Incorporation oftrans long-chain n−3 polyunsaturated fatty acids in rat brain structures and retina

During heat treatment, polyunsaturated fatty acids and specifically 18∶3n−3 can undergo geometrical isomerization. In rat tissues, 18∶3 Δ9c, 12c, 15t, one of thetrans isomers of linolenic acid, can be desaturated and elongated to givetrans isomers of eicosapentaenoic and docosahexaenoic acids. The present study was undertaken to determine whether such compounds are incorporated into brain structures that are rich in n−3 long-chain polyunsaturated fatty acids. Two fractions enriched intrans isomers of α-linolenic acid were prepared and fed to female adult rats during gestation and lactation. The pups were killed at weaning. Synaptosomes, brain microvessees and retina were shown to contain the highest levels (about 0.5% of total fatty acids) of thetrans isomer of docosahexaenoic acid (22∶6 Δ4c, 7c, 10c, 13c, 16c, 19t). This compound was also observed in myelin and sciatic nerve, but to a lesser extent (0.1% of total fatty acids). However, the ratios of 22∶6trans to 22∶6cis were similar in all the tissues studied. When the diet was deficient in α-linolenic acid, the incorporation oftrans isomers was apparently doubled. However, comparison of the ratios oftrans 18∶3n−3 tocis 18∶3n−3 in the diet revealed that thecis n−3 fatty acids were more easily desaturated and elongated to 22∶6n−3 than the correspondingtrans n−3 fatty acids. An increase in 22∶5n−6 was thus observed, as has previously been described in n−3 fatty acid deficiency. These results encourage further studies to determine whether or not incorporations of suchtrans isomers into tissues may have physiological implications. Presented in part at the 32nd International Conference on the Biochemistry of Lipids, 1991, Granada, Spain. Delta nomenclature (Δ) is used fortrans polyunsaturated fatty acids to specify the position and geometry of ethylenic bonds. Polyunsaturated fatty acids containingtrans double bonds are abbreviated giving the locations of thetrans double bonds only; e.g., 20∶5 Δ17t 20∶5 Δ5c,8c,11c,14c,17t; 22∶5 Δ19t, 22∶5 Δ7c,10c,13c,16c,19t; 22∶6 Δ19t 22∶6 Δ4c,7c,10c,13c,16c,19t.     

Dietary intake of essential and long-chain polyunsaturated fatty acids in pregnancy

The dietary intake of EFA and long-chain PUFA (LCPUFA) by women with (n=14) and without (n=31) gestational diabetes mellitus (GDM) was determined by repeated 24-h recalls. Women with GDM consumed significantly more energy as fat compared with women who had uncomplicated pregnancies; absolute dietary fat did not differ. Dietary n−3 LCPUFA was substantially lower than the current recommendation for pregnancy, whereas intake of saturated FA (SFA) exceeded it. We conclude that replacing dietary sources of SFA with those of EFA and LCPUFA, especially n−3 LCPUFA, would benefit the dietary fat profiles of all pregnant women.     

Lipase-catalyzed enrichment of long-chain polyunsaturated fatty acids

Lipase hydrolysis was evaluated as a means of selectively enriching long-chain ω3 fatty acids in fish oil. Several lipases were screened for their ability to enrich total ω-3 acids or selectively enrich either docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA). The effect of enzyme concentration, degree of hydrolysis, and fatty acid composition of the feed oil was studied. Because the materials that were enriched in long-chain ω3 acids were either partial glycerides or free fatty acids, enzymatic reesterification of these materials to triglycerides by lipase catalysis was also investigated. Hydrolysis of fish oil by eitherCandida rugosa orGeotrichum candidum lipases resulted in an increase in the content of total ω3 acids from about 30% in the feed oil to 45% in the partial glycerides. The lipase fromC. rugosa was effective in selectively enriching either DHA or EPA, resulting in a change of either the DHA/EPA ratio or the EPA/DHA ratio from approximately 1:1 to 5:1. Nonselective reesterification of free fatty acids or partial glycerides that contained ω3 fatty acids could be achieved at high efficiency (approximately 95% triglycerides in the product) by using immobilizedRhizomucor miehei lipase with continuous removal of water.     

