Which of the following is not a common cause of nail brittleness?

A high protein diet is not a common cause of nail brittleness because protein is essential for the growth and strength of nails. Nails are primarily made of a protein called keratin, and adequate protein intake supports their structure. A diet rich in protein generally promotes healthy nail growth and can help improve their overall condition, making them less prone to brittleness.

In contrast, dehydration, frequent exposure to water, and excessive moisture can contribute to nail brittleness. Dehydration can lead to dry, weak nails that are prone to breaking. Frequent exposure to water, such as washing hands or doing dishes, can strip natural oils from the nails, leading to dryness and brittleness. Excessive moisture can also disrupt the balance of the nail's hydration, causing them to swell and then contract, which may result in splitting or brittleness. Therefore, a high protein diet supports nail health rather than detracts from it.