SUMMARYWe compared within and between breastfeed changes in milk fat to short-term rates of milk synthesis and degree of breast emptying (measured using the Computerized Breast Measurement system) over two 24 h periods for five lactating mothers. The fat content (f) of fore and hind milk samples increased more steeply as the breast was progressively emptied by the infant (degree of emptying, d, range -1;f = 21 59 + 9 38d+ 70-99d2; P < 0 0001; r2 = 0-68; n = 154). For the nine individual breasts, between 41-95% of the variance of the fat content of milk was explained by degree of breast emptying. We argue that this relationship explains differences in the circadian rhythm of the fat content of milk and allows the accurate calculation of the average fat content of milk consumed by infants (37-66 g/l for the nine individual breasts). The fatty acid composition of the fore and hind milk samples was determined for four of the mothers. We observed within and between breastfeed variability in the relative proportions of the seven major fatty acids of milk fat and these changes are discussed with reference to the control of fat synthesis in the human mammary gland.

The concentration of total corticosteroids, corticosteroid binding globulin (CBG) and progesterone were determined in maternal and foetal/neonatal plasma from rats on days 5, 4, 3, 2, 1 before birth and days 0, 1, and 4 after birth. In addition free corticosteroids and adrenal weight/unit body weight were measured on the foetuses/neonates and the foetuses, respectively. Although the concentration of maternal total corticosteroids and CBG ranged from 53.0 ± 12.5 to 31.0 ± 13.1 μg/100 ml (x̄ ± sem) and 26.6 ± 2.2 to 45.1 ± 0.9 μg corticosteroid bound/100 ml plasma, respectively, the changes in the concentration of these constituents were not related to the initiation of either parturition or lactation. The concentration of total corticosteroids in foetal plasma increased significanly (P < 0.05) from day 5 (14.6 μg/100 ml) to reach peak concentrations (44.9 μg/100 ml) on day 3 before birth and then decreased to low levels (7.7 μg/100 ml) at birth. The pattern of change in foetal adrenal weight/unit body weight closely followed the pattern of change in the concentration of total corticosteroids in foetal plasma during the last 5 days of gestation. There was a significant (P < 0.05) daily decrease in the concentration of CBG in the foetuses from 4 days before until 1 day before birth, which resulted in a significant (P < 0.01) increase from 3.08 to 5.94 μg/100 ml of free corticosteroids in the foetal plasma between day 2 and day 1 before birth, respectively. This peak corresponded with a significant fall (P < 0.02) in the maternal progesterone (measured by protein binding assay) from 2.57 μg/100 ml to 0.62 μg/100 ml between day 2 and day 1 before birth. Foetal progesterone (measured by radioimmunoassay) showed the same changes as maternal progesterone but was between 25–50 % of that in maternal plasma. These findings suggest that the changes in foetal free corticosteroids and maternal progesterone are important in the initiation of parturition in the rat.

Summary Studies were undertaken to determine the progressive changes and relationships between the major constituents in the mammary secretion of breast feeding and non‐breast feeding women during the initiation of lactation. The concentration of metabolites (lactose, glucose and urea), electrolytes and proteins (total protein, α‐lactalbumin, lactoferrin, albumin. IgA, IgG and IgM) were measured in small samples of mammary secretion (0·5‐5·0 ml). Colostrum during late pregnancy contained higher concentrations of proteins and lower concentrations of metabolites than milk in established lactation. Of the electrolytes, the concentrations of sodium, chloride and magnesium were higher, whereas potassium and calcium were lower in colostrum than in milk. The osmolality of the secretion remained relatively constant over the pre‐partum and post‐partum period. These findings showed that the initiation of lactation developed in two phases, first a limited secretion of milk constituents in late pregnancy and then true induction of lactation (lactogenesis) 32‐40 h after delivery. The changes in the mammary secretion of non‐breast feeding women during the first 3 days post‐partum were similar to those observed in breast feeding women but reversed abruptly during the next 6 days, indicating the onset of mammary involution. This finding demonstrated that breast feeding is not a major factor for the initiation of lactation but is essential for the continuation of full lactation.

SUMMARY Mammary secretion (1·2–2·0 ml) was collected from the milk sinus of each mammary gland (quarter) of two pregnant heifers and eight pregnant cows (dry period 49–229 days), first at weekly intervals from 40 days before parturition, then with increasing frequency as parturition approached. The progressive changes in the concentration of lactose, glucose, casein, non-casein protein and fat in the mammary secretion were determined. Calves were separated from the cows immediately after birth and the yield and composition of milk from individual quarters were determined for 5 days after parturition. Two quarters (milked quarters) of each of a further five cows were milked throughout pregnancy, while the other two quarters (unmilked quarters) of each cow were allowed to involute (dry off) 63– 104 days before parturition. Small samples (5–10 ml) of secretion were collected from the unmilked quarters, first at weekly intervals after drying off and then every second day from 10 to 15 days before parturition. On the days that the unmilked quarters were sampled, corresponding composite milk samples were collected from the milked quarters and the progressive changes in the yield of milk, lactose and fat were determined. The changes in the concentration of lactose and glucose in the mammary secretion, during drying off in late lactation, were determined in an additional five cows. In most cows allowed a usual dry period, and in the unmilked quarters of cows in which two quarters were milked throughout pregnancy, the concentration of lactose gradually increased from about 8 to 12 days before parturition to reach levels of about half those found in normal milk just before parturition. However, in some cows the concentration of lactose was low until 1–2 days before parturition and then increased abruptly, whereas in others the concentration of lactose increased slowly from as early as 32 days before parturition. A further rapid increase in the concentration of lactose in the mammary secretion occurred between 0 and 4 days before parturition in all cows. This latter increase was accompanied by an abrupt increase in the yield of milk, lactose and fat in the milked quarters of the cows in which two quarters were milked throughout pregnancy. These observations suggest that the initiation of lactation in the cow develops in two phases, a limited secretion of milk constituents occurs in late pregnancy and then 0–4 days before parturition copious secretion (lactogenesis) occurs.

SUMMARYThe feasibility of using sequential breast volume measurements as a method of studying shortterm rates of milk synthesis in women has been established. We have developed a rapid Computerized Breast Measurement system for the determination of breast volume, based upon the Shape C Measurement System. A circle encompassing all the breast tissue is drawn in black face paint on the subject's skin. Six patterns of sixty-four horizontal light stripes are projected onto the breast and chest wall surface. A CCD camera relays video images to a computer, which produces a model of the chest by active triangulation. The volume of the breast and the chest wall segment enclosed by the circle is then calculated. The precision of the method was dependent upon the subject repositioning carefully. The coefficient of variation of replicate measurements was 166%. The accuracy of the method was established by comparing the change in breast volume before and after a breast-feed with the amount of milk removed by the infant as determined by test weighing. There was a close relationship between the removal of milk by the infant (x) and the change in breast volume (j), (r = 0 93, n = 73, t = I lOx -325). The rates of milk synthesis between breast-feeds, for six women determined on one to eight occasions, varied from II to 58 ml/h. The results show that the amount of milk available in the breast is not necessarily an important determinant of the amount of milk removed by the infant at a breast-feed.