Changes in macroingredients of guinea pig milk through lactation.

Milk samples were collected daily from English short-hair albino guinea pigs for 21 d. Analyses included protein, fat, lactose, ash, calories, and specific gravity. All components except lactose increased in concentration gradually from the beginning to end of lactation. Protein percentage began at 6% on d 1 and was 14% on d 21. The pattern of increase was either quadratic or exponential but not rectilinear. Patterns were similar for fat, ash, and calories. Fat on d 1 was 5.6% and 9.0% on d 20. Ash was .97% on d 1 and 1.4% on d 19. Calories per gram milk were 939 on d 1, increasing gradually to 1874 on d 21. Lactose dropped from 5.5% on d 1 to 5.0% on d 2. It remained at 5% until d 6, when it began a gradual decline to .5% on d 20. Thickening of milk and increasing concentrations of protein, fat, ash, and calories, as well as a marked drop in milk volume, resulted from the precipitous decline in lactose concentration of .27%/d from d 6 to 21. Because lactose is the primary controller of osmotic equilibrium and, therefore, the primary regulator of the water content of milk, its decrease in concentration explains the decline in milk volume and the concomitant increases in fat, protein, and solids from d 6 to the end of lactation.