In this study, we demonstrated that the presence and extent of fatty liver emerged as the primary health marker, and that it was significantly different between MHO and MUO adolescents. Also significant were the differences in the history of high-risk pregnancies and dietary habits between these two groups. Specifically, adolescents with MUO consumed more calories, animal protein, sodium, arachidonic acid, long-chain saturated fatty acids, and calories from ultra-processed grains compared to their MHO counterparts.
The MUO group exhibited significantly higher HFC, as measured by H-MRS, a reference standard for the noninvasive measurement of liver steatosis (42, 43), and confirmed by elevated liver enzymes and ultrasound findings. This compelling evidence suggests that liver fat may play a pivotal role in distinguishing metabolic phenotypes within the context of obesity, surpassing the influence of visceral fat, which did not significantly differ between the two groups of adolescents. These findings are consistent with previous trials in adults which demonstrated that hepatic and not visceral fat is strongly linked to obesity-related metabolic complications (33, 52). Our regression models reinforce the independent significance of HFC. Even after adjusting for various covariates, our results consistently demonstrated a substantial association between elevated HFC and metabolically unhealthy obesity, as well as with fasting plasma glucose, a primary criterion of the definition of MS (supp. Figure 2). These findings emphasize the potential clinical significance of assessing liver fat as an independent biomarker for early identification and intervention in obese adolescents at risk of metabolic disease. It is worth noting that the consensus-based MHO definition currently does not incorporate hepatic steatosis. Given our findings and those of previous studies (14, 29, 33), there is a growing rationale for considering the inclusion of hepatic steatosis in the definition of MHO.
The precise underlying mechanisms that link metabolic health and hepatic steatosis remain incompletely clarified, but the liver is recognized as playing a pivotal role in governing both carbohydrate and lipid metabolism through an intricate network of metabolic pathways. Primarily, ectopic accumulation of lipids in the liver is closely associated with metabolic dysfunction, leading to MASLD, which exerts a profound impact on the metabolic profile (53, 54). MASLD involves the accumulation of ceramides and diacylglycerols due to excessive free fatty acid accumulation, triggering insulin resistance through insulin receptor dysfunction and downstream signaling pathways (55). The capability of insulin to inhibit hepatic glucose production is compromised in the presence of MASLD-associated insulin resistance, leading to worsened glycemic control. Simultaneously, the suppression of adipose tissue lipolysis is inhibited, perpetuating a vicious cycle of insulin resistance and heightening the risk of cardiovascular complications (56, 57). In line with this perspective, another mechanistic explanation contributing to the metabolically unfavorable profile in MASLD involves the secretion of inflammatory cells and cytokines, known as hepatokines (54). Among these hepatokines are fetuin A, follistatin, HFREP1, LECT2, PEDF, and ectodysplasin, collectively exacerbating insulin resistance in skeletal muscle and adipose tissue through the activation of the c-Jun N-terminal kinase signaling pathway. This pathway is characterized by direct inhibitory phosphorylation of insulin receptor substrates, resulting in diminished insulin signaling and exacerbation of hyperglycemia (54).
Our dietary analysis revealed significant differences in nutrient intake between the two study groups. MUO adolescents had a significantly higher total calorie intake, suggesting overconsumption as a contributing factor to metabolic syndrome among them. Furthermore, their dietary patterns were characterized by elevated protein intake, particularly from animal sources, alongside increased consumption of red meat. The detrimental association of high meat consumption on health is well-documented in adults, contributing to metabolic alterations such as insulin resistance and associated diseases such as T2DM (58, 59), MS (60), cardiovascular disease (61, 62), and colorectal cancer (63, 64). A recent study revealed an association between the intake of total, red, and/or processed meat and the incidence and persistence of MASLD, along with clinically significant fibrosis in adult populations (65). This heightened risk may be attributed to several factors, including the impairment in insulin signaling induced by palmitic acid (16:0), which is the most abundant SFA found in animal-derived foods (66–69). Additionally, the consumption of SFAs has been linked to elevated circulating levels of total lipoprotein and LDL-c (70, 71). While our analysis did not find a significant difference in total SFA intake between groups (P = 0.09), there was a significant increase in the consumption of major long-chain SFAs, specifically, palmitic and stearic acid (18:0), in the MUO group. These long-chain SFAs were observed as being particularly detrimental to metabolic health compared to short-medium SFAs in a recent systematic review (72). Moreover, the detrimental effect of red meat products can be related to specific cooking methods, such as advanced glycation end products, heterocyclic amines, heme iron and other byproducts of muscle protein oxidation, adds to these risks (62, 73–75).
