“Healthy adults” is an important qualification, as is the lack of detail about just what multivitamins were being taken. As it happens, many multivitamins do not contain effective amounts or they have artificial forms of vitamins that have weak or even anti-vitamin effects.
My uncle lived to age 84. Took 30 pills a day. My dad, his brother died at 63. Both had heart disease. It had tube the vitamins as their 5 siblings didn’t make it that far.