What-is-colostrum

Protein is the most abundant macronutrient class in colostrum and serves as the primary source of its bioactive functions. Bovine colostrum contains 14%–16% total protein, approximately 3–5 times the concentration found in mature milk [1][2]. The protein fraction comprises caseins, whey proteins, immunoglobulins, bioactive peptides, and numerous trace proteins with specialized functions.

Casein accounts for approximately 30%–40% of the total protein in the first milking of colostrum, forming a micellar structure that serves as a carrier for amino acids and minerals [6]. This proportion is markedly lower than that in mature milk, where casein comprises about 80%, owing to the exceptionally high immunoglobulin content in high‑quality colostrum—immunoglobulins belonging to the whey protein fraction—which shifts the casein-to-whey protein ratio in favor of whey proteins.

Whey protein accounts for approximately 60%–70% of the total protein in colostrum from the first milking, consistent with its high IgG content [6]. The major whey proteins include β‑lactoglobulin, α‑lactalbumin, bovine serum albumin, immunoglobulins, lactoferrin, and lysozyme [6][7]. In addition to their nutritional value, many whey proteins exhibit bioactive properties, some of which are activated only upon acidification or partial digestion. Proteomic analyses have identified more than 200 distinct whey proteins in bovine colostrum, many of which are present at trace levels yet possess potent biological activities [5].

Endogenous peptides: Colostrum contains a pool of endogenous peptides generated through limited proteolysis of maternal proteins. These peptides typically range from 5 to 50 amino acids in length and exhibit diverse biological activities. Studies have identified thousands of endogenous peptides in bovine colostrum, many of which are differentially expressed between colostrum and mature milk [5].

Colostrum contains 6%–8% fat, providing a concentrated source of energy and essential fatty acids [2]. Its lipid composition comprises triglycerides (95%–98%), phospholipids (0.5%–1%), cholesterol (0.2%–0.5%), as well as trace components such as fat-soluble vitamins and carotenoids. The fatty acid profile of colostrum differs from that of mature milk, with a higher proportion of saturated fatty acids and a distinctive pattern of polyunsaturated fatty acids [2]. Colostrum is particularly rich in medium-chain fatty acids—caprylic acid (C8:0) and capric acid (C10:0)—which are rapidly absorbed and metabolized, supplying immediate energy to the neonate. The phospholipid fraction is concentrated in the milk fat globule membrane (MFGM), which contains bioactive phospholipids such as sphingomyelin, phosphatidylcholine, and phosphatidylethanolamine [6].

Colostrum contains 2.5%–3.5% lactose, significantly lower than that of mature milk (4.5%–5.0%) [1]. This relatively low lactose content may reflect the limited lactase activity immediately after birth in neonates, as well as the prioritization of immune functions over purely nutritional ones during this critical window. Lactose also serves as a primer for oligosaccharide synthesis in the mammary gland and as a reducing-end substrate for glycosyltransferases. Bovine colostrum contains 1–2 g/L of oligosaccharides, representing a 5–10-fold enrichment compared with mature milk [8]. At least 40 distinct oligosaccharides have been identified in bovine colostrum, categorized into neutral and acidic (sialylated) types. Sialylated oligosaccharides predominate, accounting for more than 70% of the total oligosaccharides in colostrum. Human colostrum is particularly rich in oligosaccharides, with concentrations of 22–24 g/L, whereas bovine colostrum contains approximately 1 g/L [6]. To date, over 200 different oligosaccharides have been characterized in human milk, a diversity far exceeding that of bovine colostrum [8].

Insulin-like growth factor (IGF) is one of the most abundant growth factors in colostrum. Bovine colostrum contains 300–500 ng/mL of IGF‑1 and 200–400 ng/mL of IGF‑2, at concentrations 10–100 times higher than those in mature milk [7]. IGFs are polypeptides with structural homology to insulin. There is 100% sequence identity between bovine and human IGFs [7]. IGF‑1 is the predominant growth factor in colostrum. In colostrum, IGF is protected from proteolytic degradation through its association with IGF‑binding proteins (IGFBPs). In addition, colostrum also contains other growth factors, such as epidermal growth factor (EGF), transforming growth factor‑β (TGF‑β), and transforming growth factor‑α (TGF‑α).

Proline‑Rich Polypeptides (PRP), also known as colostrinin, are a complex of peptides with molecular weights ranging from 500 to 3,000 Da [11]. Initially isolated from sheep colostrum, PRP has since been identified in bovine colostrum as well. The PRP complex contains approximately 25% proline and 40% hydrophobic amino acids; this unusual compositional profile contributes to its structural stability and biological activity [11].

Colostrum contains a variety of cytokines. The concentrations of cytokines in colostrum are significantly higher than those in mature milk. Both pro-inflammatory cytokines (IL‑1β, IL‑6, TNF‑α, IFN‑γ) and anti-inflammatory cytokines (IL‑10, TGF‑β, IL‑1Ra) are present.

Colostrum contains nucleotides and nucleosides. Uridine and uridine 5′-monophosphate are the most abundant nucleosides and nucleotides in colostrum. The concentration of nucleotides in colostrum peaks 24–48 hours postpartum and then gradually declines.

Recent studies have revealed that colostrum contains microRNAs (miRNAs) [12]. Colostrum harbors a higher concentration of miRNAs than mature milk. Key immune‑related miRNAs identified in bovine colostrum include miR‑142‑5p, miR‑150, miR‑155, miR‑181a, and miR‑223 [14].

Gangliosides are polar glycosphingolipids found in the milk fat globule membrane and participate in a variety of biological functions [15]. The concentration of gangliosides in colostrum is higher than in mature milk, reflecting their critical role in the development of the neonate.

Bovine colostrum contains trypsin inhibitor, a protein that protects immunoglobulin G (IgG), growth factors, and other bioactive proteins from proteolytic degradation in the gastrointestinal tract [10]. The concentration of trypsin inhibitor in bovine colostrum is approximately 5–15 times higher than in mature milk [10]. Studies have shown that the concurrent presence of casein partially shields epidermal growth factor from digestion, while the protease‑inhibitory profile of colostrum further enhances the stability and absorption of IGF‑1 [7]. In addition to trypsin inhibitor, colostrum also contains α1‑antitrypsin, α2‑macroglobulin, and other serine protease inhibitors (serpins), which together form a protease‑inhibition network [1].