Annual Review of Nutrition - Volume 16, 1996

Free radicals and antioxidants are widely discussed in the clinical and nutritional literature. Antioxidants are needed to prevent the formation and oppose the actions of reactive oxygen and nitrogen species, which are generated in vivo and cause damage to DNA, lipids, proteins, and other biomolecules. Endogenous antioxidant defenses (superoxide dismutases, H2O2-removing enzymes, metal binding proteins) are inadequate to prevent damage completely, so dietderived antioxidants are important in maintaining health. Many dietary compounds have been suggested to be important antioxidants: The evidence for a key role of vitamins E and C is strong, but that for carotenoids and related plant pigments is weaker. Interest is also growing in the role of plant phenolics, especially flavonoids. Some antioxidants can exert prooxidant effects in vitro, but their physiological relevance is uncertain. Experimental approaches to the optimization of antioxidant nutrient intake are proposed.

The development of atherosclerosis is a complex and multistep process. There are many determinants in the pathogenesis of this condition, with different factors presumably playing key roles at different times in the evolution of the atherosclerotic plaque. It has been suggested that oxidation of low density lipoproteins (LDL) by cells in the artery wall leads to a proatherogenic particle that may help initiate early lesion formation. For this reason, understanding the determinants of LDL susceptibility to oxidation is essential for developing therapeutic strategies to inhibit this process. Oxidation of LDL begins with the abstraction of hydrogen from polyunsaturated fatty acids; thus, LDL fatty acid composition undoubtedly contributes to the process of LDL oxidation. Since dietary fatty acids influence the fatty acid composition of LDL and cell membranes, the amount and type of fat in the diet may effect susceptibility of LDL and cells to oxidative damage. Additionally, since cell membrane fatty acid composition also influences cellular formation of reactive oxygen species, dietary fatty acids may help determine the prooxidant activity of artery wall cells. Both cells and lipoproteins contain a variety of antioxidants that provide protection against oxidative stress. A major source of these antioxidants is the diet. Enrichment of the diet with foods high in such antioxidants as vitamin E, β-carotene. or vitamin C, or supplementation of the diet with antioxidant vitamins, may inhibit oxidation and the process of atherosclerosis.

Thirty years ago, it was suggested that maternal intake of certain vitamins during pregnancy affected the incidence of serious fetal malformations. Subsequent research has revealed that folate (folic acid), a B vitamin, plays a crucial role in the development of the central nervous system during the early weeks of gestation, which is generally before the pregnancy is confirmed. In a significant number of embryos, an inadequate supply of folate at this time leads to a failure of the primitive neural tube to close and differentiate normally and results in neural tube birth defects (NTD). Numerous studies have confirmed the importance of an adequate intake of folate during the weeks just before and after conception. Overall, the data predict that if women consume multivitamin supplements containing folic acid during the periconceptional period, the number of children born with serious malformations (such as spina bifida and anencephaly) could be reduced by half. Although programs to increase dietary folate intake of potential mothers may be effective in reducing NTD, the only proven and practical preventive measure currently available is to take oral multivitamin supplements containing folic acid. Multivitamin supplementation has also been associated with reduced incidence of other congenital malformations.

Current research is focusing on the role of micronutrients in embryogenesis, and on methods to identify prospective mothers at increased risk for bearing a child with NTD or with other major malformations shown to occur at reduced frequency with multivitamin supplementation. Of equal importance is the development of methods to communicate current knowledge as a public health measure.

Even though the existence of a transport process for intact peptides in the brush border membrane of intestinal and renal absorptive epithelial cells has been known for almost three decades, it is only recently that the molecular nature of the proteins responsible for the transport process has been elucidated. Two peptide transporters, PEW 1 and PEPT 2, have been cloned. The cloned transporters catalyze active transport of intact di- and tripeptides and utilize a transmembrane electrochemical H⁺ gradient as the driving force. The characteristic of H⁺ coupling makes PEPT 1 and PEPT 2 unique among the transporters thus far identified in mammalian cells. In addition, the peptide transporters have immediate pharmacologic relevance because a number of peptide-like drugs are recognized as substrates by these transporters. Recently, cultured cell lines of intestinal and renal origin that express PEPT 1 and PEPT 2 have been identified. These cell lines are likely to facilitate studies on the regulatory aspects of the peptide transporters.

During vigorous exercise, carbohydrate, in the form of muscle glycogen and blood glucose, is the primary energy source, whereas fatty acids play a secondary, carbohydrate-sparing role. In the postabsorptive state, nearly all of the carbohydrate used during exercise comes from muscle and liver glycogen. The size of these glycogen stores plays a major role in determining how long vigorous endurance exercise can be performed if other causes of fatigue, such as dehydration or heat exhaustion, are avoided. In a “fight or flight” situation, individuals whose muscles are glycogen depleted are helpless, as they are unable to either run or fight. The same is true of someone who has become markedly hypoglycemic as a result of liver glycogen depletion.

