semiessential

Semiessential is a term used in nutrition to describe nutrients that the body can normally synthesize but that may need to be obtained from the diet under certain conditions. The concept is closely related to the idea of conditional essential nutrients, though some authors distinguish semiessential to emphasize variability across populations or life stages.

In humans, several amino acids are commonly described as conditionally essential. Arginine and histidine are classic examples: during rapid growth, infancy, or illness, endogenous synthesis may be insufficient, making dietary intake important. Cysteine and tyrosine are considered semiessential when the supply of their precursor amino acids, methionine or phenylalanine, is limited since their production depends on these precursors. Glutamine is also described as conditionally essential in contexts such as critical illness or major stress, where its requirement may exceed production. The classification can vary with age, health status, and metabolic conditions.

In veterinary nutrition, the term reflects species- and life-stage differences in synthesis capacity, with some amino acids deemed essential for certain animals or developmental stages. In plant science, semiessential concepts are less frequently used, but the idea can apply to nutrients that plants can synthesize only incompletely under environmental stress, affecting dietary or agricultural considerations.

The usage of semiessential is somewhat fluid; many authorities prefer the term conditionally essential for clarity. The concept highlights that dietary requirements are dynamic and depend on metabolism, growth, disease, and genetics, rather than being fixed for all individuals.