What Does 100 mL of Breast Milk Contain? An In-Depth Look

December 17, 2025 •

3 min read

Mature breast milk contains approximately 60-75 kcal per 100 mL, providing the essential energy infants need for rapid growth and development. Its composition is a dynamic, complex fluid, perfectly tailored to a baby’s changing needs beyond just macronutrients to include a variety of bioactive elements.

Quick Summary

An average 100 mL of mature breast milk delivers a balance of macronutrients like fat, carbohydrates, and protein, alongside essential vitamins, minerals, and bioactive compounds. The exact composition is dynamic, changing over time and during a single feeding to perfectly match the infant's developmental stage and immediate needs.

Key Points

Energy & Macronutrients: 100 mL of mature breast milk contains approximately 65-70 kcal, delivering a balance of fat (approx. 3.5-4.2g), carbohydrates (approx. 6.7-7.1g), and protein (approx. 0.8-1.3g).
Fat is Dynamic: The fat content is the most variable macronutrient, increasing throughout a single feeding session and varying between mothers.
Rich in Bioactive Components: Beyond basic nutrients, 100 mL of breast milk contains essential antibodies (sIgA), lactoferrin, digestive enzymes, hormones, and growth factors that protect the infant from infection and aid development.
Contains Prebiotics: Human Milk Oligosaccharides (HMOs) are complex carbohydrates in breast milk that act as prebiotics, supporting the growth of healthy gut bacteria.
Micronutrients are Highly Bioavailable: While some mineral concentrations like iron are low, their bioavailability in breast milk is high, ensuring efficient absorption by the infant.
Maternal Diet Influence: A mother's diet can influence some components, such as the fatty acid profile (e.g., DHA) and water-soluble vitamin levels in her breast milk.
Tailored for Infant Needs: The composition dynamically changes over lactation stages (colostrum, mature milk) and within feeds to meet the infant's specific and evolving developmental requirements.

Macronutrients: The Building Blocks of Growth

In a typical 100 mL serving, the macronutrient profile of mature breast milk provides a balanced mix to fuel a baby's rapid development. The fat content is the most variable and energy-dense component, fluctuating even within a single feeding session. Protein levels, while relatively low compared to cow's milk, are perfectly optimized for human infants, and carbohydrates provide steady energy.

The Dynamic Nature of Breast Milk

One of the most remarkable features of human milk is its changing composition over time. This includes key phases like colostrum, transitional milk, and mature milk, each with a distinct nutritional profile suited for the baby’s age. Furthermore, the milk's fat content increases as a feeding session progresses, with the higher-fat hindmilk appearing toward the end.

Macronutrient Breakdown (per 100 mL of Mature Breast Milk):

Fat: Around 3.5 to 4.2 grams, providing approximately 50% of the total energy.
Carbohydrates (Lactose & Oligosaccharides): Approximately 6.7 to 7.1 grams, with lactose being the primary sugar. Human Milk Oligosaccharides (HMOs) are also abundant, serving as prebiotics.
Protein (Whey & Casein): About 0.8 to 1.3 grams, with a whey-to-casein ratio that changes as lactation progresses, making it easily digestible.

Vitamins and Minerals for Comprehensive Health

Breast milk is a source of crucial vitamins and minerals, although some, like vitamins D and K, may require supplementation depending on the mother's diet and sunlight exposure. Its minerals, like iron, are highly bioavailable despite being present in low concentrations.

Vitamins: Contains a range of vitamins including Vitamin A, B6, B12, C, E, and K.
Minerals: Includes vital minerals such as calcium, sodium, potassium, and magnesium.

Bioactive Components: More Than Just Nutrition

Beyond basic nutrition, 100 mL of breast milk is packed with hundreds of bioactive substances that actively support a baby's immune system, gut health, and overall development. These factors cannot be replicated by formula and are a key reason for breast milk's superiority.

