Lactoferrin exists on an iron saturation spectrum from fully apo (0% iron-saturated) to fully holo (100% iron-saturated). Natural human lactoferrin in milk is approximately 15–20% iron-saturated (mostly apo form). Most commercial bovine lactoferrin supplements are also predominantly apo form. The iron saturation state dramatically affects its biological activity.

If I'm iron deficient, should I take apo or holo lactoferrin?

For iron deficiency specifically, holo-lactoferrin (or a lactoferrin supplement specifically formulated with bound iron) is superior for iron supplementation purposes. Standard apo-lactoferrin still improves iron status but less efficiently than holo form.

Does lactoferrin interfere with iron supplements?

Taking apo-lactoferrin alongside iron salt supplements may reduce the amount of free iron available from the supplement (lactoferrin will bind some of it). Take lactoferrin at a different time from iron sulfate or fumarate supplements.