1. n-3 polyunsaturated fatty acids decrease responses to cytokines and inflammatory agents. The present study examines how different intakes of n-6 and n-9 fatty acids influence the metabolic response to endotoxin in Wistar rats.

2. Weanling male rats were, for 4 weeks, fed diets containing 50, 100 or 200 g/kg fat in the form of maize oil (rich in linoleic acid), butter (poor in linoleic acid, rich in oleic acid) or olive oil (adequate in linoleic acid, rich in oleic acid) or standard laboratory chow. All butter and olive oil diets included 10 g/kg maize oil, in total fat, to avoid essential fatty acid deficiency.

3. Rats subsequently received 800 μg/kg Escherichia coli endotoxin or sterile saline subcutaneously. Twenty-four hours after injection, the rate of tissue protein synthesis was measured in liver, lung, kidney, tibialis muscle and spleen by the ‘flooding dose’ method. Protein and zinc concentrations were assayed in all tissues and serum albumin and caeruloplasmin measured.

4. In animals fed chow, protein synthetic rate increased by 18%, 29% and 27% in liver, lung and kidney respectively. Tissue zinc concentrations increased by 33% in kidney, and tissue protein increased by 17%, 23% and 17% in liver, lung and kidney respectively. Serum caeruloplasmin increased by 60% and albumin concentration fell by 14%.

5. In animals consuming the 50 g/kg maize oil diet, protein synthetic rate increased by 56%, 36% and 34% in liver, lung and kidney respectively. Tissue zinc concentration increased by 14%, 15% and 17% in the three tissues respectively, and tissue protein concentration increased by 7%, 9% and 51% respectively. Serum caeruloplasmin increased by 172% and albumin concentrations fell by 22%.

6. No change in any parameter occurred in response to endotoxin in rats given diets containing fat predominantly as butter (50 and 100 g/kg), or olive oil (50, 100 and 200 g/kg).

7. In animals fed maize oil diets, responses increased in magnitude in parallel with dietary intake.

8. Responses in rats fed fat (200 g/kg) predominantly as butter were similar to those seen when diets contained 50 g/kg fat as maize oil.

9. The data suggest that the metabolic response to endotoxin is enhanced by n-6 and suppressed by n-9 unsaturated fatty acids. The modulatory influence of fats on responses to inflammatory agents may depend on the relative proportions of these substances.

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