Polyunsaturated fatty acids (PUFAs) are important for neonatal growth and development in all mammalian species studied to date. Normal fetal development requires the 18carbon essential fatty acids as well as long-chain polyunsaturated fatty acids (LCPUFAs) with 20and 22-carbon–chain lengths (1). Provision of both dietary (n-3) and (n-6) PUFAs in appropriate amounts are needed to optimize development of nervous system tissue and to promote kidney, liver, and skin functions. Two LCPUFAs, arachidonic acid of the (n-6) series and docosahexaenoic acid of the (n-3) series, which are synthesized by chain elongation and desaturation of essential fatty acids, have received increasing interest recently because they are found in high concentrations in the phospholipids of normal cell membranes of the central nervous system (2). It was shown (3) that human infants who consume low LCPUFA levels also have lower LCPUFA levels in the phospholipids of the cerebral cortex. The clinical relevance of this finding is the subject of various growth and neurodevelopment studies. Because the PUFAs are transported via plasma lipoproteins (LPs), the effects of dietary fatty acids on LP metabolism may play an important role in development. In dogs, high-density lipoproteins (HDLs) are the predominant plasma LP fraction (75–85%) (4). This differs from humans, where low-density lipoproteins (LDLs) predominate. For this reason, dogs are considered to be ‘‘HDL mammals’’ (5,6) and also exhibit atherosclerosis-resistance properties (4). However, as in humans, diet can alter canine LP cholesterol distributions. For example, a study by Bauer (7) demonstrated that dogs fed a diet that is high in saturated fat (primarily from beef tallow) show an increase in both LDL and HDL fractions, and all LP fractions tend to shift toward a lower density. Dreon et. al. (8) also reported that feeding saturated fat increases both LDL and HDL concentrations in humans. Replacing saturated fat with unsaturated fat in the diet decreases LDL more than HDL in humans. Polyunsaturated fatty acids are the most potent fatty acids for reducing LDL cholesterol (9). An early study of canine LP metabolism showed that immature dogs had greater hepatic LDL receptor activity than mature dogs. This activity was undetectable by 24 mo of age. Canine liver contains two distinct LP receptors. There is an apoprotein-B,E (apo-B,E) receptor, which binds both LDL and HDL cholesterol, and an apo-E receptor, which binds only HDL cholesterol. The apo-B,E receptor is seen in immature, growing dogs, whereas only the apo-E receptor is found in adult dogs (10). This same study also revealed that feeding cholesterol can suppress apo-B,E receptors in immature dogs, whereas in mature dogs, prolonged fasting can induce expression of apo-B,E receptors. The apo-E hepatic membrane receptors remain constant in both immature and adult dogs or decrease only slightly with age (10). Thus, the possibility exists that immature dogs have greater concentrations of LDL fractions during early life, and dietary fatty acids may further modify plasma LP fractions overall. The present study was conducted to investigate plasma LP distribution changes during suckling and early adolescence of puppies born to bitches that were fed diets that varied only in fatty acid composition and were otherwise identical. 1 Presented as part of the WALTHAM International Science Symposium: Nature, Nurture, and the Case for Nutrition held in Bangkok, Thailand, October 28–31, 2003. This symposium and the publication of the symposium proceedings were sponsored by the WALTHAM Centre for Pet Nutrition, a division of Mars, Inc. Symposium proceedings were published as a supplement to The Journal of Nutrition. Guest editors for this supplement were D’Ann Finley, James G. Morris, and Quinton R. Rogers, University of California, Davis. 2 This work was supported in part by Nestle-Purina PetCare and the Mark L. Morris Professorship in Clinical Nutrition at Texas A&M University. 3 To whom correspondence should be addressed. E-mail: jbauer@cvm. tamu.edu. 4 Abbreviations used: ALA, a-linolenic acid; HDL, high-density lipoprotein; LA, linoleic acid; LCPUFA, long-chain polyunsaturated fatty acids; LDL, low-density lipoprotein; LP, lipoprotein; PUFA, polyunsaturated fatty acids.

Mammalian skin is a highly dynamic organ that is constantly adapting to changes in its environment. It provides structural, sensory, immunologic, and physiologic functions and contrib-utes an essential barrier function against potential environmental insults. The epidermis of the skin is composed of keratinocytes that undergo a highly organized maturation process that ultimately leads to desquamation. Structural components within these cells utilize dietary lipids such as linoleic acid to maintain a protective barrier. Together they form the stratum corneum, which is the outermost layer of the epidermis. Environmental, physical, and nutritional alterations can modify epidermal structure and function. In addition to keeping environmental insults outside the body, the epidermal barrier also functions to keep water and other important metabolites inside the body. In this way, metabolic processes consonant with health can be maintained. barrier include determination of hydration (HYDR) capacitance of the due to its behavior as a dielectric and assesses a thickness of the stratum