Genetic hemochromatosis is one of the most common inherited disorders in Caucasian populations. The disease frequency in Caucasian populations in Australia, Europe, and the United States is 1:300-400. The basic genetic defect remains unknown, although the hemochromatosis gene is closely linked to histocompatibility antigen (HLA) A, thus allowing early diagnosis in members of affected families. Many factors—environmental, genetic, and nongenetic in nature—influence the degree of iron loading in affected individuals. In particular, pathologic and physiologic blood loss and blood donation influence iron stores in hemochromatosis. The iron concentration in the liver is an important determinant of survival because a hepatic iron concentration in excess of 400 pmol/g dry weight is usually associated with cirrhosis. Patients with cirrhosis secondary to hemochromatosis are at risk of heptocellular carcinoma and complications of portal hypertension. The combination of improved awareness of the condition and the use of HLA typing to identify affected family members has led to earlier diagnosis and therapy, and to an improvement in overall survival

Our understanding of food allergy has made tremendous strides recently. With now-relatively uniform definitions of the clinical presentations of food allergy, the scientific literature is more discernible. In this progress, we have come to understand that the prevalence of food allergy is up to 8% in children and 2% in adults. Additionally, these studies have shown the role of specific allergens and mediators in the immunopathogenesis of food allergy. Much of the information available still relates to immunoglobulin & mediated food reactions, although other immunologic mechanisms are being studied extensively. The diagnosis and treatment of food allergy is now much more standardized. Longterm studies have shown the natural history of many of these reactions.

The enormous number of different liver functions are carried out by parenchymal and four main types of nonparenchymal cells, either alone or in cooperation. Although the liver tissue is uniform on the level of histology, it is heterogenous on the level of morphometry and histochemistry. This heterogeneity is related to the blood supply; cells located in the upstream or periportal zone differ from those in the downstream or perivenous zone in their equipment with key enzymes, translocators, receptors, and subcellular structures and therefore have different functional capacities. This is the basis of the model of metabolic zonation, according to which glucose release from glycogen and via gluconeogenesis, amino acid utilization and ammonia detoxification, protective metabolism, bile formation, and the synthesis of certain plasma proteins such as albumin and fibrinogen occur mainly in the periportal area, whereas glucose utilization, xenobiotic metabolism, and the formation of other plasma proteins such as α1-antitrypsin or α-fetoprotein occur predominantly in the perivenous zone. The morphologic and functional heterogeneity is the result of zonal differences in the activation of the cellular genome caused by gradients in oxygen, substrate, hormone, and mediator levels, in innervation, as well as in cell-to-cell and cell-to-biomatrix interactions.

Two cytoplasmic retinol-binding proteins, CRBP and CRBP II, and two cytoplasmic retinoic acid-binding proteins, CRABP-I and CRABP-II, have been well characterized. There has been significant progress in the structural analysis of these four proteins with X-ray crystallography, nuclear magnetic resonance, mutagenesis, and binding studies. In contrast, the cellular functions of these cytoplasmic vitamin A-binding proteins are less well understood. Since these proteins bind their respective ligands with high affinity, they are likely to influence retinoid signaling pathways. Analysis of retinoid metabolism in the presence or absence of these proteins provides support for the hypothesis that these proteins are involved in modulating intracellular retinoid metabolism. Molecular genetic approaches to alteration of the levels of these proteins in tissue culture cells and in whole animals have provided a powerful means toward defining the physiological roles of the cytoplasmic vitamin A-binding proteins in vivo.

The human appetite system contains central and peripheral mechanisms that interact with environmental features, especially with the physical and nutrient composition of the food supply. Foods varying in nutrient composition exert different physiologic effects, some of which function as satiety signals. High-fat diets (low food quotient) lead to high levels of energy intake. This effect is termed passive overconsumption and overcomes fat-induced physiological satiety signals. High-fat foods exert a weak effect on satiation (intra-meal satiety), and fat has a weaker effect, joule for joule, on postingestive satiety than do other macronutrients. The frequency of obesity is greater among high-fat than low-fat consumers. However, the development of obesity on a high-fat diet is not a biological inevitability. The investigation of people who resist the weight-inducing properties of high-fat diets is a key research strategy. Understanding the appetite control system suggests behavioral, nutritional, and pharmacologic strategies for modifying dietary fat intake.

How much vitamin E is enough? An established use of supplemental vitamin E in humans is in the prevention and therapy of deficiency symptoms. The cause of vitamin E deficiency, characterized by peripheral neuropathy and ataxia, is usually malabsorption—a result of fat malabsorption or genetic abnormalities in lipoprotein metabolism. Genetic abnormalities in the hepatic α-tocopherol transfer protein also cause vitamin E deficiency—defects in this protein cause an impairment in plasma vitamin E transport. Impaired delivery of vitamin E to tissues, thereby, results in deficiency symptoms. Also discussed is the use of supplemental vitamin E in chronic diseases such as ischemic heart disease, atherosclerosis, diabetes, cataracts, Parkinson's disease, Alzheimer's disease, and impared immune function, as well as in subjects receiving total parenterol nutrition. In healthy individuals, a daily intake of about 15-30 mg of α-tocopherol is recommended to obtain “optimal plasma alpha-tocopherol concentrations” (30 μM or greater).

This chapter provides an update on drug-food interactions reported in the literature during the past five years. The number of studies examining this phenomenon has increased dramatically, and many of the results of these studies have been unpredictable and spectacular. Drug-food interactions should really be considered as formulation-food interactions because of increasing evidence that a drug may be affected differently by food when it is administered in different formulations. Drug-food interactions may be classified into five categories: those causing reduced, delayed, increased, and accelerated absorption, and those in which food has no effect. While it continues to be necessary to examine drugfood interactions for specific drugs and drug formulations, additional avenues need to be explored to seek mechanistic patterns that may lead to better prediction of the nature and extent of changes in circulating drug levels due to the presense of food, and their possible clinical impact.