Two milk samples can contain similar amounts of carbohydrate while supplying different molecules. Lactose, the familiar milk sugar, is only part of the story. Milk can also contain larger sugars assembled from several linked building blocks, with differences in their branches and connections creating distinct chemical structures.
In a 2018 study led by Yuri Mineguchi, researchers identified members of that smaller carbohydrate fraction in giraffe milk. They compared the findings with samples from sitatunga, deer and water buffalo. All four shared one three-unit sugar, but the other identified structures differed.
For giraffes, the work established a chemical starting point: particular molecules that researchers can now investigate, rather than an undifferentiated category called “milk sugar.” It did not establish what those molecules do inside a calf.
Looking beyond the largest ingredient
Lactose consists of two linked sugar units, glucose and galactose. More elaborate milk carbohydrates, commonly called oligosaccharides, can extend that basic structure with additional units. The order of those units matters, as does the position and orientation of each connection.
That creates a challenge for identification. Knowing which building blocks are present is not always enough. Two molecules can contain the same types and numbers of units but arrange them differently. A chemical analysis must distinguish the structures, not merely count their ingredients.
The giraffe samples totalled 77 millilitres collected from one lactating female over a period extending from August 2016 to July 2017. The team also examined sitatunga colostrum collected one day after birth, water buffalo colostrum collected about two hours after birth and milk collected from several deer.
Colostrum is the early secretion around birth. Comparing it with milk collected across later lactation does not cleanly separate species differences from differences in timing. The study was designed to identify the structures available in these samples, with a small and uneven biological sample behind each species.
Separating a mixture before naming its molecules
The researchers first removed much of the fat and protein to isolate the carbohydrate fraction. They then passed the mixture through a column that separated components into successive fractions. The resulting traces showed large peaks corresponding to lactose and smaller peaks that warranted further investigation.
The dominant lactose peak appeared in every species. For the giraffe preparation, the estimated yield of oligosaccharides relative to lactose was about one to twelve. That is a comparison within the recovered carbohydrate fractions, not a statement that one twelfth of the entire milk was made of complex sugars.
Further chromatography separated selected fractions more finely. But a peak alone did not establish that its contents were an oligosaccharide. Some giraffe fractions that produced peaks were subsequently found not to contain the target sugars. Separation was therefore a way to obtain candidate material for identification, rather than an automatic inventory of molecules.
The team used nuclear magnetic resonance, or NMR, to examine the chemical surroundings of atoms in those separated components. Patterns of signals could be compared with reference compounds and published structural measurements. This provided information about the kinds of sugar units and how they were connected.
A shared three-unit sugar and a branched four-unit sugar
One giraffe component matched isoglobotriose. Its structure consists of the two-unit lactose foundation with an additional galactose attached in a particular orientation and position. The researchers identified the same structure in all four species examined.
The importance of the exact connection becomes clear in sitatunga. Its colostrum contained both isoglobotriose and another three-unit sugar, globotriose. Both have the same kinds of building blocks. They differ in where the additional galactose attaches. Treating them simply as “three sugars joined together” would erase the distinction the analysis revealed.
The other identified giraffe component was a branched four-unit molecule called GM2 tetrasaccharide. It includes a sialic-acid unit and an additional modified sugar attached to the lactose-based structure. Within the four-species comparison, this was identified in the giraffe material alone.
Some of its NMR signals overlapped with the water signal, making parts of the spectrum difficult to resolve. The researchers therefore also used mass spectrometry. The measured masses of the molecule with associated sodium or potassium ions provided a separate check on the proposed composition. The combination of structural signals and mass measurements supported the identification.
This is a more informative result than detecting “complex carbohydrate.” It specifies a branched molecule with connections that can guide later chemical and biological experiments.
Two identified structures are not necessarily the whole inventory
The study did not show that every giraffe produces exactly these two oligosaccharides, or that no others occur in giraffe milk. A compound present at very low abundance can be difficult to isolate. Variation among mothers and across lactation would require a broader sampling design.
A 2023 study by Chunsheng Jin and colleagues illustrates how much an inventory can expand when researchers revisit poorly studied mammals. It used detailed chromatography, tandem mass spectrometry and selective enzymes to examine milk from nine species, including bottlenose dolphins. In two dolphins, the team characterised 63 distinct oligosaccharide structures, with some present in only one individual.
The enzymes helped test proposed connections by removing particular sugar units, while the mass spectra recorded the resulting fragments. The combination could distinguish structures that simpler measurements might miss. That study did not analyse new giraffe samples, so its dolphin count is not an estimate for giraffes. It demonstrates why a first set of confirmed structures should remain a foundation for further sampling, rather than becoming a fixed limit on what a species produces.
What the chemistry makes possible
Mineguchi and colleagues discussed possible biological and evolutionary explanations for the observed milk-sugar patterns. Their experiments, however, measured structures rather than feeding outcomes, microbial growth or immune responses in giraffe calves. The presence of a particular sugar cannot establish a benefit by itself.
The next steps follow directly from what the study resolved. Researchers can measure the identified compounds in more mothers at defined stages of lactation, investigate how the mammary gland builds them and test their interactions with relevant cells or microbes. Those questions require knowing the molecules precisely enough to distinguish one arrangement from another.
The finding is therefore both modest in scale and useful in substance. The sampled giraffe milk contained a dominant supply of lactose and a smaller, chemically distinct set of larger sugars. Identifying that set reveals a part of maternal nourishment that bulk measurements of fat, protein and total carbohydrate would leave hidden.
Sources
Yuri Mineguchi, Midori Miyoshi, Epi Taufik, Ayumi Kawamura, Takuya Asakawa, Isao Suzuki, Kousaku Souma, Michiko Okubo, Tadao Saito, Kenji Fukuda, Sadaki Asakuma and Tadasu Urashima (2018). “Chemical characterization of the milk oligosaccharides of some Artiodactyla species including giraffe (Giraffa camelopardalis), sitatunga (Tragelaphus spekii), deer (Cervus nippon yesoensis) and water buffalo (Bubalus bubalis).” Glycoconjugate Journal, 35, 561–574. Primary study.
Chunsheng Jin, Jon Lundstrøm, Emma Korhonen, Ana S. Luis and Daniel Bojar (2023). “Breast Milk Oligosaccharides Contain Immunomodulatory Glucuronic Acid and LacdiNAc.” Molecular & Cellular Proteomics, 22, 100635. Related primary study.
