This is the 12004th most frequent Russian word.
здоровую
"Healthy" (feminine accusative form).
In this sentence, 'здоровую' is the accusative singular feminine form of the adjective 'здоровый', used to describe 'диету' (diet), aligning grammatically as an attribute.
Я стараюсь придерживаться здоровую диету.
I try to stick to a healthy diet.
Here, 'здоровую' again acts as an adjective in its accusative singular feminine form describing 'рыбу' (fish), indicating the state of being healthy.
На рынке мы купили здоровую рыбу.
At the market, we bought a healthy fish.
This sentence uses 'здоровую' in conjunction with 'жизнь' (life) in a slightly erroneous way, grammatically, it should be 'здоровой жизнью' in instrumental case, but this can be reflective of speech nuances.