Discussion
Our study showed that NAFLD patients with elevated intestinal
permeability have more severe disease status, manifested as more serious
liver dysfunction, hyperlipidemia, liver fat deposition, insulin
resistance, and intestinal barrier damage. Our data also showed that
serum D-lactate is positively correlated with parameters indicative of
disease severity, including ALT, AST, GGT, TBIL, IBIL, FAP, and TG.
These findings reveal a clear association between intestinal
permeability and disease severity in patients with NAFLD, which add to
accumulating evidence supporting the “gut-liver axis.” Hence,
intestinal
manifestations warrant increased attention in patients with severe NAFLD
such as NASH and NAFLD-associated cirrhosis.
However, a previous study with a small sample size (35 NAFLD patients
and 24 controls) reported no significant differences in liver
transaminases and TG between NAFLD patients with normal and increased
intestinal permeability, measured by 51Cr-EDTA
excretion testing [15]; while another study reported a correlation
between intestinal permeability and pathologic severity (portal
inflammation, fibrosis, and ballooning of hepatocytes) using the
lactulose-mannitol bowel permeability test, in line with our findings
[12]. Further, zonulin, a moderator of intestinal permeability, was
found to be positively correlated with some parameters of disease
severity, such as ALT, TG, HOMA-IR, and liver histopathology in patients
with NAFLD, hence indicating a correlation between intestinal
permeability and NAFLD severity [25]. In addition to the small
sample size of the previous study being likely inadequate to show the
relationship between intestinal permeability and disease severity in
patients with NAFLD, using different methods to detect intestinal
permeability may also lead to inconsistent results. In addition to51Cr-EDTA excretion testing and lactulose-mannitol
bowel permeability test, testing for D-lactate levels has become a
widely used method for intestinal permeability detection in recent years
[23, 24]. Owing to the simplicity of this method, the bias caused by
multiple operations can be reduced. However, given that the criteria for
defining intestinal permeability have not reached certain consensus,
other tests to evaluate intestinal permeability should be used to
confirm these findings by further studies.
There are two plausible reasons for the observed association between
intestinal permeability and severity of NAFLD. First, elevated
intestinal permeability can cause pathogenesis and progression of NAFLD.
Animal research has shown that increased intestinal permeability induced
by dextran sulfate sodium (DSS) enhances high-fat diet-induced hepatic
inflammation and steatosis in mice [26]. Subsequent to the increase
of intestinal permeability, bacterial components, particularly LPS, can
translocate into the portal vein and thus the liver, resulting in liver
inflammation and injury [27, 28]. In support of this hypothesis, a
clinical study showed that plasma antibodies against LPS were increased
in patients with NASH compared with healthy controls, and increased with
aggravated inflammation in NASH, indicating an association between LPS
exposure and the severity of NASH in humans [29]. Consistent with
this finding, our data showed a positive correlation between D-lactate
and LPS, although the difference did not reach the threshold for
statistical significance (P=0.069), suggesting that increased intestinal
permeability may lead to LPS translocation, and consequently enhanced
liver injury. The alternative explanation is that NAFLD may contribute
to the increase in intestinal permeability, as disruption of the
intestinal epithelial barrier and gut vascular barrier can be detected
in NAFLD mice induced by high-fat diet [14].
Recent studies have found that improvement of intestinal permeability by
fecal microbiota transplantation or probiotics had a therapeutic effect
on NAFLD [16, 17]. These results suggest that improvement of
intestinal permeability may promote the lipid-lowering effect of
metabolic therapy in patients with NAFLD. Accordingly, our data revealed
that NAFLD patients with elevated intestinal permeability present with
less substantial improvement in TG levels after metabolic therapy. This
study has provided evidence for the role of elevated intestinal
permeability in NAFLD progression and suggested that a combination of
treatment to improve intestinal barrier and metabolic therapy may have
better therapeutic effects on NAFLD patients, especially those with
elevated intestinal permeability.
However, we found that the improvement of clinical characteristics,
liver test parameters, and liver ultrasonographic parameters were not
significantly different between NAFLD patients with elevated and normal
intestinal permeability after one month of metabolic treatment. As these
patients received metabolic therapy, the improvement of blood lipids was
more obvious, while other parameters of disease severity did not show
significant improvement. In addition, we only analyzed the results of
the one-month-long metabolic therapy, thus the unexceptional improvement
in other parameters could likely be attributed to the short treatment
time. Therefore, a prospective and long-term follow-up study is needed
to assess whether intestinal permeability affects the improvement of
other parameters in patients with NAFLD after treatment.
The present study has some limitations. First, intestinal biopsies were
not performed as standard in our investigation, as most enrolled
patients had no gastrointestinal symptoms. Second, because this is a
retrospective study, some potential confounders (e.g., use of medication
with liver toxicity) that may lead to liver injury and steatosis were
not recorded. Third, although our sample size was larger than those of
previous studies investigating the association between intestinal
permeability and NAFLD, this was a single-center study with a limited
sample size. Only 30 patients’ clinical data were available for
analyzing the efficacy of metabolic therapy in NAFLD patients with
normal or elevated intestinal permeability; moreover, we could not
analyze whether intestinal permeability affects the improvement of other
parameters in patients with NAFLD after treatment. Therefore, our
results should be interpreted with caution, and future studies are
needed to confirm these findings.
In summary, intestinal permeability correlates with the severity of
liver dysfunction, hyperlipidemia, liver fat deposition, insulin
resistance, and intestinal barrier damage in patients with NAFLD.
Moreover, intestinal permeability may be valuable for predicting the
efficacy of metabolic therapy in patients with NAFLD.
Acknowledgement
The manuscript has been presented as a preprint.