Impact of the v=v 55 polymorphism of the uncoupling protein 2 gene on 24-h energy expenditure and substrate oxidation A Astrup 1 *, S Toubro 1 , LT Dalgaard 2 , SA Urhammer 2 , TIA Sùrensen 3 and O Pedersen 2 1 Research Department of Human Nutrition, The Royal Veterinary and Agricultural University, Frederiksberg; 2 Steno Diabetes Center and Hagedorn Research Institute; 3 Danish Epidemiology Science Centre at the Institute of Preventive Medicine, Copenhagen University Hospital, Copenhagen, Denmark OBJECTIVE: The gene that codes for a novel uncoupling protein, UCP2, has been linked to obesity in animal models. Markers encompassing the UCP2 locus have been linked to energy expenditure in humans. We studied the role of a common amino acid substitution, replacing an alanine (A) with a valine (V) at codon 55, of the coding region of the UCP2 gene for 24-h energy expenditure and respiratory quotient (RQ) in healthy subjects METHODS: 24-h energy expenditure and RQ were measured in calorimeters in 60 healthy subjects. The UCP2 polymorphism was determined by restriction fragment length polymorphism-generating polymerase chain reaction. RESULTS: Age, gender and body fat were not different between groups, the number of subjects in each groups was A=A: 35% (n  21), A=V: 48% (n  29), and V=V: 17% (n  10). Twenty-four-hour energy expenditure, adjusted for fat-free mass, fat mass, and spontaneous physical activity, was 311 kJ=d lower (95% con®dence interval: 24 ± 598 kJ=d, P  0.03) in the V=V homozygotes than in the A=A and A=V genotypes. The V=V had  20% higher 24-h spontaneous physical activity, particularly higher at night (P < 0.005). Energy expenditure due to higher spontaneous physical activity counteracted the V=V group's lower 24-h resting energy expenditure for a given body size and composition. 24-h RQ adjusted for energy balance, age, sex and spontaneous physical activity, was higher in the V=V homozygotes than in the AA and A=V groups (P < 0.05). CONCLUSIONS: Subjects with the V=V genotype of the UCP2 gene exhibit an enhanced metabolic ef®ciency and lower fat oxidation than the A=A and A=V genotypes. Keywords: energy expenditure; uncoupling protein-2; obesity; genetics; amino acid polymorphism Introduction Obesity is an increasing health problem and an improved understanding of its causes has high prior- ity. A number of studies have indicated the 40 ± 70% of the variation in body mass index and fat mass may be genetically determined. 1 Although several single- gene mutations have been shown to cause obesity in animal models, the situation in most forms of human obesity is much more complex. With rare exceptions, the most common forms of human obesity are thought to arise from the interactions of multiple genes, environmental factors, and behaviour, and obesity must be considered a multifactorial disorder. Among the genetically determined factors that may contribute to weight gain is a low energy expenditure (EE) for a given body size and composition. 2,3 In prospective studies it has been shown that a low level of EE for a given body size and composition is a risk factor for body weight gain. 4 Moreover, a meta-analysis has shown that formerly obese individuals are 5-fold more likely to have a low adjusted resting EE than matched controls. 5 It is likely that the recently discovered uncoupling protein-2, (UCP2), may be involved in the regulation of EE and fat oxidation. 6 UCP's are a group of mitochondrial proteins which dissipate the proton electrochemical gradient across the mitochondrial membrane. By this mechanism UCP may uncouple substrate oxidation from conversion of ADP to ATP, leading to generation of heat and thus increased EE. UCP1 is expressed exclusively in mammalian brown adipose tissue, which is only present in small amounts and lacks functional activity in humans. 7 By contrast, the gene encoding a novel human uncoupling protein, UCP2, is expressed in a wide range of human tissues, including important sites of thermogenesis such as skeletal muscle, heart, liver and kidney. 8 It has been demonstrated that UCP2 affects the mitochondrial activity and can, to a certain extent, uncouple the respiration. 8 Bouchard et al 9 studied potential linkage relationships between microsatellite markers encom- passing the UCP2 gene locus with resting EE and body fatness, and found that three markers were linked to resting EE in adult humans. This ®nding warranted further studies of DNA sequence variation in the gene itself, and a common amino acid *Correspondence: Arne Astrup, M.D., Research Department of Human Nutrition, The Royal Veterinary and Agricultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark. E-mail: ast@kvl.dk Received 10 November 1998; revised 31 March 1999; accepted 14 May 1999 International Journal of Obesity (1999) 23, 1030±1034 ß 1999 Stockton Press All rights reserved 0307±0565/99 $15.00 http://www.stockton-press.co.uk/ijo