Nutrition and health...

The composition of milk differs widely between species. Factors such as the type of protein; the proportion of protein, fat, and sugar; the levels of various vitamins and minerals; and the size of the butterfat globules and the strength of the curd are among those than can vary.[30] For example:

Human milk contains, on average, 1.1% protein, 4.2% fat, 7.0% lactose (a sugar), and supplies 72 kcal of energy per 100 grams.
Cow's milk contains, on average, 3.4% protein, 3.6% fat, and 4.6% lactose, 0.7% minerals[31] and supplies 66 kcal of energy per 100 grams. See also Nutritional value further on.
Donkey and horse milk have the lowest fat content, while the milk of seals and whales can contain more than 50% fat.[32][33] High fat content is not unique to aquatic mammals, as guinea pig milk has an average fat content of 46%

Milk composition analysis, per 100 grams

Constituents unit Cow Goat Sheep Water
buffalo
Water g 87.8 88.9 83.0 81.1
Protein g 3.2 3.1 5.4 4.5
Fat g 3.9 3.5 6.0 8.0
Carbohydrate g 4.8 4.4 5.1 4.9
Energy kcal 66 60 95 110
Energy kJ 275 253 396 463
Sugars (lactose) g 4.8 4.4 5.1 4.9
Cholesterol mg 14 10 11 8
Calcium IU 120 100 170 195
Saturated fatty acids g 2.4 2.3 3.8 4.2
Monounsaturated fatty acids g 1.1 0.8 1.5 1.7
Polyunsaturated fatty acids g 0.1 0.1 0.3 0.2

These compositions vary by breed, animal, and point in the lactation period.

Milk fat percentages

Cow breed Approximate percentage
Jersey 5.2
Zebu 4.7
Brown Swiss 4.0
Holstein-Friesian 3.6

The protein range for these four breeds is 3.3% to 3.9%, while the lactose range is 4.7% to 4.9%.

Milk fat percentages can be manipulated by dairy farmers' diet formulation strategies. Mastitis infection can cause fat levels to decline.