Tiia Monto [creativecommons license] via Wikimedia Commons
In many experimental protocols, rodents are given unlimited access to food. This allows the animals to overeat, resulting in substantial changes in their metabolism and physiology. Past studies have reported elevated levels of triglycerides and cholesterol; diet-induced obesity; kidney and heart dysfunction; and greater risks of pituitary, pancreatic, adrenal and thyroid tumors in association with ad libitum feeding of rodents.
Lab Anim. (NY) 41, 242 (2012).