Table 3.

Summary of variance component analysis of IGF-1 levels across age groups

Agea	N	r (MZ)*	r (DZ)*	χ 2_corr	V AD	V TW	V RS
All ages	4,471	0.55	0.31	88.8***	0.42 ± 0.06	0.10 ± 0.05	0.47 ± 0.02
16–52 years	2,086	0.52	0.28	18.8**	0.53 ± 0.04	0	0.47 ± 0.03
52–64 years	2,385	0.57	0.34	88.9***	0.56 ± 0.03	0	0.41 ± 0.02

r _(MZ) and r _(DZ) are correlations between IGF-1 levels within twin pairs by zygosity. Parameter estimates, with their corresponding standard errors, are given for best fitting and most parsimonious models in each analysis

N available sample size, χ ² _corr test for homogeneity of twin correlations, V _AD additive genetic component, V _TW common twin environment component, V _RS residual variance

*All correlations were significant at p < 0.01; **p < 0.001; ***p < 0.0001

^aAnalysis was conducted with simultaneous adjustment for age