Ad libitum is also used in psychology and biology to refer to the "free-feeding" weight of an animal, as opposed, for example, to the weight after a restricted diet. C.f., The rat's ad libitum weight was about 320 grams.
Free-fed animals are often less healthy than animals with restricted diets--as is true of humans, as well. Excess weight is highly correlated with a variety of health threatening disorders.