Dietary nucleotides do not alter erythrocyte long-chain polyunsaturated fatty acids in formula-fed term infants

There have been conflicting reports regarding the effectiveness of dietary nucleotides (NT) to regulate tissue desaturases and hence stimulate accumulation of both n−6 and n−3 long-chain polyunsaturated fatty acids (LCPUFA). The aim of this study was to examine the effect of NT-supplemented cow's milk-based formula on erythrocyte phospholipid FA status in a large randomized controlled trial involving a well-nourished infant population born at term. Formula-fed infants were allocated to control formula with an innate level of NT at 10 mg/L (n=102), or formula fortified with NT at 34 mg/L (n=98). A parallel group of breastfed infants was included as a reference. Peripheral blood samples were collected by venipuncture at 7 mon of age and erythrocyte phospholipid FA determined by capillary GC. Erythrocyte LCPUFA levels did not differe between the NT-supplemented and control formula groups and were reduced in both groups compared with breastfed infants. We conclude that there is no induction of LCPUFa accumulation in erythrocyte phospholipids of term, formula-fed infants following 7 mon of NT supplementation.     

Dietary long-chain polyunsaturated fatty acids modify heart,kidney, and lung fatty acid composition in weanling rats

The fatty acid composition of heart, kidney, and lung was studied in weanling rats fed three diets differing in their polyunsaturated fatty acid content for 0, 2, and 4 wk. The first group had a 10% w/w fat semipurified diet which consisted of a mixture of olive oil (62.5%), soybean oil (11.1%), and refined coconut oil (26.4%) and provided 18:1n-9, 18:2n-6, and 18:3n-3 in similar amounts to a maternal human milk (diet HO). The second group received 7% of HO fat and 3% fish oil (0.4% 20:4n-6 and 5% 22:6n-3 of total fatty acids) (diet FO), and the third group was fed 7% HO fat, 1.5% of the same fish oil, and 1.5% of a purified pig brain phospholipid concentrate (0.6% 20:4n-6 and 3.5% 22:6n-3 of total fatty acids) (diet FO+BPL). The experimental diets increased tissue monounsaturated fatty acids in comparison with rats at weaning. Tissue lipid content of 20:4n-6 was increased and 22:6n-3 decreased in Group HO compared with weanling rats, whereas opposite changes were observed in Group FO. Feeding diet FO+BPL increased 22:6n:3 in tissue lipids compared with diet HO, and increased 20:4n-6 content in relation to diet FO. Our results indicate that rat heart, kidney, and lung are highly responsive to dietary n-3 and n-6 long-chain polyunsaturated fatty acids during postnatal life.     

Stereospecific analysis of triacylglycerols rich in long-chain polyunsaturated fatty acids

Six oils of marine, algal, and microbial origin were analyzed for stereospecific distribution of component fatty acids. The general procedure involved preparation ofsn-1,2-(2,3)-diacylglycerols by partial deacylation with ethylmagnesium bromide or pancreatic lipase, separation of X-1,3- andsn-1,2(2,3)-diacylglycerols by borate thin-layer chromatography, resolution of thesn-1,2- andsn-2,3-enantiomers by chiral phase high-performance liquid chromatography following preparation of dinitrophenylurethane derivatives, and determination of the fatty acid composition by gas chromatography. Unexpected complications arose during a stereospecific analysis of triacylglycerols containing over 33% of either 20∶4 or 22∶6 fatty acids. Thesn-1,2(2,3)-diacylglycerols made up of two long-chain polyunsaturated acids migrated with the X-1,3-diacylglycerols and required separate chiral phase resolution. Furthermore, the enzymatic method yieldedsn-1,2(2,3)-diacylglycerols, overrepresenting the polyenoic species due to their relative resistance to lipolysis, but prolonged digestion yielded correct composition for the 2-monoacylglycerols. The final positional distribution of the fatty acids was established by pooling and normalizing the data from subfractions obtained by norman- and chiral-phase separation of diacylglycerols. The molecular species of X-1,3-,sn-1,2- andsn-2,3-diacylglycerol dinitrophenylurethanes were identified by chiral-phase liquid chromatography/mass spectrometry with electrospray ionization, which demonstrated a preferential association of the paired long-chain acids with thesn-1,2- andsn-2,3-diacylglycerol isomers.     

