An investigation of serum lipoproteins and of their contribution to milk fat in the dairy cow

1. Tritium-labelled olive-oil triglycerides were introduced into the rumens of lactating cows and the specific activities of the lipids of milk and plasma and of serum lipoproteins were measured. 2. On treatment of serum with dextran sulphate it was found that the lipid of the precipitated beta-lipoproteins consistently had a specific activity-time curve with a maximum comparable in value with, and occurring earlier than, that of the milk fat. 3. On fractionation of the lipids of these lipoproteins it was found that only the triglycerides and diglycerides had specific activity-time curves with maxima greater than that of milk fat, and on radioactivity data alone they are the only blood constituents studied that meet the requirements for being the precursor of milk fat. 4. From a consideration of abundances and the mean specific activities over the period of the experiment it is shown that the contribution of the diglycerides to the radioactivity in the milk fat must have been negligible and that only the triglycerides could have been responsible for all the radioactivity found in it. 5. Although no other fraction could alone have been responsible for all the radioactivity in the milk fat, at least one, the phospholipids, could have made some contribution. 6. It is calculated that 35-48% by weight of milk fat was derived from the beta-lipoprotein triglycerides, according to whether the phospholipids made an improbably large contribution or none at all.     

Studies on the mode of uptake of blood triglycerides by the mammary gland of the lactating goat. The uptake and incorporation into milk fat and mammary lymph of labelled glycerol, fatty acids and triglycerides

1. The mode of uptake of the precursors of milk fat by the mammary gland of the lactating goat has been examined by infusing radioactive fatty acids, glucerol or doubly labelled triglycerides into the mammary artery or jugular vein of animals surgically prepared to permit samples of arterial and venous blood to be withdrawn without disturbance to the animal. 2. Acetate was taken up by the mammary gland and incorporated into milk fat. The decrease in the specific radioactivity of blood acetate across the gland was evidence of acetate production, but there was no significant release of labelled lipid from the mammary gland. 3. When labelled long-chain fatty acids or glycerol were infused into the lactating goat, there was extensive transfer of radioactivity into milk in spite of the absence of net uptake of substrate by the mammary gland. The decrease in the specific radioactivity of each substrate across the mammary gland, however, showed that both fatty acids and glycerol were simultaneously taken up and released by mammary tissue. 4. The infusion of chylomicra and triglyceride emulsions labelled with (3)H and (14)C revealed that both glycerol and fatty acids were released during triglyceride uptake by mammary tissue. Changes in the (3)H/(14)C ratio during the transfer of triglyceride from blood into milk showed that at least 80% of the triglyceride was hydrolysed during uptake, but the potential re-utilization of both products of hydrolysis for triglyceride synthesis in mammary tissue implied that only a minimum value could be obtained from the change in the ratio. 5. The time-course of the transfer of (3)H and (14)C into milk and lymph were closely similar after the infusion of [2-(3)H]glycerol tri[1-(14)C]oleate or of a mixture of [2-(3)H]glycerol and [1-(14)C]oleate. 6. The results were consistent with the hypothesis that plasma triglycerides are extensively or completely hydrolysed during mammary uptake.     

The oxidation and utilization of palmitate, stearate, oleate and acetate by the mammary gland of the fed goat in relation to their overall metabolism, and the role of plasma phospholipids and neutral lipids in milk-fat synthesis

1. Measurements were made of milk yield, mammary blood flow and arteriovenous differences of each plasma lipid fraction, and their specific radioactivities, during the infusion of [U-¹⁴C]stearate, [U-¹⁴C]oleate, [U-¹⁴C]palmitate and [1-¹⁴C]acetate into fed lactating goats. 2. Entry rates of fatty acids into the circulation were 4·2mg./min./kg. body wt. for acetate, and 0·18, 0·28 and 0·42mg./min./kg. for stearate, oleate and palmitate respectively. Acetate accounted for 23% of the total carbon dioxide produced by the whole animal, and contributed to the oxidative metabolism of the mammary gland to about the same extent. Corresponding values for each of the long-chain acids were less than 1%. 3. There were no significant arteriovenous differences of phospholipids, sterols or sterol esters, and their fatty acid composition showed no net changes during passage through the mammary gland. 4. There were large arteriovenous differences of plasma triglycerides, and their fatty acid composition showed marked changes across the gland. The proportions of palmitate and stearate fell, and that of oleate increased. 5. Arteriovenous differences of plasma free fatty acids (FFA) were small and variable, but a large fall in the specific radioactivity of each of the long-chain acids examined indicated substantial uptake of plasma FFA, accompanied by roughly equivalent FFA release from mammary tissue. The uptake of FFA was confirmed by the extensive transfer of radioactivity into milk. The FFA of milk were similar in composition and radioactivity to the milk triglyceride fatty acids, and quite unlike plasma FFA. 6. The formation of large amounts of oleic acid (18–21 mg./min.) from stearic acid was demonstrated. 7. During the terminal stages of the [¹⁴C]acetate infusion, milk triglyceride fatty acids of chain length C₄–C₁₄ showed specific radioactivities that were 75–90% of that of blood acetate, and that of palmitate was roughly one-quarter of this value. Oleate and stearate were unlabelled. 8. The results confirmed that milk fatty acids of chain length C₄–C₁₄ arise largely from blood acetate, and palmitate is derived partly from acetate and partly from plasma triglyceride, the latter fraction being almost the sole precursor of oleate and stearate.     

