Recruitment was started in 1998 when 2640 intact twins from 22 consecutive birth years (1931-1952) were chosen via the Danish Population Register. The sample analyzed in this study includes only one from each twin pair. Both surveys included multidimensional face-to-face interviews aimed at the collection of socio-demographic information, assessment of physical, cognitive, depressive status, sensory impairments, medications, self-reported health status, and DNA sampling. Permission to collect blood samples and usage of register-based information was granted by The Danish National Committee on Biomedical Research Ethics.

Handgrip (HG) strength was measured by a handheld dynamometer (SMEDLEY's dynamometer TTM, Tokyo, Japan) while the subject was sitting with the arm close to their body. The test was repeated three times with the stronger hand; the maximum of these values was used in the analyses. When a test was not carried out, it was specified whether it was due to physical disabilities or because the subject refused to participate.

Survival analysis of UCP3 haplotypes was carried out by using AFT models as well. In particular, first, the probabilities of the individual haplotype combinations (diplotypes) were estimated by using an Expectation Maximization (EM) algorithm implemented in the haplo.stats package of R. Second, N dummy variables, corresponding to the number of different observed haplotypes, were obtained coding the reconstructed diplotype in an additive fashion (the count of a particular haplotype in the diplotype). Finally, AFT models were used for estimating the statistical significance of each haplotype, with the EM estimated probabilities used as weights. Since this approach underestimates the standard errors of rare haplotypes, they were not included in the model when their frequency was lower than 5%. As in the previous case, sex and BMI were used as adjunctive covariates in the model.

For the analysis of UCP3 genetic variability, SNPs were chosen in the genomic region 73,387,985-73,402,778 of chromosome 11, which corresponds to the coding region of UCP3 gene plus 5000 base pairs (bp) upstream and 1000 bps downstream (NCBI B36 assembly), to take into consideration possible regulatory elements of the gene. Tagging SNPs covering as much as possible of the known common genetic variation in UCP3 were chosen by analyzing HapMap genotype SNP data in HaploView software (http://www.broadinstitu- te.org/haploview/haploview, Barrett et al., 2005). In this analysis, the following criteria were adopted: Minimum Allele Frequency (MAF) of at least 5%, pairwise tagging only, r2 ≥ 0.8, LOD = 3, and a minimum distance between SNPs = 60 bp. The same program has been used for obtaining the graphical overview of Linkage Disequilibrium (LD) (definition of blocks based on confidence interval algorithm, as reported in Gabriel et al., 2002). Pairwise measures of LD between the analyzed loci were quantified by the correlation coefficient r2.