The manipulation of the fatty acid composition of farm animal adipose tissues has been of great interest in recent years due to an increasing demand for the production of meat with desirable nutritional and technological qualities [1]. In contrast, the risks associated with the consumption of red meat to human health (e.g., cancer, diabetes and coronary heart disease) are currently a controversial topic [2,3]. To reduce the risk of cancer, the World Cancer Research Fund report recommends limiting the consumption of red meat to less than 500 g per week [4]. The majority of evidence for the association of red meat with cancer shows an increase in cancer risk for consumers with the highest level of red meat consumption; however, the results of most studies have not reached the level of statistical significance [5].

Currently, the main strategies for altering the nutritional profile of meat are genetic selection and/or dietary manipulation. Selection for lean growth and against higher back fat thickness has been a major goal of the pig industry. Dietary strategies used to customize the fatty acid composition of pig fat have proven very effective because dietary fatty acids can be incorporated into pig muscle and adipose tissues with few modifications [6,7,8]. The intramuscular fat (IMF) content of the pork is one important trait that influences meat quality characteristics, such as meat tenderness, juiciness, flavor and taste [1]. The role of IMF is of particular interest in pigs because genetic selection for lean pigs has reduced its level to below 1% IMF, compared with 2%–4% IMF in pig muscle fifty years ago. Several studies have suggested a favorable relationship between IMF and meat tenderness and juiciness and recommended that a minimum level of IMF is needed to maximize meat tenderness and consequently increase consumer acceptance [1,9]. Selected monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) have a number of health benefits. The majority of the health benefits have been associated with n-3 PUFAs [10]. Several studies have investigated the effects of dietary PUFA (18:3n-3, linolenic acid; 18:2n-6, linoleic acid) source, mainly linseed, rapeseed or sunflower supplements, on pig performance and the fatty acid composition of pig tissues. It is known that feeding a linseed diet increases the content of n-3 PUFA and decreases the n-6/n-3 PUFA ratio in all tissues, whereas feeding a sunflower seed diet leads to an increase in the n-6 PUFA contents [11,12]. An increase in the tissue n-3 PUFA has been observed under these conditions and has been correlated with the direct incorporation of dietary PUFA into pig tissues [7]. Animal nutrition has a major impact on fat quality and composition; for example, protein or lysine-deficient diets are known to increase the IMF contents in pigs. Several studies have shown that the IMF content in pigs can be increased without increasing back fat deposition by feeding pigs low protein diets during the growing or finishing phase. This might be due to a tissue-specific activation of the expression of lipogenic enzymes by the reduced protein diet [13,14,15].

The present study investigated the effects of feeding reduced protein diets in combination with different vegetable oils on carcass traits, meat quality and fatty acid profile in porcine muscle. To date, feeding reduced protein diets in combination with different vegetable oils to pigs has not been investigated. Based on the current knowledge, we hypothesized that the reduced protein diet in combination with linseed or sunflower seed oil would lead to an increase in the IMF content in porcine muscle without detrimental effects on meat quality.

The results of growth performance and carcass trait measurements of male Landrace pigs are presented in Table 3. The average daily gain (ADG) was not affected by the experimental diets; however, the control group (CON) exhibited a lower ADG compared with experimental groups. The average daily feed intake (ADFI) was not affected by the diet. These results are consistent with recent studies investigating the effects of dietary protein level on growth performance and carcass traits in crossbred pigs [13,14]. Furthermore, the carcass traits, e.g., back fat, muscle area and liver and belly fat weights, were not influenced by the different experimental diets, except the hot carcass weight (Table 3).

