# Evaluation of Propylene Glycol and Essential Oil Supplementation on Growth Performance, Feed Efficiency, Serum Biochemical Indices, Hematological Parameters, and the Expression of Antifreeze IV and Lipid Metabolism-Related Genes in Nile Tilapia

## TL;DR

This study shows that adding propylene glycol and essential oils to Nile tilapia diets improves growth, stress tolerance, and immune function through effects on metabolism and gene expression.

## Contribution

The novel contribution is demonstrating how propylene glycol and essential oils improve fish growth and stress resilience through specific physiological and gene expression changes.

## Key findings

- Propylene glycol improved growth performance, feed efficiency, and immune markers in Nile tilapia.
- Essential oils upregulated antifreeze protein IV and lipid metabolism genes in a dose-dependent manner.
- Both additives reduced serum cortisol and glucose, indicating enhanced stress tolerance.

## Abstract

The effects of propylene glycol (PG) and essential oils (EOs) as functional feed additives on growth, physiological performance, and stress resilience in Nile tilapia (Oreochromis niloticus) were evaluated. PG supplementation markedly improved growth performance, feed efficiency, protein utilization, and energy conversion, indicating enhanced metabolic activity. Both PG and EOs significantly reduced serum cortisol and glucose, reflecting improved stress tolerance and hepatic modulation. PG also enhanced immune indicators, while EOs exhibited dose-dependent effects on hematological parameters and lipid metabolism. Notably, EOs upregulated antifreeze protein IV (AFPIV) and lipid metabolism-related genes, suggesting a role in thermal adaptation and lipid mobilization. PG induced higher expression of the immunoglobulin heavy chain gene (IGMH), demonstrating its immunostimulatory potential. Overall, PG improved growth and immune competence, while EOs promoted molecular adaptation to stress.

Aquaculture output, sustainability, and profitability can be enhanced by using functional feed additives. The effect of supplementation with two different dietary levels of propylene glycol (PG) and essential oils (EOs) was evaluated in Nile tilapia. A total of 150 juvenile fish were randomly allocated into five groups. Growth performance, feed utilization, serum biochemistry, hematology, and gene expression were assessed. PG supplementation significantly improved growth performance, feed conversion, protein efficiency, and energy utilization. Both additives significantly reduced cortisol and glucose levels and altered liver enzymes and lipid profiles. PG improved immunological indices, while hematological responses were dose-dependent; both EOs and PG increased hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH). Moreover, the high PG dose significantly increased platelet counts, reduced hemoglobin (Hb), and elevated hematocrit. Additionally, EOs significantly upregulated antifreeze protein IV (AFPIV) and fat metabolism-related genes in a dose-dependent manner, indicating a potential role in lipid mobilization and stress tolerance. Expression analysis of the immunoglobulin H (IGMH) gene revealed a significant increase in PG-supplemented groups, suggesting its immunostimulatory potential. Overall, PG enhanced immunity and growth performance, while EOs promoted AFPIV and fat metabolism gene expression. Therefore, PG and EO supplementation could serve as an effective functional strategy to enhance O. niloticus growth, stress adaptation, and immune resilience.