Lead (Pb)

Essentiality of Lead

Despite the remarkably high toxicity of lead, very small amounts of this metal have been proved to be essential. The essentiality of lead can be demonstrated in model studies by growth depressions, biochemical changes, and clinical symptoms occurring in the progeny of rats fed a lead-depleted diet. Now, the essential nature of lead needs to be investigated with regard to its biological functions in the body and to the mode of interaction between lead and iron.

Schwarz (1974) added 1 ppm of Pb acetate and found an increased growth rate in rats of 12% over those of the controls (1.79 versus 2.08 g/day). In further studies with rats (Reichlmayr-Lais and Kichgessner, 1984), animals consuming low dietary Pb (Further evidence for the essentiality of lead has been produced in two laboratories, and the results are suggestive but fall short of being completely convincing. Schwarz reported that rats grown in an ultraclean environment and with a diet supplying no more than 0.2 mcg lead per gram, grew better when the diet was supplemented with lead at 1 .0 or 2.5 mcg/g. Since then Reichlmayr-Lais and Kirchgessner have undertaken a series of studies in which rats were maintained through three generations on a diet containing 18 mcg lead per kilogram or the same diet supplemented with lead at I mcg/g In rats of the F1, but not the FO or F2 generation, lead deprivation significantly depressed growth, decreased liver iron stores, changed the activity of some enzymes including alkaline phosphatase, and produced hypochromic microcytic anemia. The effects were not seen in all rats of the F1 generation and were transient. In a later experiment lead depletion was again shown to retard growth and decrease carcass and tissue iron concentrations but, contradictorily, there was an increased positive iron balance.

Lead, Cadmium, and Arsenic

In the mid-1970s, Schwarz and others directed their attention to the study of the nutritional roles of lead, cadmium, and arsenic, three elements that until then had always been regarded only as poisonous. Schwarz could still report initial evidence for the biological essentiality of all three of them (Schwarz, 1977), but his death prevented completion of these studies. Although not all criteria of essentiality of Cd and Pb have been met, his observations were independently confirmed and extended by Anke et al. (1977) and Kirchgessner and Reichlmayr-Lais (1981). With arsenic, positive growth responses in young rats were observed (Schwarz, 1977), but at that time convincing evidence for the essentiality of arsenic had already been produced by two other groups (Anke et al., 1976; Nielsen et al., 1975). Nielsen’s arsenic-deficient rats exhibited rough fur, increased osmotic fragility of the erythrocytes, and abnormally enlarged spleens containing excessive amounts of iron. Anke’s As-deficient goats and pigs showed decreased fertility, low birth rates, and retarded growth. Lactating arsenic-deficient goats were also observed to die suddenly with myocardial damage. The discovery of the essentiality of arsenic and of the possible essentialities of lead and cadmium should of course not detract from the established fact that higher concentrations of these elements pose definite health hazards.

Lead has been shown to enhance the growth and improve the hematocrit and hemoglobin concentrations of iron-deficient rat pups.

(E O Uthus & F H Nielsen. Effects in rats of iron on lead deprivation. Biol Trace Elem Res 16 155, 1988). This suggests that low levels of lead (water-soluble) may be beneficial to red blood cell production under certain conditions.
Essential and Toxic Trace Elements in Human Health and Disease: An Update, pages 355-376, 1993.

Ultratrace Elements of Possible Importance for Human Health: An Update
Forrest H. Nielsen, PhD

USDA, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND 58202

In a series of reports summarized by Kirchgessner and Reichlmayr-Lais (1981), it was reported that lead deprivation depressed growth, disturbed iron metabolism, and altered the activities of several enzymes and the liver concentrations of several metabolites associated with iron status. These scientists continue to get findings which suggest that lead is necessary for optimal iron metabolism. Recently they found that lead deprivation elevated iron absorption and retention, and depressed the iron concentration in serum and various organs of rate (Reichlmayr-Lais and Kirchgessner,1988). Uthus and Nielsen (1988) found that lead enhanced growth and improved hematocrits and hemoglobin concentrations of iron-deficient pups; they suggested that the effects might have been the result of a pharmacologic action of lead. The early findings of Kirchgessner and Reichlmayr-Lais (1981) were considered as possibly being the result of a pharmacologic effect of lead on iron metabolism (Nielsen, 1984c). However, Kirchgessner et al. (1988) found that absolute lead retention increased as dietary lead increased from 25 to 225 ng/g diet, then decreased as dietary lead increased from 225 to 1,025 ng/g diet. When dietary lead was increased from 5 to -600 mcg/g diet, absolute lead retention was markedly elevated. These findings indicate that homeostatic mechanisms exist for lead, and also suggest that lead at low dietary intakes is an essential element. Furthermore, the possibility must be considered that the stressor of suboptimal iron status enhances the response to lead deprivation.