Comparison of our predicted Y_⋆ vs. [Fe/H]_⋆ relations in galaxies A–C (from top to bottom; light blue points) with the observed data of Mucciarelli et al. (2014, red squared) for blue horizontal-branch stars in the Galactic globular clusters M 30 and NGC 6397, Marconi & Minniti (2018, magenta pentagon) for a sample of RR Lyrae in the Galactic bulge, the He abundances in the Galactic open cluster NGC 6791 from McKeever et al. (2019, blue circle), and the He abundances in a sample of B-type stars within the solar neighbourhood from Nieva & Przybilla (2012, black star). The yellow star corresponds to the initial He abundance of the Sun (Serenelli & Basu 2010), for which [Fe/H] is computed assuming that the iron mass fraction scales with total metallicity, as in the solar photospheric chemical composition as derived by Grevesse & Sauval (1998).