When growing up, our nails appear to be translucent and pinkish because of the blood in the nail bed underneath them.
But as toenails — namely the big toe — suffer years of strain from the pressure of the shoe, they thicken up to protect itself.
Over time, the big toe nail (which starts off at around 2mm thick) can more than double in thickness, causing the toe nails to look less opaque and instead to take on the colour of the nail cells themselves, which are naturally yellow.
‘The body’s response to constant impact is to create an extra layer of skin or nail to protect itself,’ says Emma Ashdown, a podiatrist in Ashtead, Surrey.
‘It’s similar to the way we find calluses form on our hands if we’re doing a lot of manual work,’ she says.