The frequent association of nonalcoholic fatty liver disease with components of the metabolic syndrome such as obesity, hyperglycemia, dyslipidemia, and hypertension is well known. However, no prospective study has examined the role of the metabolic syndrome in the development of this disease. To characterize the longitudinal relationship between the metabolic syndrome and nonalcoholic fatty liver disease. A prospective observational study. A medical health checkup program in a general hospital. 4401 apparently healthy Japanese men and women, 21 to 80 years of age, with a mean body mass index (BMI) of 22.6 kg/m² (SD, 3.0). Alcohol intake was assessed by using a questionnaire. Biochemical tests for liver and metabolic function and abdominal ultrasonography were done. Modified criteria of the National Cholesterol Education Program Adult Treatment Panel III were used to characterize the metabolic syndrome. At baseline, 812 of 4401 (18%) participants had nonalcoholic fatty liver disease. During the mean follow-up period of 414 days (SD, 128), the authors observed 308 new cases (10%) of nonalcoholic fatty liver disease among 3147 participants who were disease-free at baseline and who completed a second examination. Regression of nonalcoholic fatty liver disease was found in 113 (16%) of 704 participants who had the disease at baseline and who completed a second examination. Men and women who met the criteria for the metabolic syndrome at baseline were more likely to develop the disease during follow-up (adjusted odds ratio, 4.00 [95% CI, 2.63 to 6.08] and 11.20 [CI, 4.85 to 25.87], respectively). Nonalcoholic fatty liver disease was less likely to regress in those participants with the metabolic syndrome at baseline. Ultrasonography may lead to an incorrect diagnosis of nonalcoholic fatty liver disease in 10% to 30% of cases and cannot distinguish steatohepatitis from simple steatosis. Self-reported alcohol intake may cause bias. Because all of the participants were Japanese, generalizability to non-Japanese populations is uncertain. The metabolic syndrome is a strong predictor of nonalcoholic fatty liver disease.

Nonalcoholic fatty liver disease (NAFLD) is closely associated with the metabolic syndrome. We evaluated the association among the metabolic syndrome, visceral fat accumulation, and the severity of fatty liver with a new scoring system of ultrasonographic findings in apparently healthy Japanese adults. Subjects consisted of 94 patients who received liver biopsy and 4,826 participants who were selected from the general population. Two hepatologists scored the ultrasonographic findings from 0 to 6 points. We calculated Cohen's kappa of within-observer reliability and between-observer reliability. We evaluated the predictive value of the score by the area under a conventional receiver operating characteristic curve (AUC). Within-observer reliability was 0.95 (95% CI 0.93–0.97, P < 0.001) and between-observer reliability was 0.95 (95% CI 0.93–0.97, P < 0.001). The AUC to diagnose NAFLD was 0.980. The sensitivity was 91.7% (95% CI 87.0–95.1, P < 0.001) and the specificity was 100% (95% CI 95.4–100.0, P < 0.001). The AUC to diagnose visceral obesity was 0.821. The sensitivity was 68.3% (95% CI 51.9–81.9, P = 0.028) and the specificity was 95.1% (95% CI 86.3–99.0, P < 0.001). Adjusted odds ratio of the score for the metabolic syndrome was 1.37 (95% CI 1.26–1.49, P < 0.001). The scoring system with abdominal ultrasonography could provide accurate information about hepatic steatosis, visceral obesity, and the metabolic syndrome in apparently healthy people who do not consume alcohol.

