Learn Swedish: Läkaren sa att dieten saknar näring, men vi visste inte det. - The doctor said that the diet lacks nutrition, but we did not know that.

Just like in English, when reporting a general truth or an ongoing state, Swedish often keeps the subordinate clause in the present tense even if the reporting verb is in the past. The diet still lacks nutrition right now, so saknar stays in the present tense.

Doesn't saknar usually mean "to miss" someone?

Yes, att sakna is very commonly used to mean "to miss" a person or place (for example, jag saknar dig means "I miss you"). However, its other main meaning is "to lack" or "to be without" something. In this sentence, the diet lacks nutrition (näring).

Why is inte placed after visste here? Doesn't the BIFF-rule say that inte goes before the verb?

The BIFF-rule only applies to subordinate clauses. The word men (but) is a coordinating conjunction, which means it connects two independent main clauses. Because vi visste inte det is a main clause, the negation inte comes strictly after the finite verb (visste).

Can I use the word diet to talk about my normal everyday eating habits?

Usually, no. Just like in English, en diet almost always refers to a specific, restrictive eating plan, such as for weight loss or medical reasons. If you want to talk about your general, everyday food intake, the Swedish word kost is the natural choice.