Effect of dietary ghee--the anhydrous milk fat, on blood and liver lipids in rats

Dairy products are important sources of dietary fat in India. Anhydrous milk fat, viz., ghee, is consumed as such in the diet and also is used for frying the dishes. Ghee contains high levels of saturated fatty acids and cholesterol, which are considered risk factors for cardiovascular diseases. In the present study, ghee, at levels ranging from 0.25 to 10%, was included in a nutritionally balanced AIN-76 diet fed to Wistar rats for a period of 8 weeks. The serum lipid profiles of these animals showed a dose dependent decrease in total cholesterol, low density lipoproteins and very low density lipoproteins cholesterol, and triglyceride levels when ghee was present at levels greater than 2.5% in the diet. Liver cholesterol and triglycerides also were decreased in these animals. When ghee was included as a sole source of fat at a 10% level, polyunsaturated fatty acids in the serum and liver lipids were reduced significantly. Similar results were observed when ghee was subjected to a higher temperature (120 degrees C) to generate cholesterol oxidation products and fed to the animals. Although cholesterol oxidation products were not accumulated in serum, significant amounts were accumulated in liver only when ghee was fed as a sole source of fat at a 10% level. This study revealed that the consumption of ghee up to a 10% level in the diet altered blood lipid profiles in such a manner as not to elevate the risk factors for cardiovascular diseases.     

The fatty acid composition of milk fat as influenced by feeding oilseeds

The fatty acid composition of bovine milk fat can be substantially altered by feeding lipid sources which alter the fatty acid profile of lipid entering the intestine from the rumen. As long-chain fatty acids of dietary origin can be incorporated directly into milk fat the opportunity exists to alter the ratio of short and long-chain fatty acids as well as the degree of saturation of milk fat. In practice our ability to alter the fatty acid profile of milk fat is limited not by the synthetic capacity of the mammary gland, but rather by the challenge of achieving effective protection of unsaturated dietary fatty acids from biohydrogenation in the rumen, as well as keeping the level of polyunsaturated fatty acids within the range where the organoleptic quality and shelf-life of milk and dairy products are not compromised. The fatty acid composition of oilseeds such as canola are considered desirable from a human health perspective and thus their inclusion in the diet of dairy cattle as a means of achieving a more desirable fatty acid profile in milk fat may enhance the nutritive quality of milk.     

Lipid composition of the fat body and haemolymph and its relation to lipid release in Oncopeltus fasciatus

Lipid composition of the fat body and haemolymph of male milkweed bug, Oncopeltus fasciatus, was determined. Triglycerides were the predominant lipids of the fat body while diglycerides accounted for the major lipid in the haemolymph. Sterols, sterol esters, and non-esterified fatty acids were present in both fat body and haemolymph besides triglycerides and diglycerides. Only traces of monoglycerides were detected.Gas chromatographic analysis of the fatty acids revealed a difference in the fatty acid composition between fat body and haemolymph glycerides and sterol esters. Oleate and linoleate were the predominant unsaturated fatty acids in both fat body and haemolymph lipids and in the milkweed seeds as well.When fat body was labelled in vivo and in vitro with ¹⁴C-palmitate, the fatty acid was incorporated largely into the triglycerides. When the prelabelled fat body was incubated with a medium containing haemolymph the fat body released lipids mainly as diglycerides. Some radioactivity was observed in the triglycerides and non-esterified fatty acids also.Electrophoretic analysis of the incubation medium containing the haemolymph revealed that the released lipids were bound to three haemolymph lipoprotein bands. Lipid mobilization, release, and transport in Oncopeltus are discussed in relation to studies on other insects.     

