The body has pathways that control absorption, interaction and excretion of nutrients, which it does not have for substances foreign to the body. Foreign substances can be toxic because the body has not evolved mechanisms to control or remove them. Heavy metals such as lead and cadmium can be toxic because they displace essential minerals like zinc from the many enzymes which it activates. Nutrients are the basis of our metabolism and could not be inherently toxic. The concept of U.L.s based on alleged toxicity of nutrients makes no scientific sense.
All nutrients are absolutely necessary to life.
Even when taken in large quantities, they are generally without any side effects. Even the rare adverse effects occasionally experienced after taking excessive amounts of nutritional supplements are not generally due to actual toxicities. Taking a large amount of a single nutrient could theoretically create an imbalance among other nutrients, such as by speeding up enzyme systems for which the nutrient is a co-factor. This should at first make the whole system run better as the limiting factor is increased, but could eventually result in increased need for the next most limiting nutrient, producing side effects pointing to the need for additional nutrients. These are not signs of toxicity.