Chronic kidney disease (CKD) is a frequent, under-recognized condition and a risk factor for renal failure and cardiovascular disease. Increasing evidence connects non-alcoholic fatty liver disease (NAFLD) to CKD. We conducted a meta-analysis to determine whether the presence and severity of NAFLD are associated with the presence and severity of CKD. English and non-English articles from international online databases from 1980 through January 31, 2014 were searched. Observational studies assessing NAFLD by histology, imaging, or biochemistry and defining CKD as either estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m² or proteinuria were included. Two reviewers extracted studies independently and in duplicate. Individual participant data (IPD) were solicited from all selected studies. Studies providing IPD were combined with studies providing only aggregate data with the two-stage method. Main outcomes were pooled using random-effects models. Sensitivity and subgroup analyses were used to explore sources of heterogeneity and the effect of potential confounders. The influences of age, whole-body/abdominal obesity, homeostasis model of insulin resistance (HOMA-IR), and duration of follow-up on effect estimates were assessed by meta-regression. Thirty-three studies (63,902 participants, 16 population-based and 17 hospital-based, 20 cross-sectional, and 13 longitudinal) were included. For 20 studies (61% of included studies, 11 cross-sectional and nine longitudinal, 29,282 participants), we obtained IPD. NAFLD was associated with an increased risk of prevalent (odds ratio [OR] 2.12, 95% CI 1.69–2.66) and incident (hazard ratio [HR] 1.79, 95% CI 1.65–1.95) CKD. Non-alcoholic steatohepatitis (NASH) was associated with a higher prevalence (OR 2.53, 95% CI 1.58–4.05) and incidence (HR 2.12, 95% CI 1.42–3.17) of CKD than simple steatosis. Advanced fibrosis was associated with a higher prevalence (OR 5.20, 95% CI 3.14–8.61) and incidence (HR 3.29, 95% CI 2.30–4.71) of CKD than non-advanced fibrosis. In all analyses, the magnitude and direction of effects remained unaffected by diabetes status, after adjustment for other risk factors, and in other subgroup and meta-regression analyses. In cross-sectional and longitudinal studies, the severity of NAFLD was positively associated with CKD stages. Limitations of analysis are the relatively small size of studies utilizing liver histology and the suboptimal sensitivity of ultrasound and biochemistry for NAFLD detection in population-based studies. The presence and severity of NAFLD are associated with an increased risk and severity of CKD. Please see later in the article for the Editors' Summary Chronic kidney disease (CKD)—the gradual loss of kidney function—is becoming increasingly common. In the US, for example, more than 10% of the adult population (about 26 million people) and more than 25% of individuals older than 65 years have CKD. Throughout life, the kidneys perform the essential task of filtering waste products (from the normal breakdown of tissues and from food) and excess water from the blood to make urine. CKD gradually destroys the kidneys' filtration units, the rate of blood filtration decreases, and dangerous amounts of waste products build up in the blood. Symptoms of CKD, which rarely occur until the disease is very advanced, include tiredness, swollen feet, and frequent urination, particularly at night. There is no cure for CKD, but progression of the disease can be slowed by controlling high blood pressure and diabetes (two risk factors for CKD), and by adopting a healthy lifestyle. The same interventions also reduce the chances of CKD developing in the first place. CKD is associated with an increased risk of end-stage renal (kidney) disease and of cardiovascular disease. These life-threatening complications are potentially preventable through early identification and treatment of CKD. Because early recognition of CKD has the potential to reduce its health-related burden, the search is on for new modifiable risk factors for CKD. One possible new risk factor is non-alcoholic fatty liver disease (NAFLD), which, like CKD is becoming increasingly common. Healthy livers contain little or no fat but, in the US, 30% of the general adult population and up to 70% of patients who are obese or have diabetes have some degree of NAFLD, which ranges in severity from simple fatty liver (steatosis), through non-alcoholic steatohepatitis (NASH), to NASH with fibrosis (scarring of the liver) and finally cirrhosis (extensive scarring). In this systematic review and meta-analysis, the researchers investigate whether NAFLD is a risk factor for CKD by looking for an association between the two conditions. A systematic review identifies all the research on a given topic using predefined criteria, meta-analysis uses statistical methods to combine the results of several studies. The researchers identified 33 studies that assessed NAFLD and CKD in nearly 64,000 participants, including 20 cross-sectional studies in which participants were assessed for NAFLD and CKD at a single time point and 13 longitudinal studies in which participants were assessed for NAFLD and then followed up to see whether they subsequently developed CKD. Meta-analysis of the data from the cross-sectional studies indicated that NAFLD was associated with a 2-fold increased risk of prevalent (pre-existing) CKD (an odds ratio [OR]of 2.12; an OR indicates the chance that an outcome will occur given a particular exposure, compared to the chance of the outcome occurring in the absence of that exposure). Meta-analysis of data from the longitudinal studies indicated that NAFLD was associated with a nearly 2-fold increased risk of incident (new) CKD (a hazard ratio [HR] of 1.79; an HR indicates often a particular event happens in one group compared to how often it happens in another group, over time). NASH was associated with a higher prevalence and incidence of CKD than simple steatosis. Similarly, advanced fibrosis was associated with a higher prevalence and incidence of CKD than non-advanced fibrosis. These findings suggest that NAFLD is associated with an increased prevalence and incidence of CKD and that increased severity of liver disease is associated with an increased risk and severity of CKD. Because these associations persist after allowing for established risk factors for CKD, these findings identify NAFLD as an independent CKD risk factor. Certain aspects of the studies included in this meta-analysis (for example, only a few studies used biopsies to diagnose NAFLD; most used less sensitive tests that may have misclassified some individuals with NAFLD as normal) and the methods used in the meta-analysis may limit the accuracy of these findings. Nevertheless, these findings suggest that individuals with NAFLD should be screened for CKD even in the absence of other risk factors for the disease, and that better treatment of NAFLD may help to prevent CKD. Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001680.