Quantifying Fatty Acid Composition in Adipose Tissue

Human adipose tissue is composed largely of triglycerides. Seven fatty acids predominate as follows (number of carbons:number of double bonds, typical abundance): myristic (14:0, 3%), palmitic (16:0, 19–24%), palmitoleic (16:1, 6–7%), stearic (18:0, 3–6%), oleic (18:1, 45–50%), linoleic (18:2, 13–15%), and linolenic (18:3, 1–2%) (22 (link), 23 (link)). These fatty acids account for well over 90% of the fatty acids in human adipose tissue. Odd-carbon fatty acids, longer chain fatty acids, and shorter chain fatty acids account for the remainder. Each of these less-abundant fats individually contributes much less than 1% (22 (link)).
At 7 T, 10 resonances can be resolved, designated here as A to J in alphabetic order from upfield to downfield (Fig. 1). Six resonances contribute equivalent information about triglyceride composition: the CH₃ methyl protons (labeled A, at ∼0.90 ppm), the CH₂ methylene protons α- (E, at ∼2.25 ppm) and β- (C, at ∼1.59 ppm) to the carbonyl, and the glycerol backbone CH (I) and CH₂ protons (G and H). Hence, there are only four additional informative resonances to consider: 1) bulk CH₂ methylene protons (labeled B at ∼1.3 ppm); 2) allylic CH₂ protons, α- to a double bond, at 2.03 ppm (D); 3) diallylic (also called bis-allylic) CH₂ protons at 2.77 ppm (F); and 4) olefinic, double bond -CH = CH- protons at 5.31 ppm (J), which partially overlap with the glycerol CH methine proton at 5.21 ppm (I).
It was assumed that the fatty acids detected here contain either 0, 1, or 2 double bonds. These three types of fatty acids account for ∼97–98% of total fat in humans on ordinary Western diets. Linolenic acid (18:3) is excluded in this simplification, but it contributes only ∼0.5% of the total triglycerides (22 (link)). With this assumption, f_sat + f_mono + f_di = 1 where f_sat, f_mono, and f_di refer to the fraction of fatty acids that are saturated, monounsaturated, and doubly unsaturated (or diunsaturated), respectively. The fraction that is diunsaturated, f_di, can be determined directly from the relative area of the resonance of the “bridging” diallylic protons (resonance F), with respect to the resonance of methylene protons α to COO (resonance E):
Once the f_di value is determined, one can evaluate f_mono from the relative area of proton resonance α to the double bond by:
The remaining unknown f_sat, the fraction of saturated fatty acid, is derived as f_sat = 1 − (f_mono + f_di).
Assuming that f_16C + f_18C = 1, the fraction of fatty acids that are 16 carbon versus 18 carbon can be determined from the area of the bulk methylene resonances (-CH₂-)_n:
The coefficients in front of the individual fractions are: 12 for palmitic acid (16:0), 8 for palmitoleic acid (16:1), 14 for stearic acid (18:0), 10 for oleic acid (18:1), and 7 for linoleic acid (18:2). This analysis is essentially identical to the earlier analysis (20 (link)) with the exception that a term for an unsaturated fat with three double bonds was omitted rather than assuming a low, fixed concentration.