Vitamin and mineral supplements can be pointless when it comes to improving health, a new Canadian study has suggested.
Researchers at St Michael's Hospital and the University of Toronto looked into the effects of taking the most popular vitamins and minerals on the market.
They found that folic acid may reduce heart disease and stroke, but that multivitamins, vitamin D, calcium and vitamin C ― the most common supplements — had no significant positive health effect.
Dr David Jenkins, the study's author, said: "Our review found that if you want to use multivitamins... there is no apparent advantage."
But there is disagreement among experts about how useful they are.
We asked nutritionists to help us explain the seven things you need to know about vitamins.
You can get most vitamins you need from a healthy, varied diet.
The three nutritionists we asked all agreed on one point: vitamins shouldn't replace a balanced diet and most vitamins can be consumed by eating a variety of foods.
"Some people believe that popping a multivitamin pill can make up for poor eating habits. However, according to research, nutrient-rich foods — such as fruits, vegetables, and whole grains, provide many more benefits over supplements," London nutritionist Rhiannon Lambert told HuffPost.
"It is possible to function optimally without supplements, being that we eat a balanced and well-varied diet that includes plenty of colourful foods."
Louise Joyce, a British naturopath, agrees.
She said: "Taking a multivitamin will never replace the importance of a healthy lifestyle, good quality sleep, exercise and good food."
But it's complicated, because everybody is different.
"Individual requirements can differ greatly from person to person, so what works or is essential for one person may not be best for the next," Louise Joyce told HuffPost.
She says that while most people would benefit from taking some vitamins, there is no one-size-fits-all approach. So the types and amounts of vitamins might vary from person
If you're serious about vitamins, professional advice can be a good place to start.
Make sure you do some research first, so you really know what you're looking for. It may also be worth seeking some professional advice or speaking to your doctor about what will work best for you.
"Seeking the advice of a qualified professional is the best way to understand your own personal needs and requirements," Louise Joyce said.
English nutritionist Helen Adams adds that some supplements, like zinc and magnesium, aren't as effective when taken together, which is another reason why it's best to seek advice.
For some people, vitamins can be really helpful or even essential.
Some people may have a condition, or follow a restricted diet, that leaves them in need of extra vitamins.
"For example, cutting out meat and dairy will most likely mean you will need to supplement vitamin B12 and omega 3s. These are extremely important for energy metabolism and healthy heart and brain function," Rhiannon Lambert explains.
According to Helen Adams, people taking certain medications may also have a need for extra vitamins.
"In that case," she says, "rather than going willy-nilly and thinking 'I'll take some multivitamins', it would really be good to get professional advice about what sort of thing to take and specifically what you need."
Not all vitamins are created equal.
The quality of vitamins can really vary, nutritionists told HuffPost.
Rhiannon Lambert says in fact a "large majority" of vitamins available on supermarket and pharmacy shelves have "poor bio-availability, which means that they are not very well absorbed by the body".
Helen Adams, meanwhile, explains that there can be many different types of the same vitamin. Magnesium, which can be good for energy, comes in many different forms. The most common, she says, is magnesium oxide, which is "very cheap", and is therefore often sold in supermarkets.
"But actually it's not absorbed at all, so the only thing that does is send you to the toilet. It's a laxative," she said. "So if you want it for that reason, then perfect — but if you want something that's going to support your energy, it's not the right form.
Magnesium glycinate is better absorbed, as is magnesium malate, she says.
"So this is why getting some professional advice is helpful in making sure you don't waste your money on something that doesn't work at all."
Don't always believe the hype.
Some vitamins are targeted at certain bodily functions, or at hair and nails. But make sure you look at what's in them first, before spending your money.
"Sadly there are some less-than-ideal formulations on the market — actually a lot of them. I guess this is why it's so confusing for consumers to know even where to start", Louise Joyce said.
"In an ideal world, I would like to believe that brands have the integrity to aim to make the best product possible, but it's simply not the case.
"Many synthetic forms of vitamins are a cheaper variant, and this can corrupt the quality of the end product," she added.
Some vitamins are worth paying extra for.
If you are going to buy vitamins, then good quality vitamins may be worth buying from a health-food shop, Helen Adams says. It is also worth ensuring you are taking the right amount to make a difference.
She says the RDA (recommended daily allowance) on the pack was actually developed during wartime to stop people getting things like scurvy. But today, "We're looking at making ourselves feel a lot better, so we generally need more than those levels anyway."
One supplement she says is essential is a "really good fish oil, as that's something your body doesn't make on its own (if you eat lots of oily fish, then you're fine)".
"But don't buy one that's been under the shop lights, as in capsule form they can become rancid — so it needs to be kept well."