The Short Version
No credible scientific evidence supports the assertion that warm water hydrates human cells more effectively than cold water. The only controlled human studies in the evidence pool found that cool (~16°C) water produced better overall hydration outcomes through higher voluntary intake and lower fluid losses. Claims favoring warm water rely on speculative mechanisms from non-peer-reviewed blogs and brand websites, none of which directly measured cellular hydration. Scientific consensus indicates water temperature has no meaningful effect on cellular hydration efficacy.