Mechanical Efficiency of Normal-Weight Prepubertal Boys Predisposed to Obesity YITZHAK WEINSTEIN 1,2 , TAMIR KAMERMAN 1 , ELLIOT BERRY 3 , and BAREKET FALK 1 1 The Ribstein Center for Sport Medicine Sciences and Research, Wingate Institute, ISRAEL; 2 Life Science Division, School of Physical Education, Ohalo College, Katzerin, ISRAEL; and 3 Hadassah Medical School, Jerusalem, ISRAEL ABSTRACT WEINSTEIN, Y., T. KAMERMAN, E. BERRY, and B. FALK. Mechanical Efficiency of Normal-Weight Prepubertal Boys Predisposed to Obesity. Med. Sci. Sports Exerc., Vol. 36, No. 4, pp. 567–573, 2004. Purpose: To compare 1) energy expenditure during rest and during submaximal exercise, and 2) the mechanical efficiency of normal-weight boys liable to obesity with normal-weight boys who are not liable to obesity. Methods: Two groups of prepubertal boys, aged 9 –12 yr were compared, one with both parents of normal weight (NP, 20  BMI  27, N = 20) and the other (N = 20) with one obese parent (OP, BMI  30). Results: No significant differences were found between the two groups in the anthropometric measurements (means  SD): body mass (32.9  5.4 and 31.5  3.1 kg, NP and OP, respectively), stature (141.0  6.2 and 140.0  5.5 cm, NP and OP, respectively), and body fat (16.6  3.5 and 15.1  3.5%, NP and OP, respectively). Likewise, there were no differences in the reported physical activity habits. No differences were observed in the resting metabolic rate values between the two groups (5.071  0.351 and 4.956  0.386 MJ·d -1 , NP and OP, respectively). Submaximal V ˙ O 2 at 30, 45, and 60 W was similar in the two groups (0.63  0.05, 0.78  0.06, and 0.92  0.08; and 0.63  0.06, 0.78  0.08, and 0.95  0.08 L·min -1 , NP and OP, respectively). Likewise, the mechanical efficiency, calculated at 30, 45, and 60 W was similar in both groups (19.5  2.3, 21.8  2.2, 23.4  2.5%; and 19.5  3.0, 21.9  2.9, 22.6  2.5%, NP and OP, respectively. No differences were found between groups in their V ˙ O 2peak (38.4  3.8 and 40.4  4.9 mL·kg -1 ·min -1 , NP and OP, respectively). Conclusion: These data suggest that energy expenditure during rest and exercise may not be used to predict future obesity in normal-weight prepubertal boys predisposed to obesity. Key Words: PREPUBERTAL CHILDREN, PREDISPOSITION TO OBESITY, ENERGY EXPENDITURE, SUBMAXIMAL OXYGEN CONSUMPTION, RESTING METABOLIC RATE O besity is a common condition with delayed and usually late-onset aftermath. Because the preva- lence of obesity is increasing (6,18), it is important to understand who is at risk of becoming obese and for what reasons, in order to implement preventive measures. Obesity is of substantial public concern because it is related to cardiovascular disease, hypertension, and diabetes in adult- hood, and these are leading causes of death in the Western world (29). Maintenance of normal body-mass is defined by the bal- ance between energy intake (EI) and energy expenditure (EE) (15). Obesity is caused by an imbalance of EI relative to EE (9). However, the relative contributions of EI and EE to the increased occurrence of obesity are unclear. Obesity may result from high EI, low daily physical activity, low resting EE (resting metabolic rate; RMR), and high mechan- ical efficiency. High mechanical efficiency may be defined as low EE for a given activity (9) or as a relatively low rate of metabolic fat oxidation (31). Geissler et al. (9) reported that the metabolic efficiency of formerly obese women was about 15% higher than that of normal-weight women during sitting, sleeping, and exercise, and concluded that the for- merly obese women were more metabolically “efficient” than the normal-weight women. There are no known reports on the biochemical charac- teristics of energy utilization among children which may explain their predisposition to obesity. Therefore, the fol- lowing describes such reports among adults. Chitwood et al. (3) reported that normal-weight African-American women predisposed to obesity had higher rates of carbohydrate oxidation than normal-weight white American women pre- disposed to obesity. Acheson et al. (1) suggested that oxi- dation of carbohydrates may “save” as much as 6.5% of the oxygen consumption for a given biological function. Fur- thermore, Wade et al. (26) reported that overweight men oxidized less fat during a submaximal test than normal- weight men and attributed these findings to a higher meta- bolic efficiency among the overweight men. The authors suggested that the rate of fatty acids combustion to EE is related to fatness. Studies investigating the effect of parental obesity as a risk factor for their children to develop obesity have yielded diverse results. For example, Whitaker et al. (27) reported that children of obese parents have double the risk for future obesity. On the other hand, Griffiths and Payne (13) and Griffiths et al. (14) followed up normal-weight, 4- to 5-yr- old children for 12 yr and found no differences in body Address for correspondence: Dr. Yitzhak Weinstein, The Ribstein Center for Sport Medicine Sciences and Research, Wingate Institute 42902, Israel; E-mail: itzw@macam.ac.il. Submitted for publication January 2003. Accepted for publication December 2003. 0195-9131/04/3604-0567 MEDICINE & SCIENCE IN SPORTS & EXERCISE ® Copyright © 2004 by the American College of Sports Medicine DOI: 10.1249/01.MSS.0000121958.99985.A5 567