Learn Italian: È importante che il cibo sia nutriente e abbia le proteine. - It is important that the food is nutritious and has proteins.

The phrase È importante che (It is important that) is an impersonal expression showing an opinion or necessity. In Italian, these expressions act as a trigger that requires the following verbs to be in the subjunctive mood. That is why we must use sia instead of è, and abbia instead of ha.

Both sia and abbia are used here for the third-person subject il cibo. Do these verb forms change if the subject is 'I' or 'you'?

No, they stay exactly the same! A tricky part of the present subjunctive is that the 'I', 'you' (singular), and 'he/she/it' forms are identical. For essere, all three are sia. For avere, all three are abbia. This is why Italians often include the subject pronoun (like io or tu) before a subjunctive verb if the subject is not already obvious from the context.

Why is it abbia le proteine with the article le, when the English translation just says 'has proteins'?

In Italian, you almost always need a definite article (like il or le) when talking about a broad category, concept, or substance. While English leaves the article out when speaking generally about things like proteins, Italian requires le before proteine.