Effect of lactation stage and concurrent pregnancy on milk composition in the bottlenose dolphin

Although many toothed whales (Cetacea: Odontoceti) lactate for 2–3 years or more, it is not known whether milk composition is affected by lactation stage in any odontocete species. We collected 64 pooled milk samples spanning 1–30 months postpartum from three captive bottlenose dolphins Tursiops truncatus . Milks were assayed for water, fat, crude protein (TN × 6.38) and sugar; gross energy was calculated. Ovulation and pregnancy were determined via monitoring of milk progesterone. Based on analysis of changes in milk composition for each individual dolphin, there were significant increases ( P <0.05) in fat (in all three dolphins) and crude protein (in two of three), and a decrease ( P <0.05) in water (in two of three) over the course of lactation, but the sugar content did not change. In all three animals, the energy content was positively correlated with month of lactation, but the percentage of energy provided by crude protein declined slightly but significantly ( P <0.05). At mid-lactation (7–12 months postpartum, n =17), milk averaged 73.0±1.0% water, 12.8±1.0% fat, 8.9±0.5% crude protein, 1.0±0.1% sugar, 1.76±0.09 kcal g⁻¹ (=7.25 kJ g⁻¹) and 30.3±1.3% protein:energy per cent. This protein:energy per cent was surprisingly high compared with other cetaceans and in relation to the growth rates of calves. Milk progesterone indicated that dolphins ovulated and conceived between 413 and 673 days postpartum, following an increase in milk energy density. The significance of these observed compositional changes to calf nutrition will depend on the amounts of milk produced at different stages of lactation, and how milk composition and yield are influenced by sampling procedure, maternal diet and maternal condition, none of which are known.     

Variation in the Composition of Milk of Asian Elephants (Elephas maximus) Throughout Lactation

We investigated milk nutrient composition from three Asian elephant cows over the first 3 years of lactation, including two consecutive lactations in one cow. Body mass gain is presented for three calves during the first year. Milk samples (n = 74) were analyzed for dry matter (DM), fat, crude protein (CP), sugar, ash, calcium (Ca), phosphorus (P), and potassium (K); gross energy (GE) was calculated. Concentrations of most nutrients changed over lactation: DM, fat, CP, Ca, P, and GE were positively correlated to calf age; sugar was negatively correlated to calf age. GE doubled between birth (1 kcal/g) and 2 years of age (2 kcal/g). After accounting for calf age, GE, fat, Ca, and P concentrations differed among the cows. Milk composition also differed between two lactations from the same cow. When milk nutrients were expressed on a mg per kcal basis, the pattern changes: CP, Ca, and P remained relatively constant over lactation on a per energy basis. Calf mass quadrupled over the first year of life; mass gain was linear at 0.9 kg/day. Asian elephant milk composition is variable, both across lactations and between cows, complicating efforts to determine representative values for comparative studies and for the formulation of elephant milk formulas. The fact that CP, Ca, and P were all relatively constant when expressed on a per energy basis may be of biological significance. The increase in nutrient density over lactation undoubtedly limits maternal water loss, reducing the volume of milk necessary to support the calf. Zoo Biol 32:291–298, 2013. © 2012 Wiley Periodicals, Inc.     

Macronutrient composition of longitudinal milk samples from captive aardvarks (Orycteropus afer)

Hand‐rearing and assisted‐rearing aardvarks in captivity has become commonplace and has led to success in breeding the species. However, the macronutrient content of aardvark milk past 1 month of age is unknown. A better understanding of aardvark milk composition would enhance captive management efforts. Here, we assayed milk samples from two captive individuals from 2 to 114 days postpartum (N = 21) for dry matter, fat, crude protein, total sugar, ash (total minerals), calcium (Ca), phosphorus (P), and gross energy. The body weight of one calf was measured from birth to weaning. Milk macronutrient composition was compared to that of other Afrotherian species and Xenarthra species with similar diets. Average protein, fat, and sugar concentrations of aardvark milk across lactation were 12.3%, 13.6%, and 2.5%, respectively. Ash averaged 1.9%, with Ca (0.50%) and P (0.35%) accounting for about 45% of total minerals. All measured nutrients increased over lactation except sugar, which decreased. Aardvark milk is high in energy (2.12 kcal/g) mostly derived from fat and protein and little energy from sugar. Calf growth was linear (r² = 0.995) with a mean gain of 159 g/day, achieving almost 30% of adult weight at weaning. Within Afrotheria, aardvark milk is higher in fat and protein and lower in sugar than elephant milk and more closely resembles the milk of its fellow insectivore, the elephant shrew. Aardvark milk is also similar in composition to milk of insectivorous Xenarthra species (nine‐banded armadillo and giant anteater). Aardvark milk composition is consistent with the species’ high‐protein diet, fast growth, and nursing pattern.     

