2.3. Intervention

Participants were randomly assigned to one of two fish treatment sequences (CF/EF or EF/CF). Participants in each sequence received each treatment for 8 weeks with a 6-week washout period between treatment periods. The fish species studied in the present trial was gilthead seabream (Sparus aurata L.), and the two types of fish compared were farmed fish fed with fish oil diet (CF) and farmed fish fed with olive pomace enriched diet (EF). NIREUS Aquaculture S.A. provided both types of fish. The production method of fish feed enriched with olive processing byproducts, the method to enrich farmed fish and the effect of feed on the fatty acid profile and total polar lipid (TPL) concentration in the flesh of gilthead sea bream, are described in detail elsewhere [23]. The fatty acid composition of the fish consumed in the study was not specifically analyzed for the study fish fillets. Details of the percent fatty acid composition of the fillets were as follows: 18:1ω-9 (24–25%), 20:5ω-3 (4–5%), 22:5ω-3 (2–2.6%), 22:6ω-3 (13–14%) for the CF, and 18:1ω-9 (23–23.8%), 20:5ω-3 (2.2–2.6%), 22:5ω-3 (1.7–1.9%) and 22:6ω-3 (7–7.8%) for the EF. Every week during each treatment period, participants were provided with two portions of fresh, raw, gutted and scaled gilthead seabream. Each portion weighed on average 380 g (quantity of edible fillet approximately 170 g). Participants were instructed to consume fish twice weekly (one portion at a time) on the days of their choice. At the beginning of each treatment period, participants were given oral and written instructions regarding the preparation of fish. Specifically, participants were asked to bake or grill the fish, and were allowed to add seasonings and spices of their choice during fish preparation and consumption. Participants were also asked to maintain their dietary and physical activity habits throughout the study and to refrain from any weight loss effort during that period. In particular, participants maintained their olive oil consumption pattern throughout the study with the food frequency questionnaire (FFQ) data showing similar levels of consumption at the end of each intervention period compared to the beginning of the study (30 ± 14 vs. 29 ± 15 mL/day (CF, 1st period); 30 ± 13 vs. 33 ± 19 mL/day (CF, 2nd period); 32 ± 19 vs. 33 ± 19 mL/day (EF, 1st period); 27 ± 12 vs. 29 ± 15 mL/day (EF, 2nd period); p_within-group > 0.5, for all). During the 6-week washout period, participants were not provided with any fish portion and were advised to continue their long-standing dietary habits.