One distinctive feature of the Fremen is their striking blue eyes. This transformation isn’t innate; it results from prolonged exposure to the Spice, a substance abundant in the desert. Across Frank Herbert’s “Dune” series, numerous characters exhibit this eye color change, with the Spice sometimes turning not just the irises but the entire eye blue.

The alteration of eye color is just one of many side effects from consuming Spice, and it’s not unique to the Fremen. Anyone who regularly ingests enough Spice will notice their eyes turning blue over time. While this phenomenon may seem minor within the grand scheme of “Dune,” Herbert intentionally included it to serve a purpose in his narrative.

Since “Dune” was first published in 1965, fans have speculated about the significance of characters with blue eyes. The Spice’s impact on eye color is often interpreted as a metaphor for the novel’s themes of imperialism. The Fremen, initially depicted as oppressed, eventually rise to challenge and conquer the galaxy. Some readers believe Herbert highlighted the Fremen’s blue eyes to draw parallels to the racial rhetoric used by historical imperial powers.

“Dune” has been around since 1965, and in that time the book’s fans have put a lot of thought into why so many of the novel’s characters have blue eyes. There’s no denying that the Spice’s effect on eye color has some serious thematic weight in “Dune.” Many view the story as being about imperialism, with the Fremen rising up from subjugation to eventually become militaristic dominators of the galaxy. They think that Frank Herbert chose to highlight the eye color of the Fremen to reference racist rhetoric used by real-world imperial regimes.

Someone else has a very different idea of why Herbert chose to make so many characters in his book have blue eyes. Herbert’s friend Paul Stamets has said that the sci-fi author based most of the lore surrounding the Spice on one of his favorite pastimes: taking psychedelic mushrooms. Herbert was apparently a big fan of magic mushrooms, and the Fremen’s blue eyes are meant to reflect the blue tinge that some mushrooms have.

When people ingest the Spice from Arrakis, it changes them on a physiological level, and the eyes aren’t the only part of the body it touches. The most dramatic effects of Spice are on the brain. Spice allows some people a limited ability to see the future. That’s why Guild Navigators – characters cut from Denis Villeneuve’s “Dune” – need Spice to safely travel the incredibly long distances between planets in the Imperium. Some people, like Paul Atreides, are especially affected by the Spice and can see the future on a level no other human can.

Spice also slows down the aging process, which is another reason why it’s the single most valuable substance on the entire planet. With enough Spice, people can extend their lives by decades or more. In most cases, that prolonged lifespan doesn’t come with any bizarre physical effects. In at least one famous instance, though, it did.

Paul and Chani’s son, Leto Atreides II, had a unique encounter with sand trout, infant sand worms that are also responsible for creating the Spice, during the events of “Children of Dune.” The sand trout fused to Leto’s skin and began transforming him into a human-worm hybrid. Thanks to that, Leto was able to live and rule over the empire for 3,500 years as God Emperor. Of course, Leto’s transformation wasn’t purely due to Spice, and it’s not something other users of the substance could realistically hope to replicate.