What nutrient is considered the most important for sustaining life after oxygen?

Water is deemed the most important nutrient for sustaining life after oxygen because it plays vital roles in numerous bodily functions. It is essential for maintaining hydration, which is critical for the proper functioning of cells, tissues, and organs. Water helps regulate body temperature through sweating and respiration, provides a medium for biochemical reactions, and assists in transporting nutrients and waste products throughout the body.

Without adequate water intake, the body can quickly become dehydrated, leading to serious health issues such as kidney failure, heat exhaustion, and diminished physical and cognitive performance. This underscores water's crucial role in sustaining life and overall health. In contrast, while carbohydrates, proteins, and fats are important for energy and growth, the human body can survive longer without them compared to water. Therefore, water is prioritized as a necessity for sustaining life immediately after the need for oxygen.