Short-term effects of exogenous growth hormone: effects on milk production and utilization of nutrients in muscle and mammary tissues of lactating ewes

Exogenous bovine growth hormone at a dose of 0.1 mg kg-1 liveweight increased yields of milk and milk constituents and milk fat content when injected over 5 days into ewes in mid-lactation. These changes in milk production were associated with changes in the supply to, and utilization of, nutrients by leg muscle and mammary tissues. Arterial concentrations of glucose and non-esterified fatty acids increased significantly, concentrations of lactate and 3-hydroxybutyrate tended to increase, and concentrations of triglycerides associated with very low-density lipoproteins decreased significantly. Growth hormone increased mammary uptake of non-esterified fatty acids, decreased mammary uptake of very low-density lipoproteins and tended to reduce the release of lactate from leg muscle. Oxidation of non-esterified fatty acids in the whole body and mammary tissue was increased by growth hormone and there was a tendency for reduction of glucose oxidation in mammary tissues. During injection of growth hormone, blood flow to leg muscle and mammary tissues increased as did the calculated ratio of blood flow; milk yield. These changes in blood flow, together with changes in arterial concentrations and tissue utilizations of key metabolites, were sufficient to account for the synthesis of extra milk and milk constituents.     

Lipoprotein lipase activity and its relationship to high milk fat transfer during lactation in grey seals

Lipoprotein lipase regulates the hydrolysis of circulating triglyceride and the uptake of fatty acids by most tissues, including the mammary gland and adipose tissue. Thus, lipoprotein lipase is critical for the uptake and secretion of the long-chain fatty acids in milk and for the assimilation of a high-fat milk diet by suckling young. In the lactating female, lipoprotein lipase appears to be regulated such that levels in adipose tissue are almost completely depressed while those in the mammary gland are high. Thus, circulating fatty acids are directed to the mammary gland for milk fat production. Phocid seals serve as excellent models in the study of lipoprotein lipase and fat transfer during lactation because mothers may fast completely while secreting large quantities of high fat milks and pups deposit large amounts of fat as blubber. We measured pup body composition and milk fat intake by isotope (deuterium oxide) dilution and plasma post-heparin lipoprotein lipase activity in six grey seal (Halichoerus grypus) mother-pup pairs at birth and again late in the 16-day laction period. Maternal post-heparin lipoprotein lipase activity increased by an average of four-fold by late lactation (P=0.027), which paralleled an increase in milk fat concentration (from 38 to 56%; P=0.043). Increasing lipoprotein lipase activity was correlated with increasing milk fat output (1.3–2.1 kg fat per day) over lactation (P=0.019). Maternal plasma triglyceride (during fasting) was inversely correlated to lipoprotein lipase activity (P=0.027) and may be associated with the direct incorporation of longchain fatty acids from blubber into milk. In pups, post-heparin lipoprotein lipase activity was already high at birth and increased as total body fat content (P=0.028) and the ratio of body fat: protein incrased (P=0.036) during lactation. Although pup plasma triglyceride increased with increasing daily milk fat intake (P=0.023), pups effectively cleared lipid from the circulation and deposited 70% of milk fat consumed throughout lactation. Lipoprotein lipase may play an important role in the mechanisms involved with the extraordinary rates of fat transfer in phocid seals.Abbreviations FFA free fatty acid - HL hepatic lipase - LPL lipoprotein lipase - PH-HL post-heparin hepatic lipase - PH-LPL post-heparin lipoprotein lipase - VLDL very low density lipoprotein     

Intracellular origin and secretion of milk fat globules

The cream or fat fraction of milk consists of fat droplets composed primarily of triacylglycerols that are surrounded by cellular membranes. In this review we discuss what is known about how these droplets are formed in and secreted by mammary epithelial cells during lactation. This secretion mechanism, which appears to be unique, is unlike the exocytotic mechanism used by other cell types to secrete lipids. Milk fat globules originate as small, triacylglycerol-rich, droplets that are formed on or in endoplasmic reticulum membranes. These droplets are released from endoplasmic reticulum into the cytosol as microlipid droplets coated by proteins and polar lipids. Microlipid droplets can fuse with each other to form larger cytoplasmic lipid droplets. Droplets of all sizes appear to be unidirectionally transported to apical cell regions by as yet unknown mechanisms that may involve cytoskeletal elements. These lipid droplets appear to be secreted from the cell in which they were formed by being progressively enveloped in differentiated regions of apical plasma membrane. While plasma membrane envelopment appears to be the primary mechanism by which lipid droplets are released from the cell, a mechanism involving exocytosis of lipid droplets from cytoplasmic vacuoles also has been described. As discussed herein, while we have a general overview of the steps leading to the fat globules of milk, virtually nothing is known about the molecular mechanisms involved in milk fat globule formation, intracellular transit, and secretion.     

