Endocrinology, Vitamin D and the UK’s Covid-19 Disaster

Abstract

The formation of cholecalciferol (Vitamin D3) in skin depends on solar UVB to break the B ring of 7-dehydrocholesterol. Its discovery more than a century ago resulted from the identification of rickets as due to deficient sunshine in latitudes far from the equator, exacerbated by the air pollution, factory work and indoor living. Rickets resulted from defective endocrine control of blood calcium, and was accompanied by epidemic tuberculosis from failure of the D3-dependent first-line immune system. The influenza pandemic of 2018 revealed the need for D3 to fight viruses. Half a century later the systemic hormone role of 1,25(OH)D3 of renal origin, under control of PTH, was a major stimulus to understanding the mechanism of action via the VDR-RXR heterodimer. It was soon realised that 1,25(OH)D3 is also produced and acts locally in many organs and tissues provided that there are adequate reserves of the (protein-bound) blood storage form, 25(OH)D. This is the common pool for 1-hydroxylation by any cells that need local activation of VDR for induction of specific genes. In the case of the immune system, the trigger is foreign proteins recognised as ‘non-self’. Local production and action of 1,25(OH)D, and then its local destruction by 24-hydroxylation must all occur below the ‘endocrine radar’, so as not to interfere with systemic calcium control. Coronaviruses through their ‘spike’ protein are internalised by interacting with the ACE-2 receptor, which in turn is down-regulated by Vitamin D. In the process, 25(OH)D is hydroxylated to the active 1,25(OH)D, which must later be degraded to 1,24,25(OH)D. So it is to be expected that when 25(OH)D reserves are low at the onset of infection, they will fall further, allowing virus to enter the cells and trigger a cytokine storm and other damage. Blood PTH will rise to claim any residual 25(OH)D for the dominating systemic role in calcium homeostasis. It follows that intake of vitamin D3 should always be much more than the minimum claimed by the globally-active endocrine system. Unfortunately, the UK’s Specialised Advisory Committee on Nutrition (SACN), does not recognise this. It is dominated by nutritionists, even though food sources of D3 are for most non-existent, and of D2, the vegetable substitute, highly variable. The 400IU of D3 reluctantly recommended for those ‘at risk’, based on endocrinology alone, is grossly inadequate; 4,000IU daily is needed to maintain a blood 25(OH)D at more than 30 ng/ml (75 nmol/l), and provide sufficient reserve for its many autocrine and paracrine functions. The dangers of letting the dominant endocrine function of 1,25(OH)D in ionic calcium control dictate the level of D3 supplements, have once again been underlined by the Covid-19 disaster.