I think most adults can benefit from a good multi-vitamin (one that is readily broken down in your stomach and made from natural sources) and an Omega supplement.
My cardiologist is a big fan of adults taking these. This is a doctor I respect very much.
I currently take Bluebonnet Multi One and also Nature's Bounty Omega 3-6-9 (Fish, Flax, Borage Oil combo). But, I switch brands every now and then. I have to say - I really notice more moisture in my eyes, skin and hair since beginning the Omega supp.
I just turned 35 so I probably need to get crackin' on a calcium supplement, too.
Dieting or not...having a multi on board to fill in the blanks is good. I would just research brands/types. Me personally, I wanted to avoid ones loaded with random herbs, wanted one proven to be absorbed well (no sense paying for a vitamin you are just going to poop out), and I also wanted one that was certified Kosher - I am not Jewish, but the standards to be met for Kosher cert. make me happy

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PS - Be careful not to take fish/flax oil supps or multi's on an empty stomach. May make you feel icky. But, with a little food in there you should feel fine.