Lipids in Milk and the First Steps
in Their
Digestion
Margit
Hamosh,
PhD,
Joel
Bitman,
PhD,
D. Larry
Wood,
MS,
P. Hamosh,
MD, and
N. R. Mehta,
MD
From the Department of Pediatrics and Department of Physiology and Biophysics, Georgetown University Medical Center, Washington, DC, and Agricultural Research Center, Milk Secretion and Mastitis Laboratory, Beltsville, Maryland
ABSTRACT. Human milk contains 3.0% to 4.5% fat. The fat is contained within membrane-enclosed milk fat
glob-ules. The core of the globules consists of triglycerides (98% to 99% of total milk fat) whereas the globule membrane (which originates from the mammary secre-tory cell’s Golgi and cell membranes) is composed mainly of phospholipids, cholesterol, and proteins. Milk fat con-tent and composition change during lactation. Whereas the triglyceride level rises, the phospholipid and choles-terol concentrations decrease during the transition from
colostrum to mature milk, resulting in an increase in the size ofthe milk fat globules. Digestion of milk fat depends on the consecutive action of several lipases. The first step is the partial hydrolysis of the milk fat globule core by lingual and gastric lipases in the stomach. Hydrolysis continues in the duodenum, where the bile salt-stimu-lated lipase of human milk and pancreatic lipase complete the process initiated in the stomach. Pediatrics 1984;
75(suppl):146-150; triglyceride, fat globule, gastric lipase,
lingual lipase.
Fat is the main energy source of the newborn infant. In addition to providing 40% to 50% of the total calories in human milk or formula, fats are essential to normal development because they pro-vide fatty acids necessary for brain development, are an integral part of all cell membranes, and are the sole vehicle for fat-soluble vitamin and hor-mones in milk. Furthermore, these energy-rich lip-ids can be stored in the body in nearly unlimited amounts in contrast to the limited storage capacity for carbohydrates and proteins. Before birth, glu-cose is the major energy source, whereas the fetal requirements for fatty acids are supplied mainly as
Read before the Workshop on Current Issues in Feeding the Normal Infant, Palm Springs, CA, April 8-11, 1984.
Reprint requests to (M.H.) Department of Pediatrics, George-town University Medical Center, Washington, DC 20007. PEDIATRICS (ISSN 0031 4005). Copyright © 1985 by the American Academy of Pediatrics.
free fatty acids from the maternal circulation. After birth, fat is supplied chiefly in the form of milk triglycerides.
Mature human milk has a fat content of 3.5% to 4.5%. The fat in milk is contained within mem-brane-enclosed milk fat globules. The core of the globules consists of triglycerides (98% to 99% of total milk fat), whereas the globule membrane is composed mainly of phospholipids, cholesterol, and proteins (Table 1). Milk fat content and composi-tion change during lactation. These changes are most pronounced during early lactation (colostrum, secreted at one to five days postpartum, and tran-sitional milk, days 6 to 15) and again during wean-ing. Mature milk (day 16 to the weaning period) maintains, however, a constant fat composition.
PHYSICAL PROPERTIES OF THE MILK FAT
GLOBULE
The milk fat globule has been studied most ex-tensively in bovine milk”2 and in the milk of exper-imental animals.3 The core of the globules is com-posed mainly of triglycerides. The latter contain mostly long-chain fatty acids (90% of fatty acids in mature human milk) derived from the circulation. These long-chain fatty acids reach the mammary epithelial cells after their release from lipoprotein-triglyceride at the capillary endothelium by the enzyme lipoprotein lipase.4 Short-chain fatty acids (C4 to C6) in bovine milk and medium-chain fatty acids (C8 to C,4) in human milk are synthesized de novo within the mammary gland by the fatty acid synthetase complex.5 Phospholipids are synthe-sized de novo within the mammary gland, whereas cholesterol originates both from de novo synthesis and from the circulation. The fatty acids are rees-terified to triglycerides within the endoplasmic re-ticulum.
con-tamed within endoplasmic reticulum membranes and migrate from the basal to the apical region of the cell prior to their discharge into the alveolar lumen. The droplets grow in size as they migrate to the apical cell membrane, and prior to secretion into the lumen, are enveloped by a membrane de-rived from the Golgi vesicle membranes and the apical cell membrane. These membranes provide the phospholipids and cholesterol of milk, as well as the proteins, enzymes and glycoproteins associ-ated with the milk fat globules.
The lactating mammary cells of several species contain two populations of fat droplets: relatively smaller ones (<1.5 jim) occurring throughout the cells and larger ones (>1.5 m) found mainly in the apical (secretory) region of the cell. These larger droplets, which account for almost all the fat in milk, appear to be in contact with the plasma membrane during the major part of their growth (three to four hours) (Table 2). The size and dis-tribution of fat globules in human colostrum and milk indicate the presence of three subpopulations. The diameter of the globules increases from an average of 1.5 im in colostrum to 4.0 sm in mature milk. The latter contain almost all the milk fat but amount to only 10% to 30% of total globules. In mature milk, globules less than 1 m contain only a few percent of total fat but amount to 70% to 90% of the total number of globules (Table 3). The milk fat globule membrane comprises about 2% of the total weight of milk fat. Its thickness, which varies in different areas, is in the range of 5 to 50 nm.
