Breast feeding symposium
561
NITROGEN AND PROTEIN COMPONENTS OF HUMAN MILK L. HAMBRKUS, B. LONNERDAL, E. FORSUM and M. GEBRE-MEDHIN From the Institute of Nutrition, University of Uppsala, Uppsala, Sweden
ABSTRACT. Hambrapus, L., Lonnerdal, B., Forsum, E. and Gebre-Medhin, M. (Institute of Nutrition, University of Uppsala, Uppsala, Sweden). Nitrogen and protein components of human milk. Acta Paediatr Scand, 67: 561, 1978.-The true protein content of human milk is 0.9%, in well-nourished as well as malnourished mothers. Casein constitutes only about 20% of the protein nitrogen in human milk. The remaining 80 % is derived from the whey proteins, the three dominant components being a-lactalbumin, lactoferrin and secretory IgA. a-lactalbumin is a subunit of lactose synthetase. Lactoferrin is an iron-binding glycoprotein which plays a role in the defence against gastro-intestinal infections and is probably also involved in iron transport in the gut. Secretory IgA is comparatively stable at low pH; it is resistant to proteolytic enzymes and plays an essential role in the immunological defence against gastro-intestinal infections. Lysozyme is a minor component of the whey proteins and represents an active enzyme with a bactericidal effect. The nutritional and immunological significance of the marked differences with respect to the nitrogen and protein compositions of human milk and cow’s milk should not be underestimated, but need further elucidation. KEY WORDS: Human milk, protein components. infant nutrition
Our knowledge concerning the nitrogen and protein compositions of human milk and their variation has long been very limited. Many years ago Macy and her collaborators performed a series of outstanding studies on the composition of human milk (12, 13). At that time however, the only available methods for qualitative and quantitative analysis of milk were those which were originally developed for the analysis of cow’s milk. They were therefore not always applicable to human milk, as there are physico-chemical differences between bovine and human casein which lead to different physico-chemical characteristics and subsequently different curd formation (15). This is of nutritional interest, since the curd formation influences the digestion of protein by the gastric enzymes in the stomach of the young infant. On the other hand, certain findings by Macy and her group, e.g. the high content of nonprotein-nitrogen, have oddly enough been overlooked and not taken seriously into account. Thus it has been wrongly assumed that the protein content in human milk is 1 . 1 to 1.2 %, which represents an overestimation by
20 to 25% (8). The observation that 20-25% of the total nitrogen is in the form of nonprotein-nitrogen has since been verified in our studies on larger materials on well-nourished as well as in malnourished mothers (9, 10). Table 1 shows the nitrogen composition of human milk-both colostrum and mature milk. The data given in this table were obtained in a study on milk specimens from well-nourished mothers, in which a complete analysis of the milk composition was performed (11). It is seen that non-protein nitrogen constitutes about 13 % of the total nitrogen in colostrum, but, as said before, about 25 % in mature milk. The main part of the non-protein nitrogen is derived from urea, 11 % and 33 % respectively, Table 1. Nitrogen composition of human milk The values refer to mg N per ml
Total nitrogen Protein nitrogen Non-protein nitrogen Urea nitrogen
Colostrum
Mature milk
3.60 3.13 0.47 0.05
1.71k0.31 1.2920.26 0.42k0.10 0.14f0.03 Actu Purdiatr Scnnd 67
562
Breast feeding symposium
Table 2. Protein composition of human milk The values refer to mg protein per ml
Casein Lactoferrin a-lactalbumin Secretory IgA Lysozyme Serum albumin
Colostrum
Mature milk
3.30 2.18 3.64 0.34 0.32
1 S7k0.65
1.67k0.33 1.61k0.33 1.42k0.56 0.38k0.04 0.40k0.09
other nitrogen-containing substances being creatine, creatinine, uric acid, small peptides and free amino acids. The protein composition of milk specimens obtained from the same mothers is given in Table 2. For analytical reasons no value could be obtained for the casein content of colostrum. It is quite obvious that a-lactalbumin, lactoferrin and secretory IgA constitute the dominating whey proteins in mature human milk. Casein been stated earlier to constitute about 40% of the protein in human milk and the whey proteins the remaining 60%. However, the analyses performed at our laboratory by Lonnerdal (Table 2) show that the casein
