vi: 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 30
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).