Molecular weight distributions of milk fat triglycerides from seven species

The triglyceride compositions of the milk fats of man, dog, guinea pig, cow, sheep, goat, and horse were compared by gas-liquid chromatography of the intact triglycerides and of the butyl esters of the component fatty acids. The milk fats of man, dog, and guinea pig, which were largely made up of long-chain fatty acids, showed a common pattern with major contributions made by the glycerides with 48-54 acyl carbon atoms. The milk fats of cow, sheep, and goat, which were rich in short-chain acids, showed significant proportions of triglycerides with 28-54 acyl carbon atoms. Horse milk, which contains large amounts of medium-chain fatty acids, gave a characteristic triglyceride pattern in the 26-54 carbon atoms range. The experimentally determined distributions of the molecular weights of the triglycerides of all milk fats deviated significantly from the distributions predicted by random association of the fatty acids from a single pool. The data suggest that in all species the milk fat may be formed by a partial resynthesis of preformed glycerides.     

Specific distribution of short-chain fatty acids in molecular distillates of bovine milk fat

Triglycerides with a total of 34-44 acyl carbon atoms have previously been shown to account for nearly 50% of bovine milk fat. About 80% of this material was isolated by molecular distillation and a representative fraction of these glycerides was subjected to a stereospecific analysis. Of the 29 moles % of C(4)-C(8) fatty acids present in the analyzed fraction, at least 95% were specifically attached to the glycerol molecule in the position corresponding to carbon 3 of sn-glycerol 3-(dihydrogen phosphate) (L-alpha-glycerophosphate). The distribution of the other fatty acids (C(10) or greater) did not show such marked specificity for either the 1- or the 2-position. The data support the hypothesis that the short-chain fatty acids are esterified with long-chain diglycerides, or are substituted in glycerophosphatide intermediates, during the final step in the biosynthesis of milk fat triglycerides.     

Changes in cellular composition during magnesium deficiency.

1. Concentrations and compositions of liver, serum and milk lipids of cows were measured during 6 days' starvation and serum lipids during 60 days' re-feeding. 2. The concentration of free fatty acid in serum increased fivefold during starvation. 3. The content of total lipid in liver (g/100g of liver dry matter) doubled owing to a 20-fold increase in triglyceride, an eightfold increase in cholesterol ester, a three fold increase in free fatty acid and a 20% increase in cholesterol. There were no changes in the content or composition of liver phospholipids. 4. Starvation lowered the concentrations of total lipid, phospholipid and cholesterol ester of dextran sulphate-precipitable serum lipoproteins. Total lipid and cholesterol ester concentrations in lipoproteins of d greater than 1.055 and in lipoproteins not precipitable by dextran sulphate decreased from day 4 of the starvation period and during the first 20 days' re-feeding. 5. During starvation there were decreases in percentages of stearic acid and increases in oleic acid in serum free fatty acids and triglycerides and in liver neutral lipid. 6. Throughout starvation total milk lipid yield decreased, yields and percentages of C4-14 fatty acids decreased and percentages of C18 fatty acids increased. 7. It is suggested that accumulation of triglyceride in liver may be caused by increased uptake of plasma free fatty acids without corresponding increase in lipoprotein secretion.     

The incorporation of acetate,stearate and d(?)-β-hydroxybutyrate into milk fat by the isolated perfused mammary gland of the goat

