Lipases in human milk: effect of gestational age and length of lactation on enzyme activity

Human milk contains two lipases, bile salt-stimulated lipase (BSSL) and lipoprotein lipase (LPL). In the mammary gland, LPL provides long-chain fatty acid for milk fat synthesis. LPL has no known function in milk, but has been implicated in milk fat hydrolysis during cold storage. BSSL may have an important role in infant fat digestion. The aims of the present studies were to assess (1) the methodological validity of using whole milk to analyze BSSL activity, (2) the longitudinal variation of BSSL and LPL activity in the milk of mothers delivering premature and full-term infants, and (3) the stability of BSSL and LPL activity during cold storage. Diluted whole milk and purified BSSL were shown to have similar characteristics. LPL activity was equally stable at -20 and -70 degrees C, whereas BSSL activity was higher in milks stored at -70 than at -20 degrees C (38.8 +/- 0.88 vs 33.3 +/- 0.87 U/ml milk, respectively; 1U = 1 mumol free fatty acid release/min). Levels of BSSL activity in preterm and term milk were similar. LPL activity tended to be higher in term milk. Overall, BSSL activity showed significant longitudinal variation, being highest at 1 and 3 weeks of lactation (43.2 +/- 0.04 and 42.6 +/- 1.03 U/ml milk, respectively). For LPL, the longitudinal pattern of activity depended upon the length of pregnancy. Implications for infant nutrition and mammary gland biology are discussed.     

A high dietary lipid intake during pregnancy and lactation enhances mammary gland lipid uptake and lipoprotein lipase activity in rats.

Rats fed a diet with high fat concentration produce larger amounts of milk with a higher lipid concentration than rats fed a lower fat diet. This investigation was designed to study the relationship between dietary fat intake, mammary gland lipid uptake and lipogenesis in rat dams fed, during pregnancy and lactation, one of two purified diets, with equal energy density, containing 2.5 (LL) or 20 g fat/100 g diet (HL). Milk lipid concentration and fatty acid composition were determined at d 14 of lactation. Mammary gland lipogenesis, lipoprotein lipase (LPL) activity and the uptake of [1-(14)C]triolein by the mammary gland and its transfer to the pups was measured. The intestinal absorption of oral (14)C-lipid, (14)CO(2) production and the amount of (14)C-lipid transferred to the pups (milk clot + pups carcass) were significantly higher in the HL group than in the LL group (P < 0.05). Mammary gland lipogenesis was 75% lower and LPL activity was 30% higher in the HL group (P < 0.05). Medium-chain fatty acids (C6-C14) excretion was 46% lower and that of long-chain fatty acids was 142% (P < 0.001) higher in the HL group than in the LL group. The higher milk lipid excretion in the rats fed a high-fat diet resulted from a larger uptake of dietary lipid by the mammary gland, indicated by a larger transfer of (14)C-lipid to the pups and by a higher LPL activity in the mammary gland.     

Oxidized fat reduces milk triacylglycerol concentrations by inhibiting gene expression of lipoprotein lipase and fatty acid transporters in the mammary gland of rats

Feeding oxidized fats to lactating rats causes a strong reduction of triacylglycerol concentration in the milk. The reason for this, however, has not yet been elucidated. Pregnant Sprague-Dawley rats were assigned to 2 groups of 11 rats each and fed diets containing either fresh fat (FF group) or an oxidized fat (OF group) from d 1 to d 20 of lactation. Concentrations of triacylglycerols and long-chain fatty acids in the milk and weight gain of suckling pups were lower in the OF group than in the FF group (P < 0.05). Concentrations of medium-chain fatty acids in the milk and messenger RNA (mRNA) abundance of lipogenic enzymes in the mammary gland did not differ between the 2 groups of rats. However, the OF group had a lower concentration of triacylglycerols and nonesterified fatty acids (NEFA) in plasma and lower mRNA concentrations of lipoprotein lipase and fatty acid transporters in the mammary gland than the FF group (P < 0.05). Moreover, the OF group had higher mRNA concentrations of hepatic lipase, fatty acid transporters, and several genes involved in fatty acid oxidation in the liver than the FF group (P < 0.05). The present findings suggest that a dietary oxidized fat lowers the concentration of triacylglycerols in the milk by a reduced uptake of fatty acids from triacylglycerol rich-lipoproteins and NEFA into the mammary gland. The study, moreover, indicates that an oxidized fat impairs normal metabolic adaptations during lactation, which promote the utilization of metabolic substrates by the mammary gland for the synthesis of milk.     

