Nutrients and calories in breast milk: A guide for the science-minded

The nutrients and calories in breast milk may vary according to

  • How many days a mother has been lactating
  • The time of day
  • Time elapsed since the beginning of a feed
  • The mother’s diet
  • The frequency between feedings

There are more than 200 constituents of breast milk known to science. This article describes just a few of them. As laboratory methods become more refined, new constituents are discovered.

bottle of breast milk against cheerful background

Here I provide basic nutritional information, and offer tips for assessing and improving the quality of your milk.

Elsewhere, I discuss how the food you eat affects the flavor of your milk. For details, see this article about early flavor experiences influence your baby’s food preferences.

Basic nutritional information

Asking what’s in breast milk might seem like a straightforward question. But the answer is “it depends.” It varies from woman to woman, and from time point to time point.

When researchers analyzed more than twenty studies of breast milk composition, they calculated the following average values for milk expressed by women between 2 weeks and 6 weeks postpartum (Hester et al 2012). Each 100 mL of breast milk (produced within this time frame) yields approximately

  • 65 calories
  • 6.7 g carbohydrates (primarily lactose)
  • 3.8 g fat
  • 1.3 g protein

As noted, these are averages taken at a specific point in time. In the real world, the values vary considerably.

For example, the fat content of milk produced by the same breast may vary by as much as 2g/L over 24 hours (Institute of Medicine, National Academy of Sciences 1991, p. 118).

In addition, as I note in this article, the fat content in milk can change dramatically over the course of a single feeding session.

Milk expressed at the beginning of a feed tends to be substantially lower in fat. When babies are restricted in their time at the breast, they can fill up on this “foremilk” and miss out on the richer, higher quality “hindmilk” that is produced toward the end of a feeding session.

Then there is variation between lactating women.

Some variation may reflect national differences in diet. For instance, studies suggest that mothers in India tend to have lower-fat milk than mothers in the United States (Hester et al 2012).

But there are also noteworthy differences between women living the same population:

Milk fat content may range from 2g/100mL to 5g/100mL (Institute of Medicine, National Academy of Sciences 1991).

Because fat constitutes the majority of calories in milk, these differences can have a big impact on caloric density. All else being equal, mothers producing “lighter,” lower calorie breast milk will need to nurse their babies more frequently.


Milk composition also varies over the course of lactation. Mature breast milk looks very different from colostrum, the milk produced in the first few days after birth. According to Hester and colleagues (2012), each 100 mL of colostrum yields approximately:

  • 53.6 calories
  • 5.6 g carbohydrates
  • 2.2 g fat
  • 2.5 g protein

Colostrum is low in fat and carbohydrates. As a result, there are fewer calories in breast milk for the first few days of a baby’s life.

Colostrum is yellow because it contains high levels of beta carotene (10 times more than is found in mature milk).

Colostrum also contains elevated levels of vitamin E and zinc.

Milk composition changes rapidly during the first week, increasing in fat and lactose concentration and decreasing in protein-, vitamin-, and mineral content. After the first 7 days, changes continue at a slower rate until milk reaches the “mature” stage around Day 21 (Institute of Medicine, National Academy of Sciences 1991).

Until recently, people believed that the nutritional content and calories in breast milk didn’t change much after reached the mature stage. But now we know otherwise.

It appears that total fat content continues to increase after three months postpartum. In a study tracking the same lactating mothers over time, researchers found that the fat content in milk expressed at 6 months was higher than the fat content in milk expressed at 3 months (Szabó et al 2010). Such findings are consistent with the results of a weaker, correlational study (Mandel et al 2005). And other research confirms that fat content, fatty acid composition, and energy density of milk all continue to change in the months after initiating lactation (Thakkar et al 2013).

Human breast milk in zoological perspective

What does this nutritional information mean?

In part, it reflects our status as a slow-growing species dependent on frequent infant feeds. To understand why, you need to compare human milk with the milk of other mammals.

