Jeffrey J. Zachwieja

Undermodeling affects minimal model indexes: insights from a two-compartment model

The classic (hereafter cold) and the labeled (hereafter hot) minimal models are powerful tools to investigate glucose metabolism. The cold model provides, from intravenous glucose tolerance test (IVGTT) data, indexes of glucose effectiveness (SG) and insulin sensitivity (SI) that measure the effect of glucose and insulin, respectively, to enhance glucose disappearance and inhibit endogenous glucose production. The hot model provides, from hot IVGTT data, indexes of glucose effectiveness ([Formula: see text]) and insulin sensitivity ([Formula: see text]) that, respectively, measure the effects of glucose and insulin on glucose disappearance only. Recent reports call for a reexamination of some of the assumptions of the minimal models. We have previously pointed out the criticality of the single-compartment description of glucose kinetics on which both the minimal models are founded. In this paper we evaluate the impact of single-compartment undermodeling on SG, SI,[Formula: see text], and[Formula: see text] by using a two-compartment model to describe the glucose system. The relationships of the minimal model indexes to the analogous indexes measured with the glucose clamp technique are also examined. Theoretical analysis and simulation studies indicate that cold indexes are more affected than hot indexes by undermodeling. In particular, care must be exercised in the physiological interpretation of SG, because this index is a local descriptor of events taking place in the initial portion of the IVGTT. As a consequence, SG not only reflects glucose effect on glucose uptake and production but also the rapid exchange of glucose between the accessible and nonaccessible glucose pools that occurs in the early part of the test.

Prediction of energy expenditure in a whole body indirect calorimeter at both low and high levels of physical activity

OBJECTIVES: In studies that involve the use of a room calorimeter, 24u2005h energy intake is often larger than 24u2005h energy expenditure (24u2005h EE) because of a decrease in activity energy expenditure due to the confined space. This positive energy balance can have large consequences for the interpretation of substrate balances. The objective of this study was to develop a method for predicting an individuals 24u2005h EE in a room calorimeter at both low (1.4×RMR) and high (1.8×RMR) levels of physical activity.METHODS: Two methods are presented that predict an individuals 24u2005h EE in a metabolic chamber. The first method was based on three components: (1) a 30u2005min measurement of resting metabolic rate (RMR) using a ventilated hood system; (2) measurement of exercise energy expenditure during 10u2005min of treadmill walking; and (3) estimation of free-living energy expenditure using a tri-axial motion sensor. Using these measurements we calculated the amount of treadmill time needed for each individual in order to obtain a total 24u2005h EE at either a low (1.4×RMR) or a high (1.8×RMR) level of physical activity. We also developed a method to predict total 24u2005h EE during the chamber stay by using the energy expenditure values for the different levels of activity as measured during the hours already spent in the chamber. This would provide us with a tool to adjust the exercise time and/or energy intake during the chamber stay.RESULTS: Method 1: there was no significant difference in expected and measured 24u2005h EE under either low (9.35±0.56 vs 9.51±0.47u2005MJ/day; measured vs predicted) or high activity conditions (13.41±0.74 vs 13.97±0.78u2005MJ/day; measured vs predicted). Method 2: the developed algorithm predicted 24u2005h EE for 97.6±4.0% of the final value at 3u2005h into the test day, and for 98.6±3.7% at 7u2005h into the test day.CONCLUSION: Both methods provide accurate prediction of energy expenditure in a room calorimeter at both high and low levels of physical activity. It equally shows that it is possible to accurately predict total 24u2005h EE from energy expenditure values obtained at 3 and 7u2005h into the study.