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.
Any posts made by me are purely fictional in nature and by no means is anything I say to be taken seriously. I do not grow or condone the growing of anything not legal. Any and all pictures I post are pictures widley available on the internet and any discussions I am involved in are purely hypothetical or are commentary in nature and should not constitute advice or be considered advice to assist in activities that are deemed illegal.