Immunoglobulins: Secretory immunoglobulin A (sIgA) is a predominant antibody that coats the infant’s intestinal lining, providing crucial defense against pathogens.
Lactoferrin: An iron-binding protein with potent antibacterial and antiviral properties.
Oligosaccharides (HMOs): These are complex carbohydrates that act as prebiotics, feeding beneficial bacteria in the gut and blocking pathogens from binding to the infant's intestinal wall.
Enzymes: Digestive enzymes help the baby break down nutrients.
White Blood Cells: Colostrum is particularly rich in leukocytes that fight infection.
Growth Factors and Hormones: These help in the development and maturation of the infant’s organs and systems.

Comparison: Breast Milk vs. Infant Formula (per 100 mL)


Component	Mature Breast Milk	Standard Cow's Milk-Based Formula
Energy	Approx. 65-70 kcal	Approx. 67 kcal
Fat	3.5-4.2 g (highly variable)	Approx. 3.5 g
Carbohydrate	6.7-7.1 g (mostly lactose)	Approx. 7.4 g (includes lactose, corn syrup solids)
Protein	0.8-1.3 g (whey/casein ratio changes)	Approx. 1.4 g (whey/casein ratio often fixed)
Bioactive Factors	Abundant (sIgA, lactoferrin, HMOs)	None (synthetic)
Immune Cells	Present (especially in colostrum)	None
Bioavailability of Iron	High (20%-50% absorption)	Low (4%-7% absorption)

The Impact of a Mother's Diet

While the core components of breast milk are remarkably stable, certain nutrients can be influenced by the mother's dietary intake. For instance, the fatty acid profile of milk, including levels of DHA (docosahexaenoic acid), reflects the mother's fat intake. Similarly, levels of water-soluble vitamins like B vitamins can be affected by maternal stores. Conversely, the overall macronutrient composition and many minerals are quite stable and not significantly impacted by short-term changes in maternal diet. For more details on the intricacies of breast milk composition, review the components of human breast milk from macronutrient to microbiome at.

Conclusion: A Complete and Dynamic Source of Nutrition

Ultimately, a mere 100 mL of breast milk is a powerhouse of comprehensive nutrition. It delivers perfectly portioned macronutrients for energy and growth, a wide array of vitamins and minerals for optimal development, and a complex system of bioactive compounds to support the immune system and gut health. This dynamic, ever-adapting nature of breast milk is why it is considered the biological norm for human infants, providing a far more sophisticated and responsive nutritional profile than any alternative.

Frequently Asked Questions

On average, 100 mL of mature breast milk contains between 65 and 70 kilocalories (kcal). The exact caloric value can fluctuate based on the milk's fat content, which changes throughout a feeding and over the course of lactation.

Carbohydrates, primarily in the form of lactose, are the most abundant macronutrient. Mature breast milk contains approximately 6.7 to 7.1 grams of carbohydrates per 100 mL.

Generally, 100 mL of breast milk provides sufficient vitamins and minerals for a healthy infant, with the exception of vitamins D and K, which often require supplementation. Minerals like iron are highly bioavailable, meaning the baby absorbs them efficiently.

The fat content varies significantly even within a single 100 mL serving. The milk at the beginning of a feed (foremilk) is lower in fat, while the milk toward the end (hindmilk) is higher in fat, providing a creamier, more calorie-dense feed.

HMOs are complex carbohydrates present in breast milk that act as prebiotics. They feed the beneficial bacteria in an infant's gut, supporting the development of a healthy gut microbiome, and also act as decoys to prevent pathogens from causing infection.

Yes, 100 mL of breast milk contains a robust set of immune-protective factors. These include antibodies, particularly secretory IgA, and various immune cells that protect the infant from infection.

During the transition from colostrum to mature milk, the composition changes. Colostrum, the first milk, is higher in protein and immune factors, while mature milk, which makes up a typical 100 mL serving, becomes higher in fat, carbohydrates, and overall energy.