I'm writing this at a conference so treat it as provisional as I'm on battery power and not doing my normal fact-checking. Bits in pink are bits I need to reference.
It's a while since I've really looked in any depth at the evidence for, or against, Vitamin D as something that people might want to use as a supplement. My interest in it stems from my time at Diabetes UK when there were a few questions from people about its particular value as something that might generally 'help' either in preventing diabetes from starting, or reduce complications or other helpful things.
At the time I first started looking into this I don't think there had been any intervention trials where people (with a particular type of diabetes) were split into groups, one lot given Vit D and the other not and the outcomes compared.
There were plenty of studies looking at Vitamin D status (eg how much Vitamin D or its metabolites is zinging around in your blood) and outcomes, which were interesting, but I never really felt I understood what Vit D status tells you. Vit D exists in several forms - there are one or two coming in via diet but they are immediately tweaked first by the liver to another form and then the kidney to another one (or possibly the other way around!). Your skin is also capable of rustling up a bit of Vitamin D all by itself.
So what's in your blood could be a consequence of what's in your diet or from the sun, or from what's going on with the enzymes that do this conversion. I also can't help wondering if, given that Vitamin D is a fat-soluble vitamin, gaining or losing weight makes a big difference to Vitamin D concentrations in the blood.
How good a predictor for health outcomes or diabetes complications etc is how much Vitamin D (metabolites) you have in your blood? Dunno.
But then I thought... fat soluble vitamins
I seem to remember that Vitamin A, essential for retinal goings on etc, can be a bit of a problem if you have too much of it. Vague recollections of giving polar bear livers a swerve as the content of Vitamin A can floor you. Vitamin A is fat soluble.
Vitamin E was touted as some life saving thing for people who'd had heart attacks I think? There was a trial looking at Vit E with some other vitamins or medication and it turned out that it wasn't such a great idea to use this. I mean obviously you need some of it but 'some of it being good for you' doesn't translate, not that surprisingly, to 'lots more of it is therefore also good for you'. Vitamin E is fat soluble.
Vitamin K plays an important role in blood clotting and I was surprised to learn recently that eating too much spinach (rich in Vitamin K) was perhaps contributing to unexpected INR (international normalised ratio - which is how the speed of blood clotting is measured) in someone using warfarin. High Vitamin K foods interact with warfarin, which can be potentially a bit of a problem. Vitamin K is fat soluble.
Are fat soluble vitamins something we want to be a bit more careful with, in terms of hoovering them up as supplements? I suppose to answer that I need to avoid positive confirmation bias of happening to know off the top of my head which are the fat-soluble vitamins and find out what dangers there are from all the water-soluble vitamins. To be honest I thought most of them just got peed out but I expect I should mug up on it.
Anyway, unintended consequences and all that - apparently Vitamin D supplementation can negatively affect your lipid profile. I suppose I should go and look up the reference for that, I hate it when people don't cite stuff properly...
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