Sativa plants will lose their leaves far more rapidly than indica plants will.

It is ok to lose fan leaves on a natural basis. The plant isn't jsut sucking nutes...in some cases, it's purposely dropping the leaves so light can penetrate more on the flowers.

As long as you bud growth has plenty of leaves in the bud itself, the growth will continue on the flower itself.

Its a natural process that you can interupt with more nitrogen. However, you run the risk of harsh smoke at the end.

This is part of the "ART" of the grow. Its your call.