Serum was obtained from approximately 10 individuals of each sex from each decade of life from 0 to 90 years of age. Most of the serum samples were obtained from the excess remaining after diagnostic procedures performed for hospitalized patients. The individuals were screened according to tentative diagnosis, and overt cases of diseases known to have effects upon essential fatty acid (EFA) metabolism were eliminated. Serum lipids were separated into phospholipids, cholesteryl esters, triglycerides, and free fatty acids. These lipid classes and a sample of the total lipids were subjected to fatty acid analysis by gas chromatography. The data produced were arranged by computer to compare the effects of age upon each fatty acid present and upon each parameter calculated from fatty acid composition for each fatty acid present and upon each parameter calculated from fatty acid composition for each lipid group and each sex. No significant difference between sexes was found for any individual fatty acid in any lipid, but the average total unsaturation of all lipid classes was significantly higher for males than for females at both 0 and 90 years of age. In cholesteryl esters, and to a lesser extent in phospholipids, the total polyunsaturated fatty acids, the total ω6 acids and the sum 18:2ω6 + 20:4ω6 - 20:3ω9 declined more rapidly with age in females than in males, suggesting a trend toward EFA deficiency, although the ratio 20:3ω9/20:4ω6 did not change significantly. The age and sex differences observed and the SD encountered were much smaller than the change observed in EFA deficiency, indicating that this population has utility in comparisons with suggested cases of EFA deficiency. The triene/tetraene ratio, 20:3ω9/20:4ω6 was found to be 0.1 ± 0.08 for male and female populations, indicating that a ratio above 0.2 should be considered the upper limit of normalcy. Between 0 and 4 months of age, a profound change took place in the patterns of fatty acids in serum lipids. At birth, phospholipids were low in 18:2ω6 but 20:4ω6 and 20:3ω9 were elevated. The ratio of 20:3ω9/20:4ω6 was 0.16 at birth and decreased progressively during infancy. Some of these observations suggest that at birth the infant has marginal reserves of EFA. The drastic change in composition of serum lipid shortly after birth probably reflects a change in supply of EFA from one rich in 20:4ω6 and poor in 18:2ω6 to one high in 18:2ω6 and low in 20:4ω6. This may have significance in the nutrition of premature or very young infants.
