Robert A. Hoerr

Protein Metabolism and Endurance Exercise

In brief: The use of proteins as a source of energy has been discounted in the past. However, current research has demonstrated that amino acids can contribute to whole body metabolism. The availability of amino acids can be increased by either elevating the rate of muscle protein breakdown or decreasing the rate of muscle protein synthesis. Research indicates that there is a substantial decrease in the rate of protein synthesis during exercise. We estimate that protein can provide up to 5.5% of the total caloric cost of exercise. Because essential amino acid requirements have been determined only for resting persons, the recommended maximum protein requirements may not be adequate for physically active individuals.

Phenylalanine flux in adult men: Estimates with different tracers and route of administration

The kinetics of three different tracers of phenylalanine were studied when continuously infused together either by an intravenous (IV) or intragastric (IG) route in six young healthy men during a fasting state. During IV infusion, mean flux values were 39.2 +/- 1.8, 40 +/- 3, 41.8 +/- 3.6 mumol.kg-1.h-1 for L-[ring-2H5], [15N], and L-[1-13C]phenylalanine, respectively (differences not significant). Fluxes were higher (P less than .001) during IG than IV infusion, indicating a disappearance of tracer during the first pass through the splanchnic area. Also, higher fluxes were found for [ring-2H5]phenylalanine (74 +/- 6.23 mumol.kg-1.h-1) compared with [15N] and L-[1-13C]phenylalanine (54.24 +/- 4.7 and 61.15 +/- 5.3 mumol.kg-1.h-1) during IG infusion. Proton exchange might explain this difference, possibly limiting in vivo use of this label when the tracer is to be administered by the IG route.

Alterations in Nutrient Intake and Utilization Caused by Disease

In this conference on human obesity, a focus of discussion is the disordered control of energy intake and expenditure that results in a clinically significant gain in body fat. Here the emphasis will be briefly on the effects of major systemic illness, where the disorders of energy intake and nutrient utilization often result in profound weight loss. In fact, the consequences of serious illness, including decreased appetite, lowered food intake, and possibly increased energy expenditure are synonymous with the goals for obesity therapy. There is extensive literature concerning the effects of disease on nutrient utilization, but the relationships between altered nutrient metabolism and the intake of food and nutrients are less well understood. For purposes of discussion, however, we will focus on two classes of disease; neoplasia, and injury together with sepsis. These conditions result in decreased food intake and major weight loss, often leading to cachexia. They are associated with qualitative and quantitative changes in energy substrate utilization (e.g. references 1-3). Specific examples will be used to illustrate a number of significant metabolic features that should be considered in reference to food intake regulation, because an exhaustive review of the subject is beyond our charge. Hence, a number of comprehensive reviews are provided in the references in order to give the reader an opportunity to gain relatively easy access to the more detailed literature (e .g . reference 4). Finally, it is hoped that by giving some attention to the consequences of systemic disease on food intake and nutrient utilization, the mechanisms of food intake regulation might be better understood and, in consequence, a more rational and effective approach to weight control in humans under various conditions might be achieved.