Donkey Milk Study: Gene Activity of the Mammary Gland (Fei 2022)

In the mammary gland tissue of Liaoxi donkeys, 3144 mRNAs and 293 miRNAs differed between lactation (day 60) and the non-lactating phase. Genes for caseins were approximately 17 to 18 times more active during lactation.

The study in brief

• Original title: An Integrated Analysis of Lactation-Related miRNA and mRNA Expression Profiles in Donkey Mammary Glands
• Authors: Fei Y., Gai Y., Liao Q. et al.
• Journal: Genes, 13(9), 1637 (2022)
• Country: China · Study type: Animal study

How the study was conducted

Six Liaoxi donkeys from a breeding farm in Fuxin (Liaoning Province, China; 6.3 ± 1.2 years, 280.3 ± 20.1 kg, 2.1 ± 0.4 births) were kept under the same conditions. For each group of three animals, either during lactation (60 days after birth) or in the non-lactating phase (not lactating, not pregnant), a tissue sample of approximately 100 mg was taken from the right mammary gland under general anesthesia. The tissue was histologically examined; mRNA and small RNA libraries were sequenced from the RNA.

Key figures

• 6 – Liaoxi donkeys (3 each in the lactating and non-lactating phases)
• 60 days – Time of the tissue sample after birth in the lactating group
• 3144 – Differentially expressed mRNAs (lactation vs. non-lactating phase)
• 987 / 2157 – Of which higher / lower in lactation
• 293 – Differentially expressed miRNAs (227 known, 66 new)
• 231 / 62 – Of which higher / lower in lactation

Results

Histologically, the lactating tissue showed large, dilated alveoli with milk secretion and little connective tissue. In the non-lactating phase, the alveoli were small with a narrow lumen, connective and fatty tissues increased, and the alveolar walls were thicker.

In the mRNA data (98.49–98.89% usable reads, around 90% assigned to the donkey genome), 3144 mRNAs differed between the groups: 987 were higher in lactation, 2157 lower. In addition, 536 new mRNAs were described. The genes were distributed across 340 KEGG pathways, including ECM-receptor interaction, PI3K-Akt signaling pathway, protein digestion and absorption, and arachidonic acid metabolism.

For the miRNAs, 293 were differentially expressed (231 higher, 62 lower; 227 known, 66 new). 35 of them were found only in the lactating group, 4 only in the non-lactating group. Their predicted target genes (2979) were assigned to, among others, RNA polymerase, glycosphingolipid biosynthesis, and glycerophospholipid metabolism.

Limitations of the study

• Very small groups: Only three animals of one breed from one farm were examined for each condition, and the lactating and non-lactating groups consisted of different animals. Differences between individual animals cannot thus be separated from the lactating state.
• Differently strict thresholds: For mRNAs, a correction for multiple testing (FDR) was applied, for miRNAs only an uncorrected p-value. Therefore, false-positive hits are more likely among the 293 miRNAs.
• Inconsistent terminology: The non-lactating group is called "D" in the methods section, and "N" in the results and figures. CSN1S2 is referred to in the text as "αs1-Casein-Isoform X3", although the gene is assigned to αs2-Casein in the same paragraph.
• No independent confirmation: The qRT-PCR test was performed on the same six tissue samples. The miRNA target genes are computationally predicted and were not functionally tested; the fold-change values of individual genes only appear in the discussion section.
• Milk itself was not examined: Mammary gland tissue was analyzed, not the milk. The authors' further statements on the properties of milk are based on literature; effects in humans were not the subject of the study.

Level of evidence: Animal study – describes measured values in the examined animals and their milk, no statement on effects in humans.

Our assessment

The study describes the basics of gene regulation in mammary tissue and does not provide measured values for the milk itself. Conclusions about ingredients, effects in humans, or individual commercial products cannot be derived from it.

Source: Fei Y., Gai Y., Liao Q., Zhang L., Li Z., Li B., Bai M., Li N., Deng L. (2022). An Integrated Analysis of Lactation-Related miRNA and mRNA Expression Profiles in Donkey Mammary Glands. Genes. DOI: 10.3390/genes13091637 · Summarized by the DON DONKEY'S editorial team, as of October 2026.

End of the study summary. The study was conducted independently of DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.