Vitamins that are lipid-soluble — including vitamins D, A, and E — can be harmful in extremely high concentrations, whereas water-soluble vitamins, such as the B vitamins and vitamin C, appear to be relatively benign, even in extremely high doses.

Vitamin D3 can have a negative effect on immunity if the blood levels exceed 100 ng/dL. This is why getting blood levels of vitamin D3 right is so important when supplementing.

In general, both poisons and nutrients that are stored in the body, especially in body fat, have the greatest toxicity.

Mercury is fat-soluble and is stored for a lifetime. What this means is that each time you are exposed to mercury, the amount accumulating in your fat tissue (the brain is 60 percent fat by weight) increases.

As with nutrients, the distribution of these substances is not even throughout the body. Rather, certain tissues have much higher levels than others. Yearly flu vaccines (all of which contain mercury) increase this accumulation danger.

Because dosage is so important, you would hope that scientists would know the optimum dosages. Unfortunately, that is not true.

There is a great deal we still need to learn about optimal effective and toxic doses for most compounds, even supplements.