Progress toward the production of long-chain polyunsaturated fatty acids in transgenic plants

Long-chain PUFA such as eicosapentaenoic and docosahexaenoic acids are prevalent in fish oils, and these compounds have been demonstrated to play important roles in human health and nutrition. In particular, these n−3/omega-3 long-chain PUFA provide protection from cardiovascular disease and a collection of symptoms (termed metabolic syndrome) associated with progression toward type 2 diabetes and obesity. Within Western populations, a large increase in the occurrence of these conditions represents a major public health concern. Unfortunately, both marine fish stocks and (consequentially) consumption of fish oils are in steep decline, limiting the protective role of long-chain PUFA in human health. One alternative approach to the provision of these health-beneficial FA is via their synthesis in transgenic plants. This review will describe recent advances in the production of transgenic plant oils nutritionally enhanced to produce long-chain PUFA.     

Influence of long-chain polyunsaturated fatty acids on infant cognitive function

Long-chain polyunsaturated fatty acids (LCPUFA) are important for normal visual and cortical development. In a previous study of the effects of LCPUFA on cognitive function of term infants at the age of 3 mon, we indicated that infants with evidence of reduced growth parameters at birth and impaired attention control as manifested by a late peak fixation during infant habituation assessment may benefit from LCPUFA supplementation. The aim of this prospective study was to determine whether LCPUFA supplementation and late peak fixation are related to means-end problem-solving ability in these same infants at the age of 9 mon. Term infants (58) were randomized to one of two formulas containing either LCPUFA or no LCPUFA and completed 4 mon of feeding with their formula. Cognitive function was assessed at 3 mon of age by measures of infant habituation. Infants (20 LCPUFA and 20 no-LCPUFA) completed the problem-solving assessment at 9 mon. The no-LCPUFA group had lower scores on both measures of intention and number of solutions, but neither of these differences was significant. Analysis of covariance for the effects of group and peak fixation, covaried with gestation and birth weight, showed that the number of solutions was significantly reduced in the late peak-fixation infants receiving no LCPUFA (P<0.02). Intention scores tended to be reduced in this group (P<0.06). The late peak-fixation infants who received LCPUFA had solution and intention scores similar to early peak-fixation infants receiving LCPUFA or no LCPUFA. These findings suggest that in term infants who have reduced growth parameters at birth and who show evidence of impaired attention control, information processing and problem-solving ability in infancy may be enhanced by LCPUFA supplementation. Based on a presentation at the AOCS Meeting on PUFA in Infant Nutrition: Consensus and Controversies, Barcelona, November 7–9, 1996.     

Positional analysis of triglycerides and phospholipids rich in long-chain polyunsaturated fatty acids

