The present study aimed to evaluate the potential adverse effects of the KD on liver function, specifically focusing on hepatic inflammation and lipid accumulation. The KD, characterized by its high-fat content, is known to significantly affect lipid metabolism, which can have downstream effects on liver health. Beyond its therapeutic applications in neurological diseases and cancer, the KD has recently gained traction as a weight loss strategy for obesity management [21, 22]. Consistent with previously published results [9, 10, 23], rats maintained on a KD exhibited decreased weight compared to the ND group. KDs are effective for weight loss, as evidenced by both short-term and long-term studies [24,25,26,27,28]. Despite strong evidence supporting KDs for weight-loss therapy, different theories exist regarding their exact mechanisms. The weight-loss effects are attributed to factors like appetite reduction, hormonal regulation, increased fat metabolism, and higher metabolic efficiency [29,30,31].

Our findings demonstrated a significant increase in serum Chol levels in the KD group, while TG levels remained unchanged. This aligns with the known impact of high-fat diets, but the inconsistent results in the literature suggest that the exact effects of the KD on lipid profiles may depend on various factors, including diet composition and duration. The scientific literature presents conflicting findings regarding the KD’s impact on lipid metabolism. While some investigations have noted significant increases in both TG and Chol levels [11, 12], others have reported no substantial changes or even decreases in these indicators [10, 16, 32]. For example, research by Bielohuby et al. showed elevated TG concentrations but no notable differences in Chol when comparing two distinct KD formulations [16]. In contrast, Holland et al.‘s study revealed an unexpected reduction in both TG and Chol in KD-fed rats compared to those on Western or standard diets [32]. These varying outcomes might be explained by differences in the specific proportions of macronutrients and the types of fats employed across studies, highlighting the need for additional research to elucidate how these factors affect lipid profiles in the context of the KD.

A high intake of dietary fat, particularly saturated fat, can lead to excessive fat accumulation in the liver, promoting hepatic steatosis and inflammation. In the present study, we observed a significant increase in hepatic inflammatory markers, specifically TNF-α and IL-6, along with notable signs of hepatic steatosis in the KD group. These findings are consistent with several studies that have reported increased liver inflammation and fat accumulation in response to high-fat diets, including the KD [9, 11, 12, 33,34,35]. However, not all studies agree on the inflammatory effects of the KD. While some have documented pro-inflammatory responses in the liver, others have reported anti-inflammatory or anti-steatogenic effects [13, 36]. For example, Douris et al. found that both short-term (8 weeks) and long-term (80 weeks) consumption of the KD led to hepatic steatosis and increased expression of pro-inflammatory markers [12]. Similarly, another study associated the KD with systemic inflammation and hepatic steatosis after 22 weeks [11]. Asrih et al. found increased liver inflammation and lipid accumulation in mice fed a KD, but a decrease in inflammation in white adipose tissue (WAT) after 4 weeks on the diet, compared to standard chow [33]. Additionally, research by Carmiel-Haggai et al. showed that obese rats on a high-fat diet for 8 weeks developed liver injury and steatohepatitis, along with elevated TNF-α levels [34]. Wang et al. demonstrated a clear link between elevated serum fatty acids and liver lipid content, which was associated with heightened non-alcoholic fatty liver disease (NAFLD) activity scores, liver inflammation, and injury [35]. Also, Garbow et al. observed hepatic lipid accumulation and steatosis in rats on the KD for 12 weeks without significant changes in TNF-α and IL-6 levels [9]. In contrast, Jani et al. reported anti-steatogenic effects of the KD in rats after 8 weeks [13], and Liu et al. demonstrated the anti-inflammatory effects of a calorie-restricted KD in rats after 30 days [36]. The contrasting outcomes highlight the complexity of the KD’s effects on different tissues and organs, suggesting that diet composition, duration, and caloric intake play crucial roles in determining its impact.

In addition to the inflammatory response, our study also found that serum ALT levels, a marker of liver injury, were significantly elevated in the KD group. This increase in ALT may be linked to the lipid accumulation and inflammation observed in the hepatic tissue. Several studies have similarly reported elevated ALT levels in response to the KD, further supporting the potential hepatotoxic effects of the diet [9, 12, 37]. Douris et al. observed increased ALT levels in both short-term (8 weeks) and long-term (80 weeks) KD-fed mice [12], also Garbow et al. found elevated ALT in KD-fed mice when compared to those on a Western or standard diet [9]. Notably, Ellenbroek et al. and Jornayvaz et al. reported significant increases in both ALT and AST in mice fed a classic KD [10, 11].

However, some studies have reported no significant changes or even reductions in serum liver enzyme levels in response to the KD. For example, Arsyad et al. found no significant changes in ALT and AST levels in rats fed a KD for 60 days [38], and Holland et al. reported decreased ALT levels in rats on a KD with a different macronutrient composition (20% protein, 10% carbohydrate, and 70% fat) compared with the western diet and standard diet [32]. These discrepancies may be due to differences in diet composition, duration, and fat sources used in the studies, further underscoring the importance of considering these factors when interpreting the KD’s effects on liver function.

A significant factor that may account for the discrepancies observed in various studies is the specific formulation of the KD, particularly with respect to macronutrient ratios and caloric restriction. Our investigation employed the classic KD, characterized by a composition of 90% fat, 8% protein, and 2% carbohydrates, which is consistent with numerous other studies [9, 11, 12, 33, 34]. In contrast, Jani et al. implemented a diet comprising 80% fat and 20% protein, with carbohydrates entirely excluded [13]. Additionally, Liu et al. restricted caloric intake to 20% of the required levels, although the macronutrient ratios were comparable to those of the classic KD [36].

Research indicates that rats necessitate a minimum protein intake of 12–14% for optimal growth and physiological function [39, 40]; however, most KD formulations, including the one utilized in our study, contain less than 10% protein. Diets with elevated protein levels, such as the 20% protein diet employed by Jani et al. and Holland et al. have demonstrated more beneficial effects on lipid metabolism and liver enzyme activity [13, 32].

Another critical consideration that may significantly influence the outcomes associated with the KD is the source of fat utilized in the study. The primary fat source in the present study were mutton tallow and cocoa butter. Both sources are high in saturated fatty acids (52–64%) [41,42,43,44,45]. Saturated fats are recognized for their potential to elevate serum cholesterol levels, which may lead to lipid accumulation in the liver and subsequent inflammatory responses [46].

Furthermore, the duration of KD administration is an important aspect to consider. Our study was confined to a 30-day timeframe; however, longitudinal studies have indicated that the effects of the KD may change over time. For example, Douris et al. observed that both short-term and long-term KD consumption resulted in hepatic steatosis and heightened liver inflammation [12], while Garbow et al. reported a continued increase in liver enzyme levels and lipid accumulation after 12 weeks on the KD [9]. These findings imply that the duration of KD administration may be a crucial determinant of its long-term effects on liver health.