in appropriate formulations, mixed, and are commonly extruded. Sometimes high-moisture fruits and vegetables, browse, or other supplements are fed along with dry extrusions for purposes of environmental enhancement, as described in Chapter 13. Foods used for environmental enhancement should not be used excessively and they should not dilute energy density, which can adversely limit dry matter consumption to below needs. Likewise, these foods should not distort the nutrient balance of a diet formulated to be nutritionally complete.

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Institute of Medicine. 1997. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press.

Institute of Medicine. 1998. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vitamin B₁₂, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press.

Institute of Medicine. 2000. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press.

Institute of Medicine. 2001. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press.

Jacob, R.A., and M.E. Swendseid. 1996. Niacin. Pp. 184-190 in Modern Nutrition in Health and Disease, 9^th ed., M.E. Shils, J.A. Olson, M. Shike, and A.C. Ross, Eds. Baltimore: Williams & Wilkins.

Jenkins, D.J.A., T.M.S. Wolever, and A.L. Jenkins. 1999. Fiber and other dietary factors affecting nutrient absorption and metabolism. Pp. 679-698 in Modern Nutrition in Health and Disease, 9^th ed., M.E. Shils, J.A. Olson, M. Shike, and A.C. Ross, Eds. Baltimore: Williams & Wilkins.

Knapka, J.J. 2000. Factors influencing required dietary nutrient concentrations. Lab. Anim. 29:47-50.

Mock, D.M. 1996. Biotin. Pp. 220-235 in Present Knowledge in Nutrition, 7^th ed., E.E. Ziegler and L.J. Filer, Jr., Eds. Washington, DC: ILSI Press.

National Research Council. 1989. Recommended Dietary Allowances, 10^th ed. Washington, DC: National Academy Press.