Zoologically speaking, there aren’t a lot of calories in breast milk. This is because human milk is relatively low in fat. It’s also low in protein.

Consider how humans stack up against these animals (all values are given in percentage weight—all data from Jenness 1974).

  • Human: 3.8% fat; 1% protein; 7% lactose
  • Cow: 3.7% fat; 3.4% protein; 4.8% lactose
  • Rat: 10.3% fat; 8.4% protein; 2.6% lactose
  • Dog: 12.9% fat; 7.9% protein; 3.1% lactose
  • Rabbit: 18.3% fat; 13.9% protein; 2.6% lactose

Protein is a determinant of growth rates, so the low protein content of human milk is one reason why human babies don’t grow as fast as baby rabbits or puppies (Bernhart 1961).

The low fat content of breast milk reflects, in part, our heritage as mammals that carry their babies around, rather than cache them in nests (Ben Shaul 1962).

Mammals that cache their babies leave them for long periods during the day. As a result, babies go for a long time between feedings. They need high-calorie, high-fat milk to sustain them.

When babies stick close to mother all day—-as was the case for our ancestors—-babies nurse frequently. In such frequently-feeding species, babies don’t require high fat milk.

So, compared with the milk of other mammals, human milk has fairly paltry amounts of protein and fat.

Nonetheless, that fat and protein is extremely important.

Understanding fat

By weight, fat makes up only a small portion of human milk. But it’s responsible for the majority of the calories in breast milk. And it is important for your infant’s health.

Fat is needed to metabolize many vitamins, and it affects growth rates. In addition, some types of fatty acids found in breast milk–the long-chain polyunsaturated fatty acids, or LCPs–may play important roles in brain development. The most well-known is docosahexanoic acid (DHA).

DHA helps the brain manufacture myelin, a sheath that insulates nerve fibers. In research conducted decades ago, researchers found that the brains of breastfed babies had higher concentrations of DHA than did the brains of formula-fed babies (Makrides et al 1994).

Some have speculated that a postpartum diet high in DHA contributes to the cognitive development, as measured by IQ tests. But this remains controversial. Some carefully controlled, longitudinal studies have shown no differences in developmental outcomes between formula-fed babies receiving supplemental DHA and formula-fed controls (Auestad et al 2001; Auestad et al 2003). Possibly, these studies didn’t give a high enough dose of DHA to infants. In studies where formula-fed infants were given higher levels of DHA (greater than or equal to 0.30%), supplementation had a beneficial effect on neurological development for the first 4 months of life (Hadders-Algra et al 2007).

Cholesterol is another fatty component of breast milk important for brain development. Like DHA, cholesterol is crucial to the production of myelin (Pond 2003).


There are two classes of proteins in milk-—the caseins and the wheys.

Caseins turn into clots or curds in the stomach.

The wheys remain liquid and are easier to digest.

About 60% of the proteins in breast milk are whey (Jenness 1971). That’s a lot compared with other mammals. In the cow, for instance, whey represents only 18% of milk protein (Jenness 1971). Most baby formulas are high in casein. This makes them harder to digest than breast milk.

We think of proteins as the building blocks of muscle and bone. But proteins serve a wide range of other functions, including defense against pathogens.

For instance, immunoglobulin A (IgA) is a protein that attacks respiratory viruses, bacteria, and intestinal parasites. Like other antimicrobial factors in human breast milk, it protects the respiratory and intestinal tracts of breastfeeding infants (Institute of Medicine, National Academy of Sciences 1991).