Interestingly, we observed significantly higher consumption of arachidonic acid among adolescents with MUO, which positively correlated with their intake of animal protein and red meat (76). A major role of arachidonic acid is that of a substrate for the synthesis of eicosanoids, which include prostaglandins, thromboxanes, and leukotrienes. These are formed by the metabolism of arachidonic acid by cyclooxygenase, lipoxygenase, and cytochrome P450 pathways (77–79). The resulting metabolites have many roles in inflammation, regulation of the immune response, blood clotting, and smooth muscle contraction (77–79). While the functions of arachidonic acid-derived metabolites are well-established in human health outcomes, recent reviews on the impact of arachidonic acid consumption in adults indicated no adverse effects from their increased intake(80). Also, while there was no significant difference in overall ultra-processed food intake between our two study groups, the MUO group showed a notable increase in the consumption of ultra-processed grains, including white bread and rolls, pastries, sugary breakfast cereals, and more. This observation underscores the negative impact of ultra-processed grains, emphasizing their negative contribution to health outcomes. This effect is evident both independently and when combined with a Western diet that is characterized by high consumption of red and processed meats, sugary snacks and drinks, refined grains, convenience foods, and low intake of fruits, vegetables, and whole grains (81, 82). Sodium intake was significantly higher among our adolescents with MUO compared to those with MHO, and nearly double the recommended daily upper limit of 2,300 milligrams per day (83). Sodium is widely recognized for its substantial role in blood pressure and metabolic health dysregulation, even among adolescent populations (84, 85).
Our MUO adolescents were associated with significantly elevated rates of high-risk pregnancies. Barker's hypothesis posits that adverse conditions during gestation can contribute to increased disease risk in later life (86, 87). Our findings appear to support that hypothesis.
Lastly, our evaluation of physical activity and performance revealed unexpectedly insignificant differences between the MUO and MHO groups. Contrary to prevailing beliefs (24, 88), both groups displayed similar levels of physical activity and sedentary behavior with high levels of sedentary behavior and no regular physical exercise. All participants spent an average of approximately 60% of their day in sedentary pursuits, far exceeding the recognized cutoff that leads to increased cardiovascular disease risk in adults (89). These results highlight the need for further investigation into the relationship between obesity phenotype and physical activity levels. The inclusion of hand grip strength and isometric mid-thigh pull max test in our study can help to establish a normative range for adolescents with obesity and furnish informative benchmarks for future research and clinical evaluations.
One strength of our study is our employment of advanced imaging techniques, including MRI and MRS, which are considered gold standards and references for assessing body fat distribution and HFC, respectively. This approach allowed for precise and reliable measurements, thereby enhancing the validity of our results. Moreover, the study population of adolescents with obesity provides a unique perspective in understanding the early markers of unhealthy metabolic obesity and valuable insights for developing interventions to prevent metabolic abnormalities during development. Another strength lies in our examination of a wide range of variables, from prenatal factors, such as birth weight and pregnancy conditions, to current markers, such as blood parameters, physiological measures, and dietary patterns assessed by the FFQ.
Several limitations include the relatively small sample size, which limited statistical power and may have impacted the precision of the ORs estimated in our logistic regression analysis. While efforts were made to adjust for confounding factors, it is possible that our model did not fully account for all contributors to metabolic obesity as demonstrated in this study. Therefore, caution is warranted when interpreting the independent influence of HFC, since unmeasured covariates may have influenced the observed associations. Additionally, there may be information bias in dietary self-reporting due to recall bias. Future prospective investigations with larger sample sizes in multicenter settings will be instrumental in further understanding the relationship between hepatic fat, metabolic health, and dietary factors.
In conclusion, our study findings shed light on the complex relationship between hepatic fat content and metabolic health in adolescents with obesity. We demonstrate an association between MUO and elevated hepatic fat, increased consumption of total calories, animal protein, red meat, sodium, arachidonic acid, long-chain SFAs and ultra-processed grains, along with a history of high-risk pregnancy. These findings underscore the importance of focusing on this age group for preventing metabolic abnormalities that can have lasting effects on adult health. Larger and longitudinal studies are warranted to establish the causal relationships between hepatic fat content, dietary factors, and metabolic health outcomes. Future interventions should specifically prioritize targeting the reduction of hepatic fat content, while also considering dietary modifications to lower salt intake and reduce consumption of meat and other potential contributors to metabolic abnormalities.