Deep subcutaneous adipose tissue lipid unsaturation associates with intramyocellular lipid content

Background: Obese twins have lower saturated and higher long-chain polyunsaturated fatty acids (FA) in subcutaneous adipose tissue (SAT) compared to their lean monozygotic (MZ) co-twin. Whether this holds for metabolically distinct deep (DSAT) and superficial (SSAT) depots is unknown. Here we use non-invasive magnetic resonance spectroscopy (MRS) to measure the FA unsaturation in body mass index (BMI) discordant MZ twins in DSAT and SSAT and their relationship to ectopic fat content and body fat distribution. The main finding is further confirmed in an independent cohort using standardized measurement times.

Methods: MRS and magnetic resonance imaging were used to measure DSAT and SSAT unsaturation and their relationship to intramyocellular lipids (IMCL), hepatocellular lipids (HCL) and the amount of subcutaneous (SAT) and visceral adipose tissue (VAT) in 16 pairs of healthy monozygotic twins (MZ) discordant for BMI. A second independent cohort of 12 healthy volunteers was used to measure DSAT unsaturation and IMCL with standardized measurement time. One volunteer also underwent repeated random measurements of DSAT unsaturation and IMCL.

Results: In accordance with biopsy studies SSAT unsaturation was higher in the heavier twins (15.2±1.0% vs. 14.4±1.5%, P=0.024) and associated with SAT volume (R=0.672, P=0.001). DSAT unsaturation did not differ between twins (11.4±0.8 vs. 11.0±1.0, P=0.267) and associated inversely with IMCL content (R=-0.462, P=0.001). The inverse association between DSAT unsaturation and IMCL was also present in the participants of the second cohort (R=-0.641, P=0.025) and for the repeated sampling at random of one person (R=-0.765, P=0.027).

Conclusions: DSAT and SSAT FA unsaturation shows distinct associations with obesity and IMCL in MZ twins, reflecting compartment-specific metabolic activities. The FA unsaturation in the DSAT depot associates inversely with IMCL content, which raises the possibility of cross talk between the DSAT depot and the rapid turnover IMCL depot.