Plasma oestrogen and the regulation of egg weight in laying hens by dietary fats

It has been known for a long time that increasing the lipid content of layers diets can increase egg weight, but the mechanism for this response is not known. A series of experiments was carried out to obtain information on the nature of this mechanism. They indicated that in young laying hens (up to 30 weeks of age) the increase in egg weight was caused by increases in both yolk and albumen contents of the eggs. However in older hens, the increase in egg weight was attributable solely to an increase in the albumen content of the egg. This observation suggested that dietary fat caused a stimulation of oviduct protein synthesis. Oviduct protein synthesis is regulated by oestrogen and higher plasma oestradiol concentrations were found in hens fed a high fat diet. A large-scale experiment investigated responses to a range of different dietary fats and inclusion levels. The varied responses in egg weights observed with the different diets were found to be closely related to plasma oestradiol concentrations. It is concluded that oestrogen is important in controlling egg weight and that the effect of dietary fats in influencing egg weight is mediated by an effect of the fats on oestrogen metabolism.