Characterization of milk composition in narrow‐ridged finless porpoises (Neophocaena asiaeorientalis) at different lactation stages

Lactation plays a vital role in reproductive success, evolving with ecological adaptations of mammalian life histories. Knowledge of the lactation process in odontocetes is scarce and limited to a handful of species. We investigated the changes in milk composition across different lactation stages, including prepartum mammary secretion and early, mid‐ and late lactation of narrow‐ridged finless porpoises. Prepartum mammary secretion was greenish and characterized by high protein, low sugar, and negligible fat contents. In contrast, milk fat became predominant over protein and sugar contents throughout all lactation stages. At the early lactation stage, the contents of all milk constituents except water varied greatly. At the mid‐late lactation stage, the gross milk composition was relatively stable and was composed of, on average, 67.34% ± 4.13% water, 9.96% ± 0.75% protein, 21.40% ± 3.24% fat, and 1.72% ± 0.73% sugar. Our findings provide new insight into milk composition at different lactation stages in the narrow‐ridged finless porpoise.     

Southern elephant seal (Mirounga leonina) II. Studies of milk protein fractions by gel electrophoresis

 Milk protein fractions during various stages of lactation in the southern elephant seal Mirounga leonina were analysed. Twelve milk samples were taken from ten females throughout the lactation period during 1990 and 1991 at Stranger Point, King George Island, South Shetland Islands. Milk samples were subjected to polyacrylamide gel electrophoresis (PAGE). Samples from different days of lactation gave similar qualitative electrophoretic patterns. True protein content was significantly higher (P<0.05) at the beginning of lactation, and then remained constant until weaning. Caseins and whey proteins each consisted of several protein entities (four and five distinct bands respectively). Casein constituted only about 30% of the protein nitrogen, the remaining 70% being derived from whey proteins. There was some variation in concentration of casein and whey proteins as a function of time (P<0.0.5). Received: 28 July 1993/Accepted: 25 July 1995     

Maternal diet,rather than obesity itself,has a main influence on milk triacylglycerol profile in dietary obese rats

Triacylglycerols (TG) in milk derive from different sources, and their composition may be influenced by both maternal diet and obesity. We used two rat models to ascertain potential changes in TG composition in milk associated to maternal intake of an obesogenic diet during lactation and to distinguish them from the effects attributable to maternal adiposity. Milk samples were obtained from dams fed a cafeteria diet during lactation (CAF) and from dams made obese by cafeteria diet feeding, with dietary normalization before gestation (PCaf). Levels of specific TG species in milk collected at different time points of lactation were determined by shotgun lipidomics. CAF and PCaf dams presented a greater adiposity than their respective controls. The principal component analysis of TG peaks showed a clear separation between milk from CAF dams and milk from control and Pcaf dams, already evident at 5 days of lactation. Milk from CAF dams was enriched with TG species with greater number of carbons and double bonds and reduced in TG with lower number of carbons. TG composition of milk from Pcaf dams was similar to controls, although specific differences were observed at day 5 of lactation. Thus, the intake of a cafeteria diet during lactation, rather than maternal adiposity, alters milk composition. This effect is avoided with dietary normalization before gestation, although the remaining fat reserves may also influence TG composition at initial stages of lactation. Therefore, normalization of maternal diet prior to pregnancy should be considered as a strategy for achieving optimal milk composition.     

