Basic nutritional informationAccording to a report by the United Kingdom, 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 proteinThese 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.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 proteinColostrum is low in fat and carbohydrates. As a result, there are fewer calories in breast milk for the first few days of a babys 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).Other constituentsAccording to the British study cited above, each 100mL of mature breast milk also contains an average of 76 mg iron 35 mg calcium 16 mg sodium 16 mg cholesterol 15 mg phosphorus 3.8 mg vitamin C 2.8 mg magnesiumAnd a bit in comparison to other mammels 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% lactoseProtein is a determinant of growth rates, so the low protein content of human milk is one reason why human babies dont 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.Sorry I know its a lot to read but well worth it.Oh plus theres 200 other magic ingredients unique to breastmilk that they don't even have names for!