What are the components of breastmilk?

Components of Breastmilk

Breastmilk is a complex biological fluid containing macronutrients (carbohydrates, proteins, and fats), micronutrients (vitamins and minerals), bioactive compounds (immunoglobulins, growth factors, enzymes), human milk oligosaccharides, live bacteria, and functional lipids that collectively support infant growth, immune development, and neurocognitive maturation. 1

Macronutrients

Water and Energy Content

• Water comprises 87-88% of breastmilk, with solid components making up approximately 124 g/L 2
• Average energy content is 0.62 kcal/g (2.6 kJ/g), with fat serving as the primary energy source 1, 3
• Energy density varies throughout lactation, with preliminary evidence suggesting mothers breastfeeding beyond one year produce milk with higher fat and energy content 1

Carbohydrates (7% or 60-70 g/L)

• Lactose is the dominant carbohydrate, constituting 2-8% of milk, and is enzymatically digested into glucose and galactose for intestinal absorption 1
• Human milk oligosaccharides (HMOs) represent the third largest solid component after lactose and lipids, with over 150 different structures identified 1
• HMOs are composed of five basic monosaccharides: glucose, galactose, N-acetylglucosamine, fucose, and sialic acid, with lactose at their reducing end 1
• HMO composition is genetically determined by maternal secretor (FUT2) and Lewis (FUT3) genes, remains constant within individual mothers throughout the day and week, but changes across lactation 1

Proteins (1% or 8-10 g/L)

• Colostrum contains exceptionally high protein concentrations with 88-90% being secretory IgA, which establishes infant immune function 3
• Protein concentrations decrease as lactation progresses from colostrum to transitional milk (days 7-14) to mature milk (after 2-3 weeks) 1, 3
• Lactoferrin is one of the most abundant proteins in breastmilk and functions as both a nutritional and immune component 1
• Milk contains enzymes including acidic sphingomyelinase and bile salt-stimulated lipase (BSSL) that facilitate lipid digestion 1

Fats (3.8% or 35-40 g/L)

• Fat content varies dramatically within a single feeding, with hind milk containing 2-3 times more fat than fore milk 1, 3
• Long-chain polyunsaturated fatty acids (LC-PUFAs), particularly arachidonic acid (ARA) and docosahexaenoic acid (DHA), are essential for brain development, bone growth, and cellular communication 1
• Medium-chain fatty acids are rapidly hydrolyzed by gastrointestinal lipases without requiring bile salts and are directly absorbed to the liver via portal circulation 1

Sphingolipids

• Sphingomyelins are the most abundant polar lipids in milk, present in milk fat globule membranes and exosomes 1
• Glycosphingolipids include cerebrosides (most abundant), ceramides, globosides, gangliosides, and sulfatides in descending order of concentration 1
• Sphingolipid composition varies with lactation stage, maternal diet, geographic region, and milk expression method 1

Micronutrients

Vitamins

• Breastmilk contains a wide range of vitamins with concentrations varying by time of day and stage of feeding 1
• Circadian variation occurs for most vitamins except vitamins A and E when adjusted for milk fat 1
• Within-feed variation is significant for thiamin, niacin, and vitamins A and E, but not for vitamins B6 or B12 1
• Fat-soluble vitamin concentrations (B1, B2, C) correlate with maternal dietary intake, while vitamins D and K may require supplementation despite adequate maternal nutrition 1, 2, 4

Minerals

• Mineral concentrations decline progressively across lactation 1
• Geographic location significantly impacts mineral profiles, with calcium concentrations lower in Nigeria and Gambia but higher in Japan and Sweden 1
• Iron, iodine, and zinc may be deficient in breastmilk from mothers in resource-poor countries 4

Bioactive Components

Immunological Factors

• Secretory IgA is the predominant immunoglobulin (88-90% of total), highest in colostrum, and remains stable within individual mothers from day 6 to at least day 90 postpartum 3
• Immunoglobulins modulate infant gut microbiota interactions with host cells and may influence neurodevelopment through the gut-immune-brain axis 1
• Lactoferrin contributes to innate immunity and is among the most abundant proteins in milk 1

Growth Factors and Hormones

• Breastmilk contains growth factors and signaling peptides that act on the intestinal tract, vasculature, nervous system, and endocrine system 2
• Glucocorticoids (cortisol and cortisone) peak in morning milk and reach lowest levels in evening milk 3
• Melatonin concentrations are higher in nighttime milk, supporting infant circadian rhythm development 3
• Milk cortisol increases during the first three months postpartum, while leptin declines and ghrelin increases across early lactation 3

Microbiome

• Over 200 bacterial species from approximately 50 genera have been isolated from human milk 1
• Cultivable bacteria are dominated by Staphylococcus, Streptococcus, and related Gram-positive genera, with lactic acid bacteria and Bifidobacterium also present 1
• Colostrum carries commensal bacteria including Lactobacillus and Bifidobacterium species that establish and shape the infant's immune system and gut microbiome 1
• Maternal diet influences milk microbiota composition, with fatty acid intake correlating with specific bacterial genera 1

Temporal and Individual Variations

Stage of Lactation

• Colostrum (first days postpartum) is low in fat but exceptionally high in proteins, immunoglobulins, and bioactive compounds 1, 2
• Transitional milk (days 7-14) shows decreasing immunoglobulin and protein concentrations 1, 3
• Mature milk (after 2-3 weeks) exhibits less compositional variability but undergoes subtle biologically relevant changes throughout lactation 1, 3

Circadian and Within-Feed Variations

• Day and evening milk samples are more energy-dense than night or morning samples 1, 3
• Fat and hormone concentrations change significantly during the 24-hour period 1
• Hind milk contains 2-3 times more fat than fore milk within a single feeding 1, 3

Maternal Factors

• Maternal diet significantly influences fatty acid composition, particularly PUFAs, with high-fat and high-sugar meals causing acute changes within 24 hours 1
• Maternal age, parity, delivery mode, and geographic location contribute to inter-individual variation in milk composition 1
• A lactating mother requires approximately 500 more kcal/day than a non-pregnant woman, producing an average of 780 mL/day (range 450-1200 mL/day) 1, 3

Clinical Caveats

• Maternal lactose intolerance does not affect lactose production in breastmilk, as mammary glands synthesize lactose independently of the maternal digestive system 5
• Nutrient deficiencies in maternal diet (vitamin D, iodine, iron, vitamin K in developed countries; vitamin A, B12, zinc, thiamin in resource-poor settings) are reflected in breastmilk composition 4
• Substantial individual variability exists in milk hormone concentrations across lactation, requiring individualized assessment when clinically relevant 3