12 May Does vitamin D supplementation cause fatigue?
If you are recently supplementing vitamin D, some of the anecdotal evidence online is that it can make people very fatigued indeed. This is a very interesting observation and I am not entirely sure what is causing this. Apparently the research suggests that vitamin D has no known ill effects beyond placebo. But lots of people report this tiredness. (Believe me, it is hell!)
At the start, I thought that maybe the vitamin D levels are so low that there is a kind of ‘bounce back’ effect, where our bodies feel worse before they feel better. That might be the case. Or it might be that some other deficiency is causing the tiredness, and the vitamin D supplementation has simply allowed that deficiency to come to the surface.
Other vitamins supporting vitamin D
Some of the suggested minerals and vitamins that people suggest are magnesium, Vitamin K2, and Vitamin B12. There is also discussion that perhaps the levels of calcium are too high, and this is causing the tiredness.
If you are worried about any of these other deficiencies, you can always source magnesium from dark leafy greens, broccoli, beans, salmon, etc, Vitamin K2 from grass-fed dairy produced cheese and Vitamin B12 from meat or dairy products.
My experience with vitamin D supplementation fatigue
As for me, my extreme fatigue arose on day two of supplementation and lasted for a good three-four days. It was noticeable enough to remark on it, and even to research if other people have experienced this. Sometimes a wave of fatigue would hit me in the morning after breakfast, sometimes in the afternoon after a meeting, or sometimes in the evening. In all cases, it was the kind of fatigue that makes almost anything else impossible to do. And I simply had to rest and let the fatigue pass, which it did after an hour or so on each occasion.
I am happy to report that as of day five, the fatigue really improved a lot, and I was able to get through a whole day without feeling like I needed a big break. And it is all up from there! Day six was great 🙂
Tentative advice – be patient
One of the people presenting the anecdotal evidence said that her symptoms started to appease after about a month, and suggested that maybe it is just the process of our body reabsorbing the vitamin D into our bodies. This makes sense to me, and I would hope that you don’t get so deficient in vitamin D that it takes a whole month for your body to get back into the black!
The recommended daily intake (RDI) of vitamin D
There are some studies that suggest that 400 – 800 IU is necessary to keep vitamin D levels above 20 ng/ml or 30 ng/ml, however other studies suggested that people need more like 1120 IU to 1680 IU is required to keep vitamin D levels in the healthy range. 
Particularly if you have darker skin, are not getting a lot of sunshine, are overweight or obese, or postmenopausal it is recommended that you take a somewhat higher dose of between 1000 IU and 4000 IU. If you take a much higher dose, please get your vitamin D levels checked regularly, and consult with your doctor as higher concentrations in your blood can be as damaging as lower doses. 
4000 IU is the safe upper limit. 
Bjarnadottir, A, 2017, ‘How Much Vitamin D Should You Take for Optimal Health?’, in Healthline, viewed 11 May 2020, < https://www.healthline.com/nutrition/how-much-vitamin-d-to-take#How-Much-Vitamin-D-Should-You-Take?>
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