Nonalcoholic fatty liver is associated with obesity‐related metabolic disturbances, but little is known about the metabolic perturbations preceding fatty liver disease. We performed comprehensive metabolic profiling to assess how circulating metabolites, such as lipoprotein lipids, fatty acids, amino acids, and glycolysis‐related metabolites, reflect the presence of and future risk for fatty liver in young adults. Sixty‐eight lipids and metabolites were quantified by nuclear magnetic resonance metabolomics in the population‐based Young Finns Study from serum collected in 2001 (n = 1,575), 2007 (n = 1,509), and 2011 (n = 2,002). Fatty liver was diagnosed by ultrasound in 2011 when participants were aged 34‐49 years (19% prevalence). Cross‐sectional associations as well as 4‐year and 10‐year risks for fatty liver were assessed by logistic regression. Metabolites across multiple pathways were strongly associated with the presence of fatty liver (P < 0.0007 for 60 measures in age‐adjusted and sex‐adjusted cross‐sectional analyses). The strongest direct associations were observed for extremely large very‐low‐density lipoprotein triglycerides (odds ratio [OR] = 4.86 per 1 standard deviation, 95% confidence interval 3.48‐6.78), other very‐low‐density lipoprotein measures, and branched‐chain amino acids (e.g., leucine OR = 2.94, 2.51‐3.44). Strong inverse associations were observed for high‐density lipoprotein measures, e.g., high‐density lipoprotein size (OR = 0.36, 0.30‐0.42) and several fatty acids including omega‐6 (OR = 0.37, 0.32‐0.42). The metabolic associations were attenuated but remained significant after adjusting for waist, physical activity, alcohol consumption, and smoking (P < 0.0007). Similar aberrations in the metabolic profile were observed already 10 years before fatty liver diagnosis. Conclusion: Circulating lipids, fatty acids, and amino acids reflect fatty liver independently of routine metabolic risk factors; these metabolic aberrations appear to precede the development of fatty liver in young adults. (Hepatology 2017;65:491‐500).

Prevalence of fatty liver was 29% in overweight/obese and 5% in normal-weight adults. Differences in factors associated with fatty liver were seen between these two groups: alcohol intake and smoking were more strongly and triglycerides more weakly associated in normal-weight than in overweight/obese participants.

The increase in pediatric obesity emphasizes the importance of identification of children and adolescents at high risk of fatty liver in adulthood. We used data from the longitudinal Cardiovascular Risk in Young Finns Study to examine the associations of childhood (3-18years) risk variables with fatty liver assessed in adulthood at the age of 34-49years. The findings suggest that a multifactorial approach with both lifestyle and genetic factors included would improve early identification of children with a high risk of adult fatty liver.

In a population-based sample of adults, cardiorespiratory fitness is strongly, inversely and independently related with the risk of fatty liver. Importantly, the association is evident also among obese.

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