Looking at individual studies won't determine if vitamin supplementation is good for you. They're scientifically dense and the conflicts of interest can be very hard to spot.

"Systematic review papers" are much better suited for that. This is where independent scientists gather up all the available data and re-analyze it to answer big questions.

• If you are healthy, and have a modern balanced diet, taking multi-vitamins and high-dose antioxidants may shorten your life.
• There’s no evidence to support antioxidant supplements for primary or secondary prevention of diseases.
• Beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A.
• Antioxidant supplements need to be considered as medicinal products and should undergo sufficient evaluation before marketing.
• The functions of minerals and vitamins are understood largely by their deficiency diseases so there’s no certainty what they all do or how they interact.
• Antioxidants soak up toxic, chemically-reactive by-products of metabolism called free radicals. These free radicals, left unchecked, can cause damage to DNA and may be linked to cancer.
• Your body's immune system fights infections by using free radicals to kill bugs. Several of the minerals and vitamins in excess can cause damage.