Vitamin D

Vitamin D: The Sunshine Vitamin

Imagine a world where sunlight could be bottled up, encapsulated, and consumed like any other nutrient. That’s the essence of vitamin D, often referred to as the “sunshine vitamin.”

The Role of Vitamin D in Our Bodies

Vitamin D is a group of fat-soluble secosteroids that play a crucial role in increasing intestinal absorption of calcium, magnesium, and phosphate. The two most significant compounds are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Unlike other vitamins, vitamin D is only conditionally essential due to the limited exposure to sunlight and dietary sources.

The Activation Process

Vitamin D obtained from diet or skin synthesis is biologically inactive and becomes active through two enzymatic hydroxylation steps. The activation produces calcitriol, the active form that exerts its effects via the vitamin D receptor (VDR). Calcitriol regulates calcium and phosphate concentrations, promotes bone health, and plays a major role in bone remodeling.

The Discovery of Vitamin D

How did we come to understand this vital nutrient?

The discovery of vitamin D was made in 1922 when Elmer McCollum tested modified cod liver oil, which led to the identification of the factor responsible for curing rickets. This breakthrough was a significant moment in medical history, paving the way for further research and understanding.

Government Programs and Fortification

Today, government food fortification programs and consumption of vitamin D supplements are commonly used to prevent or treat rickets and osteomalacia. The United States is one of few mandated countries with a program that started with cow’s milk. Vitamin D can be found in plant-based milks and beverages, as well as supplements.

Vitamin D Deficiency: A Global Concern

Is vitamin D deficiency just a problem for those who live far from the equator?

Vitamin D deficiency is widespread, with over one billion people globally being deficient. This condition can lead to decreased bone mineral density, osteoporosis, and fractures. However, it’s important to note that dark-skinned individuals do not necessarily exhibit ‘deficiency diseases’ despite higher serum concentration rates.

Supplementation and Upper Limits

The National Academy of Medicine revised tolerable upper intake levels in 2011 to protect against vitamin D toxicity. The maximum is at 100 μg/d (4000 IU/d). Different government organizations set different Upper Limits, but adult maximum is at 100 μg/d (4000 IU/d). Non-government authors propose a safe upper intake level of 250 μg (10,000 IU) per day.

Health Implications and Research

Does vitamin D do more than just keep our bones strong?

Vitamin D functions as an immunomodulator with antibacterial, antiviral, and anti-inflammatory effects. Low serum levels are associated with tuberculosis and acute respiratory tract infections, although supplementation trials have shown no benefit for these conditions.

Supplementation and Non-Skeletal Health

Research suggests that vitamin D supplementation may reduce the rate of moderate or severe COPD exacerbations, improve glycemic control in individuals with type 2 diabetes, and improve ADHD symptoms. However, there is insufficient evidence to recommend vitamin D supplementation to prevent or treat COVID-19.

The Evolutionary Perspective

How did our bodies evolve to need this vitamin?

Vitamin D’s role expanded from controlling metabolic genes supporting innate and adaptive immunity to regulating calcium supply for bone homeostasis in terrestrial animals. Herbivores produce vitamin D in response to sunlight, while domestic dogs and cats rely on diet due to a high activity of 7-dehydrocholesterol reductase.

The Industrial Production of Vitamin D

Vitamin D3 (cholecalciferol) is produced industrially by exposing 7-dehydrocholesterol to UVB and UVC light. Vitamin D2 (ergocalciferol) is produced using ergosterol from yeast as a starting material.

The Journey of Vitamin D Through the Body

How does vitamin D travel through our bodies?

Vitamin D is carried via the blood to the liver, where it is converted into calcifediol and then calcitriol – the biologically active form of vitamin D. The activity of calcifediol can be reduced by hydroxylation at position 24 by vitamin D3 24-hydroxylase.

The Role of the Vitamin D Receptor

Calcitriol enters the target cell, binds to the vitamin D receptor (VDR), and stimulates transcription of specific genes. It also has non-genomic actions that are too fast for the classical VDRE transcription pathway. The membrane-bound PDIA3 likely serves as an alternate receptor in this pathway.

Conclusion

Vitamin D is a complex nutrient with far-reaching effects on our health, from bone strength to immune function and beyond. Understanding its role and ensuring adequate intake can make a significant difference in preventing deficiency-related diseases. As research continues, the potential benefits of vitamin D supplementation for various conditions remain an area of active investigation.