Figure 1.
Schematic view of the trend in the use of feed additives in the 21st century.
Figure 2.
Electronic databases used.
Figure 3.
Keywords for search.
Figure 4.
Criteria for reviewing the research under study.
Table 1.
Summary of main modes of action of the major additives used in chicken diets
| Probiotics | Prebiotics | Enzymes | Betaine | L-carnitine | |
|---|---|---|---|---|---|
| Expression of pro-inflammatory cytokines in intestine | ↓ | ||||
| IgA | ↑ | ↑ | ↑ | ||
| Intestinal epithelial barrier integrity | ↑ | ↑ | ↑ | ||
| Antibodies | ↑ | ↑ | ↑ | ||
| Gut microbiota modulation | ↑ | ↑ | ↑ | ↑ | |
| Antioxidant activity | ↑ | ↑ | ↑ | ||
| Resistance to heat stress | ↑ | ↑ | ↑ | ||
| Lymphocyte proliferation | ↑ | ↑ | ↑ | ||
| Reduction of ammonia production | ↑ | ↑ | ↑ | ↑ | |
| Enhancement of nutrient absorption | ↑ | ↑ | ↑ | ↑ |