Lipoprotein lipase and the disposition of dietary fatty acids

Lipoprotein lipase (EC 3.1.1.34; LPL) is a key enzyme regulating the disposal of lipid fuels in the body. It is expressed in a number of peripheral tissues including adipose tissue, skeletal and cardiac muscle and mammary gland. Its role is to hydrolyse triacylglycerol (TG) circulating in the TG-rich lipoprotein particles in order to deliver fatty acids to the tissue. It appears to act preferentially on chylomicron-TG, and therefore may play a particularly important role in regulating the disposition of dietary fatty acids. LPL activity is regulated according to nutritional state in a tissue-specific manner according to the needs of the tissue for fatty acids. For instance, it is highly active in lactating mammary gland; in white adipose tissue it is activated in the fed state and suppressed during fasting, whereas the reverse is true in muscle. Such observations have led to the view of LPL as a metabolic gatekeeper, especially for dietary fatty acids. However, closer inspection of its action in white adipose tissue reveals that this picture is only partially true. Normal fat deposition in adipose tissue can occur in the complete absence of LPL, and conversely, if LPL activity is increased by pharmacological means, increased fat storage does not necessarily follow. LPL appears to act as one member of a series of metabolic steps which are regulated in a highly coordinated manner. In white adipose tissue, it is clear that there is a major locus of control of fatty acid disposition downstream from LPL. This involves regulation of the pathway of fatty acid uptake and esterification, and appears to be regulated by a number of factors including insulin, acylation-stimulating protein and possibly leptin.     

共轭亚油酸对小鼠乳腺脂肪合成相关基因表达的作用研究

目的研究共轭亚油酸(CLA)对小鼠乳腺脂肪合成相关基因表达的影响。方法选取32只泌乳昆明鼠,随机分为对照组(2%花生油);0.5%CLA组(0.5%CLA+1.5%花生油);1.0%CLA组(1%CLA+1%花生油);1.5%CLA组(1.5%CLA+0.5%花生油),每组8只小鼠,从泌乳第4日饲喂至第14日,每日检测母鼠采食量、体重和仔鼠窝重,分析乳成分,采用荧光定量PCR检测对照组和1.5%CLA组母鼠乳腺组织脂肪合成相关基因的表达。结果在饲喂期间各组母鼠体重没有差异,饲料添加1.5%CLA显著减少了母鼠的采食量、仔鼠窝重和乳脂肪含量(P<0.05)。与对照组相比,1.5%CLA组母鼠乳腺组织的脂蛋白脂酶(LPL)、乙酰辅酶A羧化酶(ACACA)、硬脂酰辅酶A去饱和酶(SCD)和脂肪酸合成酶(FASN)的基因表达显著降低(P<0.05),并且转录因子固醇调节元件结合蛋白(SREBF)和过氧化物酶体增殖激活受体(PPARγ)的基因表达也显著下调(P<0.01)。结论饲料中加入1.5%CLA能够减少泌乳母鼠乳脂肪含量,抑制乳腺脂肪合成相关基因的表达。     

Dietary conjugated linoleic acids lower the triacylglycerol concentration in the milk of lactating rats and impair the growth and increase the mortality of their suckling pups

Recent studies showed that conjugated linoleic acids (CLA) lower triacylglycerol concentrations in the milk of lactating animals. This study was performed to determine the reasons for this phenomenon; we also investigated whether there is a relation between altered lipid metabolism in the liver and the reduction in milk triacylglycerols in rats fed CLA. Two groups of female rats were fed diets containing 0 [sunflower oil (SFO) group] or 14.7 g/kg diet of a CLA mixture (CLA group) at the expense of sunflower oil during growth, pregnancy, and lactation. CLA-fed rats had 49 and 80% lower mRNA concentration and activity of fatty acid synthase, respectively, a 51% lower mRNA concentration of lipoprotein lipase (LPL) in their mammary glands at d 17 of lactation, and a 46% lower milk fat content than SFO rats (P < 0.05). Although CLA rats had lower concentrations of triacylglycerols in the liver than SFO rats (20.8 +/- 2.6 vs. 62.6 +/- 27.7 micromol/g, P < 0.05), concentrations of triglycerides in plasma, which are the substrates of LPL, did not differ between the groups. Moreover, the number of pups per litter, litter weights, and pup weights at d 17 of lactation were 41, 35, and 22% lower, respectively, in the CLA group than in the SFO group. In conclusion, the present study suggests that dietary CLA reduces triacylglycerol concentrations in the milk via reduced de novo fatty acid synthesis in the mammary gland and an impaired uptake of fatty acids from lipoproteins into the mammary gland. This might be the reason for reduced growth rates and an increased mortality of suckling pups.     

