The Chemistry Column

Soaking up the sun...with the liver?

Claire Gormley

Longer, warmer days mean more time to soak up the glorious vitamin D that so many of us miss during the winter months. “Get out and get your vitamin D!” my mum always used to say, and I never questioned how sunshine gave me vitamin D— even after learning that the sun’s rays are just photons and radiation. Perhaps I missed the lesson at school where we were taught about vitamin D production, or maybe I was already out the door and into the sunshine before I could take in the explanation for my mum’s advice. Today, as I was having my morning dose of vitamin C (in a yummy tangerine flavour), the thought popped into my head: how do we actually get vitamin D from the sun?

Most of our essential vitamins— vitamins which our body can’t make on its own —come from the foods we eat. Our digestive system breaks down the food and absorbs the nutrients that the body needs to perform its daily activities. Although it is widely believed that vitamin D is an essential vitamin (and we certainly do need it to develop a healthy skeleton, among other things), by definition it is not essential at all— because we make up to 90% of the vitamin D in our bodies ourselves. All we need is the power of the sun (Keane et al, 2018).

When our skin is exposed to ultraviolet B rays from the sun, a type of fat called 7dehydrocholesterol, or provitamin D, changes into pre-vitamin D3 (Keane et al, 2018). For this change to happen, enough energy from the sun must be absorbed by the provitamin D for the bond between two carbon atoms in the molecule to be broken— specifically carbon 9 and carbon 10 (Holick, Smith and Pincus, 1987). The severing of this bond produces pre-vitamin D3, a thermally unstable molecule, which undergoes an internal reaction to form vitamin D3 (Holick, Smith and Pincus, 1987). This is not where the story ends, however, because vitamin D3 in this form is inactive and cannot be used by the body (Ponchon, Kennan and DeLuca, 1969).

Activation of vitamin D3 occurs in two steps. First, vitamin D3 is transported to the liver where it is converted to 25-hydroxyvitamin D (Keane et al, 2018; Holick, Smith and Pincus, 1987; Ponchon, Kennan and DeLuca, 1969).

Scientists often measure the levels of this molecule in circulation to determine the vitamin D health of a person; recent research, although not definitive, has shown a link between vitamin D deficiency and chronic liver diseases (Keane et al, 2018). The final step of vitamin D activation occurs in the kidney, where carbon 1 is hydroxylated— meaning, a hydrogen atom bound to this carbon is replaced by a paired oxygen and hydrogen (hydroxyl group) —and becomes 1,25-dihydroxyvitamin D (Holick, Smith and Pincus, 1987). In this form, vitamin D3 behaves like a hormone, acting on bone, immune, and liver cells to regulate changes in the body (Keane et al, 2018), and ultimately contributing to all the positive effects— like improved mental health —that we associate with vitamin D and sunshine.

If you’re looking for an excuse to get outside, firing up your vitamin D factory seems like a pretty good one to me. Just don’t forget the sunscreen!

References

Holick, M.F., Smith, E., and Pincus, S. (1987) ‘Skin as the Site of Vitamin D Synthesis and Target Tissue for 1,25-Dihydroxyvitamin D3’ in Arch Dermatol, 123: 1677-1683a

Keane, J.T., Elangovan, H., Stokes, R.A., and Gunton, J.E. (2018) ‘Vitamin D and the Liver –Correlation of Cause?’ in Nutrients, 10(4): 496

Ponchon, G., Kennan, A.L., and DeLuca, H.F. ‘”Activation” of Vitamin D by the Liver’ in The Journal of Clinical Investigation, 48: 2032-2037