Purpose
To define the limits of acceptable weight loss and expectations for monitoring weight loss in research animals.
Background
Weight loss may occur in research animals in association with a variety of experimental regimens including toxicology studies, cancer studies, infectious disease studies, and studies where feed and essential nutrients are withheld, such as studies of nutritional deficiencies. Weight loss also occurs in association with many spontaneous diseases and is a prime indicator of declining clinical condition. Moderate food and calorie restriction with moderate weight loss and adequate nutrition, rather than being detrimental, has been shown to promote health and extend the life of laboratory rodents and other species. The Institutional Animal Care and Use Committee (IACUC) has accepted weight losses of up to 20% of normal or pre-study body weight as a humane limit. However, some studies (i.e., involving nutritional deficiency and tumors) may result in weight losses of greater than 20% that can be scientifically justified. In studies in which animals are performing a behavior or a task, optimal or acceptable responses might occur with weight loss of far less than an arbitrarily accepted 20% weight loss limit. Performance at lesser weight loss might be achieved with a higher preference food reward, for example.
Policy
The upper limit of acceptable weight loss in animals on experimental regimens shall generally be 20% of normal or pre-study body weight.
In studies where weight loss is expected to occur, investigators must describe the monitoring and address situations where weight loss will exceed 20%, and remedial measures that will be taken. Additionally, monitoring must be done by investigative staff trained and experienced in recognizing clinical signs of illness and distress in study animals. Weights must also be taken at least weekly under such circumstances and be readily available for review by the veterinary staff and the IACUC. Written scientific justification must be provided to the IACUC for approval of a greater than 20% weight loss. This will be allowed only if there is a veterinary determination that weight losses exceeding approved limits are not endangering animal health and well-being and the scientific justification is appropriate.
Criteria in addition to body weight measures might be used for rapidly growing animals, animals with tumors, or obese animals. Body condition scoring can be used in addition to or instead of weight measures when appropriate.
See IACUC Policies “Food and Water Restriction in Mice and Rats” or “Food and Water Restriction in Non-Rodent USDA Species” for acceptable weight loss and monitoring criteria in animals on food and fluid restriction.
Approval/Review Dates
Originally Approved: 03/22/2001
Last Reviewed/Revised by the IACUC: 12/16/2021