Dietary triacylglycerol structure and its role in infant nutrition

Human milk TG are a remarkable example of stereo-specific positioning of fatty acids with structures that are highly conserved and unusual. Not only does human milk contain high amounts of fat and 16:0, but ~70% of the 16:0 is esterified at the TG stereo-specifically numbered (sn)-2 position, with preferential positioning of 18:1(n-9) and 18:2(n-6) at the TG sn-1,3 positions. The milk TG structures and digestive lipases combine to enable efficient digestion and absorption of 16:0 by conserving 16:0 in sn-2 monoacylglycerols, which are absorbed, reassembled, and secreted in plasma conserving the original milk TG configuration; these studies are reviewed in this article. The reason why the mammary gland invests in enzymes to provide the infant with 20-25% milk fatty acids as 16:0 rather than selecting against 16:0 is unknown, yet likely has a purpose given the mammary gland capacity for 10:0, 12:0, and 14:0 synthesis. Recent advances in the development-, tissue-, and species-specific activity of enzymes of TG synthesis and knowledge that dietary TG structures are maintained postabsorption suggest that the purpose of the milk TG structures is more sophisticated than simply avoiding 16:0 malabsorption. The overall aim is to expand consideration of fatty acids in the infant diet from a simple view of average fatty acid compositions to the complex lipids and molecular structures in which fatty acids are provided to tissues during early life and the biology through which the unique features of human milk enable the infant to grow and thrive on a high-fat, high-saturated-fat milk diet.     

The effect of temperature and length of storage on bile salt-stimulated lipase and esterase in human milk

Five milk samples were analyzed for bile salt-stimulated lipase (BSSL) and bile-salt stimulated esterase (BSSE) activities immediately after collection. The samples then were divided into aliquotes and stored at 25, 4, −20 and −70 C. After 6, 12 and 24 hours of storage BSSL and BSSE activities were determined. Activities were also determined in milk after 2 and 4 weeks of storage at −20 and −70 C. There was no significant change in BSSL activity due to storage. The average activity of BSSL was 1.54 μmole free fatty acid /released/min/ml milk ±0.46 (SD). Activity from BSSE was significantly (P<0.05) increased by storage at 4, −20, and −70 C compared to 25 C. After the initial cooling or freezing of milk, measured at 6 hr, there was no additional change in BSSE activity with storage time. Bile salt-independent esterase activity increased significantly (P<0.05) due to storage. Upon collection, bile salt-independent esterase activity averaged 0.16 μmoles p-nitrophenol released min/ml milk ±0.23 (SD) compared to 1.27±1.69 (SD) in stored milk.     

Lipid Composition,Digestion, and Absorption Differences among Neonatal Feeding Strategies: Potential Implications for Intestinal Inflammation in Preterm Infants

Necrotizing enterocolitis (NEC) is a significant cause of morbidity and mortality in the neonatal population. Formula feeding is among the many risk factors for developing the condition, a practice often required in the cohort most often afflicted with NEC, preterm infants. While the virtues of many bioactive components of breast milk have been extolled, the ability to digest and assimilate the nutritional components of breast milk is often overlooked. The structure of formula differs from that of breast milk, both in lipid composition and chemical configuration. In addition, formula lacks a critical digestive enzyme produced by the mammary gland, bile salt-stimulated lipase (BSSL). The gastrointestinal system of premature infants is often incapable of secreting sufficient pancreatic enzymes for fat digestion, and pasteurization of donor milk (DM) has been shown to inactivate BSSL, among other important compounds. Incompletely digested lipids may oxidize and accumulate in the distal gut. These lipid fragments are thought to induce intestinal inflammation in the neonate, potentially hastening the development of diseases such as NEC. In this review, differences in breast milk, pasteurized DM, and formula lipids are highlighted, with a focus on the ability of those lipids to be digested and subsequently absorbed by neonates, especially those born prematurely and at risk for NEC.     