Four sources of long-chain polyunsaturated fatty acids (LCP) differing in their chemical structure (triglycerides or phospholipids) and in their origin (tuna triglycerides, fungal triglycerides, egg phospholipids, and pig brain phospholipids) were analyzed to determine the distribution of the component fatty acids within the molecule. Lipase and phospholipase A₂ hydrolysis was performed to obtain 2-monoacylglycerols and lysophospholipids, respectively, which allowed us to determine the distribution of fatty acids between the sn-2 and sn-1,3 positions of triglycerides or between the sn-1 and sn-2 position of phospholipids. Fatty acids in the LCP sources analyzed were not randomly distributed. In tuna triglycerides, half of the total amount of 22∶6n−3 was located at the sn-2 position (49.52%). In fungal triglycerides, 16∶0 and 18∶0 were esterified to the sn-1,3 (92.22% and 91.91%, respectively) 18∶1 and 18∶2 to the sn-2 position (59.77% and 62.62%, respectively), and 45% of 20∶3n−6 and only 21.64% of 20∶4n−6 were found at the sn-2 position. In the lipid sources containing phospholipids, LCP were mainly esterified to the phosphatidylethanolamine fraction. In egg phospholipids, most of 20∶4n−6 (5.50%, sn-2 vs. 0.91%, sn-1) and 22∶6n−3 (2.89 vs. 0.28%) were located at the sn-2 position. In pig brain phospholipids, 22∶6n−3 was also esterified to the sn-2 (13.20 vs. 0.27%), whereas 20∶4n−6 was distributed between the two positions (12.35 vs. 5.86%). These results show a different fatty acid composition and distribution of dietary LCP sources, which may affect the absorption, distribution, and tissue uptake of LCP, and should be taken into account when supplementing infant formulas.     

Double-bond location in long-chain polyunsaturated fatty acids by chemical lonization-mass spectrometry

For determination of the double-bond position in polyunsaturated C_24–30 fatty acids from marine organisms, methoxy derivatives were prepared. Diagnostic mass spectral fragment as well as molecular ion intensities were obtained by adjusting the ion source optics in the presence of ammonia at a lower source pressure than used conventionally. A lower detection limit was observed compared to conventional methane chemical ionization, which is a more favorable condition for capillary gas chromatography. Analysis of fatty acids from the spongeCalyx niceaensis showed the double-bond position of 8 unsaturated fatty acids, including two new ones. In addition, structural proof is provided for the presence of a new cyclopropane-containing fatty acid: 19,20-methylene-hexacosanoic acid. “Mass Spectrometry in Structural and Stereochemical Problems 262.” For preceding paper in this series, see Patterson, D.G., Haley, M.J., Midgley, J., and Djerassi, C., Org. Mass. Spectrom., submitted for publication. Recipient of a travel grant from the Queen Wilhelmina Fund, The Netherlands Cancer Foundation     

Interaction of n-3 long-chain polyunsaturated fatty acids with n-6 fatty acids in suckled rat pups

The addition of long-chain polyunsaturated fatty acids (LCP: C20, and C22) to infant formula may permit fatty acid accretion rates similar to breast-fed infants, and may have long-term outcome benefits, such as improved visual acuity and cognitive development. Although fish oil may provide a source of n-3 LCP, sources of n-6 LCP have been more difficult to identify. The present study evaluates the effects of n-3 and n-6 LCP derived from single-cell oils on liver, plasma, and brain fatty acid levels in a neonatal animal model. Newborn rat pups were suckled for 14 d by dams receiving diets containing n-3 LCP alone or combinations of n-3 LCP and increasing doses of linoleic acid (18∶2n−6) or arachidonic acid (20∶4n−6). Dietary groups received 2% n−3 LCP and 1, 2, or 5% of either 18∶2n−6 or 20∶4n−6. The 20∶4n−6 source also contained modest levels of 18∶2n−6. At the termination of the study, liver, plasma, and brain were obtained from the rat pups and the phospholipid fatty acid profiles determined. The results indicate complex interactions of n−3 and n−6 fatty acids. Groups receiving dietary 20∶4n−6 incorporated higher levels of n−6 LCP into tissues than did the groups receiving 18∶2n−6. The brain was relatively resistant to changes in fatty acid composition compared with the liver and plasma. As expected, tissue n−3 LCP levels were reciprocally related to n−6 levels. The present results document that single-cell LCP oils are bioavailable in a neonatal animal model. The use of 20∶4n−6 is a more effective means of supporting n−6 status than the use of 18∶2n−6. These results may have implications for the addition of LCP to infant formula.     

