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

© 2008 Gwen Dewar, Ph.D., all rights reserved

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.

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

According to a British report, each 100 mL of mature breast milk (i.e., breast milk produced after 21 days of lactation) yields approximately

• 70 calories

• 89.97 g water

• 7.4 g carbohydrates (primarily lactose)

• 4.2 g fat

• 1.3 g protein

These are the averages of samples taken repeatedly over a 24-hour period (Department of Health and Social Security 1988).

Within each breast, milk composition fluctuates during the day. For example, fat content may vary by as much as 2g/L over 24 hours (Institute of Medicine, National Academy of Sciences 1991, p. 118). This causes significant variation in the calories in breast milk.

To some degree, milk composition varies by diet as well. The numbers above reflect the average milk composition of British women. Analyses carried out on other populations—consuming different diets—may yield slightly different results. For instance, an analysis conducted in India produced an estimate of 3.4 g fat per 100mL (Gopalan et al 2000).

But the greatest source of variation is found between individual women.

Within a given population, individual women may range in milk fat content from 2g/mL to 5g/mL (Institute of Medicine, National Academy of Sciences 1991).

Because fat constitutes the majority of calories in breast milk, this means that some women produce more calorie-dense milk than others. Moms producing fewer calories in breast milk will need to nurse their babies more frequently.

Colostrum

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 Guthrie (1989), each 100 mL of colostrum yields approximately:

• 58 calories

• 5.3 g carbohydrates

• 2.9 g fat

• 3.7 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).

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Until recently, people believed that the fat composition and calories in breast milk didn’t change much after it reached the mature stage. But it might.

A 2005 study reported that mothers who had been breastfeeding for over a year produced significantly fattier and more caloric milk than did mothers who breastfeed for less than six months (Mandel et al 2005).

The results are intriguing, but, because the study didn’t track women from childbirth on, it isn’t known if the prolonged breast-feeders had higher-fat milk to begin with. Women who produce higher-fat milk from the beginning might have more success breastfeeding and persist longer.

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 (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.

It’s also one of the most variable constituents of breast milk.

The fat content of human milk increases during a single nursing session. As a full breast empties, the milk it produces becomes more fatty.

In addition, the fat content—-and therefore calories in breast milk—-change in response to the frequency of feeds. More frequent feedings are associated with higher fat milk.

The fat content of milk is important for your baby’s health. As the principle source of calories in breast milk, fat determines how much milk your baby needs to consume to meet his needs. Fat content is an important determinant of growth rates. It is needed to metabolize many vitamins, so too little fat is associated with vitamin deficiencies.

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. The brains of breastfed babies have higher concentrations of DHA than do the brains of formula fed babies (Makrides et al 1994). Some have speculated that DHA is partly responsible for the IQ gap between breastfed and formula-fed babies.

However, 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). At present, infant formulas do not contain cholesterol.

Protein

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 assessing 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.

You can take a series of measurements over time to observe how the calories in breast milk change. By using this method, you may be able to determine what time of day is associated with a peak in your milk fat. You can also see if the calories in breast milk increase or decrease depending on changes you make in your diet or breastfeeding schedule (see below).

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). If you don’t like fish, you can buy DHA supplements in pill form. However, I feel cautious about this. Check with your physician.

• Breastfeed on demand. This permits your baby to adjust intake in response to changes in your breast milk. In addition, more frequent feedings may boost the fat content of your milk. For more information, see my article on the infant feeding schedule.

• Let your baby take his 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 Science.com.



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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.

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.

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 last modified 6/10