Oxygen – a critical, but overlooked, nutrient
The Nutrition Society Paper of the Month for March is from the Journal of Nutritional Science and is entitled 'Oxygen – the forgotten nutrient'.
Author: Paul Trayhurn
Every nutritional scientist knows what constitutes the macro- and micronutrients, and they are extensively discussed in textbooks of nutrition. The macronutrients are considered to encompass proteins, carbohydrates and lipids – but is this the totality? There are actually two other macronutrients that are rarely, if ever, considered, one of which is water. The other is Oxygen; many in nutrition would not, however, consider this element to be a nutrient as such. In this article it is argued that O2 should be regarded as a nutrient and viewed as being within the remit of nutrition.
O2 is critical for aerobic organisms, and, indeed, it is required on an essentially continual basis with death ensuing within minutes of its deprivation (except under special circumstances such as deep sea dives by marine animals). The reason why O2 is not generally thought of as a nutrient is because of the route by which it is received. O2 is obtained, of course, through the lungs (or gills in the case of aquatic animals) rather than via the mouth and gastrointestinal tract, which is the route by which all other nutrients are delivered. But should a nutrient be restricted to something that is received only through the oral route? Dictionary definitions of a nutrient are along the lines of “any substance that is needed for the life and growth of living things” (Webster’s; on this basis, O2 is clearly a nutrient.
Unlike other nutrients which normally need to be released from the complex structure of the foods in which they are present, O2 requires no prior processing and is immediately available once taken up by the lungs. Deficiency is evident in certain environments, such as high altitudes and during deep sea dives, and in specific lung diseases. At a cellular level, O2 is essential for normal function, particularly respiration and oxidative metabolism. A lack of O2 induces key adaptations in cells through the recruitment of hypoxia-inducible transcription factors which regulate changes in the expression of a multiplicity of hypoxia-sensitive genes; these include genes encoding proteins involved in mitochondrial respiration and glucose utilisation, enabling a switch from aerobic to anaerobic metabolism.
While there are a number of similarities between O2 and what are conventionally regarded as nutrients, there are some important differences – in addition to the route of entry into the body. For example, O2 is generally freely available and there is no meaningful equivalent of the RDA (other than perhaps to ensure the maintenance of energy balance). The view that O2 is a nutrient may appear controversial – but even if provocative, it is proposed that it should be fully considered as a dimension of nutritional science.
Image with thanks to frankieleon, Flickr, reproduced under a CC license