Our meta-analysis results show that PUFAs supplementation can significantly increase IMF content but decrease meat color L* and pH 24 h in pigs (Table 3). Specially, whatever CLA or linseed and high concentration or low concentration PUFAs supplementation can improve IMF content (Fig. 2a and Fig. 3a). However, PUFAs supplementation has no effect on IMF content in finishing pigs (Fig. 4). Furthermore, CLA supplementation reduced meat color b* and linseed supplementation decreased meat color L* and pH 24 h (Fig. 2). We also found the effects of PUFAs supplementation on meat color L* and pH 24 h are concentration- and stage- dependent (Fig. 3 and Fig. 4). Overall, these data support that PUFAs supplementation are benefit for improving meat quality in pigs.
In pig production, meat quality has been declining in recent years due to the blindly pursuit of production efficiency and increase of backfat thickness. It has been reported that the content of IMF is positively related to pork quality including tenderness, flavor, and juiciness. IMF is mainly distributed in the epimysium, perimysium and endomysium of skeletal muscle which the main components are phosphoric acid and triglyceride. Previous studies found that IMF content is related to breed, sex, diet and weight at slaughter in pigs[31–33]. In our meta-analysis, we observed that dietary PUFAs supplementation can increase IMF content, not only CLA but also linseed supplementation significantly improved IMF content and CLA supplementation are more effective (Fig. 2a). Besides, we found the benefit of PUFAs supplementation on IMF content are not dependent on concentration (Fig. 3a). Hence, PUFAs supplementation can be a nutritional measure to regulate meat quality. However, only growing pigs had an increased IMF content after fed PUFAs supplementation, finishing pigs had an insignificant effect (Fig. 4a). It might because the nutrition requirements of different growth stage pigs are different. Meat color and pH are one of the most important factors that affect sensory quality of pork. Specially, low pH value (below 5.8) is often associated with pale meat color, resulting in pale, soft and exudative (PSE) pork. In contrast, high meat pH (above 6.0) often causes dark, firm and dry (DFD) pork. The alteration of meat color and pH results from the different post-mortem processes including muscle metabolism (glycolysis) and conversion rates of glycogen into lactic acid, which are affected by environmental factors, such as nutrition, breeding conditions, transport conditions, stress, weather conditions, and the methods of slaughter. However, current studies on the effects of PUFAs supplementation on meat color and pH 24 h are controversial. In our analysis, dietary PUFAs supplementation significantly decreased L* and pH 24 h but drip loss, a*, b* and pH 45 min were not influenced (Table 3). CLA is a group of positional and geometric isomers of linoleic acid with a conjugated double bond which is generally found in ruminant animals and dairy products and has many physiological functions including anti-obesity, anti-diabetic, anti-cancer and anti-hypertension. Linseed is the ripe seed of flax which is rich in n-3 PUFAs and has anti-obesity, anti-inflammatory, anti-cancer and regulating glucose and lipid metabolism effects. Even though CLA and linseed are all PUFAs, they had different effects on L*, b* and pH 24 h (Fig. 2), it might result from different fatty acids composition. We also discovered that the effects of PUFAs supplementation on L*, b* and pH 24 h are dependent on concentration and growth stage (Fig. 3 and Fig. 4). In addition, we found neither pH 24 h values above 6.0 nor pH 45 min below 5.8 in any studies. Hence, PUFAs supplementation might provide a useful strategy to improve meat quality.
As shown in Table 4, the significant heterogeneity in the drip loss and L*of pigs was primarily driven by the linseed, high-concentration and finishing pigs subgroup. Differently, the CLA, low-concentration and finishing pigs subgroup are sources of b* heterogeneity. Linseed and growing pigs subgroup are the source of a* and pH 45 min heterogeneity, respectively. Because we did not find the source of IMF heterogeneity according to subgroup analysis, we performed a sensitivity analysis through using the leave-one-out method on IMF. However, the significant heterogeneity had no change after deleted each included study, thus we think the meta-analysis results are robust and the heterogeneity did not influence the significance of pooled estimates. Besides, we used a fixed-effects model to analyze pH 24 h due to the homogeneity (I2 < 50%).
A limitation of this meta-analysis is that the effects of PUFAs supplementation duration on meat quality of pigs and whether PUFAs supplementation could affect different breeds and sex of pigs are unknown as a result of the incomplete data and we think further studies should focus on these questions. Furthermore, as SD values are important for meta-analysis and they affect many estimates, including the weight of an individual study, the 95% CIs and heterogeneity, so the lack of within-group SD might influence the results of meta-analysis. We used pooled SD as within-group SD and its might be impacted by the number of groups and SEM. To verify our finding are reliable, we checked the consistency between 95% CI of pooled estimate and the significance and tendency of included studies. Hence, our results are valid and this method is appropriate for analyzing nonruminant studies which lack of within-group SD. Besides, there is another method can be used for estimating within-group SD which is suitable for studies that reported the median, range, and the size of a sample. In a word, different approaches can be adopted to estimate within-group SD accordingly ensure the results of meta-analysis are reliable and robust.