For people on restricted diets like the elderly it's a good idea or those who are vegans as their food choices may not provide all the recommended daily amounts. Generally most people with a healthy balanced diet don't need them, but as a child I had them because children are growing and some are picky eaters.
I don't think they are necessary, but helpful in some situations. If I am traveling then I will take them as I can't guarantee my diet will be balanced if I am in a remote jungle.
The multi-vitamin works as a supplement. It is only needed if you have deficiency of some vitamins. If you already get enough of these nutrition through your regular diet, then the supplement will bring no benefit. It might be the reason why you did not feel any different.
Then again, it is a long term thing. You won't feel any different if you only skip a few days or a few weeks. And you must be in a controlled environment for accurate test results. All your diet must be the same, so is your living conditions. Otherwise the difference from not take the pills might be cancelled out by different life styles.
That is said, most of the people who take multi-vitamins are not the people who actually need it. They are already getting enough from the food. Those who do not get enough nutrition are not usually the one would buy multi-vitamins.
I eat a relatively balanced diet but I still go for vitamin supplementation to keep my body in a healthier condition. Our body goes through a normal wear and tear process and we would normally not notice this at all specially when we're young and vigorous. Taking vitamin supplements gives me an assurance that I'm helping increase my body's immunity and helping it fight unseen and undetected infection or harmful elements.
I haven't noticed any difference either whether I take a multivitamin pill or not. I'm a bit haphazard when it comes to remembering to take a pill every single day. Sometimes I get too busy and forget. I can remember to give my dogs any medication that they might need, but myself I just seem to not think about so much. I have some multivitamins that I can take, and also some glucosamine tablets. My neck grinds a bit sometimes and I take a glucosamine supplement for a few days and it helps very quickly. When I don't take it for a while the grind in my neck comes back again. This tells me that taking a daily vitamin supplement would help even if we don't really notice any difference.
I read somewhere one time that people who take a multi vitamin every day don't live as long as those who don't. I mean that sounds really weird, but I am not sure that the health effects of multi vitamins have been proven. By the same token, there are certain vitamins and supplements that probably should be taken especially if you are older. Vitamin D and Calcium are important to take.
Even if you are getting a balanced diet daily, the fact remains that the recommended daily intake (RDI) is not easy to achieve without taking supplements. An individual has to consume a large quantity of fruits and vegetables. Might be the OP was consuming foods that negatively interact with vitamins as is the case with tea.
When I was a kid, my mother used to buy all kinds of vitamin supplements for me. She was worried that I was not getting proper nutrition although the food we ate was, in my opinion, pretty well-balanced. When I grew up and lived on my own, I hardly take any vitamin supplements. However, just a couple of days ago, on my monthly regular visit to the doctor, my doctor suggested that I should consider taking some Vitamin E supplements. no, not for the anti-aging effect. It's for my skin problem. I have a skin fungal infection which has been there for months despite applying anti-fungal cream regularly. I cam back home and looked up Vitamin E. Apparently, sunflower seeds have a very high percentage of Vitamin E. Half-a-cup of sunflower seeds would supply about 60% of my daily need. So I went out to buy a big packet of sunflower seeds. I have been eating them for three days already but I don't think I have even finished half-a-cup. So, maybe I should really look into getting some Vitamin E supplements.
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There are lots of good reasons to take a multivitamin. Even the best eating plans can fall short of meeting all of the 40-plus nutrients you need each day. Most Americans fail to meet dietary recommendations for many reasons, including strict dieting, poor appetite, changing nutritional needs, or less-than-healthy food choices. Taking a once-daily multivitamin is an easy way to fill in small nutritional gaps.