libitum

Libitum is a Latin term meaning "at will" or "as desired," and is most commonly encountered in English in the phrase ad libitum, meaning "at one's pleasure" or "as much as desired." The term signals permissiveness or unconstrained quantity.

In nutrition and laboratory contexts, ad libitum refers to providing food or water without restriction. An ad libitum feeding protocol allows subjects to consume nutrients freely, unlike restricted or caloric-restriction schedules used to study metabolism, aging, and disease. In clinical nutrition, advice may be given as ad libitum, meaning the patient should eat freely unless a restriction is medically required.

In research methodology, ad libitum describes conditions where intake or exposure is not constrained, often contrasted with caloric restriction or fixed-dose regimens. While it captures natural behavior and energy balance, it can increase variability and obscure causal effects, depending on study aims.

Outside science, the phrase appears in music, theatre, and literature to indicate improvisation or freedom of execution. The abbreviation ad lib. is commonly used in scores and performance directions.