Background: The dietary supplementation with EPA (eicosapentaenoic adic; 20:5n3) and DHA (docosahexaenoic acid; 22:6n3) has been recommended because of their favourable effects on the cardiovascular system (including complications of NIDDM). Oleic acid (18:1n9) from olive oil has some analogous and complementary effects. Potential competitive relations between long-chain n-3 fatty acids (FAs) and the oleic acid would therefore mean a problem.

Aim of the study: We focused primarily on the oleic acid changes in serum phospholipids (SPL) after a supplementation with EPA and DHA.

Methods: Thirty-five patients with type 2 diabetes mellitus (NIDDM) were supplemented for 28 days with 1.7 g of EPA plus 1.15 g of DHA/day (as Maxepa^® capsules, Seven Seas^®, U. K.). After that, a 3-month wash-out control period with 21 patients followed. A fatty acid composition of serum phospholipids (SPL) was determined by capillary gas-chromatography. Values were calculated as relative percentages of all FAs.

Results: After the supplementation with the Maxepa^® capsules, there was a very strong increase in EPA, docosapentaenoic acid (22:5n3) and DHA content in SPL. It was followed by a stron decrease after the wash-out (all p<0.0001). The oleic acid SPL content after the intervention significantly decreased from 10.105±0.307% (mean ±S.E.M.) to 9.082±0.276% (p<0.0003). During the wash-out, the change was in the opposite direction (p<0.0001). When the intervention and the wash-out periods were taken together, changes in the oleic acid were inversely correlated with changes in EPA, docosapentaenoic acid and DHA (r = −0.729; r = −0.552; r = −0.629, respectively; p<0.0001; n = 56). On the background of the overall n-6 FA reduction, the decline in the arachidonic acid after the supplementation (p<0.0001) and its rise after the wash-out (p<0.0003) were similar. There were no significant changes in the saturared FA spectrum.

Conclusions: Supplementation with long-chain n-3 FAs in NIDDM patients leads to the lowering of oleic acid SPL content. Whereas the reduction of the arachidonic acid may have some desirable aspects (e. g. suppression of thromboxane TxA₂ or 4 series leukotriene production), the decline of the former is to be regarded as a potential problem. Therefore, the search for optimally balanced blends of n-3 polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs) seems to be more promising than a supplementation with only one type of FA.