1. Alterations in plasma albumin concentration and gastrointestinal permeability have been investigated in rats infected with the nematode Nippostrongylus brasiliensis and fed adequate or low protein diets.

2. Infection caused only minor changes in growth and food consumption of well nourished rats but resulted in significant reductions in those fed the low protein diet.

3. Animals in both dietary groups were able to mount an immune response beyond day 10 postinfection (p.i.) which caused expulsion of the parasites, but this was less effective in rats fed the low protein food.

4. Uninfected rats fed the low protein diet had significantly lower plasma albumin concentrations than their well nourished counterparts. Animals of both dietary groups showed a progressive reduction in plasma albumin concentration as the infection developed but values returned towards normal as the parasites were expelled.

5. The reduction in plasma albumin concentration was closely associated with increases in gastrointestinal leakage of plasma protein but losses were far greater in the protein deficiency animals. Beyond day 10 p.i. protein loss decreased in both dietary groups and by day 21 p.i. had returned to normal in well nourished animals but not those fed the low protein diet.

6. Intestinal permeability measured by the lactulose:mannitol ratio technique gave similar results to the protein loss data. Permeability increased as the infection progressed then fell as the worms were expelled but remained above control values in infected protein deficient animals.

7. Overall, animals fed the low protein diet were more severely affected by the parasite than were their well fed counterparts. The data clearly demonstrated that the combined effects of infection and dietary deficiency resulted in a more severe reduction in plasma albumin values than either factor produced alone. The results are discussed with reference to the aetiology of hypoalbuminaemia and kwashiorkor in man.

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