Effects of experimental weight perturbation on skeletal muscle work efficiency in human subjects

1 Division of Molecular Genetics, Departments of Pediatrics and Medicine, College of Physicians and Surgeons, Columbia University, New York 10032; 3 Division of Exercise Physiology, Department of Medicine, Columbia University, New York 10032; 6 Laboratory of Human Behavior and Metabolism, Rockefeller University, New York 10021; 8 Division of Cardiology, Department of Pediatrics, The New York Hospital/Cornell University Medical College, New York 10021; 5 St. Luke's/Roosevelt Hospital Medical Center, New York, New York 10025; 2 Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania 19104; 4 Laval University, Quebec, Canada G1K 7P4; and 7 Department of Endocrinology, University of Vermont College of Medicine, Burlington, Vermont 05405 Submitted 7 August 2002 ; accepted in final form 25 February 2003 Maintenance of reduced or elevated body weight results in respective decreases or increases in energy expended in physical activity, defined as 24-h energy expenditure excluding resting energy expenditure and the thermic effect of feeding, beyond those attributable to weight change. We examined skeletal muscle work efficiency by graded cycle ergometry and, in some subjects, rates of gastrocnemius muscle ATP flux during exercise by magnetic resonance spectroscopy (MRS), in 30 subjects (15 males, 15 females) at initial weight and 10% below initial weight and in 8 subjects (7 males, 1 female) at initial weight and 10% above initial weight to determine whether changes in skeletal muscle work efficiency at altered body weight were correlated with changes in the energy expended in physical activity. At reduced weight, muscle work efficiency was increased in both cycle ergometry [mean (SD) change = +26.5 (26.7)%, P < 0.001] and MRS [ATP flux change = -15.2 (23.2)%, P = 0.044] studies. Weight gain resulted in decreased muscle work efficiency by ergometry [mean (SD) change = -17.8 (20.5)%, P = 0.043]. Changes in muscle efficiency at altered body weight accounted for 35% of the change in daily energy expended in physical activity. energy metabolism; exercise; obesity; weight gain; weight loss Address for reprint requests and other correspondence: M. Rosenbaum, Div. of Molecular Genetics, Russ Berrie Medical Science Pavilion, 6 th Fl., 1150 St. Nicholas Ave., New York, NY 10032 (E-mail: mr475{at}columbia.edu ).