Biological effects and safety issues related to long-chain polyunsaturated fatty acids in infants

The purpose of this workshop at the American Oil Chemists’ Society Symposium, “PUFA in Infant Nutrition: Consensus and Controversies”, was to enumerate the safety issues raised by the prospect of supplementing infant formulas with long-chain polyunsaturated fatty acids (LC-PUFA), to evaluate the evidence that these concerns are problematical, or theoretically problematical, and to identify the safety issues most in need of resolution. This was approached by reviewing briefly the known biological effects of LC-PUFA and how these effects might give rise to concerns about safety of LC-PUFA as components of infant formulas. Some of these issues were then discussed in more detail by invited participants, all of whom had submitted abstracts concerning the issue discussed. The pertinent aspects of all issues discussed during the workshop are summarized. In addition, since the symposium was held over 2 yr ago, an addendum summarizing additional data reported since the symposium that either support or refute issues discussed during the workshop also is included. Summary of workshop “Biological Effects and Safety Aspects of PUFA Related to Infants”, held at the AOCS symposium: PUFA in Infant Nutrition: Consensus and Controversies, November 7–9, 1996, Barcelona, Spain.     

Production of triglycerides enriched in long-chain n-3 polyunsaturated fatty acids from fish oil

Processes that combine enzymic and physical techniques have been studied for concentrating and separating eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish oil.Candida rugosa lipase was used in hydrolysis reactions to concentrate these acids in the glyceride fraction. By controlling the degree of hydrolysis, two products have been obtained, one enriched in total n-3(∼50%), the other enriched in DHA and depleted in EPA (DHA∼40%, EPA∼7%). The glyceride fraction from these reactions was recovered by evaporation and converted back to triglycerides by partial enzymic hydrolysis, followed by enzymic esterification. Both reactions were carried out withRhizomucor miehei lipase. DHA-depleted free fatty acids from aC. rugosa hydrolysis were fractionated to increase the EPA level (∼30%) and re-esterified to triglycerides by reaction with glycerol andR. miehei.     

Dietary linoleic acid and polyunsaturated fatty acids in rat brain and other organs. Minimal requirements of linoleic acid

Starting three weeks before mating, 12 groups of female rats were fed different amounts of linoleic acid (18∶2n−6). Their male pups were killed when 21-days-old. Varying the dietary 18∶2n−6 content between 150 and 6200 mg/100 g food intake had the following results. Linoleic acid levels remained very low in brain, myelin, synaptosomes, and retina. In contrast, 18∶2n−6 levels increased in sciatic nerve. In heart, linoleic acid levels were high, but were not related to dietary linoleic acid intake. Levels of 18∶2n−6 were significantly increased in liver, lung, kidney, and testicle and were even higher in muscle and adipose tissue. On the other hand, in heart a constant amount of 18∶2n−6 was found at a low level of dietary 18∶2n−6. Constant levels of arachidonic acid (20∶4n−6) were reached at 150 mg/100 g diet in all nerve structures, and at 300 mg/100g diet in testicle and muscle, at 800 mg/100 g diet in kidney, and at 1200 mg/100 g diet in liver, lung, and heart. Constant adrenic acid (22∶4n−6) levels were obtained at 150, 900, and 1200 mg/100 g diet in myelin, sciatic nerve, and brain, respectively. Minimal levels were difficult to determine. In all fractions examined accumulation of docosapentaenoic acid (22∶5n−6) was the most direct and specific consequence of increasing amounts of dietary 18∶2n−6. Tissue eicosapentaenoic acid (20∶5n−3) and 22∶5n−3 levels were relatively independent of dietary 18∶2n−6 intake, except in lung, liver, and kidney. In several organs (muscle, lung, kidney, liver, heart) as well as in myelin, very low levels of dietary linoleic acid led to an increase in 20∶5n−3. Dietary requirements for 18∶2n−6 varied from 150 to 1200 mg/100 g food intake, depending on the organ and the nature of the tissue fatty acid. Therefore, the minimum dietary requirement is estimated to be about 1200 mg/100 g (i.e., the level that ensures stable and constant amounts of arachidonic acid).     

