Provision and delivery of oxygen

In mammals, and other higher animals, there is a requirement for what is in effect the continuous, or virtually continuous, delivery of O₂. This contrasts with other nutrients, the provision of which is episodic, most mammals being ‘periodic feeders though constant metabolisers’. A relative lack of O₂ occurs in specific situations, either continuously in the case of terrestrial species living at high altitudes, or acutely and temporarily such as with aquatic animals undergoing deep-sea dives. Even some terrestrial animals living at sea level may be periodically exposed to low levels of O₂, such as the naked mole-rat in its underground burrows⁽ Reference Park, Reznick and Peterson ^{1 )}. When acute or chronic O₂ deficiency is part of the ecological niche or environmental circumstances to which a species is customarily exposed, selective adaptations have evolved. For example, in the naked mole-rat resistance to near anoxic conditions is sustained by utilising fructose as a fuel in glycolysis, thereby bypassing the key regulatory glycolytic enzyme phosphofructokinase⁽ Reference Park, Reznick and Peterson ^{1 )}.

Once taken up by the lungs, O₂ is distributed in essence immediately and directly to tissues and cells throughout the body, needing no prior processing before being made available. In contrast, the classical nutrients normally require release from the complex structures of the foods in which they are present, as well as processing to a form that can be transferred out of the interior of the gastrointestinal tract (as with polysaccharides and TAG). In addition, in many cases once food has been digested the nutrients released are transported from the gastrointestinal tract via substrate-specific transporters (e.g. Na⁺-dependent glucose transporter and amino acid transporters) rather than moving passively across cell membranes.

Following transfer from the lungs (or gills) into the circulation the handling of O₂ becomes more similar to that of other nutrients. O₂ is transported to tissues and cells by the specific Fe-containing metalloprotein, Hb, located in the cytoplasm of erythrocytes in vertebrates. This has parallels with the delivery of a number of other nutrients, such as lipids and retinol, to the sites where they are required. Not all nutrients are transferred directly to the site of action – many, including glucose (as glycogen), fatty acids (as TAG), and vitamins such as retinol and vitamin D, are first stored prior to being delivered to the sites where they are required – with the liver and white adipose tissue being key storage organs. O₂ is stored to a limited extent in muscle, bound to myoglobin, for local use only within the tissue, and this is especially evident in marine mammals that undergo apnoea when diving⁽ Reference Ordway and Garry ^{2 )}.

Metabolic functions

Despite not being considered within the remit of nutritional science in whole-body terms, at a cellular level O₂ is recognised as a critical factor without which respiration and other key metabolic processes cannot take place. Oxidative metabolism, particularly the catabolism of fatty acids and glucose with the production of ATP through oxidative phosphorylation and mitochondrial respiration, requires a continuous supply of O₂. Metabolic pathways central to this include glycogenolysis, glycolysis, lipolysis and the citric acid cycle, and involve cytochrome enzyme systems within the mitochondria⁽ Reference Frayn ^{3 )}.

The number of mitochondria in a cell, and whether there is a highly developed cristae structure within these organelles, varies according to the extent to which each cell type undergoes oxidative metabolism and consumes O₂. Brown adipocytes in rodents adapted to the cold, for example, have large numbers of mitochondria with densely packed cristae, reflecting the exceptionally high levels of fatty acid oxidation and O₂ consumption needed for thermoregulatory heat production (thermogenesis)⁽ Reference Cannon and Nedergaard ^{4 )}.

Oxygen deficiency states

The complete absence of O₂ leads to death within minutes in man and other mammals. In addition to environmental circumstances in which a relative lack of O₂ occurs related to the ecological niche of a species, there are certain disease states, primarily lung diseases such as pulmonary fibrosis and emphysema, where the provision of O₂ to the body as a whole is impaired⁽ Reference Kulkarni, O'Reilly and Antony ^{5 )}. There are also states of cyclic O₂ lack, as in obstructive sleep apnoea, which is one of the disorders particularly associated with obesity⁽ Reference Bonsignore, McNicholas and Montserrat ^{6 )}. In each of these cases the overall availability of O₂ is limited, though not necessarily to a specific tissue. O₂ deficiency can be ameliorated, both acutely and chronically, whether in lung disorders such as chronic obstructive pulmonary disease or in medical emergencies, by increasing the provision through O₂ therapy.

