The GLOBAL OBJECTIVE of my research program is to understand the nutritional costs of maintenance functions of amino acids in growing neonates, especially in the face of a challenge. In particular, I intend to describe the amino acid requirements for these non-growth functions.
My research program can be divided into three areas:
1) Nutritional costs of creatine synthesis.
2) Neonatal methyl metabolism.
3) Early nutritional insults and long term consequences on health.
1) NUTRITIONAL COSTS OF CREATINE SYNTHESIS In piglets, both arginine and methionine are essential amino acids and have several important metabolic roles other than protein synthesis. For example, both amino acids are necessary for creatine synthesis which is in high demand in young growing pigs. Arginine is converted to guanidinoacetic acid (GAA) in the kidney and then methylated by methionine to form creatine in the liver. We have previously demonstrated that when dietary arginine or methionine is limiting, creatine synthesis is particularly impaired. In fast growing animals, the demand for creatine cannot be met by new synthesis and it has been estimated that only three-quarters of creatine needs can be synthesized, with the rest of creatine coming from the diet. Creatine and GAA can potentially be very effective as growth-promoting feed supplements, especially in all-grain diets which are typically creatine-free. They can also be particularly effective as supplements in milk-based weanling diets due to their low arginine content; if creatine or GAA are supplemented to these diets, then arginine is free to be used for other functions. The use of creatine and GAA in the swine industry has enormous potential as supplements in young pigs, given they can ‘spare’ arginine and methionine early in life and perhaps improve growth and health.
Our overall objective is to evaluate the interactions among methionine, arginine, creatine and GAA.
1) How much dietary arginine and methionine are required to synthesize creatine?
2) Can we spare methionine and arginine for growth by adding creatine to the diet?
3) Which organs are involved in creatine metabolism and how is GAA and creatine in the blood used?
Significance: Understanding the metabolic fate of essential amino acids is critical to defining nutritional requirements and allows us to modify amino acid supply to pigs.
2) NEONATAL METHYL METABOLISM Methionine is an essential amino acid that is necessary for the synthesis of cysteine and taurine as well as for the supply of labile methyl groups used in methylation reactions. These methyl groups are needed in nutritionally significant amounts, inter alia, for the synthesis of creatine, phosphatidylcholine (PC), and for regulating gene expression via DNA methylation. Methyl groups can also be provided by choline (via betaine) and serine (via folate). Because folate, choline and methionine are all essential nutrients, the dietary availability of folate and choline can influence the amount of methionine needed. In growing animals, this balance is more precarious as growth requires even more amino acids to accommodate expanding protein and methyl pools. Without adequate folate and betaine/choline, more methionine is needed to maintain methylation reactions which in turn, can limit methionine’s availability for protein synthesis and growth. Furthermore, a scarcity of methyl groups will eventually impede methylation reactions and lead to changes in metabolism for the growing organism. Indeed, changes to DNA methylation can have permanent effects on gene expression which can lead to chronic diseases in adulthood. Thus, elucidation of the transmethylation pathways and their relationship to dietary nutrients in neonatal metabolism will not only optimize infant health, but also adult health.
1) What is the methionine requirement for methylation reactions and how is methionine partitioned between protein synthesis and methylation demands?
2) Which methylation pathways are preserved over decreasing methionine intakes?
3) What is the role of dietary methyl donor supply on DNA methylation in neonatal piglets?
Significance: Ultimately, this research will help define the methionine and methyl requirements to optimize nutrition for infants fed metabolically stressful diets such as casein-based formula (high methionine), soy-based formula (low choline and creatine-free), breastmilk from folate- or choline-deficient mothers, as well as nutrient-poor post-weaning diets.
3) EARLY NUTRITIONAL INSULTS AND LONG TERM CONSEQUENCES ON HEALTH Recently, human epidemiological studies have been able to link small birth weight and increased postnatal growth to several chronic adult diseases such as obesity, hypertension, coronary heart disease, diabetes and osteoporosis. Small fetuses that are adapted to a ‘thrifty’ environment in utero resulting in their smallness seem to over-compensate for the relative nutritional ‘abundance’ in postnatal life. This concept of nutritional “mismatch” between pre- and post-natal environments is often referred to as the early origins of adult disease. It is most likely that the susceptibility to chronic disease is what is being programmed. In other words, it is our tolerance to stressors such as poor diet and lack of exercise which is determined early in life. Only recently, have researchers developed animal models to try and understand the mechanisms by which early programming occurs. Changes in maternal homocysteine metabolism have been implicated which suggests that altered methionine and methyl metabolism seems to be involved in the early origins hypothesis; these changes should be observable in the neonates who are permanently programmed. Indeed, the abnormal methylation of DNA has emerged as a leading hypothesis to explain how environmental influences early in life could permanently affect neonatal metabolism. It is surprising that important and permanent gene expression control can be so easily manipulated by our diet.
Research Questions: We have recently developed a pig model of developmental origins of adult disease using Yucatan miniature pigs.
1) What are the compensatory limits of methyl donor pathways in small birthweight neonates?
2) What is the role of early nutritional stress (such as intrauterine growth restriction or TPN feeding) on programming the risk for chronic diseases later in life?
Significance: Because many infants are exposed to dietary extremes of methyl-related nutrients, most infants are vulnerable to perturbed methyl metabolism and we need more information on the ‘optimal' balance of these nutrients and the potential for deleterious outcomes from imbalance. Furthermore, we need to better understand the long term consequences of low birthweight or TPN feeding in infants and how to enhance recovery by modulation of TPN ingredients.