The origin of fatty acids in milk has not been elucidated in detail. We investigated the contribution of dietary α-linolenic acid (ALA) to human milk fat, its oxidation and endogenous conversion to long-chain polyunsaturated fatty acids. Ten lactating women were given (13)C-ALA orally, and breath and milk samples were collected for a five-day period, while dietary intakes were assessed. 37.5 ± 2.7 % (M ± SE) of the tracer was recovered in breath-CO2, and 7.3 ± 1.1 % was directly transferred into milk. About 0.25 % of the tracer was found in milk long-chain polyunsaturated fatty acids. Combining intake and milk data, we estimate that about 65 % of milk ALA is directly derived from maternal diet. Thus, the major portion of milk ALA is directly derived from the diet, but dietary ALA does not seem to contribute much as a precursor to milk n-3 long-chain polyunsaturated fatty acids within the studied time period.

The fractional conversion rates of plasma phospholipid α-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) to docosahexaenoic acid (22:6n-3) and arachidonic acid (20:4n-6), respectively, and the fractional rates of incorporation of 22:6n-3 and 20:4n-6 into plasma phospholipids were determined in 27 healthy 3-wk-old term infants who had received formulas with ≈16% of fat as 18:2n-6 and 0.4% (n=6), 1.0% (n=11), or 3.2% (n=10) as 18:3n-3 from birth. The infants were given a single dose of both [U-13C] 18:2n-6 and [U-13C]18:3n-3 with a feeding, and blood samples were collected 8, 12, and 24 h afterward for determination of the isotopic enrichments of the [M+18] isotopomers of plasma phospholipid fatty acids by negative chemical ionization gas chromatography/mass spectrometry. A simple precursor/product compartmental model was used to estimate fractional rates of conversion and incorporation. All infants converted 18:3n-3 to 22:6n-3 and 18:2n-6 to 20:4n-6. Although the fractional rate of conversion of 18:3n-3 to 22:6n-3 did not differ among groups, the fractional rate of incorporation of 22:6n-3 into the plasma phospholipid fraction was greater in infants who received 3.2% vs. 0.4% or 1.0% 18:3n-3 (4.1±2.2 vs 1.6±1.5 or 2.0±1.0% of the plasma phospholipid 22:6n-3 pool daily). The fractional rate of conversion of 18:2n-6 to 20:4n-6 was less in infants who received the 3.2% 18:3n-3 intake (0.4±0.3% of the plasma phospholipid 18:2n-6 pool daily vs. 1.1±0.7% and 0.8±0.5% in those who received 0.4 and 1.0% 18:3n-3, respectively). The fractional rate of incorporation of 20:4n-6 into plasma phospholipid also was less in the 3.2% vs. the 0.4 and 1.0% 18:3n-3 groups (2.7±1.4% vs. 5.9±2.6 and 4.4±1.7%, respectively, of the plasma phospholipid 20:4n-6 pool daily).

Polyunsaturated fatty acids in human milk may derive from diet, liberation from maternal body stores, or endogenous synthesis from precursor fatty acids. The contribution of each of these sources has not been studied in detail. Although maternal diet is a key factor affecting human milk composition, other factors such as gestational age, stage of lactation, nutritional status, and genetic background are known to influence the fat content and fatty acid composition in human milk. Both linoleic and α-linolenic acids, the essential fatty acids, are present in human milk, as are several other n−6 and n−3 longer chain polyunsaturated fatty acids that are required for optimal growth and development of infants. The fatty acid profile of human milk from lactating women of different countries is remarkably stable, but there is variability in some of the components, such as docosahexaenoic acid, which is mainly due to differences in dietary habits. Tracer techniques with stable isotopes have been valuable in assessing the kinetics of fatty acid metabolism during lactation and in determining the origin of fatty acids in human milk. Based on these studies, the major part of polyunsaturated fatty acids in human milk seems not to be provided directly from the diet but from maternal tissue stores.