The O₂ tension (pO₂) of inspired air at sea level is 160 mmHg and in alveolar blood it is approximately 104 mmHg, while the general level of oxygenation in tissues is of the order of 40–50 mmHg⁽ Reference Brahimi-Horn and Pouysségur ^{7 –} Reference Trayhurn ^{9 )}. However, some tissues have a markedly lower pO₂, examples including the retina, thymus and spleen, with a pO₂ of 2–25, 10 and 16 mmHg, respectively⁽ Reference Brahimi-Horn and Pouysségur ^{7 –} Reference Trayhurn ^{9 )}.

As well as low levels of O₂ being characteristic of certain tissues under normal circumstances, local deprivation also occurs in specific pathological situations. These include the site of wound healing, in the heart in ischaemic disease, in tumours, and in white adipose tissue depots in obesity⁽ Reference Brahimi-Horn and Pouysségur ^{7 –} Reference Semenza, Agani and Feldser ^{10 )}. The pO₂ of solid tumours can be so low that those cells at the centre may be effectively anoxic. In the case of white fat, a reduced pO₂ has been documented in white adipose tissue depots of obese rodents⁽ Reference Ye, Gao and Yin ^{11 –} Reference Rausch, Weisberg and Vardhana ^{13 )}, the pO₂ being >3-fold lower than in lean animals⁽ Reference Ye, Gao and Yin ^{11 ,} Reference Sun, Halberg and Khan ^{14 )}. Adipose tissue hypoxia in obesity is considered in part to reflect the considerable size of enlarged white adipocytes in relation to the normal diffusion distance of O₂ in tissues⁽ Reference Trayhurn ^{9 ,} Reference Trayhurn and Wood ^{15 )}. This hypoxic state is linked to inflammation and fibrosis, and is considered to be a key factor underlying the changes in adipose tissue function that lead to the development of the major obesity-associated diseases, particularly insulin resistance and the metabolic syndrome⁽ Reference Trayhurn ^{9 ,} Reference Trayhurn and Wood ^{15 ,} Reference Halberg, Khan and Trujillo ^{16 )}.

Metabolic and cellular adaptations to oxygen deficiency

Part of the response to a chronic deficiency of O₂ in a tissue is the stimulation of angiogenesis in order to extend the vasculature. At the level of the cell, a local deficiency of O₂ leads to extensive metabolic changes⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Brahimi-Horn and Pouysségur ^{8 ,} Reference Semenza ^{17 ,} Reference Semenza ^{18 )} (Fig. 1). Glucose and lipid oxidation, oxidative phosphorylation and mitochondrial respiration fall, and there is a compensatory increase in substrate flux through anaerobic pathways⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Brahimi-Horn and Pouysségur ^{8 ,} Reference Semenza ^{18 )}. In particular, the rate of glycolysis is greatly increased with lactate being the end product rather than pyruvate⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Brahimi-Horn and Pouysségur ^{8 ,} Reference Semenza ^{18 )}; under aerobic conditions pyruvate is oxidised via acetyl CoA and the citric acid cycle. Elevated rates of glycolysis are driven by increases in glucose uptake through the recruitment of GLUT1, the basal facilitative transporter, and raised levels of key glycolytic enzymes⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Brahimi-Horn and Pouysségur ^{8 )}. Tumours have, of course, long been recognised to produce substantial quantities of lactate, reflecting their marked hypoxic state⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Semenza ^{19 )}. Similar observations have been made on white adipocytes maintained under hypoxic conditions⁽ Reference Trayhurn ^{9 ,} Reference Trayhurn ^{20 )}.

