2.1. Epidemiologic perspective
As the prevalence of MAFLD and CKD increases, interest in and research on these diseases with increased prevalence has also increased. Accordingly, studies investigating the association between MAFLD and CKD have been actively conducted. Among them, the majority of studies have investigated whether MAFLD affects the occurrence of CKD. In 2014, a meta-analysis of 20 studies (11 cross-sectional and 9 longitudinal) reported that the presence and severity of MAFLD were associated with the risk and severity of CKD [
11]. Several years later, a meta-analysis that included nine longitudinal studies reported that MAFLD was associated with an approximately 40% increase in the risk of incident CKD [
12]. These analyses defined CKD as an eGFR < 60 mL/min/1.73 m
2 and/or proteinuria regardless of a cause such as DKD.
To date, no meta-analysis has reported the association between MAFLD and DKD, which is the leading cause of CKD. Several longitudinal studies have been conducted on this topic. In 2008, Italian researchers reported a higher prevalence of DKD in T2D patients with MAFLD in a large cohort study using a cross-sectional design [
13]. Shortly thereafter, they published a longitudinal analysis following up 1760 patients with T2D and normal or near-normal kidney function and without overt proteinuria to find the occurrence of DKD, defined as overt proteinuria and/or eGFR < 60 mL/min/1.73 m
2 for 6.5 years [
14]. As a result, MAFLD, diagnosed by liver ultrasound, was associated with an increased incidence of DKD [hazard ratio (HR) 1.69; 95% confidence interval (CI) 1.3 to 2.6; P < 0.001]. Consistent results have also been reported in another study that recruited an Asian population [
15]. In that study, the cumulative incidence of DKD was higher in patients with T2D and MAFLD, and the liver fat content showed a positive relationship with albuminuria and a negative relationship with eGFR. In 2022, Korean researchers demonstrated the relationship between liver fibrosis and DKD [
16]. Initially, they failed to show the differential risk of incident DKD between the MAFLD and non-MAFLD groups; however, among the T2D patients with MAFLD, advanced liver fibrosis was significantly associated with DKD (HR 1.75; 95% CI 1.15 to 2.66; P=0.009). Similar results were reported by Chinese researchers [
17]. They showed an association between liver fibrosis and DKD incidence and progression in older patients with T2D through both cross-sectional and longitudinal designs. In the case of type 1 diabetes (T1D), another study reported an association between MAFLD and DKD [
18]. The result was similar to that of patients with T2D (HR 2.85; 95% CI 1.59 to 5.10; P<0.001). In addition to the studies mentioned above, several longitudinal and cross-sectional studies have reported a significant association between MAFLD and DKD [
19,
20,
21,
22,
23].
Table 1 summarizes the previous studies that have investigated the association between MAFLD and DKD.
Although considerable evidence has been accumulated, a clear causal relationship between MAFLD and DKD has not yet been identified. Most previous studies adjusted for important risk factors, such as age, sex, body mass index (BMI), glycated hemoglobin (HbA1c), and comorbidities; however, it might be insufficient to adjust for factors shared by MAFLD and DKD, including insulin resistance and abdominal obesity. In addition, studies on the histological findings of MAFLD and DKD are scarce. Studies on changes in DKD according to the progression or improvement of MAFLD, and vice versa, are also lacking. Large-scale studies that address these limitations are required to clarify the association between MAFLD and DKD.
2.2. Pathophysiological mechanisms linking MAFLD and DKD
Based on the epidemiological evidence described above, MAFLD and DKD are considered to be closely related. Although the precise mechanisms linking MAFLD and DKD are not fully understood, several potential factors and mechanisms may link them.
First, as can be inferred from the names, MAFLD and DKD share broad areas of metabolic dysfunction, such as obesity, insulin resistance, hypertension, dyslipidemia, and diabetes [
30]. Insulin resistance is one of the most important pathogenic mechanisms underlying MAFLD and T2D. Insulin resistance, which is also widely known as a fundamental pathological factor in metabolic syndrome, increases lipolysis in adipose tissue, thereby increasing the plasma concentration of free fatty acids [
31,
32,
33]. This induces an excessive accumulation of hepatic triglycerides, resulting in the development and progression of MAFLD. Insulin resistance also results in compensatory hyperinsulinemia, which increases hepatic fatty acid uptake and inhibits β-oxidation, thereby leading to
de novo lipogenesis and aggravation of MAFLD. Lipid overload in the liver contributes to the formation of lipotoxic lipids, which leads to the activation of inflammatory responses, mitochondrial dysfunction, and oxidative stress [
33]. Progression to hepatic inflammation, that is, the metabolic-associated steatohepatitis state, is not limited to the inflammatory response in the liver but also increases systemic inflammatory responses [
34]. Delivery of pro-inflammatory mediators to the kidneys via systemic circulation may act as initiating or aggravating factors for kidney diseases. Inflammatory processes play an important role in the initiation and development of DKD [
35,
36]. In addition, an increase in oxidative stress leads to a reduction in the level of antioxidant factors in the kidney, such as the Klotho protein [
37]. Similar to the other mechanisms discussed below, insulin resistance, MAFLD, and DKD interact with each other, making it difficult to determine the precedence relationship. Insulin resistance is an important pathogenic mechanism of MAFLD that can affect the kidney through various pathways, such as systemic inflammation and oxidative stress. On the other hand, insulin resistance can directly affect kidneys via its pro-inflammatory features and hemodynamic alteration, and vice versa; insulin resistance is known to be a common and very early alteration in CKD, which includes the concept of DKD [
38].
