The problem with anyone falling back onto the 'you are ok if you eat a varied diet' argument, is that they never say what that varied diet should include. Usually health care professionals who say this are just covering up lack of knowledge. What they mean is a general, catch-all, vague diet which would include whole grains, fruits, vegetables and the Eatwell plate. Lots of room for malnutrition there! I could eat nothing but weetabix, strawberries and carrots, and think i ate well.
Likewise talking about 'a normal diet'. I mean, what the heck is that?!? What if my 'normal diet' is 5 pints a night and a takeaway to follow?
Assuming that a blood test is ok because it 'falls within normal limits' does not take into account the individuals whose personal requirements fall outside the statistical middle ground - they do exist. I have a mother who has had to continually fight a series of doctors. They all want to take her of her thyroid medication. No matter that she feels awful without it, her bloods are a fraction inside the 'normal range' so they think she doesn't need it.
I'm afraid the only opinions on dietary supplements that I trust, are those based on personal experience. If a person says 'I tried Vit x, y or z, and I felt so much better!' Then I think it was a good decision for them. If someone else says 'I tried herb a, b or c, and it made no difference', then that was a waste of their money. But they couldn't know that til they had tried it, could they?
Even the results of a well conducted, unbiased medical study will not tell you how your body will react to a supplement. You are an individual, with a unique body, lifestyle, medical history and diet. It might make you feel nothing, or worse, or better for the first time in decades.
Anyone with half a brain can read up on their supplement of choice, and make a decision based on their personal circumstances, the safe levels to supplement, their symptoms. Even a blood test should only be a guideline.
And don't get me started on the nutritional inadequacies of produce grown on over-farmed, toxically fertilised land, that then get harvested early and chilled or gassed for weeks before they arrive at the supermarket, where they are bought, sit in our kitchens for days and start to wrinkle before we over-cook them.