Resistance of certain long-chain polyunsaturated fatty acids of marine oils to pancreatic lipase hydrolysis

When whale oil triglycerides were subjected to pancreatic lipase hydrolysis, eicosapentaenoic and docosahexaenoic acids were found mainly in the di- and triglyceride products, suggesting that they are in the 1,3-positions but resistant to the action of the lipase. Their presence in the 1,3-positions was confirmed. Their resistance to pancreatic lipase hydrolysis was demonstrated by analysis of the products of the enzyme action on: (a) a concentrate of highly unsaturated whale oil triglycerides; (b) the latter after randomization; and (c) synthetic 1,2-di-octadecenoyl-3-eicosapentaenoyl glycerol. Docosapentaenoic acid was also shown to be present in the 1,3-position of whale oil triglycerides but was not lipase resistant. It is postulated that the presence of a double bond near the carboxyl group exercises an inhibitory effect, or that the location of the double bonds in the resistant acids places their terminal methyl groups close to the carboxyl, producing a steric hindrance effect.     

Addition of long-chain polyunsaturated fatty acids to formula for very low birth weight infants

Thirty-four premature infants who were appropriate for gestational age and weighing less than 1500 g at birth were fed “preemie” SMA-24 formula, “preemie” SMA-24 formula manufactured to contain C₂₀ and C₂₂ ω6 and ω3 fatty acids (LCPE-SMA), or expressed milk (EBM). Blood samples were drawn from a small arm vein during the first week of life and after 28 days of feeding. The fatty acid content of plasma phospholipids was determined. Infants fed SMA-24 had a high content of 18∶2ω6 in plasma phospholipids. Feeding LCPE-SMA normalized plasma phospholipid levels of C₂₀ and C₂₂ ω6 and ω3 fatty acids to be similar to levels of C₂₀ and C₂₂ ω6 and ω3 fatty acids found in infants fed EBM, and significantly higher than characteristic levels for infants fed SMA-24. Feeding LCPE-SMA or EBM results in a balanced incorporation of C₂₀ and C₂₂ ω6 and ω3 fatty acids into phospholipids derived from the liver or perhaps the small intestine. Based on a paper presented at the Symposium on Milk Lipids held at the 1990 AOCS Annual Meeting, Baltimore, MD, April 1990.     

Dietary intakes and food sources of omega-6 and omega-3 polyunsaturated fatty acids

Both n−6 and n−3 polyunsaturated fatty acids (PUFA) are recognized as essential nutrients in the human diet, yet reliable data on population intakes are limited. The aim of the present study was to ascertain the dietary intakes and food sources of individual n−6 and n−3 PUFA in the Australian population. An existing database with fatty acid composition data on 1690 foods was updated with newly validated data on 150 foods to estimate the fatty acid content of foods recorded as eaten by 10,851 adults in the 1995 Australian National Nutrition Survey. Average daily intakes of linoleic (LA), arachidonic (AA), α-linolenic (LNA), eicosapentaenoic (EPA), docosapentaenoic (DPA), and docosahexaenoic (DHA) acids were 10.8, 0.052, 1.17, 0.056, 0.026, and 0.106 g, respectively, with longchain (LC) n−3 PUFA (addition of FPA, DPA, and DHA) totaling 0.189 g; median intakes were considerably lower (9.0 g LA, 0.024 g AA, 0.95 g LNA, 0.008 g EPA, 0.006 g DPA, 0.015 g DHA, and 0.029 g LC n−3 PUFA). Fats and oils, meat and poultry, cereal-based products and cereals, vegetables, and nuts and seeds were important sources of n−6 PUFA, while cereal-based products, fats and oils, meat and poultry, cereals, milk products, and vegetable products were sources of LNA. As expected, seafood was the main source of LC n−3 PUFA, contributing 71%, while meat and eggs contributed 20 and 6%, respectively. The results indicate that the majority of Australians are failing to meet intake recommendations for LC n−3 PUFA (>0.2 g per day) and emphasize the need for strategies, to increase the availability and consumption of n−3-containing foods.     

