In the balance studies, daily intakes of 0.23 μg/kg resulted in a balanced molybdenum metabolism in breast-fed infants. The dietetically treated patients receiving supplemented diets retained 3.2 μg/kg. This exceeded the complete intake of breast-fed infants more than 13-fold. Although the ranges within the groups are considerable, there was no correlation of total molybdenum intake and excretion between breast-fed infants and infants with PKU (Fig. 1).
The plasma concentrations reflected the results obtained in the balance trials. With the exception of one extreme value registered in one breast-fed participant at 4 months, the results in infants with PKU were above those of the healthy infants. The initial median plasma concentration was 0.04 μg/l in breast-fed infants compared with 2.5 μg/l in the infants with PKU. The values were 1.48 μg/l (range, 1.22–3.65 μg/l) in infants with PKU and 1.41 μg/l (range, 0.47–2.09 μg/l) in breast-fed infants at 1 year of age. With a molybdenum supply predominantly from supplementary food, the differences in plasma concentrations were negligible (Fig. 2).
Human milk is regarded as the preferred feeding in the first 4 to 6 months of infancy (20,22). Many dietetically treated infants with inborn errors of metabolism cannot be nourished in accordance with these recommendations. Their supply of essential nutrients, however, should be equivalent to that of healthy breast-fed infants, when evaluated by comparative studies.
In conservative trace element balance studies, human milk supplied the breast-fed infant daily with 0.23 μg molybdenum/kg (Fig. 1). This was sufficient to maintain balanced results. This intake was similar to the results of other intake studies (13). In infants with PKU, however, not only the daily molybdenum intake of 6.6 μg/kg, but also the retention of 3.2 μg/kg was considerably higher. Alexander et al. (19) reported a daily retention of 2 μg/kg in two infants with PKU, which was comparable to the retention in infants in our study. Neither the nature of the disease nor knowledge gained from previous studies suggests that the high retention is due to special needs of this group. Unnecessary storage is more likely. Because it has been shown in adults that molybdenum retention is dependent on urinary excretion (23), supplementation leading to substantial quantitative retention of this trace element may be specific to early infancy, related to the physiological development of renal function.
Molybdenum intake in infancy is predominantly dependent on four aspects of nutrition:
1. The route of application (enteral or parenteral). Parenteral products may contain considerable amounts of molybdenum by contamination (24). Supplementation in infancy is recommended only in long-term total parenteral nutrition (0.25 μg/kg per day) (25).
2. The choice of nutrition modifies the intake, in that there is a low concentration in human milk (14), a wide range of concentrations in infant formulas (17), and a high concentration in special foods for medical purposes, supplemented in accordance with dietetic directions (17,18).
3. The time of the introduction of supplementary foods outlined by the nutritional concept pursued (e.g., 20) limits the importance of formulas or human milk as exclusive nutritive sources.
4. The molybdenum content of supplementary foods depends on the choice and origin of products (26).
While pastry and fruits do not contain much molybdenum, not only animal products but also vegetables have higher molybdenum concentrations (26). The range of molybdenum concentrations in water is considerable; mineral waters contain 0.22 to 21.95 μg/l (27). The upper limit in drinking water was set at 70 μg/l by the World Health Organization (28,29). Diets in infants with PKU are thus not necessarily prone to render an insufficient supply of molybdenum.
Longitudinal plasma studies underline the importance of the nutritional supply in early infancy (Fig. 2). The different ranges assessed for the groups were dependent on nutrition rather than age. The initial results of infants with PKU exceeded those of breast-fed infants by far during a period of more than 4 months and reflected the molybdenum retention observed in parallel balance trials (Fig. 1). The plasma concentration in breast-fed infants decreased with age until the introduction of supplementary foods. There was no difference between the groups at the age of 1 year.
Systematic data on molybdenum plasma concentrations in relation to nutrition in infancy are scarce. Cord blood and blood samples drawn from the respective mothers rendered a significant correlation between the molybdenum concentrations (37). The mean result of 1.44 μg/l, however, was not confirmed by other investigators who found 0.7 μg/l (range, 0.4–1.6 μg/l) (38). Values of 0.6 μg/l in breast-fed infants and 1.9 μg/l in infants fed breast milk and formula were observed (39). Concentrations assessed at the end of the first year in the present study are equivalent to those in patients with PKU (1.33 ± 0.5 μg/l) and healthy children aged 2 to 12 years (1.75 ± 0.8 μg/l) (40). Recent studies revealed a mean molybdenum concentration of 1.6 and 1.9 μg/l in infants with PKU at the ages of 3 and 6 months, respectively (41). The molybdenum intake in formula-fed preterm infants correlated with urinary excretion as well as with plasma and urine concentrations (16).
Molybdenum was not completely excreted in long-term ammonium tetrathiomolybdate-treated sheep but was widely distributed in many organs, including the brain and pituitary (42). The response of nonruminants on molybdenum excess is partly suggestive of an effect on copper use (43,44). The reaction of molybdate with sulfide generated by bacterial reduction of sulfate within the gastrointestinal tract may lead to the formation of thiomolybdates reacting with copper. Based on the fact that large amounts of sulfide can be generated within the human colon on high sulfate intake (45), high human intakes of molybdenum (46) may be sufficient for thiomolybdate formation and effects on copper utilization. Therapeutic applications of ammonium tetrathiomolybdate in adults have been recommended to enhance copper excretion in the treatment of Wilson's disease (120–240 mg molybdenum per day) (47,48)). Though molybdenum intakes provided by formulas are considerably lower, copper metabolism should be monitored in recipients of long-term molybdenum-supplemented diets.
In conclusion, molybdenum supplementation of diets in PKU leads to a substantial retention, exceeding the respective results of healthy breast-fed infants by far. This is reflected by increased plasma concentrations in the first months of life. There remains a deficit in data to support the necessity of molybdenum supplementation and exclusion of interactions with other trace metals. The present practice of molybdenum fortification of diets used in early infancy should be revised in light of the actual findings.
The authors thank the patients' families for their participation and cooperation. The study was funded by the Deutsche Forschungsgemeinschaft (Si 514-1). The authors thank Milupa Co. (Friedrichsdorf, Germany), Hipp AG (Pfaffenhofen, Germany), Fürst Bismarck Quelle (Aumühle, Germany), and SHS GmbH (Heilbronn, Germany) for providing a continuous supply of defined nutritional products.
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