Explorers going to the North or South Poles must carry green vegetables and fruit juices with them. Why is this necessary?

Explorers travelling to the north pole or the south pole will travel for a long time without access to fresh fruits or green vegetables. Not getting fresh fruits or vegetables for long periods can cause deficiency diseases such as scurvy. To prevent this, explorers going on long journeys to the north pole or the south pole carry with them large stocks of green vegetables and fruit juices.