Learn Hardcore German: Ich achte auf meine Gesundheit.

The verb achten in German is a prepositional verb that always requires auf to link it to its object. Without auf, achten has no direct object and the sentence ich achte meine Gesundheit would be ungrammatical. You must say ich achte auf meine Gesundheit.

What case does meine Gesundheit take after auf, and how can I tell?

In the phrase achten auf, auf is one of the Wechselpräpositionen (two-way prepositions) but in this fixed idiom it always takes the accusative. Gesundheit is a feminine noun, and both its nominative and accusative forms look identical (die Gesundheit). With the possessive mein it appears as meine Gesundheit in both cases—but grammatically it’s accusative here because of auf.

Is auf in achten auf a separable prefix like in aufstehen?

No. In aufstehen, auf- is a separable prefix attached to the verb stem. In achten auf, auf is a preposition, not a prefix. It remains directly before its object (meine Gesundheit) and does not move to the end of the clause.

Why is the verb achte placed in second position in the sentence?

German main clauses follow the Verb-Zweitstellung (V2) rule: the finite verb must be the second element. Here Ich is the first element, so achte comes in second position. The rest of the sentence (auf meine Gesundheit) follows.

How does the word order change if I begin with Auf meine Gesundheit?

When you front Auf meine Gesundheit, you still keep the finite verb in second position. The sentence becomes:
Auf meine Gesundheit achte ich.
Notice that ich (the subject) now follows the verb.

Is ich achte auf meine Gesundheit reflexive? Do I need a reflexive pronoun like mich?

No, it’s not reflexive. Achten auf is a transitive prepositional verb taking a direct object in the accusative; no reflexive pronoun is used. You simply have the subject (ich), the verb (achte), and the prepositional object (auf meine Gesundheit).

What are some synonyms or alternative ways to express the same idea?

You could say:

Ich kümmere mich um meine Gesundheit. (I take care of my health.)
Ich passe auf meine Gesundheit auf. (I watch over my health; less common.)
Ich lebe gesund or Ich lebe gesundheitsbewusst. (I live healthily / consciously.)
Ich bleibe in Form. (I stay in shape.)
Each variant has its own nuance, but all convey attention to one’s health.

Why is Gesundheit capitalized in German?

All nouns in German are capitalized. Gesundheit is a noun, so it always begins with a capital letter.