## 4 Discussion
Accumulation of minerals--particularly P, Cu, Mn, Zn, and Fe--in the poultry litter has raised public concern due to its negative impacts on the environment and crop yield [70; 71]. For example, excess P in poultry manure may cause eutrophication, leading to the overgrowth of aquatic plants, such as algae [72; 73]. Excess manure P in soil may also contaminate ground water [74]. Also, using livestock and poultry manure containing high trace mineral content as fertilizer may increase toxicity risks to plants and soil microorganisms reducing crop yield [70; 75]. This has raised concerns for the current practice of oversupplying inorganic minerals in poultry diets and necessitated mineral sources that can be incorporated at lower levels but still satisfy birds' nutritional requirements [76]. This problem may be solved by replacing inorganic with organic minerals in poultry diets, which are more bioavailable and digestible than inorganic mineral sources [66]. In the current study, feeding food waste-based diets that contained more organic minerals than the control diets reduced the excretion of nine minerals but also increased the excretion of three minerals (P, Na, and Cr) into the environment. The reduction in the amounts of various minerals in the excreta of hens offered food waste-based diets may be attributed to the lower intake but higher or similar mineral digestibility of these minerals in food waste diets compared to the control group. In contrast, the higher P, Na, and Cr levels in the excreta of hens fed the food waste-based diets were likely associated with the higher intake but similar mineral digestibility of these minerals in the food waste group compared to the control group in the current study. Thus, feeding food waste-based diets may have both positive and negative effects on mineral excretion. Reducing inclusion rates or replacement of food waste streams with high Na (salt), P, and Cr content (e.g., hospital and nursing home meal, pub and restaurant meal, and oyster shell meal) with waste streams without high contents of these minerals may help to solve this issue and provide a more balanced mineral profile, leading to further improvements in the nutrient digestibility, laying performance, and environmental outcomes. Additionally, analyzing phytate-P levels in the food waste streams and mixed complete diets and optimizing phytase enzyme inclusions for food waste-based diets are necessary to improve P digestibility and reduce P excretion. Other nutritional strategies, such as adjusting dietary Ca to P ratios and/or supplementation of vitamin D, may also help to increase P digestibility in the food waste diets [77]. Gut pH may be influenced by various nutritional factors, including feed form and particle size, dietary fiber, protein, Ca, and Na levels [78; 79; 80]. Generally speaking, lower gut pH is more favorable as it may facilitate nutrient digestibility and diminish the growth of acid-sensitive pathogenic bacteria [81]. In this study, the pH values were relatively unchanged throughout the gastrointestinal tract with the exception of the ileum, which showed higher pH in hens fed the food waste-based diets compared to those fed the control or 50:50 blend diets at week 63. It could be seen that the food waste-based diet used in the last period of this study (weeks 59-63) was generally more complex and contained higher levels of CP, Na, and P but a lower level of fiber compared to the control diet during the same period. In a recent review, [PERSON] et al. [80] indicated that increased intestinal pH values in birds are often associated with a reduction in dietary fiber. Additionally, higher dietary CP levels may also raise intestinal pH by increasing the amounts of undigested protein entering the hindgut, which may in turn also increase microbial fermentation of nitrogenous waste, resulting in greater ammonia production [82; 83]. Previous research has shown that ammonia production increased intestinal pH [84; 85].