Other constituents

According to studies cited by the Institute of Medicine of the U.S. National Academy of Sciences (1991), each liter of mature breast milk also contains

  • cholesterol in concentrations ranging from 100 to 150 mg/L
  • calcium in concentrations ranging from 254 to 306 mg/L
  • sodium in concentrations ranging from 140 to 220 mg/L
  • phosphorus in concentrations ranging from 188 to 262 mg/L
  • vitamin C in concentrations of 50 to 60 mg/L (assuming the mother consumes more than 100mg vitamin C each day)
  • magnesium in concentrations of about 35 mg/L

and much smaller amounts of zinc, pantothenic acid, nicotinic acid, iodine, and vitamin A and copper. Breast milk contains trace amounts of other vitamins and minerals (including E, K, D, and the B vitamins) and a host of hormones, growth factors, and antiinfectious agents (Institute of Medicine, National Academy of Sciences 1991).

What about iron?

Iron concentrations may range from 0.2 to 0.9 mg/L, and they seem to depend on the mother’s iron status and the length of time a woman has been breastfeeding. In one study of Finnish mothers, milk expressed in the first weeks postpartum had about twice the iron content as milk produced 9 months later (Siimes et al 2008).

Calories in breast milk: A rough guide for observing changes in your own milk

Precise measurements of the calories in breast milk require fancy lab equipment. But you can get a rough idea of how the calories in breast milk change over time by collecting milk from a breast pump. A study by Wang et al (1999) has demonstrated that calories in breast milk can be estimated from the amount of cream that floats to the top of collected milk.

The researchers used a centrifuge to separate the cream from the milk. You probably don’t have one, but you can still observe the amount of cream that forms at the top of a cup of freshly expressed milk.

After pumping, put the milk in a transparent container (so you can observe the cream column from the side). Leave in a cool place for 12-24 hours. This will permit the cream to rise. For best results, the container should be relatively shallow, so the cream doesn’t have to travel very far.

Measure the length of the cream column. The higher the column, the more fat and calories in breast milk.

By conducting multiple experiments, you may be able to notice changes in fat content over time.

Improving the quality of your breast milk

There is no convincing evidence that you can significantly change the lactose content of your milk.

But studies suggest that diet and breast feeding practices can influence the protein-, vitamin-, and fatty acid composition of breast milk (Institute of Medicine, National Academy of Sciences 1991; Woolridge 1995).

Here are some ideas for increasing the quality–and calories–in breast milk.

• Take prenatal vitamins. Don’t take additional supplements without consulting a doctor. Some vitamins—like A and D—can have toxic effects in high doses.

• Don’t skimp on protein. Although protein levels remain pretty stable across a wide range of diets, populations subsisting on very low-protein diets are associated with low protein breast milk.

• Limit saturated fats. Western diets are very high in saturated fats. Babies who consume breast milk high in saturated fat may be at increased risk of developing high blood pressure and high cholesterol levels later in life (Leeson et 2001; Mott et al 1990).

• Increase your DHA intake. The DHA levels in American breast milk are among the lowest in the world. The populations with the highest DHA levels are those that eat significant quantities of fish (Brenna et al 2007). But it’s also possible to boost your DHA levels by consuming certain plant products, or taking plant-based omega-3 supplements.

• Breastfeed on demand. This permits your baby to adjust intake in response to changes in your breast milk. For more information, see my article on the infant feeding schedule.

• Let your baby take his or her time at each breast. As noted above, premature breast-switching can rob babies of higher-fat “hind” milk.

If you found this article on the nutrients and calories in breast milk helpful, check out the other breastfeeding articles at Parenting

References: Nutrients and calories in breast milk

For very extensive information on the nutrients and calories in breast milk, I recommend Nutrition During Lactation. This volume is available online.

Auestad et al. 2001. Growth and Development in Term Infants Fed Long-Chain Polyunsaturated Fatty Acids: A Double-Masked, Randomized, Parallel, Prospective, Multivariate Study. Pediatrics 108 (2): 372-381

Auestad et al 2003. Visual, Cognitive, and Language Assessments at 39 Months: A Follow-up Study of Children Fed Formulas Containing Long-Chain Polyunsaturated Fatty Acids to 1 Year of Age. Pediatrics 112 (3): e177-e183

Ben Shaul DM. 1962. The composition of the milk of wild animals. International Zoo Yearbook 4: 333-342.

