I do see a lot of people who want to cover all their bases when it comes to their health. I do find this admirable on some level, but I also find a lot of people spending time and money trying to cover things that do not need to be covered. Honestly, when it comes to spending money I have a tendency to roll my eyes and say "it's your bank account" and move on. When it comes to potentially doing harm to one's health, however, I tend to be a little bit more forceful (when people ask) about really making sure you need whatever you have been taking/supplementing with, particularly when a simple blood test can tell you whether or not you need to take it.
Such is the case with vitamin D supplementation as indicated by this particular study, which indicated that elevating the vitamin D levels beyond a certain point might actually confer some harm. (Link) Your body normally works pretty hard to keep blood levels of vitamin D (and pretty much everything else) within a certain range, and more of something is usually not better, especially when that more might put you at risk for hardening of the arteries. Naturally, there should be some more studies done on the potential harm of elevated vitamin D levels, but if a simple blood test could let know whether or not you should spend the money on a supplement and/or potentially increase a risk, why not do it?
I'm sure some of you are going to remind me that in this day and age not everybody has insurance and/or not all insurance plans will cover a blood test. So you will need to consider if you are part of the high risk for low vitamin D groups as outlined in the article:
1) A female greater than 75 years of age.
2) You have a diagnosis of chronic kidney disease (even if you are not requiring dialysis yet).
3) A female who is post-menopause.
4) If you are obese, meaning that you have a waist to hip ration of > 0.90 for men and > 0.85 for women (Instructions here: http://www.bmi-calculator.net/waist-to-hip-ratio-calculator/)
I would also add you should consider yourself at risk if you have celiac disease or any other inflammatory bowel condition (as you may have trouble absorbing nutrients if your condition is not controlled) and/or if you have known liver disease.
If you have the blood test done and are put on supplementation per your provider's recommendations, make sure that you follow up with a blood test at least 3 months later and follow up with your physicians recommendations later. In the meantime, it doesn't hurt to make sure that you also get enough sunlight (as your body knows when to shut off production of vitamin D from sunlight) and eat foods like salmon, sardines, mackeral (or vitamin D fortified foods), to make sure that you get enough vitamin D from the usual sources.
Oh, and I guess I should also mention that "normal" levels pf vitamin D are considered to be between 20 nanograms/milliliter to 50 ng/mL. Supplements are usually recommended if you are below 20 ng/mL, but deficiency is usually diagnosed at <12 ng/mL.
Take home message--before assuming that you need to supplement, consider your evidence-based risk factors and use the tools available to you (including blood tests) to determine your actual need for supplementation.