I hate to say it, mdamon, but it really depends. As a general rule, vitamins and minerals should be taken with meals. They’re cofactors in enzymatic processes. And there is a FLURRY of enzymatic processes going on in conjunction with digesting and absorbing and storing or utlizing the food you’ve eaten. So that’s easy.
Some supps NEED to be taken on an empty stomach. One that comes to mind is something like Wobenzym, which is a proteolytic enzyme combination. If you take it with protein, it will digest the protein you’ve eaten. In reality, you want to take it on an empty stomach so that systemically it’s used to break down damaged protein structures and suppress inflammation. So there’s an example of a supp that NEEDS to be taken on an empty stomach.
Some non-vitamin supps are better taken with fat containing meals. I’m thinking of DHEA, pregnenolone, and CoQ10.
In general, amino acids that are being used therapeutically need to be taken on an empty stomach. Otherwise they compete for uptake, resulting in your receiving little to no benefit.
If you’re taking herbs for a medical condition, I’d recommend you follow the advice of your healthcare practitioner. If taken with food, the result/effect is almost one of timed release. If taken on an empty stomach, the effect is faster and stronger.
I do remember reading something Cy Wilson said one time, and that is that a lot of manufacturers test their supps on an empty stomach to take out any element of variability. So that’s how the label usually reads. But that doesn’t mean you wouldn’t derive the full benefit if you took them with a meal.
So the bottom line as I said when I was starting out is that it depends. Ask the manufacturere what they recommend and ask them why. Even then, research it yourself until you’re satisfied.