Energy intake and milk production in mink (MUSTELA vison)‐effect of litter size

Energy intake and milk production were measured in 12 mink dams raising litters of 3, 6 and 9 kits one to four weeks post partum by means of balance experiments and measurements of milk intake of the kits by the water isotope dilution technique. The dams were fed ad libitum on a conventional wet mink diet (DM: 323 g/kg; CP: 173 g/kg; ME: 4.4 MJ/kg). Milk samples collected from dams with corresponding litter sizes and lactation weeks, and body composition of kits nursed by these dams, were analysed for content of DM, ash, N and fat. The ME and drinking water consumption were higher in dams nursing 9 kits than in dams nursing 3 kits. The N and water balances as well as the live weight of dams were not affected by litter size. Daily milk production was higher in dams nursing 9 kits than in dams nursing 3 kits. The DM, N and fat content of the milk increased during lactation, but were not affected by litter size. Individual kit live weight was higher in litters of 3 than in litters of 6 and 9 kits four weeks post partum. The DM and fat content of the kits were lowest in kits from litters of 9 kits, whereas these kits had the highest protein content. Daily ME for maintenance of kits and the efficiency of utilisation of ME in milk for body gain were estimated to 356 kJ/kg^0.75, k_p ≈0.53 and k_f ≈0.71, respectively. In conclusion, daily milk production increased with increasing litter size, but not in proportion to the number of kits, indicating that milk production limits the growth rate of the young. In the fourth week of lactation, milk production was not different between dams nursing 6 or 9 kits, indicating a maximum capacity.     

Effects of dietary energy intake during gestation and lactation on milk yield and composition of first,second and fourth parity sows

Abstract

In order to determine the effects of a varied level of dietary energy intake during pregnancy and lactation on milk yield and composition, first, second and fourth parity sows (Large White × German Landrace) were provided with energy at a level of either: (i) 100% of ME requirement (MEreq) during pregnancy and lactation, (ii) 120% MEreq during pregnancy and 80% during lactation, and (iii) 80% MEreq during pregnancy and 120% during lactation. In spite of equal target levels feed analysis revealed that gestating first parity sows with 120/80 treatment combination and lactating sows of 80/120 treatment combination received 25, and 11 – 17% more digestible N than in the respective 100/100 treatment combination. Irrespective of this 120/80 sows responded with the highest milk DM, fat, and energy contents, and the lowest lactose concentrations whereas protein levels where not affected, irrespective of parity (p < 0.05). Milk yield of sows in 1st and 4th lactation was 85 and 106% of that in 2nd lactation, respectively. Average milk composition was 18.1% DM, 4.9% protein, 6.8% fat, 5.6% lactose, and 0.8% ash. Milk composition changes ceased at day 7 of lactation with a reduction of milk GE and protein, and an increase of lactose content. Concentrations of threonine, arginine, valine, leucine, tyrosine, phenylalanine, cystine, and tryptophan, as well as stearic, oleic, and linoleic acid were higher in colostrum than in milk at later lactation stages. In contrast, laurine, myristic, palmitic, and palmitoleic acids were lower concentrated in colostrum. In conclusion, these results illustrate the importance of body reserve mobilization for milk production in sows and indicate that low energy supply during gestation cannot be compensated by higher energy supply during lactation.     

The milk of the fur-seal, Arctocephalus tropicatis gazella; in particular the composition of the aqueous phase

Milk and plasma samples from the Southern fur-seal (Arctocephalus tropicalis gazella) , collected from animals shot between 0 and 26 days post-partum , have been analysed. The aqueous phase of milk (ultracentrifugate) was hyperosmotic to plasma by about 35 m-osmol/kg water but contained no lactose. The osmolality of milk could be accounted for by the ionic content, of which sodium and potassium (in approximately equal amounts) were the main cations. The fat content of the milk was very high (approximately 40%), except in very early lactation (0 and 1 day post-partum). The concentrations of chloride, calcium, magnesium, phosphate and citrate in the aqueous phase are given. Major differences between the composition of fur-seal milk and that of other mammals are pointed out, and discussed in relation to water conservation, energy and fat supply, and prolonged milk storage in this snecies.     

Milk production and composition in reindeer (Rangifer tarandus): effect of lactational stage

Milk yield and composition of major milk constituents were measured in captive, nursing reindeer. Registration of milk production was performed during two successive lactations (2001 and 2002). The milk yield was significantly affected by week of lactation (P<0.001) and by individual (P<0.001). The lactation curve had an asymmetrical peak 3 weeks postpartum and the milk yield at peak lactation was 983 g/day (range 595-1239). The length of lactation varied from 24 to 26 weeks and average total milk production was 99.5 kg. From peak lactation the milk production decreased linearly (P<0.001) until milk production was terminated. Mean values for content of major milk constituents were 15.5% fat, 9.9% protein and 2.5% lactose. The content of fat and protein increased markedly with the lactation stage (P<0.001), while lactose showed a slight decrease (P<0.001). The milk composition was significantly affected by stage of lactation (P<0.001). There was a marginally significant decrease in protein:fat ratio (P=0.06) as protein was substituted by fat with stage of lactation. The caloric value of the milk averaged 8.7 kJ/g and increased significantly with the stage of lactation (P<0.001). The overall increase in milk gross energy content during lactation was 67.6%. The energy output averaged 7996 kJ/day at peak lactation and decreased significantly during the course of lactation (P=0.002).     