In hospitals, human milk is subjected to heat treatment to reduce risk of transmission of infectious agents such as human immunodeficiency virus (HIV), hepatitis B, cytomegalovirus, and bacterial contamination, especially during feeding of banked milk to preterm infants. Fat losses due to heat treatment have been extensively studied in cow milk but have received little attention in human milk. We studied the effect of human milk pasteurization and sterilization on total fat content available to the infant as well as on fatty acid composition. Milk samples from 12 mothers (days 5-35 of lactation) were divided into three equal parts: one remained fresh, one was pasteurized (62.5degreesC for 30 min), and one was sterilized (120degreesC for 30min). Fat content was determined gravimetrically, and the contribution of 30 fatty acids was determined by gas chromatography. For investigation of loss of available fat in sterilized milk, milk was collected from two additional mothers and analyzed with a modified extraction method. Total fat content was the same in fresh, pasteurized, and sterilized milk. The available fat content was 3.1 +/- 0.4g/dL (mean SE) in fresh human milk, 3.1 +/- 0.4 g/dL in pasteurized human milk, and 2.7 +/- 0.3 g/dL (P < 0.00 1 vs. fresh) in sterilized human milk because of formation of a surface skin and fat adherence to the vial wall after sterilization. The fatty acid composition of 10 saturated, 10 monounsaturated, and 10 polyunsaturated fatty acids of both the n6 and n3 series was not affected by pasteurization. In sterilized milk there was a slight decrease of linoleic acid (C18:2n6; -0.7% vs. fresh; P = 0.006) and arachidonic acid (C20:4n6; -2.5%; P = 0.045). Pasteurization and sterilization do not affect total fat content of human milk, but sterilization may reduce available fat content by >10%. Fatty acid composition of human milk is not changed by pasteurization, but is slightly changed by sterilization.

The origin of polyunsaturated fatty acids (PUFA) in human milk has not been studied in detail. Diet, liberation from maternal stores and endogenous synthesis from precursors may contribute to PUFA present in human milk. Other factors influencing lipid content and fatty acid composition such as gestational age, stage of lactation, nutritional status and genetical background are known. In a series of in vivo studies using stable isotope methodologies we investigated the metabolism of PUFA during lactation. With this techniques the transfer of single dietary fatty acids into human milk, the oxidation and the deposition in tissues were estimated. Our studies demonstrate that the major part of PUFA in human milk seems not to be derived directly from the maternal diet but from body stores. Nevertheless diet is important, because long term intakes affect composition of body stores.

Microorganisms have been receiving increased attention as sources of novel lipids. Those that accumulate more than 20-25 % of their biomass as oil may be termed oleaginous and their oils single cell oils (SCOs), unicellular oils or microbial oils. For the lipid accumulation in yeasts, moulds and eukaryotic algae, but not in bacteria, the presence of enzyme ATP-cytrate lyase is of vital importance. This enzyme serves to produce acetyl-CoA, which is the substrate for fatty acid biosynthesis. Nitrogen limitation is the most frequently used condition to favour lipid accumulation. Oleaginous organisms differ from nonoleaginous ones in being able to convert carbon from the growth medium into the intracellular lipid, after the nitrogen has been depleted from the medium, provided that the supply of carbon stays plentiful. Biosynthetic pathways of n-6 and n-3 polyunsaturated fatty acids from the saturated and monounsaturated precursors with the chain elongations and desaturations are presented. The suitability of an microalgal triglyceride-SCO highly enriched in docosahexaenoic acid (DHASCOŇ) as a source for nutritional supplementation for formula milk is compared to fish oil. Some safety evaluation studies of SCOs are presented. For the safe use of SCOs in infant formulas even further safety studies should be performed. By growing microalgal strains in a medium containing D-[1-13C]glucose, SCOs enriched with the stable isotope 13C can be produced. Some examples of recent research and diagnostic applications of 13C-labelled SCOs to study fatty acid metabolism are outlined. In conclusion, SCOs in combination with stable isotopes have become indispensable to study metabolic pathways.

BACKGROUND Human milk is frequently heat treated in hospitals to reduce bacterial contamination, particularly in banked milk fed to preterm infants. Pasteurization and sterilization may induce oxidative losses of unsaturated lipids and vitamins and may inactivate enzymes and immunologic factors. This study was designed to examine the effects of pasteurization and sterilization on milk fat content available to the recipient infant and on fatty acid composition. METHODS In fresh, pasteurized (62.5 degrees C for 30 minutes), and sterilized (120 degrees C for 30 minutes) milk samples (5 ml) of 12 mothers (days 5-35 of lactation), fat content was determined gravimetrically and the contribution of 30 fatty acids was determined by gas-liquid chromatography. RESULTS The coefficients of variation for measurements of milk fat content were 0.7% and of fatty acids accounting for more than 0.09% of weight, 0.1-3.0%. Available fat content was 3.1+/-1.4 g/dl (mean +/- SD) in fresh human milk and 3.1+/-1.4 g/dl (not significant) in pasteurized human milk. Fat content declined to 2.7+/-1.1 g/dl (p < 0.001 vs. fresh) in sterilized human milk, because of increased fat adherence to the container surface after sterilization. The percentage composition of saturated, monounsaturated, and polyunsaturated fatty acids of the n-6 (C18:3, C20:2, C20:3, and C22:4) and the n-3 series (C18:3 C20:5, C22:5, and C22:6) was not affected by thermal treatment. Milk sterilization caused a slight decrease of linoleic (-0.7% vs. fresh milk; p = 0,006) and arachidonic (-2,6%; p = 0.045) acids. CONCLUSIONS Pasteurization of human milk does not influence fat content and composition, but sterilization may reduce available fat content by more than 10%, whereas there are only slight changes in fatty acid composition.

