Complex sentence: By the late 17th century, it had become normally as a supplementary rather than a major food because the main diet was around butter, milk, and grain products, it became a base food of the poor, especially in winter.
Split into simpler sentences: By the late 17th century, it had become common as a supplementary rather than a major food because the main diet was around butter, milk, and grain products. It became a base food of the poor, especially in winter.