AN EVALUATION OF THE NUTRITIONAL, METABOLIC AND IMMUNE STATUS IN OCCUPATIONAL LEAD TOXICITY

Abstract

The modulating effect of nutritional factor on lead (Pb) toxicity and the adverse effects of Pb metabolism and immune function were evaluated in occupational Pb exposure. One hundred and thirty-seven (137) subjects comprising of 86 Pb workers and 51 appropriately matched controls were studied. The Pb workers were classified into 3 exposure categories according to severity of exposure based on prevailing air Pb levels (PbA) at the work sites; (a) low PbA <500µg/m³), (b) moderate PbA 500-200µg/ m³), (c) severe (PbA >2000-4000µg/m³). Blood and urine sample were collected randomly from all subjects. Anthropometric indices, socio-demographic factor, and dietary intake were not significantly different between Pb workers and controls (P>0.05). Blood lead level (BLL) of the Pb workers were significantly higher than in controls (P<0.001). The BLL did not correlate significantly with severity and duration of exposure. Ninety-five percent of Pb workers had BLL >40µg/dL (1.93µmol/L) (the upper limit currently acceptable in occupational exposure) 70% of the Pb workers had BLL >55µg/dL(2.70µmol/L) (a level indicative of severe poisoning), 38% of Pb workers had BLL (2.90µmol/L) (a level considered indicative of the need to exclude the worker from further exposure), the rest of the workers had BLL <40µg/dL. The BLL in controls was 2 to 3 fold current acceptable levels. Total and ionized Ca were significantly lower in Pb workers than in controls (P<0.01; P<0.001 respectively). In contrast, zinc (Zn) and copper (Cu) were significantly higher in Pb workers than in controls (P<0.001; P<0.001 respectively). The Zn level In the low exposure group (a) which had the highest BLL was significantly lower than in the medium and severe exposure groups ("b" and "c") (P<0.01; P<0.001 respectively). Indices of iron (Fe) homeostasis such as serum Fe, TIBC, transferrin and percent Fe saturation did not differ significantly between Pb workers and controls (P>0.05). However, group "a" had significantly lower Fe level than groups "b" and "c" P<0.001; P<0.001 respectively). Vitamin C excretion rate was significantly lower in Pb workers than in controls (P<0.02). Serum K⁺ level employed to assess erythrocyte membrane status was significantly elevated in Pb workers compared with controls (P<0.01), reportedly due to inhibition of Na⁺ -K⁺ATPase. Urate level was also significantly higher in Pb workers than in controls (P<0.001) and positively correlated with BLL (r + 0.24; P<0.026). Cholinesterase (ChE) employed to assess acetylcholine metabolism significantly inhibited in Pb workers compared with controls (P<0.005). Erythrocyte protoporphyrin (EPP) and porphobilinogen (PBG), were similar in Pb workers and controls, probably due to the synergism of Cu and Zn on the haem pathway enzymes. There was also no alteration in RNA metabolism as assessed by the absence of basophilic stippling in Pb workers probably due to non sigibificant inhibition of pyrimidine 5 nucleotidase (P5N). Both total and bone form (isoenzyme) of alkaline phosphatese (ALP, B-ALP) were similar in Pb workers and controls (P>0.05). The ALP and B-ALP were however, lower in group "a" (group with significantly decreased Zn level) than groups "b" and "c" (P40,05, (P<0.05, respectively). Some indices of immune function such as total lymphocyte count (TLC), lgA, IgG were significantly depressed in Pb workers compared with controls (P<0.01; P<0.01 and P<0.001 respectively). IgM level was however, not significantly depressed (P>0.05). Additionally, IgA was negatively correlated with BLL (r-028, P<0.009), suggesting immunosuppression. In contrast, total globulins and C-reactive protein (C-RP) levels were significantly elevated in Pb workers compared with controls (P<0.001; P<0.001 respectively); probably indicating presence of inflammatory state in Pb workers. Some indices of haematotoxicity Hb, PCV, MCHC were significantly decreased in Pb workers compared with controls (P<0.001; P<0.001 and P<0.001 respectively). Of the indices of renal function, creatinine, urea urinary microalbumin (index of subclinical renal damage) and urinary protein, only urinary protein was significantly elevated in the Pb workers compared with controls (P<0.001). Multiple regression and principal component analyses suggest among others that total globulin, IgA, IgG, TLC. Urate ionized calcium, and ChE were the most significant and informative parameters related to BLL. High lead burden exists in Pb workers and unexposed subjects in this environment. The burden in Pb workers is associated with alterations in Ca, ascorbate, urate and acetylcholine metabolism; modulated by the synergistic antioxidant effects of Cu and Zn and causes immunosuppression, erythrocyte membrane impairment, unconventional haematotoxicity and inflammatory disorders, These findings will enhance the understanding and management of Pb toxicity.