Lifestyle factors and lipid metabolism in non-alcoholic fatty liver disease, the metabolic syndrome, and atherosclerotic cardiovascular disease

Background: Non-alcoholic fatty liver disease (NAFLD) and the metabolic syndrome (MetS) have become major health concerns due to the global obesity epidemic. Both conditions increase the risk of atherosclerotic cardiovascular disease (ASCVD), and their mechanisms partially lie in dysregulated lipid metabolism. Obesity is the leading risk factor for NAFLD and the MetS, but not all obese individuals develop these metabolic abnormalities. The risk may be modulated by lifestyle factors, which also remain the cornerstone of the treatment of NAFLD, the MetS, and ASCVD. However, their role has not been fully elucidated. Aims: The present series of studies was undertaken to better understand the role of lifestyle factors and lipid metabolism in NAFLD, the MetS, and ASCVD. Specifically, we wished to determine i) whether the characterization of lifestyle factors can help identify obese patients who have advanced NAFLD with fibrosis (NAFLD-fibrosis), ii) how physical training using a weighted hula-hoop influences abdominal obesity and the associated metabolic parameters compared to walking, iii) how the lipid composition of the liver is related to the lipid composition and aggregation susceptibility of low-density lipoprotein (LDL) particles, and iv) how overfeeding different macronutrients influences the proatherogenic properties of LDL particles. Participants and methods: We cross-sectionally examined a group of 100 (Study I), and 40 (Study III) obese individuals. Fifty-five abdominally obese participants underwent a cross-over designed trial addressing different forms of physical activity (PA) (Study II), and 36 participants underwent a three-week overfeeding intervention comparing three different diets (Study IV). The participants were randomized to hula-hooping for six weeks using a 1.5 kg weighted hula-hoop followed by energy expenditure matched walking for another six weeks or vice versa (Study II), and to daily consuming an extra 1000 kilocalories from either saturated fat, unsaturated fat, or simple sugars (Study IV). Physical and biochemical parameters were measured during clinical visits (Studies I–IV), and liver histology was assessed from liver biopsies (Studies I, III). Lifestyle factors (PA, diet, sleep, fatigue, smoking) were assessed using questionnaires and face-to-face interviews, and PA was also assessed objectively using accelerometer recordings (Study I). The lipid compositions of the liver (Study III) and isolated LDL particles (Studies III–IV) were measured using ultra-high performance liquid chromatography–mass spectrometry. The aggregation susceptibility of LDL particles was measured ex vivo by inducing aggregation with human recombinant acid sphingomyelinase (Studies III–IV). The proteoglycan-binding of plasma lipoproteins and the concentrations of oxidized LDL were also measured (Study IV). Abdominal obesity was measured using dual-energy X-ray absorptiometry (Study II). Results: Low levels of self-reported moderate-to-vigorous PA, high red meat intake, low relative carbohydrate intake, and smoking independently were associated with the presence of NAFLD-fibrosis in obese participants. The characterization of these lifestyle factors, in addition to known physical, biochemical, and genetic risk factors, significantly improved the identification of patients with NAFLD-fibrosis (Study I). Weighted hula-hooping decreased abdominal fat % and lowered LDL-cholesterol in abdominally obese individuals in comparison to walking (Study II). Walking, on the other hand, decreased blood pressure and increased high-density lipoprotein (HDL)-cholesterol. The fatty acid total carbon number and double bond compositions of the triglycerides, sphingomyelins, and phosphatidylcholines in the liver were strongly associated with those in LDL particles. Hepatic dihydroceramides and ceramides correlated positively with the concentrations of the corresponding sphingomyelin species in LDL, as well as with increased LDL aggregation susceptibility (Study III). Overfeeding saturated fat increased the content of sphingolipids and saturated triglycerides in LDL as well as LDL aggregation susceptibility. Overfeeding unsaturated fat reduced the proteoglycan-binding of plasma lipoproteins, whereas no changes were observed during carbohydrate overfeeding (Study IV). Conclusions: Lifestyle factors, that is, both diet and PA, are associated with and modulate the features of NAFLD, the MetS, and ASCVD in obese individuals. Assessing these lifestyle habits helps estimate the risk of advanced NAFLD in obese patients. Weighted hula-hooping reduces abdominal obesity, and excess intake of saturated fat modifies the composition of LDL particles, increasing their aggregation susceptibility. The lipid composition of the liver and LDL particles are closely related, and patients with aggregation-susceptible LDL particles show changes in hepatic sphingolipid metabolism. In line with current treatment recommendations, the present series of studies supports the metabolic and cardiovascular benefits of PA, also in their untraditional forms, and the replacement of dietary saturated fat with unsaturated fat in cases of obesity.