Gelsolin expression in sheep milk somatic cells during lactation

The identification of genes involved in phenotypes related to milk quality is important for both economic and health aspects in livestock production. The aim of this study was to assess the level of gelsolin gene expression in two breeds of dairy sheep – Sarda and Gentile – with pronounced differences in quantitative and qualitative milk traits. Gelsolin, a type of actin-modulating proteins is involved in the processes of actin remodeling during cell growth and apoptosis; therefore a role of this protein in mammary changes during lactation was here hypothesized. Individual milk samples were collected three times during lactation from 26 ewes of the two breeds. The differential gene expression of gelsolin in the two breeds and the three lactation times was estimated by quantitative PCR on RNA extracted from milk somatic cells. Correlations of gelsolin gene expression with milk yield and quality and days of lactation were also estimated. The results showed that gelsolin gene expression was significantly higher in the Sarda compared to the Gentile at each lactation stage, in agreement with the longer lactation duration and the higher daily milk yield of the first breed. Significant correlations of gelsolin gene expression were found with milk fat content in Sarda breed (−0.46, P<0.05). Gelsolin expression analysis confirmed the link between gelsolin gene function and milk fat content of sheep.     

Milk secretion in the rat: progressive changes in milk composition during lactation and weaning and the effect of diet.

1. Progressive changes in the composition of milk from rats has been studied from day 0 to 20 of lactation and for 3 days following separation of the dams and pups at day 20 post partum. 2. The changes in concentration of Na, K and lactose suggested that secretion both prepartum and following weaning occurred by a paracellular mechanism whereas a transcellular pathway existed during established lactation. 3. The concentration of total protein and casein increased gradually throughout lactation. In contrast, the concentration of serum albumin increased and transferrin decreased markedly during early lactation. The fat content of milk declined 3-fold within 5 days of birth but the concentration of Ca, Mg and inorganic P increased. The concentration of each of these milk constituents remained constant during established lactation. 4. Following weaning the pronounced decline in lactose, K and inorganic P was negatively correlated with an increase in all other milk constituents except fat. 5. Rats fed a low energy diet produced milk with a lower fat content but with an unaltered concentration of protein and carbohydrate. The growth rate of these litters was similar for the first 5 days of lactation when compared to litters from dams fed a high energy diet. The growth rate of litters thereafter and following weaning was greater for rats fed a high energy diet.     

Does the milk of callitrichid monkeys differ from that of larger anthropoids?

The generalization that anthropoid primates produce dilute milks that are low in protein and energy is based primarily on data from large monkeys of the families Cebidae and Cercopithecidae, as well as humans. The marmosets and tamarins (Callitrichidae) are not only much smaller in body size, but also typically raise multiple offspring during a relatively brief lactation. We hypothesized that selection for small body size and high reproductive rate might favor secretion of milk of higher energy and protein concentrations. To test this hypothesis, 46 milk samples collected from 10 common marmosets (Callithrix jacchus, ca. 350 g) were assayed for dry matter (DM), crude protein (CP), fat, and sugar; and gross energy (GE) was calculated from these constituents. We also assayed five samples collected from three golden lion tamarins (Leontopithecus rosalia, ca. 700 g) and six samples collected from a single pygmy marmoset (Cebuella pygmaea, ca. 150 g) over two lactation periods. All samples were collected between days 10 and 57 post partum, representing mid lactation for these species. The milks of these three species were similar, containing 14.0%, 16.1%, and 13.7% DM; 2.7%, 2.6%, and 2.9% CP; 3.6%, 5.2%, and 3.7% fat; 7.4%, 7.2%, and 7.8% sugar; and 0.76, 0.90, and 0.82 kcal/g for common marmosets, golden lion tamarins, and the pygmy marmoset, respectively. These species produced milks with energy values that were within the range reported for large anthropoids, albeit with slightly higher protein concentration. However, milk composition did vary substantially among individual common marmoset females, especially in the proportion of milk energy derived from fat. In contrast, CP as expressed as a percent of GE was remarkably constant among common marmoset females. Callitrichid milk appeared to be similar to that of larger anthropoid primates in GE, but was higher in CP and in the proportion of GE from CP. However, the small sample sizes for the golden lion tamarin and the pygmy marmoset, and the wide variation in milk composition found among common marmoset females cautions against definitively characterizing the milks of callitrichids from these data.     

