I. Geurden, P. Bergot, P. Coutteau, P. Sorgeloos
This study investigates possible roles of phospholipids (PL) in diets of young fish. These were start-feeding larval carp, Cyprinus carpio, or newly-weaned marine fish, European sea bass, Dicentrarchus labrax, and turbot, Scophthalmus maximus, fed dry diets with different PL compositions.
Each experiment confirmed the growth-promoting effect of PL, irrespective the origin, i.e. vegetable or animal. For the early carp larvae, PL-deficient diets also provoked a drastic mortality. In a first part, we saw that the positive PL effect in se was not explained by the nutritional (energy, essential fatty acids, choline, inositol) and physical (emulsifier) particularities of the PL molecule.
The second part showed that the response of the fish varied according to the polar head group of the PL. Phosphatidylcholine (PC) increased the initial growth of carp larvae, but caused severe body deformities, whereas phosphatidylinositol-rich fractions provided an optimal survival. The PL class composition (% PL) of the carp was unaffected by that of the diet, but related to the size of the fish.
In a third part, we demonstrated a specific role of PC in stimulating the intestinal neutral lipid absorption. Histological examination of the anterior intestine of one week old carp showed that dietary PC stimulated the export of lipoproteins from the enterocytes into the blood, in contrast to observations with PL-rich or PL-free diets. Similarly, adding soybean PC to diets having an equal amount of neutral lipid highly unsaturated fatty acids (HUFA), stimulated the uptake of these HUFA by the turbot. The larval response was also changed by modifying the fatty acids of the diet PL, i.e. hydrogenation
and hydrolysis. Hydrogenated PC reduced the growth of carp as compared to native soybean PC, but histological analysis of the enterocytes did not distinguish both PC sources. Also the fatty acid analysis of eye, brain and muscle of turbot fed iso-HUFA diets, indicated that hydrogenated PC stimulated the dietary neutral lipid absorption as efficiently as native PC. A further increase of DHA uptake, albeit moderate, was observed when DHA was provided by PC instead of neutral lipid. The latter HUFA-acyl-carrier effect raised the question on the digestion of PL in fish, also questionned by the inferiority of dietary hydrolysed sn-1 acyl lysoPL as compared to non-hydrolysed PL.
Some conclusive explanation for the mechanisms behind the PL requirements of larval fish are given, together with some intriguing points requiring further research.
(Laboratory for Aquauculture & Artemia Reference Center, Rozier 44, 9000 Gent, Belgium)