Comparisons of Diets Used in Animal Models of High-Fat Feeding

Animal models are invaluable resources for biomedical research, including research on the effects of diet on metabolism and disease. Usually, great care is taken to ensure comparable genetic backgrounds and environmental conditions when performing studies using animal models, since this minimizes introduction of variability that can confound detection of treatment-related phenotypic differences. However, many papers using animal models draw conclusions about dietary effects from comparisons of natural-ingredient chow with defined diets, despite marked differences in micro- and macronutrient content. When comparing the effects of a chow diet with a defined high-fat diet, the effects of the dietary fat will be confounded with the effects of other components in the diets. This issue is highlighted by a limited literature survey that was conducted to identify common problems in the use and reporting of rodent diets.All original research papers identified by the keywords “mouse high fat” published in 2007 in five high-impact journals were included in this evaluation. Of the 35 papers examined, only 14% (5 papers) compared diets using identical nutrients differing only in relative amounts of fat and carbohydrate (Figure 1Figure 1). Specific details regarding the dietary comparisons made were often lacking, and frequently conclusions were drawn from comparisons of defined high-fat diets to chow.Figure 1Diet Comparisons in Recent Research PapersPie chart showing the percentage of 35 original research papers evaluated that used appropriate diet comparisons (14%), that compared chow and defined high-fat diets (43%), and that presented insufficient information to evaluate diet comparisons (34%). In the “chow and defined” category (9%), both diet types were used but no direct comparison was made between them. The journals examined were Cell Metabolism (7 papers), Diabetes (the first 11 of 36 papers), The Journal of Clinical Investigation (12 papers), Nature (2 papers), and Nature Medicine (3 papers).View Large Image | View Hi-Res Image | Download PowerPoint SlideRegular chow is composed of agricultural byproducts such as ground wheat, corn, or oats, alfalfa and soybean meal, a protein source such as fish, and vegetable oil and is supplemented with minerals and vitamins. Thus, chow is a high-fiber diet containing complex carbohydrates, with fats from a variety of vegetable sources. Chow is inexpensive to manufacture and is palatable to rodents. In contrast, defined high-fat diets consist of amino acid-supplemented casein, corn starch, maltodextrose or sucrose, and soybean oil or lard, also supplemented with minerals and vitamins. Fiber is often provided by cellulose. Chow and defined diets may exert significant separate and independent unintended effects on the measured phenotypes in any research protocol.Two important differences between chow and defined diets are the phytoestrogen content from soy, which is high but variable in chow diets but absent from defined diets (reviewed in Thigpen et al., 2004xThigpen, J.E., Setchell, K.D., Saunders, H.E., Haseman, J.K., Grant, M.G., and Forsythe, D.B. ILAR J. 2004; 45: 401–416Crossref | PubMedSee all ReferencesThigpen et al., 2004), and from sucrose, which is used as a carbohydrate source in defined diets but is absent from chow. Dietary phytoestrogens influence food and water intake, anxiety-related behaviors, locomotor activity, learning and memory, fat deposition, blood insulin, leptin and thyroid levels, and lipogenesis and lipolysis in isolated rat adipocytes (Torre-Villalvazo et al., 2008xTorre-Villalvazo, I., Tovar, A.R., Ramos-Barragan, V.E., Cerbon-Cervantes, M.A., and Torres, N. J. Nutr. 2008; 138: 462–468PubMedSee all References, Lephart et al., 2004axLephart, E.D., Porter, J.P., Lund, T.D., Bu, L., Setchell, K.D., Ramoz, G., and Crowley, W.R. Nutr. Metab. (Lond). 2004; 1: 16Crossref | PubMed | Scopus (34)See all References; reviewed in Lephart et al., 2004bxLephart, E.D., Setchell, K.D., Handa, R.J., and Lund, T.D. ILAR J. 2004; 45: 443–454Crossref | PubMedSee all ReferencesLephart et al., 2004b). Sucrose is 50% fructose, and there is abundant evidence that fructose can exacerbate weight gain and contribute to insulin resistance and dyslipidemia (reviewed in Stanhope and Havel, 2008xStanhope, K.L. and Havel, P.J. Curr. Opin. Lipidol. 2008; 19: 16–24Crossref | PubMed | Scopus (121)See all ReferencesStanhope and Havel, 2008). Other effects that differ between chow and defined diets and may be related to either phytoestrogen or fructose content include bone-related changes, plasma estradiol, urinary calcium and corticosterone (Tou et al., 2005xTou, J.C., Arnaud, S.B., Grindeland, R., and Wade, C. Exp. Biol. Med. (Maywood). 2005; 230: 31–39PubMedSee all ReferencesTou et al., 2005), and solution taste preferences (Tordoff et al., 2002xTordoff, M.G., Pilchak, D.M., Williams, J.A., McDaniel, A.H., and Bachmanov, A.A. J. Nutr. 2002; 132: 2288–2297PubMedSee all ReferencesTordoff et al., 2002).When comparing the effects of chow with a defined high-fat diet, the effects of the dietary fat will be confounded with the effects of other components that differ between the diets. Just as it is essential that mouse strains be specified, constituents of experimental diets must be specified (Thigpen et al., 2004xThigpen, J.E., Setchell, K.D., Saunders, H.E., Haseman, J.K., Grant, M.G., and Forsythe, D.B. ILAR J. 2004; 45: 401–416Crossref | PubMedSee all References, Tordoff et al., 1999xTordoff, M.G., Bachmanov, A.A., Friedman, M.I., and Beauchamp, G.K. Science. 1999; 285: 2069PubMedSee all References, Everitt and Foster, 2004xEveritt, J.I. and Foster, P.M. ILAR J. 2004; 45: 417–424Crossref | PubMedSee all References). Specifics to be considered are the objectives of the study and whether the endpoints are affected by several diet components, the maintenance diet, or the palatability of the diet (Thigpen et al., 2004xThigpen, J.E., Setchell, K.D., Saunders, H.E., Haseman, J.K., Grant, M.G., and Forsythe, D.B. ILAR J. 2004; 45: 401–416Crossref | PubMedSee all ReferencesThigpen et al., 2004). Although this correspondence focuses on high-fat diet comparisons, these issues are applicable to any specific nutrient comparisons.