Date Published: January 21, 2014
Publisher: Public Library of Science
Author(s): Zemeng Feng, Xiaoli Zhou, Fei Wu, Kang Yao, Xiangfeng Kong, Tiejun Li, Francois Blachier, Yulong Yin, Dmitri Boudko.
The Chinese population has undergone rapid transition to a high-fat diet. Furthermore, monosodium L-glutamate (MSG) is widely used as a daily food additive in China. Little information is available on the effects of oral MSG and dietary fat supplementation on the amino acid balance in tissues. The present study aimed to determine the effects of both dietary fat and MSG on amino acid metabolism in growing pigs, and to assess any possible interactions between these two nutrients.
Four iso-nitrogenous and iso-caloric diets (basal diet, high fat diet, basal diet with 3% MSG and high fat diet with 3% MSG) were provided to growing pigs. The dietary supplementation with fat and MSG used alone and in combination were found to modify circulating and tissue amino acid pools in growing pigs. Both dietary fat and MSG modified the expression of gene related to amino acid transport in jejunum.
Both dietary fat and MSG clearly influenced amino acid content in tissues but in different ways. Both dietary fat and MSG enhance the absorption of amino acids in jejunum. However, there was little interaction between the effects of dietary fat and MSG.
As an umami flavor, monosodium L-glutamate (MSG) has been widely used as a food additive. The worldwide production and consumption of MSG has recently increased, and it is even expected to further increase in the next years (http://www.ihs.com/products/chemical/planning/ceh/monosodium-glutamate.aspx). MSG is considered as safe among food additives by the Joint Expert Committee on Food Additives of the United Nations Food and Agriculture Organization and World Health Organization . When MSG is consumed, L-glutamate (Glu) is released in the small intestine lumen and metabolized notably by the enterocytes . The regular daily consumption of MSG presumably modify the metabolism of nutrients, notably the metabolism of AAs in the body , and protein synthesis regulation and proteolysis , . Only few studies have reported the effects of daily oral MSG supplementation on AA metabolism.
Dietary fat and protein are two of the main macronutrients required for life. Many published work had tested the effects of each nutrients independently, however, a few studies are available on the effect of combination of dietary fat and protein. The metabolism is a systemic network, and it would be misleading to evaluate the effects of one nutrient while ignoring the effects of others in plausible nutritional situation. As early as 1956, Mellinkoff et al. found that a diet that contained very few short-chain saturated fats and relatively abundant long-chain unsaturated fats could significantly reduce Leu and Val and increase Pro, Cys, Arg and Asp in plasma . Dietary fatty acids, particularly n-3 fatty acids, can spare AAs for protein and peptide synthesis . On the contrary, AAs, especially those containing sulfur, can also modulate lipid metabolism . As the Chinese economy is developing, the Chinese population, including children and adolescents, has undergone a rapid transition to a high-fat diet . In parallel with these trends, there has recently been a large increase in the consumption of MSG as an umami food additive in China. MSG can facilitate the gastric emptying of a protein-rich meal, and plays an important role in protein digestion . Conversely, MSG has been shown to increase the stomach antral area in human volunteers fed with a normo-proteic diet  and to slow gastric emptying in preterm piglets ; suggesting that the effects of MSG on the stomach physiology depends on nutritional conditions. In the present study, the effects of dietary fat and MSG on AA metabolism in growing pigs were determined, along with the interaction between these two factors.
In conclusion, the effects of dietary fat and MSG, both alone and in combination, on the AA in the circulating and tissue pool were determined. Little interaction was found between the effects of dietary fat and MSG. Both dietary fat and MSG enhanced the absorption of AAs in jejunum. Dietary fat can enhance the AA pool in plasma and muscle, while MSG enhances AA pool in the kidney. The results of the present study will help to further uncover the effects of dietary fat and MSG addition on human amino acid metabolism, with predictable consequences for the optimization of animal feeding and human nutrition.