Association at the FADS1 locus. SNPrs174547 associates most strongly with the NMR signal at chemical shift δ = 2.801 ppm (P = 3.7 × 10-35) (top). When ratios between NMR intensities are tested, the strength of association increases by 59 orders of magnitude for the ratio between NMR intensities at 2.801 ppm and 2.017 ppm (P = 1.1×10-94) (Table 1). A similar increase in the strength of association has been observed in previous GWAS for this locus with ratios between phospholipids containing C20:3 and C20:4 fatty acids, such as PC aa C36:3 and PC aa C36:4 . The FADS1 codes for a delta-5 fatty acid desaturase; C20:3 and C20:4 fatty acids are their substrate-product pair. The correlation plot between these lipid species and the NMR intensities (bottom) indicates that the region around δ = 2.801 ppm correlates more strongly with C20:4 fatty acid-containing lipids, while the region around δ = 2.017 ppm more with C20:3 lipids (bottom insets). The ratio between intensities at δ = 2.801 ppm and δ = 2.017 ppm is therefore a likely proxy for the ratio between three- and four-fold desaturated log chain fatty acids.