Nutrient Requirements. First of all, the essential requirements of nutrients for each animal species need to be fulfilled. These nutrient requirements are described in the documents of the National Research Council. In case one does not live up to these requirements, a negative impact on animal welfare and on the validity of experimental results are obtained. E.g. when examining the influence of dietary linoleic acid concentration on tumour development, a control group without any linoleic acid will cause premature disease and death (De Wille 1993, Cancer Letters 69, 59), as linoleic acid is an essential nutrient. As linoleic acid is essential for the development of cell membranes, tumours cannot develop when linoleic acid is left out of the control diet.

Feeding methods. Ad libitum feeding of rodents has many reproducible negative effects as compared to restricted feeding: higher cancer incidence at an earlier age, shorter survival time and more degenerative diseases of the kidney and the heart. Due to these effects, restricted feeding is performed in long-term toxicology studies in some laboratories. Unfortunately, this often leads to individual housing, which is in direct contrast with the essential needs of social living animals. Therefore, alternative ways of feeding are indicated. Ad libitum feeding of minipigs leads to excessive obesity in females. This may give a good model for studying obesity, but when obesity research is not the objective, ad libitum feeding is undesirable from an animal welfare point of view.

Nutrition and animal models. The selection of ingredients versus the species of animals needs careful evaluation. Fish oil is known to induce liver damage and atherosclerosis in the herbivorous rabbit. A relatively high dietary phosphorus level will induce nephrocalcinosis in rats and rabbits. Enrichment. The way of feeding can be an important tool in enriching the environment. Feeding rabbits a restricted amount of food closer to the natural feeding time significantly reduced stereotyped behaviour, also as compared to near ad libitum feeding. Searching and working for food could contribute to animals welfare and more research should address this big potential for enrichment.