Human milk docosahexaenoic acid (DHA) may be derived from maternal diet, liberation from body stores and endogenous synthesis from precursors. To assess the effects of DHA supplementation on human milk fatty acids and the transfer of DHA from a dietary supplement into human milk, healthy breastfeeding mothers (age 21-35 yr, body mass index 20-26) were given from 4 to 6 weeks postpartum an oil rich in DHA (DHASCO: C12:0 5.4%, C14:0 19.4%, C16:0 17.5%, C18:1n-9 9.5%, C22:6n-3 45.1%) (n = 5) or a placebo (n = 5). Dietary intakes were followed by protocols at home.

An alternative pathway of ω3 and ω6 fatty acid metabolism has been described in isolated rat hepatocytes and human fibroblasts. This alternative pathway, which is independent of Δ4 desaturation, involves elongation of C22:5ω3 and C22:4ω6 to C24 fatty acids,Δ6 desaturation of the C24 fatty acids and subsequent β oxidation of the desaturated products to C22:6ω3 and C22:5ω6. To determine whether this alternative pathway is operative in the human infant and also to obtain additional information concerning endogenous conversion of C18:3ω3 and C18:2ω6 to longer chain more unsaturated fatty acids, presence of [M + 18] isotopomers of ω3 and ω6 fatty acids in the plasma phospholipid fraction of term and preterm infants after administration of [U-13C]18:3ω3 and [U-13C]18:2ω6 was determined by negative chemical ionization gas chromatography/mass spectrometry. [M + 18] isotopomers of the following ω3 fatty acids were detected: C18:3, C18:4, C20:3, C20:4, C20:5, C22:4, C22:5, C22:6, C24:4 (two infants only), C24:5, and C24:6. [M + 18] isotopomers of ω6 fatty acids detected included only C18:2, C18:3, C20:2, C20:3, and C20:4, but sensitivity was insufficient to detect [M + 18] isotopomers of C22 and C24 ω6 fatty acids. Presence of[M + 18] isotopomers of C24:5ω3 and C24:6ω3 indicates that these fatty acids were synthesized endogenously from C18:3ω3. This plus thein vitro data strongly suggests that infants use the recently described alternative pathway in endogenous synthesis of C22:6ω3. However, involvement also of Δ4 desaturation cannot be excluded. Detection of [M + 18] isotopomers of C20:3ω3, C20:2ω6, and C22:4ω3 suggests that C18:3ω3, C18:2ω6, and C20:4ω3 are elongated as well as desaturated. The specific fate of these elongation products and their importance in endogenous synthesis of ω3 and ω6 long chain polyunsaturated fatty acids remain to be determined.

Data obtained with stable isotope methodology have demonstrated that preterm and term infants can convert LA and ALA, respectively, to AA and DHA. In addition, they have clarified the pathways by which infants convert LA and ALA to LCPUFA and have demonstrated the importance of factors such as the dietary LA/ALA ratio and postnatal age on biosynthesis of AA and DHA. Further work is needed to clarify the role of other influential factors on endogenous synthesis of LCPUFA and to determine the absolute amounts of endogenous LCPUFA synthesis. Such data are necessary to define more precisely the LCPUFA requirements of growing infants.

Endogenous conversion of both 18:3ω3 and 18:2ω6 varies as a function of diet and age (Sauerwald et al. Pediatric Res, 1995) but there also is substantial variation within diet and age groups. We hypothesized that this latter variation is due to individual differences in conversion of the two fatty acids. To test this hypothesis, the fractional rates of conversion (FRC) of 18:3ω3 and 18:2ω6 and the fractional rates of incorporation(FRI) of 22:6ω3 and 20:4ω6 into plasma phospholipid (PL) were determined at 43 and 56 wk postmenstrual age in 11 preterm and 27 term infants fed formulas with 0.4%, 1% or 3.2% of fat as 18:3ω3 and correlations among FRC of 18:3ω3 and 18:2ω6 as well as FRI of 22:6ω3 and 20:4ω6 were determined by regression analysis. FRCs and FRIs were determined by a precursor/product compartmental model based on changes in enrichments of [M+18] fatty acids in plasma PL following administration of[U-13C]-18:3ω3 and -18:2ω6. The correlation coefficients(r) among FRCs and FRIs in 10 term infants on the same diet at 43 and 56 wk are shown: Table In all infants studied at both times, 55% of the variation in FRC of 18:2ω6 was explained by FRC of 18:3ω3 but only 12% of the variation in FRI of 20:4ω6 was explained by FRI of 22:6ω3. Although inherently obvious (ie, 18:3ω3 and 18:2ω6 are converted by the same desaturases and elongases), this is the first demonstration of such a correlation. It indicates that not only diet and age but individual differences in activity of desaturases and elongases may be important determinants of ω3 andω6 fatty acid status during infancy.

