Which is more beneficial for health

Weight management and satiety
Since water adds no calories, this version offers fullness with minimal energy intake. When soaked in milk, chia’s fiber combines with protein and fat, both of which have strong appetite-regulating effects.
So, for someone looking for low-calorie fullness and pre-meal appetite control, chia seeds in water is the better choice. And for someone who’s aiming for sustained satiety and reduced cravings, chia seeds in milk is ideal.

Hydration efficiency
Reviews describe chia seeds as able to absorb up to about 10–12 times their weight in liquid. When soaked in plain water, the soluble fibre and mucilage of chia seeds absorb significant amounts of liquid, forming a gel that can hold many times the seed’s weight in water.

When soaked in milk, chia seeds still hydrate, but the presence of proteins, fats, and other solids interferes with the maximum water absorption and gel formation.
For hydration, chia seeds in water are more efficient.