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Milk produced by horses, mules, donkeys and zebras has been lined up against other mammals as part of a study conducted by Chinese and American scientists.
The milk production of equids appeared alongside a long list of mammals, including humans, in a table produced by the researchers. It showed that human milk was much fattier than mare’s milk, while the latter had a considerably higher ratio of protein.
Guoyao Wu and his colleagues collated data from previously published studies to produced their table.
Milk comprises water, protein, lipids, carbohydrates (mainly lactose), minerals and vitamins. It is the secretory product of mammary epithelial cells in all lactating mammals.
The research team, writing in the Journal of Animal Science and Biotechnology. described the lactating gland as a highly organized organ.
Their paper, entitled “Amino acids and mammary gland development: nutritional implications for milk production and neonatal growth”, said lactation was an integral component of the highly successful reproductive strategy of mammals.
Development of the mammary gland, which is affected by the provision of amino acids, is closely linked to the reproductive cycle of all mammals.
At present, they said, there was a general lack of information about the metabolism of amino acids in lactating mammary tissue.
More research on amino acid metabolism in mammary tissue would not only advance knowledge of lactation biology, but have practical implications for improving the efficiency of livestock production to sustain animal agriculture worldwide, they wrote.
“Results from animal studies will also have important implications for optimizing milk synthesis by lactating women as well as infant growth and development in both developed and developing countries.”
The table below provides a breakdown of the composition of mature milk in domesticated and wild mammals.
Values are given in grams per kilogram (g/kg) of whole milk. Non-protein nitrogen (NPN) refers to total nitrogen in the milk, less the nitrogen contained in protein. (Nitrogen content in milk protein is 15.67%)
The amount of non-protein nitrogenous substances (g/kg whole milk) is calculated as the amount of nonprotein nitrogen (g/kg whole milk) multiplied by 6.25. Caseins include α S1-casein, αS2-casein, β-casein, γ-casein, and k-casein. Whey proteins include α-lactoalbumin, β-lactoglobulin, serum albumin, immunoglobulins, lactoferrins, lysozymes, amino acid oxidases, xanthine oxidase, and other enzymes
Ca refers to calcium, Carb to carbohydrates, DM to dry matter, and NPN to non-protein nitrogenous substances (including free amino acids, small peptides, urea, ammonia, uric acid, creatine, creatinine, and other low-molecular weight nitrogenous substances). “X” denotes a lack of data in the literature.
The sources cited in the table can be found in the full study.
Guoyao was joined in the research by Reza Rezaei, Zhenlong Wu, Yongqing Hou, Fuller Baze. The researchers are variously affiliated with Texas A&M University, China Agricultural University and Wuhan Polytechnic University.
Amino acids and mammary gland development: nutritional implications for milk production and neonatal growth
Reza Rezaei, Zhenlong Wu, Yongqing Hou, Fuller W. Bazer and Guoyao Wu.
Journal of Animal Science and Biotechnology 2016 7:20 DOI: 10.1186/s40104-016-0078-8
The full study, published under a Creative Commons License, can be read in full here.