## 4 Discussion
This is interesting and consistent with the results of our previous research where feeding food waste-based diets with higher fiber and NSP levels compared to the conventional control diets decreased dry matter digestibility and tended to decrease energy digestibility of laying hens at 43 weeks of age [23]. Similarly, other studies have shown higher dry matter and energy digestibility in birds fed diets with lower fiber or NSP levels [55; 56; 57]. This is understandable as increases in dietary fiber and/or NSP levels--particularly soluble NSP--may increase digest viscosity, resulting in reduced nutrient utilization in the gut [58; 59; 60]. Additionally, it has been suggested that energy originating from fat may be more digestible than that from carbohydrates, as it may reduce competition between glucose and amino acids for absorption sites in the small intestine, resulting in increased nutrient utilization [61]. Thus, food waste diets may deliver an advantage by providing proportionally more energy as fat rather than carbohydrates. Due to the variability in fiber composition, use of commercial food waste diets will requiring monitoring of fiber levels within ingredients and supplementation with NSPase enzymes accordingly. The idea of long-life laying hens that can produce 500 eggs or more in a single production cycle of 100 weeks has become a reality thanks to advances in hen genetics [62]. However, despite the benefits of improving the utilization of scarce resources and reducing waste and the carbon footprint, the extension of the laying cycle may also induce challenges for hen health, particularly with respect to bone metabolism, as large amounts of Ca are mobilized from the skeleton for eggshell formation, which may increase the incidence of bone fractures and osteoporosis during the late laying phase [63; 64]. Nutritional strategies that could improve the accumulation of medullary bone in the early laying cycle may help to maintain eggshell quality and skeletal integrity in aging laying hens [65]. In the current study, hens offered food waste-based diets and 50:50 blend diets had increased mineral digestibility for various minerals, which explains the improved bone ash and mineral levels in the bones of hens that received these diets compared to the control group. Thus, it is speculated that feeding food waste-based diets would be even more advantageous for egg production and hens' health and welfare during the late laying phase when the hens' physiological conditions and bone mineralization status worsen. Besides the minerals provided in the vitamin-mineral premix, the mineral sources in the food waste-based diets mainly originated from the organic sources presented in fish offal and spent brewers' grain blend, pub and restaurant meal, recycled meat and bone meal, hospital and nursing home meal, vegetable and fruit meal, and oyster shell meal. In contrast, the minerals presented in the control diets were mainly from inorganic sources, such as limestone and di-calcium phosphate. It is widely accepted that organic minerals are more bioavailable and digestible compared to inorganic mineral sources [44; 66; 67]. This may be due to the effects of organic minerals in diminishing the complexation of trace minerals with phytate, fiber, and macro minerals, such as Ca and/or P, leading to fewer antagonistic reactions with these dietary components during feed digestion [68]. This may explain the higher mineral digestibility in hens fed the food waste-based diets and 50:50 blend diets compared to those fed the control diets in the current study. Nevertheless, despite the increased mineralization, the bone breaking strength did not differ between the dietary treatments in the current study. This may be explained by the fact that bone strength in birds is not only influenced by bone mineral density but also by organic factors, such as collagen crosslinkage in bone [69].