Rat liver outer mitochondrial carnitine palmitoyltransferase activity towards long-chain polyunsaturated fatty acids and their CoA esters

The activity of the overt form of rat liver mitochondrial carnitine palmitoyltrasferase or CPT₀ (EC 2.3.1.21) towards different fatty acid substrates was studied. The following non-esterified fatty acids (NEFA) and their CoA esters in the presence of 1% bovine serum albumin (BSA) were tested: 16∶0, 18∶0, 18∶1, 18∶2, 18∶3ω3, 20∶4, 20∶5ω3 and 22∶6ω3. The data fit a square hyperbolic model for enzyme catalysis (p<0.001, non-linear regression). Asymptotic V_max and K_0.5, substrate concentration at one-half V_max, were calculated using total concentrations of acyl-CoA, or unbound concentrations of NEFA. BSA was found to act as a true substrate reservoir for NEFA in that the dissociation of the NEFA-BSA complex was 10–330 times faster than the CPT₀ reaction. Regardless of form (NEFA or CoA ester), 18∶3ω3 gave the highest, while 22∶6ω3 and 18∶0 gave the lowest rates of acylcarnitine synthesis. Except for 18∶3ω3 and 18∶2, V_max for NEFA was generally lower than for acyl-CoA, with the greates differences observed for 20∶4, 20∶5ω3 and 22∶6ω3, suggesting that acyl-CoA synthesis may also be important in the control of the entry of these fatty acids into the mitochondria. The data provide an enzymatic rationale for the relatively low content of 18∶3ω3 in esterified lipid.     

Incorporation of radioactive polyunsaturated fatty acids into liver and brain of developing rat

The incorporation of radioactivity from orally administered linoleic acid-1-¹⁴C, linolenic acid-1-¹⁴C, arachidonic acid-³Hg, and docosahexaenoic acid-¹⁴C into the liver and brain lipids of suckling rats was studied. In both tissues, 22 hr after dosing, 2 distinct levels of incorporation were observed: a low uptake (from 18∶2-1-¹⁴C and 18∶3-1-¹⁴C) and a high uptake (from 20∶4-³H₈ and 22∶6-¹⁴C). In adult rats, the incorporation of radioactivity into brain lipids from 18∶2-1-¹⁴C and 20∶4-³H was considerably lower than the incorporation into the brains of the young rats. In the livers of the suckling rats, the activity from the 18 carbon acids was associated mostly with the triglyceride fraction, whereas the activity from the 20∶4-³H₈ and 22∶6-¹⁴C was concentrated in the phospholipid fraction. In the brain lipids, the activity from the different fatty acids was associated predominantly with the phospholipids. In the liver and brain phospholipid fatty acids, some of the activity in the 18∶2-1-¹⁴C and 18∶3-1-¹⁴C experiments was associated with 20 and 22 carbon polyunsaturated fatty acids; however, radioactivity from orally administered 20∶4-³H₈ and 22∶6-¹⁴C was incorporated intact into the tissue phospholipid to a much greater extent compared with the incorporation of radioactivity into 20∶4 and 22∶6 in the experiments where 18∶2-1-¹⁴C and 18∶3-1-¹⁴C, respectively, were administered. Possible reasons for these differences are discussed. Rat milk contains a wide spectrum of polyunsaturated fatty acids, including linoleate, linolenate, arachidonate, and docosahexaenoate. During the suckling period in the rat, there is a rapid deposition of 20∶4 and 22∶6 in the brain. The results of the present experiments suggested that dietary 20∶4 and 22∶6 were important sources of brain 20∶4 and 22∶6 in the developing rat.