Alterations in the milk yield and composition during different stages of lactation cycle in elite and non-elite Karan-Fries cross-bred cows (Holstein Fresian x Tharparkar)

Alterations in the milk constituents throughout the lactation cycle have been studied in 20 Karan-Fries cross-bred cows (Holstein Fresian x Tharparkar) divided into two groups, viz. elite (n = 10) and non-elite (n = 10). About 100 ml of composite milk samples (from all the quarters) were collected through hand milking in sterile tubes from the day of calving at 15-day interval till 300 days of lactation cycle. Different milk constituents (viz. fat, protein, lactose, Solid not fat) were estimated by automatic milk analyzer. The daily milk yield varied significantly (p < 0.01) between group and different days of lactation cycle. Milk fat percentage of milk did not differ significantly between groups and different days of lactation cycle. Milk protein and lactose percentage did not differ significantly between groups but differed significantly (p < 0.001) between days. Solid not fat (SNF) percentage of milk was significantly (p < 0.01) higher in elite cows compared to non-elite cows and also varied significantly (p < 0.05) during different days of lactation cycle. In both the group of cows, milk protein, fat, and SNF percentage was highest and lactose percentage was lowest up to 1st week of lactation cycle and rapidly declined thereafter due to the transition of the colostrum into milk. After 2nd week of lactation, all the milk constituents under study were almost remained unaltered till the end of the cycle. The above stated investigation not only substantiates the already existing information of lactation stage-specific alteration in milk constituents but also depict the exact point of transition of these constituents during the lactation cycle so the nutritional and managemental interventions could be carried out in proper time.     

Relationship between milk composition and pup's growth in mice

The effect of lactation numbers (first, second, and third) on milk composition and the relationship between milk composition and pup's growth were examined in four strains of mice (SHN, SLN, C3H/He, and GR/A). Although the percentage of milk fat and moisture increased and decreased, respectively, through the first to third lactations in C3H/He mice, no differences in milk composition among lactation numbers were observed in the other mouse strains. In pooled data derived from the four strains of mice, there was no significant correlation between milk composition and body weight or growth rate of pups on Day 12 or 20 of the first lactation. In contrast, the amount of milk fat and moisture was the positively and negatively, respectively, correlated with the pup's growth parameters during the second and third lactations. This was reflected by a lower fat content and a higher moisture content in the milk of GR/A mice, especially evident during the second and third lactations and reduced pup's growth, when compared with the other mouse strains. Milk lactose and ash content was not correlated with pup's growth parameters. The results of our study stress the importance of milk fat for the growth of mouse pups.     

Asian elephant (Elephas maximus) milk composition during the first 280 days of lactation

Milk samples (n = 10) taken during the first 280 days of lactation from one Asian elephant were examined for nutrient composition including total solids, protein, fat, ash, α-tocopherol, and retinol levels. Total solids averaged 19.7 ± 2.7% SD (range 15.0–23.3). Percent protein remained fairly stable throughout this portion of lactation and averaged 3.4 ± 0.3% (range 3.0–.4.0). Ash content averaged 0.54 ± 0.03%. Milk fat and fat soluble vitamin levels varied considerably with a suggestion of a cyclic pattern. Fat content of milk averaged 7.6 ± 2.6% (range 3.9–.12.1); α-tocopheral levels averaged 0.33 ± 0.12 μg/ml; and retinol levels averaged 0.46 ± 0.1 μg/ml. © 1994 Wiley-Liss, Inc.     