LIPID CLASSES
There are few studies in which reliable tech-niques have been applied to sampling, extraction, separation, and analysis of lipid classes in human milk.7’8 Most of the studies have been reviewed in detail recently.9”#{176}
Total fat content increases gradually from cobs-trum (2.0%) through transitional (2.5% to 3.0%) to mature milk (3.5% to 45%)8 Cholesterol content
is highest in colostrum and decreases to lower levels in transitional and mature milks; it is distributed as 87% free cholesterol and 13% cholesteryl esters (Table 4). Phospholipids show a similar decrease from high levels in cobostrum to lower levels in mature milk (Table 4). The decline in phospholipid and cholesterol levels is consistent with an increase in the fat globule size6 and thus a decrease in the amount of membrane lipids (containing about 60% of milk triglyceride and 85% of milk cholesterol).
The higher cell content (3 x 106/mL) might also contlibute to the higher phospholipid and choles-terol levels of cobostrum than of mature milk, which
TABLE 1. Composition of Human Milk Fat* Percentaget Glycerides (3.0-4.5 g/dL)
Triglycerides 98.70%
Diglycerides 0.01%
Monoglycerides 0
Free fatty acids 0.08%
Cholesterol (10-15 mg/dL) Phospholipids (15-20 mg/dL)
Sphingomyelin 37.00%
Phosphatidylcholine 28.00%
Phosphatidylserine 9.00% Phosphatidylinositol 6.00% Phosphatidylethanolamine 19.00% * Data from Bitman et al.8 Mature milk from mothers of term infants. Glycerides are the major component of the
core of milk fat globules; cholesterol and phospholipids
are the major components of the milk fat globule mem-brane.
t Percent in lipid class (glycerides and phospholipids,
respectively).
TABLE 2. Formation of Milk Fat Gbobules*
Cell Position Rabbit Cow Cat Rat
Basal 0.8 0.8 0.6 0.7
Apical 1.4 1.8 1.6 1.3
Protruding #{189} 1.9 2.3 2.0 2.4
Protruding >#{189} 3.2 2.7 2.5 3.5
Volume increase (apical 11.9 3.4 3.8 19.5 to >#{189})
* Data from Stemberger and Patton.3 Position and size (mean diameter in micrometers) of fat globules in 200 to
350 cells from each species.
TABLE 3. Milk Fat Globules in Human Milk* Colostrum Mature Milk Fat content (%) 2.6 3.3
No. of globules/mL of 6 x 1010 1.1 x 106
milk ‘
Av diameter 1.8 4.0
Surface area of fat/mL 0.097 0.054 of milk (m2)
* Data adapted from Ruegg and Blanc.6
TABLE 4. Changes ing Lactation*
in Huma n Milk Compositio n
Dur-Lipid Class Lactation Day
3 7 21 42 84
Total fat (g/dL) Triglycerides (%)t Cholesterol (%) Phospholipid (%) 2.04 97.60 1.30 1.10
2.89 3.45 3.19
98.50 98.70 98.90
0.70 0.50 0.50 0.80 0.80 0.60
4.87
99.00
0.40 0.60 * Data from Bitman et al.8
t Lipid class as percentage of total lipid. Data are means of 8 to 41 milk specimens at the different lactation times.
FATTY ACID COMPOSITION TABLE 5. Fatty Acid Composition of Human Milk*
More than 98% of the fat in human milk is
present in 1 1 major fatty acids from C100 to C20:4 (Table 5). Medium-chain fatty acids amount to 10% of the total fat in mature milk of mothers of term infants, but contribute 17% of total fatty acids in milk produced by mothers of premature infants.8
Saturated fatty acids constitute 42% and unsat-urated fatty acids account for 57% of total lipid. Linoleic acid concentrations are higher in recent studies8 than in earlier reports 7,9,10 and reflect the
higher intake of polyunsaturated fats by the Amen-ican population. Essential fatty acid bevels are higher in cobostrum and transitional milk than in mature milk.8 Long-chain polyunsaturated fatty acids derived from linoleic acid (20:2w6, 20:3, 20:4, 22:5w6) and from linolenic acid (20:5, 22:5w3, 22:6) show a similar trend throughout lactation. The level of these fatty acids is significantly higher in cobs-trum and milk of mothers of premature infants.8 These fatty acids are important in brain develop-ment, cell proliferation, myelination,’2 and retinal function.’3 Among all the lipid classes, cholesteryl ester has the highest content of unsaturated fatty acids (70% of which 28% is binoleic acid).