Fig. 1. Schematic illustration of the key cellular responses to oxygen deficiency based on white adipocytes. The effect of low oxygen tension on gene expression, glucose uptake and utilisation, lipid metabolism and the production of selected adipokines is shown. PAI-1, plasminogen activator inhibitor-1; angptl4, angiopoietin-like protein-4; VEGF, vascular endothelial growth factor; MMP, matrix metalloproteinases; FA, fatty acid; HIF-1, hypoxia-inducible factor-1; TF, transcription factors (additional to HIF-1, hypoxia-inducible factor-1); GLUT1, facilitative glucose transporter 1; MCT1, monocarboxylate transporter-1.

The range of metabolic changes resulting from low pO₂ extends well beyond the augmentation of glycolysis. In the specific case of white adipose tissue, microarray studies have indicated that the expression of approximately 1300 genes is altered in adipocytes exposed to 1 % O₂ (Fig. 1) compared with those incubated under ‘normoxic’ conditions (21 % O₂)⁽ Reference Mazzatti, Lim and O'Hara ^{21 )}. In addition to glucose utilisation, lipolysis and lipid oxidation, the pathways and functions altered in fat cells in response to low pO₂ include cell-to-cell signalling and interaction, amino acid metabolism, and cell death⁽ Reference Mazzatti, Lim and O'Hara ^{21 )}. This is reflected in changes in the amounts of encoded transporters, enzymes, and key proteins such as adipokines – including those associated with the inflammatory response⁽ Reference Trayhurn ^{9 ,} Reference Trayhurn ^{20 )}. Cells not only respond to major differences in pO₂, but again, as illustrated in adipocytes, they appear to carefully titrate small variations in pO₂ with alterations in gene expression and glucose utilisation⁽ Reference Wood, Stezhka and Trayhurn ^{22 )}.

The cellular sensing of O₂ deficiency is initiated at the cell membrane primarily through K⁺ ion channels⁽ Reference Kemp and Peers ^{23 )} and the intracellular response is transmitted by hypoxia-sensitive transcription factors which regulate the expression of hypoxia-sensitive genes⁽ Reference Brahimi-Horn and Pouysségur ^{7 ,} Reference Brahimi-Horn and Pouysségur ^{8 ,} Reference Semenza ^{17 –} Reference Semenza ^{19 )}. The most important of these transcriptional signals are the hypoxia-inducible factors (HIF), particularly HIF-1 which is termed the ‘master regulator of O₂ homeostasis’⁽ Reference Semenza ^{18 )}. HIF-1 consists of two subunits – HIF-1β, which is constitutively expressed, and HIF-1α which is continuously synthesised and degraded but is stabilised when pO₂ is low, this enabling the formation of the functional transcription factor⁽ Reference Brahimi-Horn and Pouysségur ^{8 ,} Reference Semenza ^{17 ,} Reference Semenza ^{18 )}. The transcription of multiple genes is directly regulated by HIF-1, including GLUT1, glycolytic enzymes, vascular endothelial growth factor (VEGF), angiopoietin-like protein-4 and the adipocyte hormone leptin⁽ Reference Brahimi-Horn and Pouysségur ^{7 –} Reference Trayhurn ^{9 ,} Reference Trayhurn ^{20 )}. VEGF is, of course, a key angiogenic signal, the growth of the vasculature being central to the delivery of O₂ as well as of other nutrients.

Coda

O₂ has been a forgotten, or at the very least highly neglected, nutrient. It is absolutely critical for all aerobic animals, and for most higher species is required on a continuous basis. It is essential for cellular respiration and for a host of other metabolic processes. States of deficiency are recognised and can be ameliorated. Cells have the ability to adjust to acute or chronic changes in O₂ availability, this involving alterations in the expression of a multiplicity of hypoxia-sensitive genes regulated by key transcription factors.

Despite the similarities between O₂ and other nutrients, there are some differences beyond the route of delivery. There is no meaningful equivalent of the RDA, and in most circumstances O₂ is both abundant and freely available, and requires no prior processing. In contrast to many other nutrients, excess is difficult to achieve though toxicity is evident in artificially induced hyperoxaemia. It is argued that O₂ should be viewed as firmly residing within the purview of nutritional science.