Milk composition and volume during the onset of lactation in a diabetic mother

Milk volume and composition were examined in a diabetic mother on days 3-7 postpartum. By day 5 milk volume produced and concentrations of sodium, potassium, chloride, lactose, protein, calcium, magnesium, and citrate were within limits of a reference population. Fat content of the milk was slightly lower. Free fatty acids were 2% of total lipid on day 3 but increased to 23% on days 4-7, suggesting impaired esterification in the mammary gland. Total milk lipoprotein lipase increased approximately fourfold during days 4-5. Other changes were 1) low cholesterol content, only one-fifth of normal milk; 2) decreased medium-chain fatty acids, suggesting impairment of fatty acid synthesis in the mammary gland; 3) increased oleic acid; and 4) high concentrations of polyunsaturated fatty acids, suggesting increased chain elongation. These results suggest that diabetes produces changes in lipid metabolism in the mammary gland that alter the composition of milk produced by the diabetic mother.     

Effect of orange juice,folic acid,and oral contraceptives on serum folate in women taking a folate-restricted diet.

The effect of folate intake from orange juice on serum folate was evaluated in 60 women (age 20-39) during 9 weeks of a folate-restricted diet. Twenty-one were users of oral contraceptives (OCA). Folate intake from the restricted diet was 159 +/? 5 micrograms/day, as assessed by dietary surveys. Serum folate of women taking OCA was lower than in nonusers at the inception of the study (P less than 0.01). During the initial 2 weeks of restricted diet, serum folates decreased significantly (13.8 +/? 1.8 to 8.5 +/? 0.4 ng/ml; P less than 0.002). This decrease was further prevented by supplementation of the diet for 7 weeks with 100 micrograms/day of total folate from reconstituted frozen orange juice or synthetic folic acid (PteGlu). Both folate supplements were effective (P less than 0.05) in increasing serum folate (9.4 +/? 1.0 to 14.5 +/? 1.4 ng/ml, orange juice; 8.4 +/? 0.7 to 20.5 +/? 5.8 ng/ml, folic acid). Serum folates were similar in women taking either orange juice or folic acid. Serum folate of nonsupplemented women decreased from 10.2 +/? 0.8 to 8.3 +/? 0.4 ng/ml (P less than 0.05). No difference between serum folates of OCA users and nonusers was detected during the restricted diet or folate supplementation. These data indicate that folate in reconstituted orange juice was as available as folic acid, and that utilization of both folate forms and folate in a mixed diet was unaffected by oral contraceptives.     

Metabolic adaptations in pregnancy and their implications for the availability of substrates to the fetus

During the first two-thirds of gestation, the mother is in an anabolic condition, increasing her fat depots thanks to both hyperphagia and enhanced lipogenesis. During the last third of gestation, the mother switches to a catabolic condition. Glucose is the most abundant nutrient crossing the placenta, which causes maternal hypoglycemia despite an increase in the gluconeogenetic activity. Adipose tissue lipolytic activity becomes enhanced, increasing plasma levels of FFA and glycerol that reach the liver; consequently there is an enhanced production of triglycerides that return to the circulation in the form of very low density lipoproteins (VLDL). Glycerol is also used as a preferential gluconeogenetic substrate, saving other more essential substrates, like amino acids, for the fetus. Under fasting conditions, fatty acids are converted into ketone bodies throughout the beta-oxidation pathway, and these compounds easily cross the placental barrier and are metabolized by the fetus. An enhanced liver production of VLDL-triglycerides together with a decrease in adipose tissue lipoprotein lipase (LPL) and an increase in plasma activity of cholesterol ester transfer protein causes both an intense increment in these lipoproteins and a proportional enrichment of triglycerides in both low and high density lipoproteins. Maternal triglycerides do not cross the placenta, but the presence of LPL and other lipases allows their hydrolysis, releasing fatty acids to the fetus. Under fasting conditions, the maternal liver uses circulating triglycerides as ketogenic substrates. Around parturition there is an induction of LPL activity in the mammary glands, driving circulating triglycerides to this organ for milk synthesis, allowing essential fatty acids derived from the mother's diet to become available to the suckling newborn.     

