Tuesday, May 10, 2011

Nutrients and calories in breast milk

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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


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


ColostrumMilk

ColostrumMilk 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).


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.


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.


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 suggestions 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.

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