Introduction
The expansion of aquaculture raises sustainability concerns due to its reliance on finite marine resources, driving aquafeed industry to seek alternative efficient lipid sources aligned with a circular bioeconomy. Medium-chain fatty acids (MCFAs; 6-12 carbons) have shown potential benefits in fish nutrition as they are highly soluble and rapidly oxidized for energy (Liu et al 2025). The available MCFAs sources (≈ 40% of total fatty acids) derived from plant oils (e.g. palm kernel oil and palm kernel fatty acid distillate, a by-product of crude palm kernel oil refining, with more than 60 % of free fatty acids (FFA) or insects such as the black soldier fly. When evaluating new feed ingredients, assessing their digestibility and their effect on growth performance are key steps. Although faecal digestibility overall provides useful estimates of lipid utilization, understand the fatty acid (FA) absorption dynamics along the gastrointestinal tract (GIT) can give deeper insight into bioavailability and lipid efficiency. However, such information related to MCFAs is limited, particularly beyond Atlantic salmon (R��sj�� et al 2000). This study investigated different MCFAs-rich lipid sources in diets for rainbow trout (Oncorhynchus mykiss), focusing on the FA absorption dynamics along the digestive tract.
Materials and Methods
The study was conducted at the Experimental Facility of DISAFA, University of Turin (Italy). Four isonitrogenous (38.5% CP) and isolipidic (25% EE) extruded diets were formulated: a Control (CON) diet containing 7% fish oil and 15% rapeseed oil (RO), and three test diets in which 50% of the RO was replaced with different MCFA-rich oils: Black soldier fly oil (BSFO), palm kernel oil (PKO),and palm kernel fatty acid distillate (PKFAD). Two trials using rainbow trout were performed: a) 18-week growth trial (12 tanks, 32 fish/tank, 3 replicates/diet), and b) 8-week digestibility trial (12 tanks, 20 fish/tank, 3 replicates/diet). Faeces were collected according Choubert method (1982). Apparent digestibility coefficients (ADCs) were assessed using yttrium oxide (0,1 % of the diet) as inert marker. At the end the experimental period, the content of stomach, pyloric caeca (PC), anterior (AI) and posterior intestine (PI) from 5 fish/tank were collected and stored at ���20 °C for analysis.
Results
Partial replacement of RO by MCFA-rich oils significantly affected final body weight (FBW). Fish fed the BSFO diet showed the highest FBW (741.69 g), followed by PKFAD diet (713.65 g), significantly higher than CON and PKO diets (689.67g and 673.94g; P≤0.001). No differences were found in feed conversion ratio, daily feed intake and daily weight gain and specific growth rate among treatments (P>0.05). The FA digestibility varied along the GIT. In the stomach, all the FAs showed negative ADC values across all diets. In the PC, total FA (TFA) digestibility was 60%, with differences only in MCFAs among treatments: PKFAD showed higher ADC (71.53%) than PKO (57.55%) and BSFO diets (53.68%)( P<0.01). Except for PKFAD, these values are lower than for monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs). In the AI, TFA digestibility increased to 85–87%. MCFA digestibility was higher in MCFA-rich diets (91%) than in CON (46.92%)( P < 0.01), reaching levels similar to MUFA and PUFAs for all experimental diets. Digestibility of long-chain saturated FA (LCSFAs, ≥14C) was lower in PKFAD diet (71.75%) than in BSO (80.22%) and PKO (80.25) (P=0.022). In the PI, TFA digestibility was higher in CON fish (93.32%) than in PKFAD (88.24%)(P < 0.05). No significant differences were observed among MCFA-rich diets for MCFAs and PUFAs. In faeces, TFA digestibility reached the 95-97 %, highest in CON diet (98.28 % and the lowest for PKFAD (95.24%) (P<0.01). Overall, the PC region constitute the main absorption site for all FA (64–69% of total absorption for TFA). However, its contribution was greater in PKFAD diet for all FAs, mainly MCFAs (79 %), but not for PUFA.
Discussion
This study shows that the dietary inclusion of 7% of MCFA-rich oils supports growth in rainbow trout comparable to, or improved, than the control diet, suggesting that these oils can provide an adequate energetic value similar to RO. Unlike previous results (Verge-M��rida et al., 2022), high FFA content did not impair growth, as shown by the higher FBW in PKFAD-fed fish versus PKO and CON. However, growth differences were not consistently reflected in digestibility results. As reported in Atlantic salmon (R��sj�� et al, 2000), the PC is the main site for FA absorption, especially in FFA-rich diets, although the AI also contributed substantially (35%). The final lower overall digestibility in PKFAD diet, will be related to its high FFA content and reduced absorption of LCSFA present in the diet, particularly from the AI onwards, linked to their hydrophobicity, high melting point and greater tendency to form insoluble soaps that limits their absorption (Verge-M��rida et al. 2022). In relation to MCFAs, the higher digestibility for PKFAD diet than for BSO and PKO diets in PC disappear in the AI, where ADC values were similar to PUFA. The shorter chain length of MCFAs enhances solubility and allows more direct intestinal absorption without or with less dependence on bile-mediated micelle formation (��enesiz and ��iftci, 2020), being more evident and faster when present as FFA. Overall, this study confirms the potential of MCFA-rich oil as viable energy source in trout diets and highlight distinct FA absorption pattens along the GIT.
Acknowledgment
This work was carried out thanks to the financial support provided by Ministerio de Ciencia, Innovaci��n y Universidades of the Spanish Government (R&D&I project PID2020-115688RB-C21, funded by MCIN/AEI/10.13039/501100011033) and AQUAEXCEL3.0 Transnational Access programme (EU), proposal PID35208.
References
��enesiz, A. A., & ��iftci, ��. (2020). Modulatory effects of medium chain fatty acids in poultry nutrition and health. World's Poultry Science Journal, 76(2), 234-248. https://doi.org/10.1080/00439339.2020.1739595
Liu, H., Zhou, W., Cai, C., Feng, F., Cai, H., & Yang, H. (2025). Biological function of medium-chain fatty acids and their application in aquatic animals: A review. Animals, 15(15), 2294.
R��sj��, C., Nordrum, S., Olli, J. J., Krogdahl, ��., Ruyter, B., & Holm, H. (2000). Lipid digestibility and metabolism in Atlantic salmon (Salmo salar) fed medium-chain triglycerides. Aquaculture, 190(1-2), 65-76.
Trull��s, C., Fontanillas, R., Tres, A., & Sala, R. (2015). Vegetable re-esterified oils in diets for rainbow trout: Effects on fatty acid digestibility. Aquaculture, 444, 28-35.
Verge-M��rida, G., Barroeta, A. C., Ferrer, C., Serrano, T., Guardiola, F., Soler, M. D., & Sala, R. (2022). Olive pomace and soybean-sunflower acid oils as alternative fat sources in European seabass (Dicentrarchus labrax) diets: Effects on performance, digestibility and flesh fatty acid composition and quality parameters. Animals, 12(9), 1198.