percentage in human milk might be as low as 20%. This discrepancy might be due to the fact that a valid method for quantitative estimation of casein is still lacking. At present casein is determined as the protein fraction which precipitates at pH 4.6. However, if the nitrogen contents of the various human whey protein fractions, i.e. a-lactalbumin, lactoferrin, lysozyme, serum albumin and the immunoglobulins (SIgA, IgA, IgM, IgG), are substracted from the total nitrogen content there is only about 20% left to be made up by casein. Our findings indicate that earlier values of 40 % are due to the co-precipitation of other protein fractions together with casein. a-lactalbumin is a protein which is synthesized by the mammary gland. It is one of the two proteins which form the lactose-producing enzyme, lactose synthetase (4). When milk specimens obtained from various mammals are compared, it is obvious that the alactalbumin content is related to the lactose content (4). As human milk is specially rich in lactose, it also represents the milk that has the highest a-lactalbumin content. There is no direct correlation, however, between the lac-
Table 3. Nitrogen and protein contents of human milk and their variation during lactation The values refer to mg per ml Duration of lactation (months)
Totul nitrogen Swedish mothers Ethiopian priv. Ethiopian non-priv.
0-0.5
0.5-1.5
1.5-3.5
3.5-6.5
3.05 3.14
1.93 2.89 2.50
1.61 1.97 1.77
1.48
0.46 0.46 0.43
0.41 0.41 0.36
0.38
1.94
1.65 1.89 1.67
I .39
3.37 2.64 3.26 3.72 3.58
2.78 2.92 2.76
1.69
6.5-
1.74
Non-protein nitrogen
Swedish mothers Ethiopian priv. Ethiopian non-priv.
0.43 0.46
0.34
0.33
Lactoferrin
Swedish mothers Ethiopian priv. Ethiopian non-priv.
3.53 3.75
1.72
I .48
a-luctulbumin
Swedish mothers Ethiopian priv. Ethiopian non-priv. A
561
NITROGEN AND PROTEIN COMPONENTS OF HUMAN MILK L. HAMBRKUS, B. LONNERDAL, E. FORSUM and M. GEBRE-MEDHIN From the Institute of Nutrition, University of Uppsala, Uppsala, Sweden
ABSTRACT. Hambrapus, L., Lonnerdal, B., Forsum, E. and Gebre-Medhin, M. (Institute of Nutrition, University of Uppsala, Uppsala, Sweden). Nitrogen and protein components of human milk. Acta Paediatr Scand, 67: 561, 1978.-The true protein content of human milk is 0.9%, in well-nourished as well as malnourished mothers. Casein constitutes only about 20% of the protein nitrogen in human milk. The remaining 80 % is derived from the whey proteins, the three dominant components being a-lactalbumin, lactoferrin and secretory IgA. a-lactalbumin is a subunit of lactose synthetase. Lactoferrin is an iron-binding glycoprotein which plays a role in the defence against gastro-intestinal infections and is probably also involved in iron transport in the gut. Secretory IgA is comparatively stable at low pH; it is resistant to proteolytic enzymes and plays an essential role in the immunological defence against gastro-intestinal infections. Lysozyme is a minor component of the whey proteins and represents an active enzyme with a bactericidal effect. The nutritional and immunological significance of the marked differences with respect to the nitrogen and protein compositions of human milk and cow’s milk should not be underestimated, but need further elucidation. KEY WORDS: Human milk, protein components. infant nutrition
Our knowledge concerning the nitrogen and protein compositions of human milk and their variation has long been very limited. Many years ago Macy and her collaborators performed a series of outstanding studies on the composition of human milk (12, 13). At that time however, the only available methods for qualitative and quantitative analysis of milk were those which were originally developed for the analysis of cow’s milk. They were therefore not always applicable to human milk, as there are physico-chemical differences between bovine and human casein which lead to different physico-chemical characteristics and subsequently different curd formation (15). This is of nutritional interest, since the curd formation influences the digestion of protein by the gastric enzymes in the stomach of the young infant. On the other hand, certain findings by Macy and her group, e.g. the high content of nonprotein-nitrogen, have oddly enough been overlooked and not taken seriously into account. Thus it has been wrongly assumed that the protein content in human milk is 1 . 1 to 1.2 %, which represents an overestimation by
