Oxygen-A Critical, but Overlooked, Nutrient

Abstract

Gaseous oxygen is essential for all aerobic animals, without which mitochondrial respiration and oxidative phosphorylation cannot take place. It is not, however, regarded as a "nutrient" by nutritionists and does not feature as such within the discipline of nutritional science. This is primarily a consequence of the route by which O₂ enters the body, which is via the nose and lungs in terrestrial animals as opposed to the mouth and gastrointestinal tract for what are customarily considered as nutrients. It is argued that the route of entry should not be the critical factor in defining whether a substance is, or is not, a nutrient. Indeed, O₂ unambiguously meets the standard dictionary definitions of a nutrient, such as "a substance that provides nourishment for the maintenance of life and for growth" (Oxford English Dictionary). O₂ is generally available in abundance, but deficiency occurs at high altitude and during deep sea dives, as well as in lung diseases. These impact on the provision at a whole-body level, but a low pO₂ is characteristic of specific tissues includings the retina and brain, while deficiency, or overt hypoxia, is evident in certain conditions such as ischaemic disease and in tumours - and in white adipose tissue in obesity. Hypoxia results in a switch from oxidative metabolism to increased glucose utilisation through anaerobic glycolysis, and there are extensive changes in the expression of multiple genes in O₂-deficient cells. These changes are driven by hypoxia-sensitive transcription factors, particularly hypoxia-inducible factor-1 (HIF-1). O₂ deficiency at a whole-body level can be treated by therapy or supplementation, but O₂ is also toxic through the generation of reactive oxygen species. It is concluded that O₂ is a critical, but overlooked, nutrient which should be considered as part of the landscape of nutritional science.

Keywords: adipocyte; adipose tissue; hypoxia; hypoxia-inducible factor-1 (HIF-1); mitochondria; oxygen deficiency; respiration.

Figure 1

Schematic representation of some of the central cellular responses to hypoxia (oxygen deficiency) in white adipocytes. The figure illustrates adaptations that are universal to all cell types, particularly the increase in glucose utilisation through anaerobic glycolysis and the reduction in respiration and oxidative phosphorylation (ox phos). Adaptations that are specific to adipocytes are also shown, primarily those relating to lipid utilisation and the production of adipokines as key secretory proteins of these cell types; in some of the examples, such as MT-3 (metallothionein-3), only major changes at the gene expression level have been formally documented. angptl4, angiopoietin-like protein-4; enzy, enzyme; FA, fatty acid; GLUT1, facilitative glucose transporter 1; HIF-1, hypoxia-inducible factor-1; MCT1, monocarboxylate transporter-1; MIF, macrophage migration inhibitory factor; MMPs, matrix metalloproteinases; PAI-1, plasminogen activator inhibitor-1; TF, transcription factors (additional to HIF-1); VEGF, vascular endothelial growth factor. Adapted from (1).