"We will eat well therefore we will be healthy"

Translation: Tutakula vizuri kwa hiyo tutakuwa na afya

[saynave]

is the 'na' here necessary?

[machieng]

yes. Either that, or you can say 'tutakuwa wenye afya', but there has to be some connector there, since 'afya' is a noun and not an adjective

[AGreatUserName]

Yep, as machieng said, afya is the noun "health".

Tutakuwa afya = We are health. (?) That doesn't make much sense.

Tutakuwa na afya = We are with health. = We have health. = We are healthy.