1. Mammary glands of lactating goats were perfused for 12.5-15hr. with heparinized whole blood and infused with a substrate mixture of glucose, acetate and amino acids (and sometimes chylomicra) containing either [1-(14)C]acetate, d(-)-beta-hydroxy[1-(14)C]butyrate or [U-(14)C]stearate. 2. There was a substantial net uptake of acetate by the glands and transfer of radioactivity into milk fat. Acetate was extensively utilized for the synthesis of milk fatty acids of chain length up to C(14) and to a smaller extent for the synthesis of palmitate. 3. There was a small and variable net uptake of stearate and beta-hydroxybutyrate and negligible oxidation of these substrates. However, tissue uptake was demonstrated by a substantial fall in specific radioactivity across the glands and an extensive transfer of radioactivity into milk fatty acids. 4. With beta-hydroxybutyrate the labelling of milk fat was very similar to that with acetate, but the distribution of radioactivity suggested a cleavage into C(2) fragments of about 40%. 5. Labelled stearate gave rise to highly labelled stearate and oleate in the milk fat. Small amounts of radioactivity were detected in stearate of plasma triglycerides and oleate of plasma free fatty acids. 6. In experiments where there was a decline in milk-fat secretion late in perfusion, the milk fatty acids showed a marked decline in the proportion of stearate and oleate and a rise in the proportion of myristate and palmitate. This did not occur in experiments where milk-fat secretion was maintained at a higher level. 7. The present results confirm that there is a large pool of long-chain fatty acids in mammary tissue that can act as an endogenous source of these substrates.     

Identification of the conjugated linoleic acid isomer that inhibits milk fat synthesis

Conjugated linoleic acids (CLA) are octadecadienoic fatty acids that have profound effects on lipid metabolism. Our previous work showed that CLA (mixture of isomers) markedly reduced milk fat synthesis. In this study, our objective was to evaluate the effects of specific CLA isomers. Multiparous Holstein cows were used in a 3x3 Latin square design, and treatments were 4-day abomasal infusions of 1) skim milk (control), 2) 9,11 CLA supplement, and 3) 10,12 CLA supplement. CLA supplements provided 10 g/day of the specific CLA isomer (cis-9,trans-11 or trans-10,cis-12). Treatments had no effect on intake, milk yield, or milk protein yield. Only the 10,12 CLA supplement affected milk fat, causing a 42 and 44% reduction in milk fat percentage and yield, respectively. Milk fat composition revealed that de novo synthesized fatty acids were extensively reduced. Increases in ratios of C(14:0) to C(14:1) and C(18:0) to C(18:1) indicated the 10,12 CLA supplement also altered Delta(9)-desaturase. Treatments had minimal effects on plasma concentrations of glucose, nonesterified fatty acids, insulin, or insulin-like growth factor-I. Overall, results demonstrate that trans-10,cis-12 CLA is the isomer responsible for inhibition of milk fat synthesis.     

Composition of plasma fatty acids and non-cholesterol sterols in anorexia nervosa

Anorexia nervosa is a model of simple starvation accompanied by secondary hyperlipoproteinemia. The pattern of plasma fatty acids influences the levels of plasma lipids and lipoproteins. The concentration of plasma lathosterol is a surrogate marker of cholesterol synthesis de novo, concentrations of campesterol and beta-sitosterol reflect resorption of exogenous cholesterol. The aim of the study was to evaluate fatty acids in plasma lipid classes and their relationship to plasma lipids, lipoproteins, cholesterol precursors and plant sterols. We examined 16 women with anorexia nervosa and 25 healthy ones. Patients with anorexia nervosa revealed increased concentrations of total cholesterol, triglycerides, HDL-cholesterol, campesterol and beta-sitosterol. Moreover, a decreased content of n-6 polyunsaturated fatty acids was found in all lipid classes. These changes were compensated by an increased content of monounsaturated fatty acids in cholesteryl esters, saturated fatty acids in triglycerides and both monounsaturated and saturated fatty acids in phosphatidylcholine. The most consistent finding in the fatty acid pattern concerned a decreased content of linoleic acid and a raised content of palmitoleic acid in all lipid classes. The changes of plasma lipids and lipoproteins in anorexia nervosa are the result of complex mechanisms including decreased catabolism of triglyceride-rich lipoproteins, normal rate of cholesterol synthesis and increased resorption of exogenous cholesterol.     

Fatty acid composition of the milk lipids of Nepalese women: correlation between fatty acid composition of serum phospholipids and melting point.

