Teelobhain,
You don't have to take orange juice. You can just munch on fruits such as oranges, grapefruit, strawberries, kiwi. You can also get it from tomatoes, peppers,broccoli, and Brussels sprouts A multivitamin shouldn't make you gain weight, it doesn't have a significant amount of calories and nor does it contain fat. It'll help compensate for any deficiencies in your diet. You can even squeeze some lemon juice in water and take a multivitamin with that.