Changes in Nutritional Metabolism During Weaning

Abstract

In all marsupial species that have been studied, milk composition changes greatly during lactation. For example, in Macropus eugenii, the milk is high in carbohydrate and low in lipid until the start of weaning but then changes to become high in lipid and low in carbohydrate during the next few weeks (Chapter 4). While the young is totally dependent on milk it is provided with energy largely in the form of simple carbohydrate (sugars), which is readily utilisable and which can be aminated and incorporated into the amino acid pool. During weaning, the form of energy supply changes to lipids derived from the milk, and volatile fatty acids produced by microbial fermentation of the complex carbohydrates (cellulose and starch) present in herbage. Many fore-stomach microorganisms of eutherians do not readily tolerate a diet high in sugars (Schwartz and Gilchrist 1975), largely because bacteria rapidly ferment the sugars to lactate, which causes the pH of the fore-stomach contents to fall to levels that are not well tolerated by fore-stomach protozoa (Mackie et al. 1978). Since a balanced population of bacteria and protozoa in the fore-stomach is necessary for optimal digestion of herbage, it is likely that the decline in milk sugar allows the establishment of this mixed population of microorganisms. The milk lipids will not be fermented in the fore-stomach and so will be fully available to the young, providing the increased energy required at the time of pouch exit. Nevertheless, this radical nutritional change is a major hurdle for the young. Since the sugar content of the milk must fall before fermentation of plant cell walls can begin and a supply of the gluconeogenic fatty acid, propionate, becomes available, the young must supply its glucose requirements by gluconeogenesis from protein. At this time the young is still growing rapidly, so there will be competition for protein between growth and energy requirements. It is of considerable interest that new proteins appear in the milk at this time, particularly the so-called late lactation protein described by Nicholas in Chapter 6, and these cause an increase in the total concentration of protein in the milk. No function has yet been ascribed to late lactation protein, which forms about 25% of milk protein by Day 250 in M. eugenii, but it is tempting to speculate that the increase in milk protein may provide extra substrate for gluconeogenesis.