The present study elicited several findings. We revealed a significantly positive association between RBC count, MCHC and the risk of NAFLD. On the whole, every one-unit (1012/L) increase in RBC count was associated with a 50% increased risk for NAFLD while everyone-unit (g/dL) increase in MCHC was related with a 20% increased risk for NAFLD. Participants in higher quartiles of RBC counts and MCHC had a higher risk for NAFLD. Moreover, RDW was positively associated with advanced liver fibrosis and every one-unit (%) increase in RDW was related with a 30% increased risk for NAFLD. Participants in higher quartiles of RDW had a higher risk for advanced liver fibrosis. However, no correlation were detected between RBC counts and advanced liver fibrosis, MCHC and advanced liver fibrosis, RDW and NAFLD.
A limited number of studies have focused on the association between RBC and NAFLD, and the results remain controversial and inconclusive. Wang H et al. conducted a large-scale cross-sectional study of 8618 southern Chinese adults, discovering higher RBC count was positively associated with a higher risk of ultrasonography-diagnosed fatty liver disease[10], but their results were puzzled by alcohol consumption. A cohort study that included 27,112 subjects with up to 5 years of follow-up in China indicated increased RBC count might precede the onset of NAFLD[11]. The same results were found in another study[19]. However, Danny Issa et al. concluded no significant association was detected between the RBC count and severity degree of NAFLD depended on the histopathological features [20]. This inconsistency among these studies might be due to the heterogeneity among sample sizes, study designs, samples from different regions, and methods to detect NAFLD. We innovatively found the intimate relationship of MCHC and NAFLD, since no literature has previously reported. In the present study, we revealed RBC count and MCHC were positively related with the risk of NAFLD but not liver fibrosis in the general American population.
In the present study, we revealed RDW was positively associated with advanced liver fibrosis, but not associated with NAFLD. Several previous studies investigating the correlation of RDW with NAFLD and advance fibrosis have been also published over the last decade [12, 13, 21]. A retrospective study including 24,547 subjects from Korea analyzed the association between RDW values and the degree of fibrosis in NAFLD[13]. NAFLD was diagnosed on abdominal ultrasonography and the degree of liver fibrosis was determined according to BARD and FIB-4 scores. After adjusting for age, hemoglobin level, mean corpuscular volume, history of hypertension, history of diabetes, and high-sensitivity C-reactive protein, elevated RDW was found independently associated with advanced fibrosis in NAFLD. Although the ethnicity, sample size, diagnostic approach, and adjusted confounding factors are different, the result was congruent with that in our study. Yang et al collected data from 1637 normal control individuals and 619 NAFLD patients for routine medical check-up in their hospital and found that patients with NAFLD defined by abdominal ultrasonography were more likely to have high levels of RDW (β = 0.301, P < 0.01) analyzed by binary linear regression analysis. The independent variable was NAFLD and the dependent variable was the level of RDW in their study. The inconsistency in this study with our result might mainly caused by the differences in ethnicities of participants, sample size and statistical approach. More detailed study should be carried on to determine the correlation of RDW with NAFLD and advance fibrosis in general population of Americans.
Previous literature has preliminarily explained how RBCs function in the pathogenesis of NAFLD and progression. RBCs are well equipped with antioxidant systems, which can protect liver tissue from injury and regulate cardiovascular homeostasis while NAFLD is frequently accompanied by iron overload, inflammation, and oxidative damage[22].Oxidative stress in metabolic syndrome and NAFLD could enhance compensatory RBC count, functioning to remedy liver function. Iron is proved vital to the creation of hemoglobin and red blood cell, therefore iron overload and higher ferritin levels in NAFLD might also lead to the increase of compensatory RBC count in circulation[23]. Besides, the capacity of the liver to degrade aging RBCs in NAFLD patients might be lower. Besides, the liver is an important organ for metabolism and regularly produces various pro-oxidant reactive species. When RBCs flow through liver tissues, the liver functions to eliminate aging RBCs, resulting in the release of iron and triggering oxidative stress[10]. However, further thorough studies focusing on the mechanism for higher RBC count in NAFLD are urgently needed with fatty liver biopsy samples from patients or in animal and cell models.
Due to the nationally representative nature of NHANES, the strength of our findings lies in the large size of the study samples. Focusing on the correlation of RBC count and the risk of NAFLD, this study included 3563 participants, the largest number of study samples in our perspective. However, there were some shortcomings. First, NAFLD was confirmed based on CAP detected by FibroScan® but not the gold standard liver biopsy and many participants lacking available VCTE data were excluded, which might cause bias in including NAFLD participants. Moreover, the accuracy needs further checked with CAP ≥ 285dB/m to diagnose NAFLD. Second, patients with known hepatitis B and C as well as those with significant alcohol intake were ruled out. Nevertheless, several other liver diseases might have been underestimated and not been considered (e.g. AIH, cholestatic diseases). Third, in a large sample size, even minor and non-significant differences come as significant p value. Forth, though we have adjusted several important covariates, other potential factors might introduce the bias to induce the conclusion not credible.