20 to 25% (8). The observation that 20-25% of the total nitrogen is in the form of nonprotein-nitrogen has since been verified in our studies on larger materials on well-nourished as well as in malnourished mothers (9, 10). Table 1 shows the nitrogen composition of human milk-both colostrum and mature milk. The data given in this table were obtained in a study on milk specimens from well-nourished mothers, in which a complete analysis of the milk composition was performed (11). It is seen that non-protein nitrogen constitutes about 13 % of the total nitrogen in colostrum, but, as said before, about 25 % in mature milk. The main part of the non-protein nitrogen is derived from urea, 11 % and 33 % respectively, Table 1. Nitrogen composition of human milk The values refer to mg N per ml
Total nitrogen Protein nitrogen Non-protein nitrogen Urea nitrogen
Colostrum
Mature milk
3.60 3.13 0.47 0.05
1.71k0.31 1.2920.26 0.42k0.10 0.14f0.03 Actu Purdiatr Scnnd 67
562
Breast feeding symposium
Table 2. Protein composition of human milk The values refer to mg protein per ml
Casein Lactoferrin a-lactalbumin Secretory IgA Lysozyme Serum albumin
Colostrum
Mature milk
3.30 2.18 3.64 0.34 0.32
1 S7k0.65
1.67k0.33 1.61k0.33 1.42k0.56 0.38k0.04 0.40k0.09
other nitrogen-containing substances being creatine, creatinine, uric acid, small peptides and free amino acids. The protein composition of milk specimens obtained from the same mothers is given in Table 2. For analytical reasons no value could be obtained for the casein content of colostrum. It is quite obvious that a-lactalbumin, lactoferrin and secretory IgA constitute the dominating whey proteins in mature human milk. Casein been stated earlier to constitute about 40% of the protein in human milk and the whey proteins the remaining 60%. However, the analyses performed at our laboratory by Lonnerdal (Table 2) show that the casein
percentage in human milk might be as low as 20%. This discrepancy might be due to the fact that a valid method for quantitative estimation of casein is still lacking. At present casein is determined as the protein fraction which precipitates at pH 4.6. However, if the nitrogen contents of the various human whey protein fractions, i.e. a-lactalbumin, lactoferrin, lysozyme, serum albumin and the immunoglobulins (SIgA, IgA, IgM, IgG), are substracted from the total nitrogen content there is only about 20% left to be made up by casein. Our findings indicate that earlier values of 40 % are due to the co-precipitation of other protein fractions together with casein. a-lactalbumin is a protein which is synthesized by the mammary gland. It is one of the two proteins which form the lactose-producing enzyme, lactose synthetase (4). When milk specimens obtained from various mammals are compared, it is obvious that the alactalbumin content is related to the lactose content (4). As human milk is specially rich in lactose, it also represents the milk that has the highest a-lactalbumin content. There is no direct correlation, however, between the lac-
Table 3. Nitrogen and protein contents of human milk and their variation during lactation The values refer to mg per ml Duration of lactation (months)
Totul nitrogen Swedish mothers Ethiopian priv. Ethiopian non-priv.
0-0.5
0.5-1.5
1.5-3.5
3.5-6.5
3.05 3.14
1.93 2.89 2.50
1.61 1.97 1.77
1.48
0.46 0.46 0.43
0.41 0.41 0.36
0.38
1.94
1.65 1.89 1.67
I .39
3.37 2.64 3.26 3.72 3.58
2.78 2.92 2.76
1.69
6.5-
1.74
Non-protein nitrogen
Swedish mothers Ethiopian priv. Ethiopian non-priv.
0.43 0.46
0.34
0.33
Lactoferrin
Swedish mothers Ethiopian priv. Ethiopian non-priv.
3.53 3.75
1.72
I .48
a-luctulbumin
Swedish mothers Ethiopian priv. Ethiopian non-priv. A
