 Oxygen, O2, is essential for aerobic organisms, just like traditional nutrients. However, O2 is not usually considered a nutrient because it is taken into the body through the lungs or gills instead of the digestive system. When O2 levels are low, such as at high altitude or while scuba diving, unique adaptations occur. Lack of O2 can lead to emphysema, and without O2, cells must switch to anaerobic glycolysis. Hypoxia-sensitive transcription factors are activated, leading to changes in gene expression. Therefore, O2 should be included in the scope of nutrition research alongside other essential macronutrients. This article was authored by Paul Trehan.