Lipoprotein lipase activity in skeletal muscle and brown adipose tissue of pregnant and lactating rats

Changes in lipoprotein lipase (LPL) activity during pregnancy and lactation were followed in skeletal muscles and interscapular brown adipose tissue (BAT) of rats fed two diets differing in energy density (high carbohydrate or high fat). Rats were decapitated after 7, 19 or 21 d of pregnancy or after 3 or 12 d of lactation. Virgin rats and females separated from their litter just after delivery were used as nonpregnant and nonlactating controls, respectively. Blood was collected for determination of plasma glucose, triglycerides (TG) and nonesterified fatty acids (NEFA). Soleus, extensor digitorum longus (EDL), diaphragm and interscapular BAT were rapidly removed and frozen in liquid nitrogen for LPL activity measurement. LPL activity was not significantly higher in muscles and BAT of virgin rats fed the high fat diet than in those of rats fed the high carbohydrate diet. No significant change of skeletal muscle LPL activity was observed during pregnancy, regardless of the diet fed. Although BAT exhibited a transitory hypertrophy during pregnancy, its LPL activity was not significantly altered; during lactation BAT lost weight and its LPL activity dropped sharply when either diet was fed, leaving more TG available for milk production.     

Evidence of enhanced storage capacity in adipose tissue of genetically fat chickens

The storage capacity for plasma triglyceride in abdominal adipose tissue has been investigated in two lines of chickens selected for either high or low adiposity (fat line and lean line, respectively). Adipose tissue cellularity and lipoprotein lipase (LPL) activity were estimated in 2- and 5-wk-old birds. In 2-wk-old birds, cellularity and LPL and glycerophosphate dehydrogenase (GPDH) activity were evaluated in the stromavascular fraction. At both ages studied, the fat line exhibited a twofold increase in mature fat cell number and a marked hypertrophy. LPL activity per cell did not differ with genotype, regardless of the nutritional state; however, LPL activity per whole fat pad was higher in fat birds. In 2-wk-old fat chickens, the stromavascular fraction was characterized by an increase in cell number and a decrease in LPL activity, while GPDH was unchanged. Finally, adipocyte hyperplasia in the fat line appeared to reflect an excessive proliferation of precursor cells. The higher LPL activity in fat birds resulted mainly from cell hyperplasia, rather than from a greater intrinsic activity of adipocytes. Consequently, enhanced fatty acid uptake by adipose tissue represents a major factor in determination of adiposity in the chicken.     

Lipoprotein lipase activity and chylomicron clearance in rats fed a high fat diet

The relationships of tissue and plasma lipoprotein lipase (LPL) activities to tissue uptake and plasma clearance of 14C-labeled chylomicron-triglyceride (14C-CM-TG) were studied in female rats fed isoenergetic and isonitrogenous control (12% kJ from fat) or high fat diets (72% kJ from fat) for 8 wk. Animals fed the high-fat diet had higher levels of fasting plasma triglycerides and lower LPL activities in heart, renal adipose tissue and post-heparin plasma. Changes in LPL activities of skeletal muscles varied among muscles with higher values in the soleus and plantaris (32-61%) and no differences in the gastrocnemius. The lower LPL activity in renal adipose tissue was associated with lower uptake of fatty acids from 14C-CM-TG by adipose. Fatty-acid uptake from labeled TG was not associated with tissue LPL activity in other tissues. Clearance of 14C-CM-TG from plasma and the half-lives of 14C-CM-TG were similar in both dietary groups. These data indicate that tissue and plasma LPL activities are not a direct index of uptake of fatty acids by tissues or clearance of chylomicron triglycerides.     

Milk and plasma lipid composition in a lactating patient with type I hyperlipoproteinemia

This report describes studies on the plasma and milk lipid composition of a patient with primary Type I hyperlipoproteinemia who had been followed through her second pregnancy. Post-partum she lactated, supplying milk for assay. It was abnormal in the low content of its total lipid and in the bizarre composition of its fatty acids. The proportion of long chain fatty acids was unusually low, and that of medium chain fatty acids unusually high. Furthermore, the fatty acids of the patient's milk differed greatly from those of her plasma triglycerides. This was in marked contrast to normal nursing mothers' milk, in which the fatty acid composition is comparable to that of plasma triglycerides. The patient's milk fatty acids were shorter in chain length and deficient in essential fatty acids. During the time of lactation, the patient remained hyperlipidemic and her post-heparin plasma had no lipolytic activity. These data and the differences between the plasma and milk fatty acids suggested that in the patient the circulating triglyceride fatty acids did not enter the mammary gland. Without preformed fatty acids entering it from plasma or adipose tissue, the lactating breast apparently synthesized fatty acids de novo. These newly synthesized fatty acids were of medium, rather than long chain length. This accounted for the abundance of medium chain length triglycerides in the patient's milk. The studies suggested that the deficit of lipoprotein lipase in Type I hyperlipoproteinemia extended to the mammary gland.     

