyperphosphatemia are common electrolyte disturbances in perinatal asphyxia (PA). Different reasons have been proposed for these electrolyte disturbances. This study investigated the effect of the urinary excretion of calcium (Ca), magnesium (Mg), and phosphorus (P) on the serum levels of these substances in babies who were treated using therapeutic hypothermia for hypoxic ischemic encephalopathy (HIE) caused by PA. This study sheds light on the pathophysiology that may cause changes in the serum values of these electrolytes.MethodsThis study included 21 healthy newborns (control group) and 38 patients (HIE group) who had undergone therapeutic hypothermia due to HIE. Only infants with a gestational age of 36 weeks and above and a birth weight of 2000 g and above were evaluated. The urine and serum Ca, Mg, P, and creatinine levels of all infants were evaluated at 24, 48, and 72 h.ResultsThe lower serum Ca value and the higher serum P value of the HIE group were found to be statistically significant compared to the control group (p<0.05). There was no significant difference in serum Mg values between the groups. However, hypomagnesemia was detected in five patients from the HIE group. The urine excretion of FeCa and FeMg at 24 h, and FeP excretion at 48 and 72 h were found to be significantly higher in the HIE group compared to the control group.ConclusionsThis study determined that the urinary excretion of Ca, Mg, and P has an effect on the serum Ca, Mg, and P levels of infants with HIE.     

Fractional calcium absorption is increased in girls with Rett syndrome

BACKGROUND: Rett syndrome (RTT), an X-linked neurodevelopmental disorder primarilyaffecting girls, is characterized in part by osteopenia and increased risk of skeletal fractures. We hypothesized that decreased intestinal calcium (Ca) absorption relative to dietary Ca intake and increased renal Ca excretion might cause these problems in RTT. OBJECTIVE: We measured fractional Ca absorption, urinary Ca loss, dietary Ca intake, and the hormonal factors regulating Ca metabolism to determine whether abnormalities in Ca balance might relate to poor bone mineralization in RTT girls and to evaluate the contribution of these factors to the overall dietary Ca needs of RTT girls. STUDY DESIGN: Ten RTT girls and 10 controls, matched for age, sex, and pubertal status, were given a 3 day constant Ca diet that mimicked their habitual intakes. At the end of each dietary period, girls received single doses of Ca (intravenous) and Ca (oral). Fractional urinary excretion of Ca, Ca, 24 hour urinary Ca, and urinary cortisol excretion were determined. Serum Ca, phosphorous, alkaline phosphatase, vitamin D metabolites, parathyroid hormone (PTH), and osteocalcin were measured in the postabsorptive state. Bone mineral content (BMC) was measured by dual-energy x-ray absorptiometry. RESULTS: Fractional Ca absorption was significantly higher in RTT than in control girls (mean +/- SDp, 52 vs. 33 +/- 13%). Dietary Ca intake (mean +/- SDp, 1,100 vs. 1,446 +/- 440 g/d) and net Ca absorption (mean +/- SDp, 513 vs. 362 +/- 306 mg/d) did not differ significantly between RTT and controls, respectively. Although urinary Ca excretion did not differ between groups, the increased urinary Ca:creatinine ratio (mean +/- SDp, 0.39 vs. 0.23 +/- 0.38) was consistent with clinical hypercalcuria and paralleled the significantly increased urinary cortisol excretion (mean +/- SDp, 3.1 vs. 1.7 +/- 1.1 mg/kg lean body mass per day) in the RTT girls. BMC was significantly lower in RTT than in controls (mean +/- SDp, 527 vs. 860 +/- 275 g). Serum Ca, P, alkaline phosphatase, vitamin D metabolites, PTH, and osteocalcin concentrations did not differ between the groups. CONCLUSION: Fractional Ca absorption showed a compensatory increase in the presence of adequate dietary Ca intakes, mild hypercalcuria, and pronounced bone mineral deficits in RTT girls. Whether supplemental dietary Ca could enhance fractional Ca absorption and improve bone mineralization in RTT girls is unknown.     

A metabolic balance study in term infants fed lactose-containing or lactose-free formula

Lactose-free (L-) formulas are recommended for infants with conditions affecting lactose digestion. Cows' milk protein-based formulas containing other carbohydrate sources are most often used for such infants. This study compared fat absorption and absorption and retention of nitrogen, calcium, phosphorus, and magnesium in term infants fed either a L- or standard lactose-containing (L+) bovine milk protein-based formula. Data from three single-centre, double-blind, randomized, parallel-group metabolic balance studies were combined. After a 4-7-d equilibration period on either L- or L+ formula, a 72-h balance study was performed. Twenty infants received L- and 21 received L+ formula. Besides the L- group having a higher percentage of males (65%) and the L+ group a higher percentage of females (52.4%), other baseline measurements were similar. The majority of nutrient balance data was similar between the two groups. Exceptions were relative nitrogen absorption, calcium intake and calcium retention, magnesium retention, and faecal phosphorus excretion, all of which were significantly higher in the L- group. Vitamin D supplementation did not significantly affect either calcium or phosphorus data. This new L- formula provided similar nutrients and is a suitable alternative to a L+ formula in term infants requiring L- feedings.     

