Iron requirements are especially high during early stages of life as infants need a large amount of iron to meet the demands of blood production and growth and development [1,2]. As iron deficiency (ID) in infants and young children often leads to irreversible damage on neural, intestinal and immune system development [[3], [4], [5], [6], [7]], the iron supplement is necessary for infants. To prevent iron deficiency (ID) in infants and young children, the fortified dose of iron in infant formula milk powder tends to be closer to the higher dose of established nutritional standards [8]. However, high doses of iron fortification in infant formula powders are unnecessary and even risky. Iron supplementation for infants and young children who are not deficient in iron can lead to damaged immune system, and impaired brain development [[9], [10], [11], [12], [13]].

The liver, as the main organ of iron storage, is particularly sensitive to damage stemming from iron overload [14]. Excessive iron intake can promote ferroptosis of liver cells, damage liver structure and functions [14,15]. Studies in adult animal or weaned animal models have reported that high iron intake could lead to iron overload in the liver, which in turn causes liver damage and fibrosis [14,[16], [17], [18]]. However, liver cells are more sensitive to imbalance iron metabolism in infants, which may exacerbate fat production and insulin resistance, and promote the development of non-alcoholic fatty liver disease (MAFLD) or steatohepatitis (NASH) [[19], [20], [21]]. Thus, it is necessary to establish a model of suckling animals to clarify the influence of different doses of iron on the liver during suckling.

The liver is one of the important immune organs and plays a core role in the development and function maintenance of the immune system, especially in infants whose immune systems are not yet fully developed. Iron plays a crucial role in immune function. Iron is an essential element for various immune cells to maintain their functions and differentiation. Studies have shown that iron deficiency can lead to abnormal mitochondrial function and reduced proliferation capacity, significantly affecting the function of CD8+T cells and macrophages, and causing impaired immune responses [22,23]. Also, iron can regulate the polarization of macrophages, the recruitment of neutrophils, and the activity of natural killer (NK) cells [24]. Iron can participate in the regulation of pro-inflammatory REDOX reactions, thereby promoting the polarization of M1-like (pro-inflammatory) macrophages [25]. Research has found that excessive iron intake can increase the pro-inflammatory environment of the liver and affect the activation of the hepatic innate immune system [26]. This is because excess iron can promote the production of reactive oxygen species (ROS), leading to oxidative stress and lipid peroxidation, enhancing M1 polarization, intensifying inflammatory responses, and ultimately damaging liver immune function [[27], [28], [29]]. Accumulating evidence has demonstrated that dysregulated nuclear factor-κB (NF-κB) signaling which is a key transcription factor that regulates inflammatory responses, plays a critical role in liver inflammation and metabolic disturbances [30,31]. Iron-induced ROS generation has been shown to activate NF-κB signaling, thereby amplifying inflammatory responses and disrupting hepatic homeostasis [[32], [33], [34]]. Importantly, while transient NF-κB activation may represent an adaptive response to oxidative stress, sustained or excessive activation can exacerbate hepatic inflammation and oxidative injury [35,36]. Recent studies have increasingly recognized the involvement of NF-κB signaling in iron-related liver injury; however, most existing research has focused on adult models, the mechanistic evidence for the effects of iron overload during suckling remains limited.

Therefore, due to the high susceptibility of the liver to excessive iron during suckling, it is necessary to investigate the effect of different iron doses on liver, as well as the damage of liver caused by excess iron during suckling. In this study, suckling rats with different dose of iron supplement were utilized to explore the effects on liver structure and functions, revealing the underlying mechanism by which excessive iron disrupts the liver homeostasis during suckling.