Milk was collected from 36 Nepalese women, 15 to 32 years of age, in order to investigate relationships between the proportions of intermediate chain-length (C10-C14) fatty acids and critical n-3 and n-6 polyunsaturated fatty acids in the milk lipids they were producing. Serum was also obtained from these lactating women and the fatty acid composition of their serum phospholipid fraction was determined and compared with that of the corresponding milk lipid fraction. Compared to women in technologically advanced parts of the world, the serum phospholipids of the Nepalese women contained nutritionally adequate proportions of linoleic acid (LA) (16.8%), alpha-linolenic acid (ALA) (0.53%), arachidonic acid (AA) (5.69%), and docosahexaenoic acid (DHA) (1.42%). However, although the milk lipids contained adequate proportions of ALA (1.81%), AA (0.43%), and DHA (0.23%), the lipids contained low to moderate percentages of LA (mean, 9.05%). Positive correlations were observed between the proportions of AA (P=0.001, r=0.50) and ALA (P=0.03, r=0.36) in the serum phospholipids and milk lipids of the women. As the proportion of C10-Cl4 fatty acids in the milk lipids increased from 10% to 40%, there was preferential retention of three critical n-3 and n-6 fatty acids (ALA, AA, and DHA) at the expense of two relatively abundant nonessential fatty acids, namely stearic acid and oleic acid. In addition, using fatty acid melting point data and the mol fraction of the 9 most abundant fatty acids in the milk, we estimated the mean melting point (MMP) of the milk lipids of the Nepalese women. The MMPs ranged from 29.3 to 40.5 degrees C (median, 35.5 degrees C). These results indicate that: 1) the levels of AA and ALA in the blood of lactating mothers influence the levels of these fatty acids in the milk they produce; 2) when the mammary gland produces a milk that is rich in C10-Cl4 fatty acids, it somehow regulates triglyceride synthesis in such a way as to ensure that the milk will provide the exclusively breast-fed infant with the amounts of the critical n-3 and n-6 fatty acids it requires for normal growth and development; and 3) the melting point of the milk lipid fraction is determined mainly by the mol % of the intermediate chain-length (C10-C14) fatty acids, oleic acid, linoleic acid, and alpha-linolenic acid.     

Changes in mammary uptake of free fatty acids,triglyceride, cholesterol and phospholipid in relation to milk synthesis during lactation in goats

Uptake of free fatty acids (FFA), triglycerides (TG), cholesterol (CHOL) and phospholipids (PL) was measured in both mammary glands of dairy goats during lactation. Arterial concentrations of TG, CHOL and PL as well as arterio-venous difference (AV) and extraction rate (E) for TG were highest in goats with the highest dietary feed intake. AV were linearly related to arterial concentrations for the four lipid classes, and arterial concentrations of CHOL were linearly related to output of lactose, protein and fat in milk. Arterial supply, and not mammary synthetic activity, is the main determinant of mammary FFA, TG and CHOL uptake.     

Polyunsaturated fatty acids in maternal plasma and in breast milk

In order to explain processes underlying the transfer of fatty acids from the maternal compartment into human milk, the lipid content and the fatty acid composition of maternal plasma and milk have been analyzed in breastfeeding mothers at 1 day and 3 months of lactation.The rise in milk lipids occurring during the study period was concomitant with a fall in plasma total fat content, mainly due to the decrease of triglycerides. Significant correlations between plasma and milk fatty acids at the two time points were observed only for linoleic (LA, 18:2 n-6) and (alpha;-linolenic acid (alpha LNA, 18:3 n-3), while for arachidonic (AA, 20:4 n-6) and docosahexaenoic acid (DHA, 22:6 n-3) correlations were found only at one day and 3 months, respectively.These data suggest that levels of the n-6 and n-3 18C polyunsaturated fatty acids in milk are closely dependent on their concentrations in maternal plasma, in turn related with the dietary intake, while the accumulation of AA and DHA in milk is the result of a sequence of transfer and metabolic processes.     

The pattern of fatty acid synthesis in lactating rabbit mammary gland studied in vivo

The biosynthesis of fatty acids has been studied in lactating rabbits at 6h after intravenous injection of sodium [1-(14)C]acetate. The specific radioactivities of the individual fatty acids (C(6:0) to C(14:0)) and the proportions of these fatty acids synthesized were similar in mammary tissue and milk. Hexanoic acid had the highest specific radioactivity, and the C(8:0)-C(14:0) fatty acids had similar specific radioactivities, which were about five times those of C(16) and C(18) acids. No radioactivity was detected in fatty acids of chain length C(14) in these tissues were similar to those of the long-chain fatty acids in the milk and mammary gland. The results show that the C(4:0)-C(14:0) fatty acids are synthesized within the mammary gland rather than by fatty acid uptake from circulating blood or by oxidation of long-chain fatty acids within the gland. We conclude that de novo synthesis of esterified fatty acids in vivo by this tissue has a high degree of chain-length specificity.