Substrate-specific mitochondrial bioenergetics and metabolic flexibility in visceral adipose tissue across obesity-related glycemic states

Obesity is characterized by the expansion of metabolically dysfunctional visceral adipose tissue (VAT) and chronic low-grade inflammation, which contribute to the development of insulin resistance. However, to what extent VAT bioenergetics and substrate preferences differ across distinct obesity-related glycaemic status profiles remains poorly understood. The present study aimed to assess substrate utilization and metabolic flexibility of VAT from individuals with obesity across different glycaemic statuses, namely euglycemia, prediabetes, and type 2 diabetes (T2D). VAT from individuals with obesity with euglycemia (n = 11), prediabetes (n = 12) and T2D (n = 6) was harvested during bariatric surgery. VAT bioenergetics and mitochondrial substrate preferences under the influence of pyruvate, glutamine and fatty acids metabolism inhibitors, were assessed using the Seahorse Analyser. VAT expression of OB, GLS1, MPC1, CPT1a, HK2, PDHA1, RPII, β-actin genes was quantified by qRT-PCR. VAT basal mitochondrial respiration was similar across glycaemic groups. In VAT from euglycemic individuals, pyruvate inhibition decreased spare respiratory capacity. In contrast, in VAT from individuals with prediabetes and T2D, ATP-linked respiration increased when glutamine was the sole substrate available, which in individuals with prediabetes, was accompanied by an increase of non-mitochondrial oxygen consumption. There were no differences in the expression of genes related to energy metabolism across groups. These exploratory findings suggest that substrate-dependent VAT respiratory responses may vary across obesity-related glycaemic states despite the absence of detectable basal respiratory differences. The response observed under glutamine-only conditions raises the possibility of compensatory metabolic adaptations in prediabetes, although their mechanisms and potential alteration in T2D remain to be established.