Type 2 diabetes mellitus (T2DM) develops more frequently and at younger ages in men compared to women, even at lower levels of body fat. This sex dimorphism may be partly explained by differences in insulin action, with women often showing greater insulin sensitivity in skeletal muscle and liver. The role of adipose tissue in this sex difference is unclear. Adipose tissue is crucial for insulin regulation of energy homeostasis due to insulin's effects on fatty acid metabolism. Less efficient insulin inhibition of lipolysis (triglyceride breakdown) and/or stimulation of lipogenesis (triglyceride synthesis) could lead to elevated circulating fatty acids and contribute to insulin resistance. Therefore, sex dimorphism in insulin's action on adipose lipid metabolism may have a different impact on T2DM pathogenesis than insulin resistance in liver and muscle. Traditional methods for assessing adipose insulin action are cumbersome and unsuitable for large-scale studies. AdipolR, a simple index calculated from fasting fatty acid and insulin levels, offers a potential alternative. This study used AdipolR in a large cohort to investigate sex differences, considering the strong influence of obesity on insulin action. To explore cellular mechanisms, the study also examined basal lipolysis and insulin's action on lipolysis and lipogenesis in isolated subcutaneous adipocytes, as well as mRNA expression of genes involved in the canonical insulin signaling pathway.

Existing literature indicates that men are more susceptible to developing T2DM at younger ages and lower body fat percentages compared to women. Differences in insulin action between sexes are implicated, with studies showing higher insulin sensitivity in women's skeletal muscle and liver. However, the contribution of adipose tissue to these sex differences remains an area of investigation. The importance of adipose tissue's role in insulin's regulation of energy balance is recognized, particularly concerning its impact on fatty acid metabolism. Inefficient insulin action on lipolysis and lipogenesis in adipocytes can lead to elevated circulating fatty acids, thus contributing to systemic insulin resistance. Therefore, investigating the effects of insulin on adipose tissue's lipid metabolism might provide unique insights into the pathogenesis of T2DM that differ from insulin resistance found in muscle and liver. Traditional methods for assessing adipose tissue insulin sensitivity are resource-intensive, making it challenging to conduct large-scale studies. Recently developed indirect techniques, such as the AdipolR index (calculated from fasting insulin and fatty acid concentrations), provide a more convenient approach for evaluating insulin action in adipose tissue in larger populations. This study intends to build on these advances by leveraging the AdipolR index to explore sex differences in adipose tissue insulin resistance and the underlying cellular mechanisms.

The study utilized data from two cohorts: KAROLINSKA and DiOGenes. KAROLINSKA, a single-center cohort (Stockholm, Sweden) recruited between 1993 and 2020, included 2344 women and 787 men with AdipolR data. Participants were considered body weight stable for at least 3 months. Subcutaneous adipose tissue biopsies were collected from obese participants (BMI ≥ 30 kg/m²) to assess insulin action on lipolysis and lipogenesis in isolated adipocytes. The DiOGenes cohort, a pan-European study, was used solely for gene expression analysis in subcutaneous adipose tissue. This cohort comprised 115 men and 234 women with obesity, with gene expression data obtained from baseline examinations. A subset of DiOGenes participants (175 women, 109 men) also underwent RT-qPCR validation. In KAROLINSKA, AdipolR was calculated, along with HOMA-IR (Homeostasis Model Assessment for Insulin Resistance). Physical activity was assessed using a four-graded scale. In both cohorts, clinical chemistry measurements were performed. Adipocyte lipolysis was assessed by measuring glycerol release in the presence and absence of insulin. Lipogenesis was evaluated by measuring glucose incorporation into lipids. Insulin sensitivity (pD2) and responsiveness (maximal effect) were determined. RNA sequencing was performed on subcutaneous adipose tissue from the DiOGenes cohort to assess the mRNA expression of genes involved in insulin signaling. RT-qPCR was used for validation in a subset of participants. Statistical analyses included Wilcoxon's two-sample test, analysis of covariance (ANCOVA), and Spearman correlation.

Men showed significantly higher AdipolR values than women only when obesity was present (p < 0.0001). This sex difference persisted across various subgroups, including those with different activity levels, presence/absence of cardiometabolic disease, and nicotine use. In obese individuals, men's adipocytes showed significantly lower insulin sensitivity (pD2, approximately tenfold lower) and maximal antilipolytic effect (10% lower) compared to women (p ≤ 0.005). Basal lipolysis rates were significantly higher in men than women (p > 0.0001). Insulin's effects on lipogenesis were similar between sexes. RNA sequencing analysis of the canonical insulin signaling pathway in obese participants revealed significantly lower expression of IRS1 in men compared to women (p < 0.0001), confirmed by RT-qPCR (60% higher IRS1 expression in women, p < 0.0001). Analysis of other genes related to basal lipolysis showed sex dimorphism in the expression of CIDEA, PDE3B, and testosterone receptors (higher in men).

This study demonstrates that in obesity, men exhibit more pronounced adipose tissue insulin resistance than women. This appears to be driven by a less efficient insulin-mediated inhibition of adipocyte lipolysis, an elevated basal lipolysis rate, and decreased expression of IRS1, a critical component of insulin signaling. The observed sex difference in AdipolR and antilipolysis was independent of various cofactors, including age, cardiometabolic disease status, BMI, nicotine use, and physical activity level. The findings suggest that interventions aimed at improving insulin sensitivity, reducing basal lipolysis, or targeting IRS1 expression may be particularly beneficial for reducing the risk of future glucose intolerance and T2DM in obese men. The tenfold difference in insulin sensitivity (pD2) for antilipolysis in obese men compared to women suggests that early steps in the canonical insulin signaling pathway are pivotal for these sex differences.

In conclusion, this study reveals that adipose tissue insulin resistance is more prominent in obese men than in obese women. This sex difference is largely attributable to a less efficient inhibition of lipolysis, a higher basal lipolysis rate, and reduced adipose expression of IRS1. These findings highlight potential therapeutic targets for improving insulin sensitivity and reducing the risk of T2DM in obese men. Future research should explore the longitudinal relationship between these factors and the development of glucose intolerance and T2DM, and investigate depot-specific differences in adipose tissue insulin action.

The study primarily focused on abdominal subcutaneous adipose tissue, neglecting potential depot-specific differences in adipose tissue function. The sample size was larger for women than men, potentially introducing bias. The cross-sectional nature of the study limits causal inferences. Menstrual cycle status was not considered, and the study population was not randomly selected.