1. Introduction
Soybean, palm, rapeseed (canola) and linseed oils are commonly used in poultry feed [
1], and palm oil is the cheapest source of oil in the market. As of the first quarter of 2023, the average price for crude palm oil was MYR 3909/ton, RBD palm olein was MYR 4140/ton, crude palm kernel oil was MYR 3865/ton (Malaysian Palm Oil Board) and soybean oil (SBO) was around MYR 5498/ton (World Bank). Including palm oil and palm kernel oil (PKO) in the poultry diet is more cost-effective as it could deliver similar productivity compared to SBO. Dietary supplementation of palm oil, PKO and SBO in laying hens had similar laying performance in terms of hen-day egg production, egg weight, feed intake and feed conversion ratio [
2,
3].
Palm oil has a balanced fatty acid composition where the fraction of saturated fatty acids (SFA) is almost proportional to the fraction of unsaturated fatty acids (USFA). Palmitic acid (44–45%) and oleic acid (39–40%) are the dominant SFA and USFA, respectively, together with linoleic acid (10–11%) and a minute amount of linolenic acid [
4]. PKO is rich in medium-chain fatty acids (MCFA), mainly lauric (C12:0) and myristic (C14:0) acids and contains up to 80% SFA. SBO is polyunsaturated fatty acid (PUFA)-rich oil extracted from soybean and undergoes refining, bleaching and deodorization to produce refined SBO.
Table 1 summarizes the fatty acid and saturation profiles of the oils [
3].
In addition, palm oil has beneficial components present in small amounts, such as carotenoids responsible for the reddish-orange color and vitamin E in the form of tocopherols and tocotrienols. Tocotrienols are potent antioxidants at higher levels in palm oils than in other vegetable oils [
5].
Figure 1 summarizes the fatty acid, carotenoids and vitamin E properties of the oils. The oxidation of lipids during processing and storage is caused by variations in chemical reaction mechanisms, including autoxidation and photosensitized oxidation. The oxidative stability of oils is the ability of oils to resist oxidation at processing and storage, which can be described as the periods to achieve the critical point of oxidation, which can be a sensory change or acceleration of the oxidative process [
6,
7]. Determining oxidative stability is a crucial indicator of oil quality and shelf life, where oxidized oils lead to less acceptable or unacceptable oil for both consumers in household use and industrial use as food ingredients [
6,
8]. The oxidation of oils affects the essential fatty acids, particularly polyunsaturated fatty acids (PUFA), and generates toxic compounds and oxidized polymers as oxidation products [
6].
The fatty acid composition of the oil is a vital factor in determining oxidative stability [
9]. The higher the amount of USFA fraction in oil, the greater and faster the oxidation process occurs compared to an SFA fraction in oil [
10]. PUFAs, such as linolenic and linoleic acids, have a higher oxidation rate than monounsaturated fatty acids (MUFA), such as oleic acid. Free fatty acids (FFA) are another important criterion in determining the stability of oils. FFA are susceptible to autoxidation compared to esterified fatty acids in triglyceride form, making them a prooxidant in oils [
6,
11]. Therefore, the presence of high FFA in oils affects their stability. In preventing FFA formation, sterilizing fresh fruit bunches during milling deactivates the hydrolytic enzymes involved and accelerates the reaction that produces FFA. Crude oils contain some amount of FFA, and the refining process removes it to maintain oil stability.
Diet has an important role in modulating gut health by maintaining microbial balance and preventing oxidative stress in the gut. Oxidation is a normal process in the cells. Still, the production of free radical compounds and imbalance in the anti-oxidation process leads to excess radical compounds, causing oxidative stress, which is detrimental to the cells [
12]. Antioxidants are required to prevent oxidation by controlling the free radical’s formation level and safely eliminating it from the cells. Antioxidant compounds may be acquired from the diet or antioxidant enzymes produced by the body’s cells. There are several antioxidant enzymes produced by the body, of which superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) are the main antioxidant enzymes responsible for the detoxification of free radicals [
12,
13]. The SOD enzyme converts superoxide radicals into hydrogen peroxide and oxygen, whereas CAT and GPX convert hydrogen peroxide into water [
14]. The antioxidant enzymes convert superoxide radicals and hydrogen peroxide into water and oxygen as final products and safely eliminates them from the body. Oils containing medium-chain fatty acids (MCFA) had higher antioxidant activity [
15]. Higher saturated fatty acids (SFA) and antioxidant compounds, such as carotenes and tocotrienols in palm oils, including crude palm oil (CPO) and red palm oil (RPO), are expected to reduce oxidation in the body. A lower oxidation level would reduce the body’s requirement and production of antioxidant enzymes [
13,
16].
Palm oils contain natural and potent antioxidants in the form of β-carotene and vitamin E. Dietary palm oil containing carotenoids and vitamin E and dietary palm kernel oil (PKO) having a high fraction of medium-chain fatty acids (MCFA) are expected to modulate the gut lumen environment and affect antioxidant enzyme regulation, mucosal immune response and barrier function in laying hens. Several studies showed that dietary polyunsaturated-rich fatty acids (PUFA) were beneficial to gut health and improved immune status in poultry [
17,
18,
19].
Despite the excellent properties and benefits of naturally occurring antioxidant compounds in palm and kernel oils, no studies have explored their potential, particularly in investigating their dietary influence on antioxidant enzyme systems and lipid peroxidation in poultry. Hence, this study investigated the antioxidant properties of CPO, RPO, refined palm oil (RBD), PKO and SBO and their dietary effects on antioxidant enzyme regulations in the liver, serum and intestinal mucosa. In the current study, SBO containing high USFA was used as a control to palm and kernel oils for the measurement of the relative expression of genes.
5. Conclusions
In summary, free fatty acid and acid values were present in a significant amount in lesser refined oils, such as CPO, RPO and PKO, compared to RBD and refined SBO. The CPO and RPO had higher carotenoids in the form of β-carotene, which was reflected in the redness and yellowness of the oils. High antioxidant compounds, such as TPC, TFC and TTC, are present in palm oils and PKO and reduce the oxidation of oils. Refined oil had lower FFA and AV, and the presence of antioxidant compounds in the oil helped to reduce the degree of oxidation and the presence of oxidation products. The liver, serum and jejunal mucosal antioxidant enzyme activities were lower in palm oils, particularly CPO and RPO. The jejunal mucosa antioxidant enzymes were downregulated in palm oils and PKO, but there was no difference between oils in liver antioxidant enzymes. It is associated with a lower requirement for producing antioxidant enzymes in dietary oils with higher antioxidant compounds and capacity. It is suggested that dietary supplementation with CPO is cost-effective and preferred because of the high content of antioxidant compounds, and it contributes to better antioxidant protection in laying hens at a lower price compared to other oils.