Second, expansion and inflammation of the adipose tissue is another common metabolic dysfunction shared by MAFLD and DKD [
10]. This encompasses unhealthy diet, obesity, and insulin resistance. Expanded or inflamed adipose tissue releases large amounts of fatty acids into the systemic circulation [
10,
39,
40], which are transported to the liver and cause MAFLD. Inflamed adipose tissue also secretes various pro-inflammatory cytokines such as tumor necrosis factor-α, interleukin-6, resistin, and monocyte chemoattractant protein-1 and increases oxidative stress. MAFLD and systemic inflammation affect the kidneys. Furthermore, renal fat accumulation increases in obese individuals, which may result in local adverse reactions [
41,
42]. Adiponectin is a representative adipokine that is dysregulated [
10,
43]. The mechanisms by which adiponectin improves insulin resistance and inhibits reactive oxygen species (ROS) through adenosine monophosphate-activated protein kinase activation have a protective effect against MAFLD and podocyte injury [
44]. In contrast, fetuin-A, a hepatokine, induces insulin resistance and suppresses adiponectin production in the adipose tissue [
45,
46]. The interplay between pro-insulin-resistant fetuin-A and pro-insulin-sensitive adiponectin is considered a common pathogenic mechanism in MAFLD and DKD [
10,
43,
44,
47]. Recently, perturbation of the gut microbiota has emerged as a common pathophysiology in MAFLD and DKD [
10,
30,
43,
48,
49]. Genetic, environmental, and nutritional factors can influence the composition of gut bacteria [
50]. In particular, an unhealthy diet adversely alters the intestinal flora. This change, the so-called intestinal dysbiosis, causes the production of gut-derived toxins and promotes their absorption by damaging the intestinal barrier integrity [
43]. Increased absorption and accumulation of toxins lead to systemic inflammation, ectopic fat deposition, and insulin resistance. Renal excretion of the toxins and their metabolites can also damage the kidneys [
30].
Hyperglycemia can adversely affect both the liver and kidneys in patients with T2D. High concentrations of glucose in the plasma are delivered to the liver and used for
de novo lipogenesis [
51]. Hyperglycemia induces glomerular hyperfiltration, which initiates and propagates kidney damage in T2D [
9,
36]. Alteration of glucose metabolism in the kidney also promotes inflammatory responses and fibrotic changes, wherein advanced glycation end products and ROS are the major mediators [
36]. Furthermore, in T2D, tubular glucose reabsorption and renal gluconeogenesis are usually increased [
52,
53]. These changes demand an increased consumption of oxygen, leading to proximal tubule damage, which increases vulnerability to hypoxia [
54,
55,
56,
57].
Activation of the renin-angiotensin system (RAS), particularly angiotensin II production, plays an important role in the pathogenesis of MAFLD and CKD [
43,
58]. In the liver, RAS activation promotes
de novo lipogenesis, insulin resistance, and production of pro-inflammatory cytokines [
59,
60], thereby leading to the development of MAFLD. In the kidney, RAS activation induces renal fat accumulation, inflammatory processes, and vessel constriction, which cause and worsen CKD or DKD [
60,
61]. In contrast, the progression of DKD also activates RAS; decreased renal blood flow and glomerular filtration rate in the progressive stage of DKD promote renin secretion, thereby activating RAS [
62].
Finally, MAFLD and DKD share genetic susceptibilities [
30,
49]. A representative example is the genetic polymorphism in
PNPLA3 [
63], which encodes patatin-like phospholipase domain-containing protein 3 (PNPLA3) and is known to have lipase activity [
64]. The
PNPLA3 gene is expressed in the liver and kidneys. PNPLA3 rs 738409 polymorphism has been associated with poor renal outcomes and MAFLD severity [
63,
65,
66,
67]. Other genetic abnormalities, such as transmembrane 6 superfamily member 2
(TM6SF2) and glucokinase regulator
(GCKR) polymorphisms, have also been reported to be associated with the risk of both MAFLD and CKD [
65,
68,
69,
70]. A schematic diagram summarizing the pathophysiological mechanisms linking MAFLD and DKD is shown in
Figure 1.