Why do some supplements like stimulants work better on an empty stomach but other supplements like vitamins work better when taken with meals?

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I believe that vitamins are taken with meals because it helps with vitamin absorption as your body will break down the vitamins along with your food. But if that is the case, why should stimulants like caffeine (for the purpose of exercise such as pre workout) be taken on an empty stomach to maximize efficacy?

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It depends on whether the supplements are soluble in fat or not. For example, vitamin D is soluble in fat, so it’s better to take it with a fatty meal, as this will help it cross from your gut to your bloodstream faster. Caffeine, on the other hand, is water-soluble, so it doesn’t need any help crossing from the gut to the bloodstream. In this case, it’s absorbed faster on an empty stomach when there isn’t much else getting in the way.