Serum levels of unbound free fatty acids. I: Normative data in term newborn infants.

Free fatty acids (FFA) play essential roles in maintaining physiologic homeostasis in the newborn infant. Most of the FFA in serum is carried in complex with albumin, but a small fraction remains unbound in the aqueous phase.

This study's goal is to report the values of serum levels of unbound free fatty acids (FFAu) in pregnant women and their newborn infants at term gestation.

The measurements were made possible by the availability of the fluorescent probe for unbound FFA, acrylodated intestinal fatty acid binding protein (ADIFAB). Twenty-two mother-infant pairs were enrolled in the study. Maternal levels were obtained immediately before delivery, cord levels at the time of delivery, and infant levels after 24 hours of age.

The level of FFAu measured in maternal samples was 11.8 +/? 4 nM, in cord samples 9.2 +/? 4 nM, and in infants 13.9 +/? 3 nM. These population averages are considerably greater than those observed in healthy adults (7.5 +/? 2.5 nM). No correlation was found between cord levels and birthweight, gestational age, labor duration, mode of deliver, and infant or maternal temperature.

This investigation is the first to measure FFAu in a group of mothers and their infants and provides the technique for future investigations of the biologic activity of free fatty acids.     

Opposite contributions of trunk and leg fat mass with plasma lipase activities: the Hoorn study

OBJECTIVE: Lipoprotein lipase (LPL) and hepatic lipase (HL) are essential in hydrolysis of triglyceride-rich lipoproteins. LPL activity is negatively, whereas HL activity is positively, associated with total body fat. We determined the associations of trunk and leg fat mass with plasma LPL and HL activities in a cross-sectional study. RESEARCH METHODS AND PROCEDURES: LPL and HL activities were determined in post-heparin plasma in a sample of 197 men and 209 women, 60 to 87 years of age. A total body DXA scan was performed to determine trunk and leg fat mass. RESULTS: In women, but not in men, trunk fat mass was negatively associated with LPL activity, whereas leg fat mass was positively associated, after mutual adjustment and adjustment for age. Standardized betas (95% confidence interval) for trunk and leg fat mass were -0.24 (-0.41; -0.08) and 0.14 (-0.02; 0.31), respectively (interaction by sex, p = 0.03). Larger trunk fat mass was associated with higher HL activity in men [0.48 (0.28; 0.68)] and women [0.40 (0.24; 0.56)]. A negative association of leg fat mass and HL activity was observed in men, although not statistically significant [-0.13 (-0.33; 0.06)], and in women [-0.28 (-0.38; -0.18)]. DISCUSSION: Abdominal fat is associated with unfavorable and femoral fat with favorable LPL and HL activities in plasma.     

Fatty acids and retinyl esters of rat milk: effects of diet and duration of lactation

This study was initiated to explore the quantitative and qualitative differences in milk total fatty acids and milk retinyl esters when either hydrogenated or nonhydrogenated fat is fed during pregnancy and lactation. Rats were fed diets containing 10% by weight of corn oil or partially hydrogenated corn oil. Milk was collected on d 1, 8 and 14 of lactation and analyzed for protein, total fatty acids, fatty acid pattern, and retinyl ester pattern. Whereas diet produced no quantitative differences in milk protein or total fatty acids, the pattern of milk fatty acids varied significantly. Rats fed corn oil produced milk having more medium-chain saturated fatty acids, less long-chain monoenoic fatty acids, and more polyunsaturated fatty acids compared to those fed hydrogenated corn oil. Rats fed hydrogenated corn oil produced milk fat having 21-26% of the trans fatty acid, elaidic acid. Significant differences were also observed with duration of lactation: medium-chain fatty acids increased three to fourfold between d 1 and 8, where cis-monoenes and polyunsaturated fatty acids declined. The pattern of milk retinyl esters strongly reflected, but was not identical to, that of total milk fat. Comparing d 14 milk from rats fed corn oil with that from rats fed hydrogenated corn oil, medium-chain esters of retinol constituted 24 and 11% of total retinyl esters, whereas saturated long-chain fatty acid esters constituted 52 and 44%, respectively. trans Fatty acid esters of retinol comprised 24% of vitamin A esters in milk of rats fed hydrogenated fat. These data provide evidence that the composition of milk retinyl esters, as well as that of total milk fat, is determined both by the type of fatty acids from diet and from diet-related differences in de novo synthesis of fatty acids within the mammary gland and other tissues.