VITAMIN D

When vitamins become hormones

Vitamin D research has expanded to include the extra-skeletal domain.

In Summary

• Vitamin D is the only one of the 13 vitamins that can be made by the human body, although small amounts of vitamin K are produced by bacteria that reside in our intestines.

• As it can also be derived from skin synthesis, it is not an essential dietary factor, therefore technically not a vitamin.

Vitamins.
Vitamins.
Image: COURTESY

One of my earliest childhood memories was the daily ritual of having to swallow cod liver oil, in liquid form, from an unfairly large spoon.

I have no doubt that this was, and still is, the daily reality of growing up in Kenya in the 80s.

At the time, the importance of vitamin D revolved around its skeletal utility, which was to prevent rickets and other bone disorders. We were aware that vitamin D was synonymous with bone health.

 

Rickets peaked in the 1600s industrial revolution. Worryingly, there are now reports that it’s coming back, particularly among breastfed children.

Fast forward 40 years and vitamin D research has expanded to include the extra-skeletal domain. In the last decade, the resurgence of interest has largely been birthed following a deluge of evidence linking vitamin D deficiency to a plethora of diseases and disorders – from viral infections to cancer; coupled with the incontrovertible rise and rise of vitamin D deficiency worldwide.

 

Vitamin D is one of four fat-soluble vitamins—A, D, E and K. Vitamin D is the only one of the 13 vitamins that can be made by the human body, although small amounts of vitamin K are produced by bacteria that reside in our intestines.

As it can also be derived from skin synthesis, it is not an essential dietary factor, therefore technically not a vitamin. It is now considered a hormone, and a crucial one at that, with a multitude of roles and functions.

While we’ve known for years that poor diets contribute to many diseases, we’ve only recently discovered that this sunshine vitamin impacts every single cell in our body.

Which means that it impacts our DNA. Ergo, a deficiency can contribute to a wide variety of disorders.

Vitamin D is largely synthesised from cholesterol underneath our skin by ultraviolet (UVB) radiation, part of the natural energy produced by the sun. Dietary sources of vitamin D exist, but they are inadequate and inferior to solar-sourced vitamin D.

 

From an evolutionary point of view, vitamin D was a pre-requisite to becoming a land animal, hundreds of millions of years ago.

Early vertebrates did not have access to calcium as they did in the sea. Vitamin D helps the body absorb calcium, which together with vitamin K, have an important role in building strong bones.

Vitamin K, although less well known, keeps calcium inside the bones and away from the blood vessels, which could result in the calcification of arteries, contributing to heart disease.

From the skin, vitamin D is further processed in the liver and then the kidneys, which make the active vitamin D hormone.

Your vitamin D status can be easily measured in a lab. Levels below 25nmol/L are considered low, and national guidelines recommend that doctors aim for at least this target.

This target level, however, has since been subjected to intense scrutiny, with good quality observational studies associating higher vitamin D levels (of at least 40nmol/L), with significantly lower rates of diseases. This suggests that the guidelines may be worryingly conservative.

Modern equatorial humans are getting less sunlight than their recent and historical ancestors.

We spend our lives in buildings and vehicles. We wear more clothes for modesty and cultural reasons. We’ve shied away from organ meats, bones and bone marrow, in favour of leaner muscle meat.

We know that indigenous Maasai who are often partially clothed have much higher vitamin D levels than those who live in the cities (fully clothed).

Over 60 per cent of us have insufficient vitamin D levels. Most of my patients have inadequate levels – and, interestingly, not a single one who has type 2 diabetes has an adequate vitamin D status.

We’ve been informed that vitamin D deficiency is rising. In the same breath, we are being advised to lower our cholesterol, limit our intake of eggs, eat only low-fat dairy and limit our sun exposure so as to reduce our risk of developing skin cancer – measures that inadvertently impact our access to this critical vitamin.

Yet, we know that the dangers of vitamin D deficiency far outweigh those of excess sun exposure.

This is just the beginning of the fascinating vitamin D story. Join me next week as I continue to discuss how it is inextricably linked to human health and why we ought to be very deliberate about getting enough of it.

 

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