General audience and those dealing with metabolism and MR spectroscopy.The regulation of adipose tissue fatty acid (FA) composition is largely unknown. Previous studies on adipose tissue FA composition have used invasive sampling procedures and primarily have not taken into account the metabolically more active deep subcutaneous adipose (DSAT) depot. Novel magnetic resonance spectroscopy (MRS) methods allow non-invasive fat depot specific analysis of the tissue FA composition [1]. A recent twin study found higher unsaturation in the adipose tissue of obese twins compared to their leaner monozygotic (MZ) co-twin [2]. This study, however, used invasive biopsy for sampling FA composition and did not differentiate between the DSAT and superficial adipose tissue (SSAT) compartments. The present study now uses non-invasive MRS to examine whether MZ twins discordant for BMI display depot specific differences in adipose tissue unsaturation (DSAT and SSAT), and how the unsaturation relates to body fat distribution and ectopic fat. The main finding of the twin study is further confirmed in a general population study.For the twin study, a clinical 1.5-Tesla MR imager (Avanto, Siemens, Erlangen, Germany) was used to determined the FA unsaturation of DSAT and SSAT and their relationship to ectopic fat content (intramyocellular lipids, IMCL, and hepatocellular lipids, HCL) and body fat distribution (subcutaneous, SAT, and visceral adipose tissue, VAT) using non-invasive MRS and MRI in 16 pairs of healthy MZ discordant for BMI. Heavy to lean twin differences was tested using a paired t-test. Intra-pair resemblance (IPR) and fat depot associations were analyzed using correlation analysis. For the general population study, a 3.0 Tesla MR imager (Achieva, Philips, Best, The Netherlands) was used to measure spectra from the DSAT and IMCL in 12 healthy volunteers, and also in one volunteer using random repeated sampling (N=8). Detailed methods have been previously described [1,3]. DSAT was sampled from the abdomen, SSAT from lower limb and IMCL from the tibialis anterior muscle. Adipose tissue and liver spectra were analyzed using jMRUI and muscle spectra using LCmodel. The unsaturation was calculated as the ratio of the olefinic (5.3 ppm) to methylene (1.3 ppm) resonances and the IMCL was referenced to muscle creatine content.The results for twin to twin differences and IPR are shown in table 1 (see Figures). In correlation analysis BMI was strongly associated with SAT, VAT and HCL content, as expected. In accordance with the previous twin study, the SSAT unsaturation was higher in the heavier twins and associated with SAT volume (R=0.672, P=0.0012). DSAT unsaturation did not differ between twins and associated inversely with IMCL (R=-0.479, P=0.0056). The inverse association between DSAT unsaturation and IMCL content was confirmed in a general population study (R=-0.6407, P=0.0248) and for the random repeated sampling of one person (R=-0.7653, P=0.0269), see Figure 1 for scatter plot. Adjusting for age, sex or BMI did not change results. The heavier twins had higher SAT, VAT and HCL. Also the unsaturation in SSAT was higher in the heavy twins. This may seem contradictory to the impact of dietary fat, but is in line with a recent observation in adipose tissue biopsies of another set of MZ twins discordant for BMI [2]. Despite the BMI difference, HCL, VAT, SAT, and SSAT unsaturation in the twins still showed IPR>50%. However, neither IMCL nor DSAT unsaturation differed between the discordant co-twins nor showed IPR. For IMCL, the lack of IPR is to be expected, as IMCL levels show significant intraday variation due to both physical activity and dietary intake [3,4]. The lack of a difference for DSAT unsaturation and the lack of IPR in the twins implicate that DSAT unsaturation displays, like the IMCL depot, a relatively fast response to environmental factors. This is in line with the close association observed between DSAT unsaturation and IMCL in twins, in the general population study and for the repeated sampling of one volunteer.Our observations challenge the way adipose tissue FA composition is viewed. Instead of being a slow responding energy pool, the DSAT depot FA respond on a timescale comparable to IMCL and participate in skeletal muscle crosstalk.