Nutrients and calories in breast milk: A guide for the science-minded
© 2008 - 2012 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
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
- 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
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
Then there is variation between lactating women.
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
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
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.
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 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
We think of proteins as the building blocks of muscle and bone.
But proteins serve a wide range of other functions, including defense
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).
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
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;
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
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