Nutrition is well-recognized as a necessary component of educational programs for physicians. This is to be valued in that of all factors affecting health in the United States, none is more important than nutrition. This can be argued from various perspectives, including health promotion, disease prevention, and therapeutic management. In all cases, serious consideration of nutrition related issues in the practice is seen to be one means to achieve cost-effective medical care. These modules were developed to provide more practical knowledge for health care providers, and in particular primary care physicians. This module discusses the cyclic mechanisms involved in storage and mobilization of protein, fat, and carbohydrates. It is designed to illustrate the relevance of biochemistry in daily clinical practice as it relates to patient nutrition. Included are learning goals and objectives, a self-check of achievement with regard to goals, and references. Appendices include discussions of the regulation of glucose uptake by the liver, and the molecular mechanism of glucagon and insulin action on enzyme regulation. (CW)