Learn Italian: Penso che il pasto sia nutriente. - I think that the meal is nutritious.

We use the subjunctive mood because the sentence starts with Penso che (I think that). In Italian, verbs expressing opinions, thoughts, or personal beliefs trigger the subjunctive in the clause that follows them. sia is the third-person singular present subjunctive form of the auxiliary verb essere (to be).

Is sia only used for third-person subjects like 'he', 'she', or 'it'?

Actually, in the present subjunctive, the verb forms for I, you (singular), and he/she/it are exactly the same! Therefore, sia can mean 'I am', 'you are', or 'he/she/it is' depending on the context. In this sentence, the subject is clearly il pasto (the meal), so we know it means 'it is'.

Since il pasto is masculine, shouldn't the adjective end in -o?

Not in this case. nutriente belongs to a group of adjectives that naturally end in -e in their dictionary form. These adjectives use the exact same ending for both masculine and feminine singular nouns. If you were talking about multiple meals (i pasti), the ending would change to an -i (nutrienti).