The importance of 20:4ω6 and 22:6ω3 during early development is well known, but little quantitative data are available concerning their endogenous synthesis and incorporation into different tissues. We have addressed some of these issues in 5-day old piglets. After a bolus intravenous dose of [U-13C]-18:3ω3 and - 18:2ω6, plasma, liver and brain samples were collected serially for 120 h and the concentrations as well as the enrichments of the [M+18] isotopomers of ω3 and ω6 LC-PUFA of various lipid fractions of each were determined by conventional GC and negative chemical ionization GC/MS, respectively. Peak enrichment of [M+18] 18:3ω3 and 18:2ω6 as well as 22:6ω3 and 20:4ω6 occurred in plasma triglycerides (TG) within 1 h. Peak enrichment of [M+18]18:3ω3 in plasma phospholipids (PL) occurred after 2-4 h and that of 18:2ω6 after 8-12 h; peak enrichments of [M+18]22:6ω3 and 20:4ω6, however, were not reached until 24-36 h. Enrichment of [M+18] 18:2ω6 and 20:4ω6 was higher in liver (28.1±3.4 and 37.9 ± 14.8 μg/g, respectively) vs. brain PL (1.3±0.6 and 2.1±1.0 μg/g, respectively) at 24 h and remained higher at 48 and 72 h, suggesting that 20:4ω6 is synthesized predominantly in liver. Enrichment of [M+18]22:6ω3 was higher in liver vs. brain PL at 24 h (16.2±2.4 vs. 1.4±0.7 μg/g) but enrichment in liver PL decreased thereafter whereas that in brain PL increased, suggesting that 22:6ω3 also is synthesized predominantly in liver. Enrichment of [M+18] 22:5ω3 was similar in liver and brain PL at 24 h (5.4±1.1 vs 4.9±3.6 μg/g tissue) but enrichment in liver decreased to 1.03±0.26 μg/g at 72 h while enrichment in brain remained unchanged, suggesting that this FA may be a precursor for synthesis of 22:6ω3 in brain. Other [M+18] metabolites of 18:3ω3 and 18:2ω6, including the C24 FA involved in alternative pathways of 22:6ω3 and 22:5ω6 synthesis, were detected in plasma, liver, and brain PL. Thus, in piglets, endogenous synthesis of LC-PUFA is apparent in plasma TG and PL within 1 h but maximum incorporation into plasma PL occurs only after 24-36 h. The higher enrichment of 18:3ω3, 18:2ω6, 20:4ω6 and 22:6ω3 in liver vs. brain PL suggests that liver is the primary site of LC-PUFA synthesis. However, the higher uptake of 22:5ω3 than 22:6ω3 in brain, despite the predominance of 22:6ω3, may reflect substantial synthesis of 22:6ω3 from 22:5ω3. The presence of [M+18] isotopomers of C24 PUFA in all tissues suggests that the alternative pathways of LC-PUFA synthesis involving this FA are operative in both liver and brain.

Journal of Pediatric Gastroenterology and NutritionVolume 21, Issue 3 p. 358-358 Abstracts: PDF Only EFFECT OF α-LINOLENIC ACID INTAKE AND POSTMENSTRUAL AGE ON ARACHIDONIC AND DOCOSAHEXAENOIC ACID BIOSYNTHESIS IN PRETERM INFANTS T U Sauerwald, T U Sauerwald Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorP L Hachey, P L Hachey Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorH M Chen, H M Chen Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorR E Anderson, R E Anderson Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorC L Jensen, C L Jensen Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorW C Heird, W C Heird Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this author T U Sauerwald, T U Sauerwald Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorP L Hachey, P L Hachey Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorH M Chen, H M Chen Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorR E Anderson, R E Anderson Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorC L Jensen, C L Jensen Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this authorW C Heird, W C Heird Depts. of Peds, and Ophthal., USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TXSearch for more papers by this author First published: 01 October 1995 https://doi.org/10.1002/j.1536-4801.1995.tb11930.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue3October 1995Pages 358-358 RelatedInformation