Bioavailability of urea nitrogen for the low birthweight infant

Previous studies have demonstrated increased retention (40%) of dietary urea nitrogen by term infants recovering from infection compared to healthy infants (13%), possibly due to a higher requirement for nitrogen. Since low birthweight infants also have a high requirement for nitrogen, the bioavailability of urea nitrogen was investigated in low birthweight infants using 15N,15N-urea. Four low birthweight infants (gestational age = 30 +/- 2.2 weeks [mean +/- SD], birthweight = 1.4 +/- 0.3 kg) were fed formula enriched with 15N,15N-urea. Metabolic balance studies (72 hours) were performed with urine and fecal collections. Nitrogen was quantitated by Kjeldahl analysis and 15N,15N-urea by gas chromatography/mass spectrometry. Mean nitrogen intake was 489 +/- 32 mg/kg/d, with 75% nitrogen absorption and 62% nitrogen retention. Maximum urinary enrichment was 8%. Cumulative 15N,15N-urea excretion was 72%, resulting in 28% retention. Thus the bioavailability of urea nitrogen for low birthweight infants appears to be intermediate between compromised and normal term infants.     

Renal aspects of calcium and phosphorus metabolism in preterm infants

The aim of this study is to emphasize renal aspects of calcium and phosphorus metabolism from our data of more than 200 metabolic balance studies carried out in preterm infants. Renal production of 1,25-dihydroxyvitamin D increased rapidly after birth provided the concentration of the substrate, 25-hydroxyvitamin D, is adequate. The gut of preterm infants is able to respond to the active metabolite of vitamin D. Mean plasma phosphate threshold for tubular reabsorption of phosphate is high, about 2.1 mmol/l or 6.5 mg/dl. The low fractional excretion of phosphate cannot be explained by immature parathyroid function nor by renal unresponsiveness to parathormone, at least after the first days of life. It is probably due to regulating factors related to the high rate of growth. Because of reduced glomerular filtration rate, a too high phosphorus intake may result in hyperphosphatemia. Conversely, a too low phosphorus intake will lead to a phosphate depletion syndrome characterized by marked increase in urinary calcium excretion, no urinary phosphate, and hypophosphatemia. Preterm infants with chronic metabolic acidosis are able to acidify urine so that titratable acid is directly related to urinary excretion of phosphate. Clinical implications are that calcium:phosphorus ratio in milk must be adapted according to net bone and soft tissue retention.     

Parenteral nutrition for infants: effect of high versus low calcium and phosphorus content

Calcium (Ca) and phosphorus (P) homeostasis were determined in 18 infants (birth weight, 2,810 +/- 135 g; gestational age, 37.4 +/- 0.5 weeks; mean +/- SEM) who received high or low Ca and P content (Ca, P) parenteral nutrition (PN) with a fixed, low dose of vitamin D (25 IU/dl). Nine infants were randomized into low (standard) Ca, P (20 mg Ca and 15.5 mg P/dl) and nine into high Ca, P (60-80 mg Ca and 46.5-62 mg P/dl) PN, and then were studied for up to 6 weeks. The high Ca, P group had stable serum 1,25 dihydroxyvitamin D [1,25(OH)2D], which consistently remained within the normal range (less than 116 pg/ml). Tubular reabsorption of phosphorus (TRP) also was stable and remained consistently less than 90%. The low Ca, P group had elevated and higher 1,25(OH)2D (p = 0.03) than the high Ca, P group. The mean serum 1,25(OH)2D concentration rose from 32 to 112, 115, and 133 pg/ml over a period of 6 weeks. TRP also was higher (p = 0.02) and remained consistently greater than 90%. There were no significant differences between groups in serum parathyroid hormone, calcitonin, Ca, Mg, P, alkaline phosphatase, vitamin D binding protein, and 25 hydroxyvitamin D concentrations; urine Ca/creatinine and Mg/creatinine ratios, and fractional excretion of sodium (Na). Thus, a "high" Ca (60 mg/dl) and P (46.5 mg/dl) content in PN solutions can result in stable serum 1,25(OH)2D and TRP, presumably reflecting minimal stress to Ca and P homeostatic mechanisms without further increase in urinary Ca excretion.     

Zinc absorption and exchangeable zinc pool sizes in breast-fed infants fed meat or cereal as first complementary food.

BACKGROUND: The aims of this study were to compare the absorption efficiency of zinc from rice cereal and meat, with and without human milk, in 7-month-old breast-fed infants and to compare the size of exchangeable zinc pools in the infants according to the assigned complementary food. METHODS: Fractional absorption of zinc was measured in male infants using extrinsic labeling with a stable isotope of zinc in a test meal of either pureed beef (n = 9) or iron-fortified infant rice cereal (n = 9). The effect on fractional absorption of the addition of human milk to each complementary food was measured in each infant with a second oral zinc isotope. Fractional absorption was measured using fecal monitoring of isotope excretion, and exchangeable zinc pool size was calculated from isotopic enrichment in urine. RESULTS: Fractional absorption of zinc did not statistically differ between the beef (0.41 +/- 0.11) and cereal (0.36 +/- 0.05) test meals, although the trend showed that beef had higher fractional absorption than cereal. The higher intake of zinc from the beef versus cereal test meal resulted in a 16-fold greater amount of absorbed zinc ( P = 0.0002). The addition of human milk caused significant decreases in fractional absorption of zinc (0.07 +/- 0.02, P = 0.01) and absorbed zinc (0.04 +/- 0.01 mg, P < 0.0001). The size of the exchangeable zinc pool did not differ according to group but was strongly correlated with mean daily zinc intake ( r = 0.72, P = 0.003). CONCLUSIONS: These results confirm that meat as a complementary food for breast-fed infants can provide a rich source of dietary zinc that is well absorbed. The significant positive correlation between zinc intake and exchangeable zinc pool size suggests that increasing zinc intake positively affects metabolically available zinc.