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Comparative Nutrition and Metabolism: Explication of Open Questions with Emphasis on Protein and Amino Acids

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Comparative Nutrition and Metabolism: Explication of Open Questions with Emphasis on Protein and Amino Acids Comparative nutrition and metabolism: Explication INAUGURAL ARTICLE of open questions with emphasis on protein and amino acids David H. Baker* Department of Animal Sciences and Division of Nutritional Sciences, University of Illinois at Urbana–Champaign, Urbana, IL 61801 This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 3, 2005. Contributed by David H. Baker, October 28, 2005 The 20th century saw numerous important discoveries in the almost completely absorbed (e.g., K, Na, Cl, I, F), some have nutritional sciences. Nonetheless, many unresolved questions still Ϸ70–90% absorption efficiency (P, Se, B, As), some have remain. Fifteen questions dealing with amino acid nutrition and Ϸ30–40% (Ca, Mg, Cu), some have Ϸ15–20% (Fe, Zn, Ni), metabolism are posed in this review. The first six deal with the some have Ϸ5% (Mn, V), and some have Ͻ1% (Cr). Ratios of functionality of sulfur amino acids (methionine and cysteine) and one element to another in a meal also are known to substantially related compounds. Other unresolved problems that are discussed affect absorption efficiency; this is illustrated by excess dietary include priorities of use for amino acids having multiple functions; Zn reducing Cu and Fe absorption and excess dietary P reducing interactions among lysine, niacin and tryptophan; amino acid Mn absorption (4, 5). Moreover, certain food ingredients in the contributions to requirements from gut biosynthesis; the potential vegetable category can bind mineral elements and reduce their for gluconeogenesis to divert amino acids away from protein bioavailability: phytic acid (inositol hexaphosphate) binds not synthesis; the unique nutritional and metabolic idiosyncrasies of only P, but also Ca, K, Zn, and Fe, and oxalates in foods like feline species, with emphasis on arginine; controversies surround- spinach can strongly bind Ca and Fe. ing human amino acid requirements; and the potential for mater- With vitamins, whether existing in foods as free vitamins or SCIENCES AGRICULTURAL nal diet to influence sex ratio of offspring. coenzymes, cooking, baking, and other food processing proce- dures can markedly reduce the bioavailability of vitamins like amino acid deficiency ͉ sulfur amino acids ͉ arginine ͉ lysine ͉ niacin thiamin and folacin that contain a free amino group (3). Heat processing also reduces the bioavailability of certain protein- reat progress was made in the nutritional sciences during the bound amino acids that contain a free amino group (lysine) via G20th century. By 1948, all of the essential vitamins had been the same mechanism (Maillard reaction); this occurs promi- discovered, and their synthesis was accomplished (1). The last of nently when cookies, cakes, or bread is baked and when bread is the amino acids found in food proteins (i.e., threonine) was toasted (3). isolated in 1935 (2). Defined functions for dietary essential Molecular genetic approaches are being used today to engi- mineral elements (Ca, P, K, Na, Cl, Mg, I, Zn, Fe, Cu, Mn, Cr, neer food crops to have increased concentrations of specific Se, F) and ultra trace elements (Si, Ni, Sn, V, B, As) also were nutrients (6). ␤-Carotene is the direct vitamin A precursor in discovered. Specific dietary requirements for essential nutrients plant-derived food products. The entire biosynthetic pathway of were subsequently listed for animals by National Research ␤-carotene biosynthesis in plants is now known (7), and its first Council (NRC) and for humans by Food and Nutrition Board application was in the development of transgenic rice (i.e., committees of the National Academy of Sciences. Numerous golden rice) that contained a substantial increase in ␤-carotene Nobel Prizes were awarded for this body of work, particularly for (8). Biosynthetic pathways for ␣-tocopherol (vitamin E) and the discovery of vitamins and their role in nutrition and disease. folacin have been established as well (6, 9, 10). Plant geneticists World-wide, deficiencies of vitamin A, iron, and high-quality have also found ways to engineer food crops to have increased protein are generally considered the most serious problems in levels of lysine and iron, and decreased levels of phytic acid (6, human nutrition. Prominent questions today are whether a role 10, 11). exists for nutrients beyond their role in preventing specific Everyone seems to be interested in their own nutrition and deficiencies. Thus, considerable research effort is being ex- health, and this has led to a plethora of so-called ‘‘nutritional pended into the role of certain carotenoids in preventing mac- supplements’’ being made available (without prescription) to the ular degeneration and cataracts, specific vitamin E isomers as general public. Those of us who consider ourselves nutritional antioxidants, various hydroxylated vitamin D products for pro- professionals find this alarming. Indeed, many of these supple- motion of gut absorption of Ca and P as well as for treatment of ments have no demonstrated efficacy or safety data to justify osteoporosis and psoriasis, pharmacologic Se and conjugated their use. Also, several essential nutrients (and nonnutrients) linoleic acid for cancer prevention, and selected amino acids at available for sale to the general public are toxic at higher dose pharmacologic dose levels for a host of clinical conditions. In levels, e.g., Se, V, cysteine, and vitamin A. addition, obesity and adult-onset diabetes have become serious My research program has focused on using animal models and problems in developed countries such as the U.S. Terms such as chemically defined diets to study nutrition and disease problems glycemic index, soluble vs. insoluble (and fermentable vs. non- that affect both animals and humans. In this review, I will fermentable) fiber, and indigestible starch have entered our describe and briefly discuss my own personal list of ‘‘15 vexatious vocabulary. Bioavailability assessment of food nutrients has advanced to the point where we now have reasonable estimates of digestibility Conflict of interest statement: No conflicts declared. and absorbability of most nutrients in a food matrix. Vast Abbreviations: NRC, National Research Council; SAA, sulfur amino acid; DRI, Dietary differences exist in bioavailability both within and among nu- Reference Intake. trient groups (3, 4). Within dietary essential mineral elements, *E-mail: [email protected]. for example, some ingested elements in inorganic form are © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0509317102 PNAS ͉ December 13, 2005 ͉ vol. 102 ͉ no. 50 ͉ 17897–17902 Downloaded by guest on September 30, 2021 questions’’ that have intrigued me over the course of my career convert some of the dietary cystine to lanthionine (26), a as an academic scientist. crosslinked sulfur compound that has minimal SAA bioactivity The first six questions deal with sulfur compounds and sulfur (27). Thus, protein-bound cystine has a low bioavailability (28). This amino acids (SAA, i.e., methionine and cysteine). The role of could be important clinically, because undigested cystine will pass these compounds in protein synthesis, transmethylation, synthe- to the colon where sulfate-reducing bacteria may degrade it to sis of glutathione, taurine, CoA, and phosphoadenosine-5Ј- sulfides, and sulfides have been found noxious to colonic epithelial phosphosulfate as well as in ameliorating various inflammatory cells (29, 30). Still, the link between undigested SAA, particularly conditions have had longstanding emphasis in my laboratory. cystine, and colonic inflammation has not been firmly established. Clearly, the elegant research contributions of the late Vincent du Vigneaud (12), an Academy member and Nobel Laureate, Question 4: Are There Components of Foods and Feeds Other than provided great inspiration for the nutrition work on SAA done Methionine, Choline, Betaine, Folacin, and Serine That Have Methyl in my laboratory. Sulfur amino acid work is of great practical Donating Capacity? Many foods and feedstuffs (e.g., soybean relevance to animal nutrition in that well over 90% of SAA meal) contain significant and measurable quantities of S- production is used to fortify diets for animals, particularly methylmethionine (SMM), an analog of S-adenosylmethionine poultry. Poultry diets around the world are based on corn and (SAM). As such, it may be capable of replacing (or sparing) soybean meal, and these diets for poultry, without fortification, SAM in biological methylation reactions such as choline bio- are deficient in SAA. synthesis from phosphatidylaminoethanol and creatine synthesis from guanidinoacetate. Our recent work using the chick as an Questions animal model showed that SMM does indeed have choline- Question 1: Why Does the Addition of Methionine, Alone, to a sparing activity (31). However, the methylation reaction in which Protein-Free Diet Increase Nitrogen Retention, Protein Accretion, and homocysteine is converted to methionine prefers betaine as the Growth? Several investigators have reported that methionine methyl donor. Thus, methionine sparing by SMM was found to supplementation of a protein-free diet reduces body weight loss occur only when choline and betaine were deficient in the diet. and improves nitrogen balance in rats (13), chickens (14), pigs Dimethylsulfoniopropionate is another sulfur compound (15), and dogs (16). Our own work (17) has confirmed the earlier present in foods (32). Can consumption
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