Variation in body composition of female big brown bats (Eptesicus fuscus) during lactation

Most small mammals support the nutritional requirements of milk production by increasing food intake. However, when nutrient intake is low, maternal body reserves may be mobilized to maintain adequate milk output. We examined patterns of body composition, including dry matter, fat, protein, and mineral content in big brown bats, Eptesicus fuscus, during lactation. Concentrations of fat and phosphorus were markedly lower in lactating mothers during week three of lactation than during the first two weeks, but these constituents rebounded to previous levels in the fourth and fifth week. Rapid recovery from fat depletion suggests that females are able to adjust to changes in demands for energy. The decrease in phosphorus during mid-lactation suggests bone demineralization, but an interspecific comparison of adult concentrations of minerals prevalent in bone suggests that mineral concentrations may never reach critically low levels in reproductively active females.     

An Odor Timer in Milk? Synchrony in the Odor of Milk Effluvium and Neonatal Chemosensation in the Mouse

Mammalian newborns exhibit avid responsiveness to odor compounds emanating from conspecific milk. Milk is however developmentally heterogeneous in composition as a function of both evolved constraints and offspring demand. The present study aimed to verify whether milk odor attractivity for neonates is equally distributed along lactation in Mus musculus (Balb-c strain). Therefore, we exposed pups varying in age to milk samples collected from females in different lactational stages. The pups were assayed at postnatal days 2 (P2), 6 (P6) and 15 (P15) in a series of paired-choice tests opposing either murine milk and a blank (water), or two samples of milk collected in different stages of lactation [lactation days 2 (L2), 6 (L6), and 15 L15)]. Pups of any age were able to detect, and were attracted to, the odor of the different milk. When milk from different lactational stages were simultaneously presented, P2 pups oriented for a similar duration to the odors of L2 and of L6 milk, but significantly less to the odor of L15 milk. Next, P6 pups roamed equivalently over L2 and L6 milk odors, but still less over the odor of L15 milk. Finally, P15 pups explored as much L15 milk odor as the odors of both L2 and L6 milk. This developmental shift in milk attractivity is discussed in terms of changing chemosensory properties of milk and of shifting chemosensory abilities/experience of pups.     

Jennet milk production during the lactation in a Sicilian farming system

In Italy, the interest for jennet milk production has recently developed. An 18-month-long experiment was carried out on a jennet farm near Milo (CT), where 24 jennets, which derived from the Ragusana breed, were tested for milk yield and composition over an entire lactation period. The jennets were fed with hay and concentrate in a large paddock. From the 28th post-foaling day to the end of the lactation, the jennets were machine-milked twice a day with an in-between milking interval of 5 h. The milk amount from each jennet was recorded every 3 weeks and individual samples were collected and analyzed for fat, protein, casein, non-proteic nitrogen, lactose and somatic cell count. This study showed that jennets at Sicilian latitudes are not seasonal polyestrous. The daily milk yield, the length of lactation and the milk characteristics varied depending on the foaling season. The total average milk production was 490 ± 36 kg in 295 ± 12 post-foaling days, considering two milking records per day. During the lactation, milk yield decreased constantly from 1.98 to 1.28 kg/jennet per day. When looking at the jennet milk quality during lactation, the percentage of fat and protein decreased, while the lactose percentage increased, according to a tendency apparently unique for equines when compared to the ruminants. When looking at the productive season, spring generally gave the best qualitative and quantitative results. Based on these results, jennet milk yield and quality could be improved; furthermore, jennet milk production may turn out to be a profitable business.     

Changes in the yield, and carbohydrate, lipid and protein content of milk during lactation in the rat

The milk yield and composition was studied during the first three lactations of a group of rats. Milk yield increased steadily throughout the three lactations, but was somewhat lower during the first than subsequent lactations. Protein concentration was similar during all three lactations and varied little with stage of lactation. In contrast the lactose concentration, which was reasonably constant for the first 8 days post partum, increased thereafter two-fold by the end of the period studied in all three lactations. However, the N-acetyl-neuraminyl lactose concentration showed somewhat reciprocal changes. Considerable variations in the triacylglycerol concentration was found during the first lactation but few changes were observed during subsequent lactations. The free fatty acid concentration was at all times low and showed no significant changes during or between lactations. At most stages of lactation in raw milk, the major fatty acids are palmitate, oleate and linoleate. However, as lactation progresses there is an increase in the proportion of medium-chain saturated fatty acids and a corresponding decrease in the proportion of long chain unsaturated fatty acids in milk fat. Clearly the composition of milk is not invariable but changes both during and between lactations. Such changes may be expected to have some influence on the metabolism of the offspring.