Selected meat quality parameters of the longissimus muscles of pigs are presented in Table 4. A number of studies have demonstrated that IMF and subcutaneous fat content might be manipulated independently through dietary means. For example, feeding a low protein diet increases the level of IMF with much smaller effects or no effects on subcutaneous fat content in pigs [9,13,14]. Inadequate dietary protein and/or lysine limit protein synthesis and increase the amount of energy available for fat deposition, resulting in higher IMF. In contrast, the IMF level in the Landrace pigs of the present study was not affected by the reduced protein level or by the use of linseed or sunflower seed oil in the diet. The IMF content ranged between 1.2% and 1.4% (Table 4). One possible explanation for unaffected IMF contents could be the smaller difference of protein level in the diets (19.4%–19.6% HPD) vs. 15.0%–15.7% in the RPD groups compared with other studies (10%–13% low protein vs. 18%–23% for the high protein diet) [13,14]. No significant effects were observed between the different diets for muscle color (L*, a*, and b*), consistent with the results of recent studies by Alonso et al. [9] and Guo et al. [14]. However, one other study detected higher L*, a* and b* color values in the longissimus muscles of pigs fed low protein diets due to higher IMF contents and higher concentrations of 12:0 and 14:0 in the muscle, which makes the IMF less translucent and leads to a greater color saturation [19]. Neither reduced protein level nor dietary vegetable oil supplementation affected shear force (WBSF), pH value or cooking loss of the longissimus muscle (Table 4). Our results indicate that the pigs of all diet groups were in good condition pre-slaughter, resulting in normal pH development with a tendency toward better water holding characteristics in the muscles of HPD-LO pigs (lower drip loss). The lack of significant differences in IMF contents between the dietary treatments can contribute to the similar shear forces measured in the longissimus muscle (Table 4). Additionally, comparable muscle shear force seems to be a reflection of a uniform growth and development pattern, indicating similar sizes of muscle fibers and IMF contents.

The effects of dietary PUFA supplements, mainly linseed, rapeseed or sunflower seed oils, cake or seeds, on pig performance and fatty acid composition of tissues have been investigated intensively. It is known that feeding a linseed diet increases the n-3 PUFA content and decreases the n-6/n-3 fatty acid ratio in all tissues, whereas feeding sunflower seed leads to an increase in the n-6 PUFA contents due to the direct incorporation of the dietary PUFA into pig tissues [7,11,20,21]. The single fatty acid concentrations of the longissimus muscle in pigs are shown in Table 5, and the selected total fatty acid concentrations are presented in Figure 1. The muscle fatty acid concentrations of the present study appeared to be significantly affected by the diet, resulting in significantly higher n-3 FA concentrations (up to 113 mg/100 g muscle) in linseed oil-containing high- and reduced protein diets, compared to sunflower seed oil diets; only docosahexaenoic acid (22:6n-3, DHA) was not significantly increased. In most of the previous studies, the use of n-3 PUFA-rich vegetable oils showed no or only small effects on the intramuscular DHA level [7,15,21,22]. Feeding linseed oil-containing HPD- and RPD diets significantly decreased the concentration of single and total n-6 PUFA in longissimus muscle of pigs compared with the sunflower seed oil-containing fed groups and the control group (Table 5 and Figure 1). Consequently, the high level of n-3 PUFAs, which are beneficial for human nutrition, and the lower level of n-6 PUFAs in pigs fed linseed oil-containing diets caused a beneficially low n-6/n-3 PUFA ratio in the muscle compared with sunflower seed oil containing diets and the control diet. The n-6/n-3 PUFA ratio was 1.8:1 in muscle of pigs fed linseed oil containing HPD- and RPD diets, which corresponds to the n-6/n-3 PUFA ratio recommended by the German Nutrition Society (≤5:1) [23]. The concentrations of single saturated and total saturated fatty acids (SFA) were not affected by either the protein content or the type of oil supplement (Table 5 and Figure 1). Other studies showed that feeding a low protein diet with an increased IMF level resulted in elevated concentrations of 14:0, 16:0 and total SFAs, as a reflection of the higher IMF contents [19]. Additionally, oleic acid (18:1cis-9), the most abundant fatty acid in pig muscles, and total monounsaturated fatty acids (MUFAs) were not affected by the different dietary treatments.

In contrast to our hypothesis, a reduced protein diet in combination with linseed or sunflower oil did not lead to an increase in the IMF content of the longissimus muscle of male Landrace pigs. However, diets supplemented with linseed oil allow the production of pork meat with a high n-3 fatty acid content and low n-6/n-3 PUFA ratio, which is beneficial for human nutrition. Additionally, feeding pigs a reduced protein diet in combination with linseed or sunflower oil had no detrimental effects on muscle meat quality.