FACTORS THAT AFFECT THE AMOUNT AND
COMPOSITION OF MILK FAT
Fat is the most variable component of milk. In addition to the changes in the concentration and composition of milk fat that are associated with the stage oflactation and the length ofpregnancy, there are diurnal and in-feed variations of fat concentra-tion. The increase in fat content during the feeding bed to the suggestion that this change might regu-bate food intake in breast-fed infants so as to help prevent obesity.’4 Fat content rises during the day; early morning milk has the lowest fat content. Minerals, trace elements, and enzymes that asso-ciate with the cream fraction of milk have similar diurnal variations. Nutrient content might also vary in the milk secreted from the right or left breast at the same feeding.
In contrast to the changes in fat concentration, the fat composition of mature human milk is re-markably constant.’4 Only drastic changes in the diet, such as consumption of excessively large amounts of polyunsaturated fats, carbohydrates, or severe limitation of total food intake result in the increase of linobeic acid, palmitic acid, and medium-chain fatty acid levels, respectively.
PREDUODENAL DIGESTION OF MILK FAT
Fat digestion requires adequate lipase activity and bile salt levels, the former for the breakdown
Fatty Acid % of Total Fat Decanoic 10:0 0.97 ± 0.28 Lauric 12:0 4.46 ± 1.17 Myristic 14:0
15:0
5.68 ± 1.36 0.31 ± 0.07 Palmitic 16:0 22.20 ± 2.28 Palmitoleic 16:1
17:0
3.38 ± 0.39 0.49 ± 0.36 Stearic 18:0 7.68 ± 1.85 Oleic 18:1 35.51 ± 2.73 Linoleic 18:2 15.58 ± 1.99 Linolenic 18:3
20:0 20:2 20:3
1.03 ± 0.21
0.32 ± 0.11 0.18 ± 0.20 0.53 ± 0.15
Arachidonic 20:4 20:5 21:0 22:4 22:5w6 22:5w3 22:6
0.60 ± 0.29
0
0.17 ± 0.12 0.07 ± 0.16 0.03 ± 0.08 0.11 ± 0.15 0.23 ± 0.14 * Data from Bitman et al.8 Values are means ± SEM of
fatty acids in mature milk of mothers of term infants:
name, chain length, and degree of unsaturation (number of double bonds).
of triglyceride, the latter for emulsification of fat prior to and during bipolysis. In the newborn, and especially the premature infant, pancreatic lipase and bile acid bevels (the major components of intes-tinab fat digestion) are low. The efficient fat ab-sorption in the newborn depends on alternate mechanisms for the digestion of dietary fat. Of special importance is intragastnic lipobysis, in which lingual and gastric bipases compensate for bow bevels of pancreatic lipase’5 (Table 6), whereas the prod-ucts of bipobysis, fatty acids, and monoglycenides, compensate for low bile salt levels by emulsifying the lipid mixture.’6 The breast-fed newborn infant
may depend on an additional compensatory
en-zyme: the bile-salt stimulated bipase of human milk, and enzyme found only in the milk of primates (Table 6).’’
Lipase in Gastric Aspirates
Origin Lingual serous glands,
gastric mucosa
Ontogeny Present from 24 wk of gestation
Site of action Stomach (duodenum)
Characteristics pH optimum pH stability Rate
Reaction products Bile salts
Molecular weight Function
3.0-6.0 >2.2
MCT> LCT, FA un-saturated> satu-rated
FFA, DG, MG
20% to 40% stimula-tion
46,000-48,000
Hydrolysis of 50% to
70% of ingested fat
90,000-125,000
Hydrolysis of 30% to 40% of milk fat
* Abbreviations used are: MCT, medium-chain triglyceride; LCT, long-chain triglyceride; FA, fatty acids; FFA, free fatty acids; DG, diglyceride; MG, monoglyceride.
ACKNOWLEDGMENTS
These studies were supported by National Institutes
of Health Contract HD-2-2816 and Grant HD-10823.
We thank Marguerite Starry for secretarial help.
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TABLE 6. Compensatory Digestive Lipases in the Newborn
membrane.23 Indeed as much as 15% of core tn-glyceride is hydrolyzed without producing any change in the microscopic appearance of the milk fat gbobules.23 Lingual lipase hydrolyzes medium-and short-chain triglycerides at higher rates than long-chain triglycerides. Recent animal studies have shown that short- and medium-chain fatty acids are absorbed directly through the gastric mu-cosa, suggesting that these products of intragastnic lipolysis appear rapidly in the circulation. Hydrol-ysis of milk fat by lingual lipase also produces relatively large amounts of monolauryl,24 a sub-stance with antibacterial, antiviral, and antifungal activity, suggesting that antibacterial agents are formed in the infant’s stomach during fat hydroly-sis.
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Human Milk
Bile-Salt-Stimulated Lipase Mammary gland
(hu-man and gorilla only)
Present after term and preterm (26-36 wk) delivery
Intestine
7.0-9.0
>3.5
MCT = LCT,
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FFA, glycerol Obligatory
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