Bernhart FW. 1961. Correlations between growth-rate of the suckling of various species and the percentage of total calories from protein in the milk. Nature 191: 358-360.

Brenna TJ et al. 2007. Docosahexaenoic and arachidonic acid concentrations in human breast milk worldwide. Am J Clinical Nutrition 85: 1457-1464.

Department of Health and Social Security. 1988. Present day practice in infant feeding: third report: report of a Working Party of the Panel on Child Nutrition, Committee on Medical Aspects of Food Policy: Report on Health and Social Subjects 32. Her Majesty’s Stationery Office, London.

Gopalan C, Ramasastri BV, and Balasubramanian SC. 2000. Nutritive value of Indian foods. Hyderabad, India: NIN, ICMR.

Guthrie AH. 1989. Introductory Nutrition. St. Louis, MO: Times Mirror/Mosby College Publishing.

Hadders-Algra M. Bouwstra H, van Goor SA, Dijck-Brouwer DA and Muskiet FA. Prenatal and early postnatal fatty acid status and neurodevelopmental outcome. 2007. Journal of Perinatal Medicine 35 Suppl 1: S28-34.

Hester SN, Hustead DS, Mackey AD, Singhal A, and Marriage BJ. 2012. Is the macronutrient intake of formula-fed infants greater than breast-fed infants in early infancy? Journal of Nutrition and Metabolism: 891201.

Institute of Medicine, National Academy of Sciences. 1991. Nutrition during lactation. Washington, DC: National Academy Press.

Jenness 1974. Biosynthesis and composition of milk. Journal of investigative dermatology. 63: 109-118.

Leeson CPM, Katterhorn M, Deanfield JE and Lucas A. 2001. Duration of breastfeeding and arterial distensibility in early adult life: population based study. BMJ 322: 643-7.

Makrides et al 1994. Fatty acid composition of brain, retina, and erythrocytes in breast- and formula-fed infants. Am Journal Clinical Nutrition 60: 189-194.

Mandel D, Lubetsky R, Dollberg S, Barak S, Mimouni FB. 2005. Fat and energy contents of expressed human breast milk in prolonged lactation. Pediatrics 116:e432-e435.

Mott GE, Jackson EM, McMahan CA, McGill HZ. 1990. Cholesterol metabolism in adult baboons is influenced by infant diet. J Nutrition. 120:243–251

Pond WG. 2003. Dietary Fatty Acids and Cholesterol in Normal Brain Development. Comments on Theoretical Biology, 8(1): 37-68.

Siimes MA, Vuori E, Kuitunen P. 2008. Breast milk iron–a declining concentration during the course of lactation. Acta Pædiatrica 68(1): 29 – 31.

Szabó E, Boehm G, Beermann C, Weyermann M, Brenner H, Rothenbacher D, Decsi T. 2010. Fatty acid profile comparisons in human milk sampled from the same mothers at the sixth week and the sixth month of lactation. J Pediatr Gastroenterol Nutr. 50(3):316-20.

Thakkar SK, Giuffrida F, Cristina CH, De Castro CA, Mukherjee R, Tran LA, Steenhout P, Lee le Y, Destaillats F. 2013. Dynamics of human milk nutrient composition of women from Singapore with a special focus on lipids. Am J Hum Biol. 25(6):770-9

Wang CD, Chu PS, Mellen BG, and Shenat JP. 1999. Creamotocrit and the nutrient composition of human milk. Journal of Perinatology 19(5): 343-346.

Woolridge MW. 1995. Baby-controlled breastfeeding: Biocultural implications. In: Breastfeeding: Biocultural perspectives. P. Stuart-Macadam and KA Dettwyler (eds). New York: Aldine deGruyter.

Content of “Nutrition and calories in breast milk last modified 2012

image of bottle by istock/ kieferpix