People here in northern Maine, as in my native Sweden, don’t get a whole lot of natural sunlight a good part of the year. As a kid, I had to swallow a daily spoonful of cod liver oil to get the extra vitamin D my mother and many others believed we all needed. Some years later, that fell out of fashion as it turned out that too much vitamin A, also found in that particular dubious marine delicacy, could be harmful.
This is how it goes in medicine: Things that sound like a good idea often turn out to be not so good, or even downright bad for you.
Other vitamins, like B12, can also cause harm: Excess vitamin B12 can cause nerve damage, just as deficiency can.
Both B12 and D can be measured with simple blood tests, but the insurance industry doesn’t pay for screening. That is because it hasn’t been proven that testing asymptomatic people brings any benefit. In the case of B12, it is well established that deficiency can cause anemia and neuropathy, for example. But here is no clear evidence what the consequences are of vitamin D “deficiency”. A statistically abnormal result is not yet known to definitely cause a disease or clinical risk, in spite of all the research so far, but we’re staying tuned.
This is what I tell my patients:
I don’t recommend supplements or vitamins because, as a physician, my job is to only recommend pills that are scientifically proven to treat or prevent disease. Prescription medications also have quality controls to make sure you get what you’re supposed to.
For example, we have proof that aspirin is aspirin and it can cut a person’s risk for blood clots. But fish oil capsules can have varying quality and composition, so they may or may not be as good for you as eating salmon.
I make a distinction in my practice between what we know works and what we believe might work. I feel I owe my patients that honesty. I work from the assumption that most people don’t want to pop unnecessary pills, so I keep my recommendations to the tried and true.
If I go out on a limb and recommend unproven remedies, I risk losing my credibility with my patients also when I recommend well established life-saving medications as a well trained, critically thinking and experienced physician.
A Country Doctor Writes: 
You are a prodigious educator when you pass on your opinions to other doctors and even to patients. Patients will have to become better informed nowadays when everything online is open to wild salesmanship and delusional advertising. I may have taken in by this myself. I’m an old retired psychiatrist used to sitting in one chair for 10 hrs a day. Osteoarthritis has caught up with me, Now I have started doing physical therapy and planning some healthier activity (having the time is a great advantage of retirement). My question is whether or not collagen supplements can build up the strength in my skin, muscles, and bone, as the label claims—? (I understand the obtaining a Hollywood contract is too much to expect.)
Hi, I wanted to reply to this last Sat, and your site let me know I was hacked-My electronic life is cured right now,-BUT BACK to this article.
Sometimes the patient is somehow right about supplements. As a military dependent, I had military medical care. I was diagnosed with IBS-D when I was in my mid-20s. I had wonderful USN gastroenterologists that went with the British and provided care and tracking of the prisoners, my personal GI went to Belsen, and later because the Japanese were more brutal than the Nazis. My husband was enlisted, but I was always treated very well by military doctors, better than most of the current civilians.
The USN doctors always suspected that I didn’t absorb nutrients very well, and they did encourage supplements. They recommended Slimfast, as that was the best of what was available in the mid 1960s. It was too heavy for me to digest and sat like a brick in my stomach. They encouraged me to be physically active, and that made a great difference in my health being more than survival.
I got off of taking supplements and I had a bad vitamin B6 deficiency. I had another deficiency in 2004-B6 and B12. The civilian neurologist that took care of me, told me not to be bullied by doctors and take the B supplements.
Medical science caught up with me and the WW 2 military doctors and the ‘Nam military doctors. I was scanned because of a bleeding diverticulitis. I have Igg4 and my pancreas has calcification. My current GI told me that normally he’d tell me to start taking vitamin supplements, but I’m already doing that. At least my family doctor has stopped nagging me and calling me a liar-because I’m an old woman. I feel OK, and do yard work, walk my coonhounds. I do hire a handyman for things that I can’t do, like get on the roof-help prune trees.
I mostly agree with you, except for this. Thank you for sharing your wisdom and experience.
Agree 100%. Thank you for confirming what the evidence shows and few too practitioners are willing to say.