Three zones from deficiency to toxicity
Most nutrients follow a dose-response with three broad zones. Below the requirement, function is impaired and deficiency symptoms appear. Across an adequate range (roughly from the recommended intake up toward the tolerable upper level) the body works normally and benefit plateaus. Above the upper level, intake stops helping and risk of harm rises. The generic curve sketches this rise, plateau, and rise again.
The plateau in the middle is the key idea: once needs are met, more is generally not better, and for many nutrients it is eventually worse.
Why every nutrient draws its own shape
The curve is a template, not a fixed picture. Water-soluble vitamins that the body excretes readily have a wide, forgiving safe range, while fat-soluble vitamins and some minerals that accumulate have a much narrower one with a closer toxicity threshold. The distance between the requirement and the upper limit (the safety margin) differs sharply from nutrient to nutrient.
This is general educational information, not medical advice. Tolerable upper limits exist to prevent harm, so supplement doses near or above them should be set by a qualified professional.