The alarming increase in obesity and its related metabolic health complications, such as type 2 diabetes, has evolved into a global pandemic. Obesity is mainly characterized by excessive accumulation of adipose tissue, primarily due to an imbalance between energy intake and expenditure. Prolonged positive energy balance leads to the expansion of existing adipocytes (hypertrophy) and/or an increase in preadipocyte and adipocyte number (hyperplasia) to accommodate excess energy intake. However, obesity is not solely defined by increases in adipocyte size and number. The turnover of adipose tissue cells also plays a crucial role in the development and progression of obesity. Cell turnover encompasses the processes of cell proliferation, differentiation, and apoptosis, which collectively regulate the overall cell population within adipose tissue. Lipid turnover represents another critical factor that influences how adipose tissue stores and releases energy. Our understanding of adipose tissue lipid turnover in humans remains limited due to the slow rate of turnover and methodological constraints. Nonetheless, disturbances in lipid metabolism are strongly associated with altered adipose tissue lipid turnover. In obesity, there is a decreased rate of triglyceride removal (lipolysis followed by oxidation), leading to the accumulation of triglycerides over time. This review provides a comprehensive summary of findings from both in vitro and in vivo methods used to study the turnover of adipose cells and lipids in metabolic health and disease. Understanding the mechanisms underlying cellular and lipid turnover in obesity is essential for developing strategies to mitigate the adverse effects of excess adiposity.

Litter size is a biological variable that strongly influences adult physiology in rodents. Despite evidence from previous decades and recent studies highlighting its major impact on metabolism, information about litter size is currently underreported in the scientific literature. Here, we urge that this important biological variable should be explicitly stated in research articles. Below, we briefly describe the scientific evidence supporting the impact of litter size on adult physiology and outline a series of recommendations and guidelines to be implemented by investigators, funding agencies, editors in scientific journals, and animal suppliers to fill this important gap.

Childhood obesity increases the risk of developing metabolic syndrome later in life. Moreover, metabolic dysfunction may be inherited into the following generation through non-genomic mechanisms, with epigenetics as a plausible candidate. The pathways involved in the development of metabolic dysfunction across generations in the context of childhood obesity remain largely unexplored. We have developed a mouse model of early adiposity by reducing litter size at birth (small litter group, SL: 4 pups/dam; control group, C: 8 pups/dam). Mice raised in small litters (SL) developed obesity, insulin resistance and hepatic steatosis with aging. Strikingly, the offspring of SL males (SL-F1) also developed hepatic steatosis. Paternal transmission of an environmentally induced phenotype strongly suggests epigenetic inheritance. We analyzed the hepatic transcriptome in C-F1 and SL-F1 mice to identify pathways involved in the development of hepatic steatosis. We found that the circadian rhythm and lipid metabolic process were the ontologies with highest significance in the liver of SL-F1 mice. We explored whether DNA methylation and small non-coding RNAs might be involved in mediating intergenerational effects. Sperm DNA methylation was largely altered in SL mice. However, these changes did not correlate with the hepatic transcriptome. Next, we analyzed small non-coding RNA content in the testes of mice from the parental generation. Two miRNAs (miR-457 and miR-201) appeared differentially expressed in the testes of SL-F0 mice. They are known to be expressed in mature spermatozoa, but not in oocytes nor early embryos, and they may regulate the transcription of lipogenic genes, but not clock genes, in hepatocytes. Hence, they are strong candidates to mediate the inheritance of adult hepatic steatosis in our murine model. In conclusion, litter size reduction leads to intergenerational effects through non-genomic mechanisms. In our model, DNA methylation does not seem to play a role on the circadian rhythm nor lipid genes. However, at least two paternal miRNAs might influence the expression of a few lipid-related genes in the first-generation offspring, F1.

Nutritional challenges and obesity can contribute to the transmission of metabolic diseases through epigenetic mechanisms. Among them, DNA methylation stands out as a potential carrier of information because germline cytosine methylation responds to environmental factors and can be transmitted across generations. Yet, it remains unclear whether inherited DNA methylation plays an active role in the inheritance of metabolic phenotypes or solely influences expression of a few genes that cannot recapitulate the whole metabolic spectrum in the next generation offspring. Previously, we established a mouse model of childhood obesity by reducing litter size at birth. Mice raised in small litters (SL) developed obesity, insulin resistance, and hepatic steatosis. The offspring (SL-F1) and grand-offspring (SL-F2) of SL males also exhibited hepatic steatosis. Here, we aimed to investigate whether germline DNA methylation could serve as a carrier of phenotypic information, hepatic steatosis, between generations. Litter size reduction significantly altered global DNA methylation profile in the sperm of SL-F0 males. Remarkably, 8% of these methylation marks remained altered in the sperm of SL-F1 mice and in the liver of SL-F2 mice. These data suggest that germline DNA methylation is sensitive to environmental challenges and holds significant heritability, either through direct germline transmission and/or through sequential erasure and reestablishment of the marks in the following generations. Yet, DNA methylation did not strongly correlate with the hepatic transcriptome in SL-F2 mice, suggesting that it does not directly drive phenotypes in the F2. As an alternative, germline DNA methylation could potentially influence the phenotype of the next generation by modulating the expression of a reduced number of key transcription factors that, through an amplification cascade, drive phenotypic outcomes in subsequent generations.

Childhood obesity represents a major public health challenge, increasing long-term risks such as type 2 diabetes and cardiovascular disease. Lifestyle inte