Learn German: Isst du kein Protein, verlierst du Muskeln. - If you eat no protein, you lose muscle.

In German, you can drop the word wenn (if) to form a conditional clause. When you do this, the conjugated verb moves to the very beginning of the sentence. Isst du kein Protein means the exact same thing as Wenn du kein Protein isst.

Why does the second half of the sentence also start with a verb (verlierst)?

The entire first clause (Isst du kein Protein) occupies the first position in the overall sentence. Because German strongly requires the conjugated verb to sit in the second position of a main clause, verlierst has to come immediately after the comma.

The English translation uses the singular "muscle". Why does German use the plural Muskeln?

In English, "muscle" is often used as an uncountable mass noun (like "building muscle"). In German, der Muskel is strictly countable. To talk about your overall muscle mass or losing muscle in general, you have to use the plural Muskeln.

Why is it kein Protein instead of nicht Protein?

Protein is a noun. In German, you negate nouns that have no article (like uncountable mass nouns) using kein. You generally use nicht to negate verbs, adjectives, or specific nouns with definite articles.