Submitted to: Poultry Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/21/2003
Publication Date: 12/1/2003
Citation: Carew, L.B., McMurtry, J.P., Alster, R.A. 2003. The effects of methionine deficiencies on plasma levels of thyroid hormones, insulin-like growth factors I and II, liver and body weights, and feed intake in growing chickens. Poultry Science 82:1932-1938. Interpretive Summary: Soybeans are a major ingredient in poultry dietary rations. However, this legume does not contain sufficient levels of the essential amino acid methionine, which must be added to chicken diets to meet their dietary requirements. Normal blood levels of insulin-like growth factors and the thyroid hormones are important to maintaining normal growth in domestic birds. In mammals, the secretions of these hormones are greatly influenced by dietary changes, especially deficiencies in the essential amino acids, such as methionine. These dietary effects have not been investigated in poultry. Therefore this study was conducted to evaluate the effect of feeding a diet deficient in methionine on circulating levels of insulin-like growth factor and thyroxine. Broiler chicks were fed a diet marginally deficient in methionine. The results of this study demonstrate that as in mammals, methionine is essential to not only maintaining normal growth patterns in chickens, but also to maintaining normal blood circulating levels of insulin-like growth factor II and triiodothyronine. The results of this study will be of interest to other scientists.
Technical Abstract: A deficiency of methionine (Met) at 0.25% of the diet, or 50% of the National Research Council's recommended level, has been reported to cause elevations in plasma triiodothyronine (T3) in growing broiler chickens. In the present study, plasma levels of thyroid hormones as well as insulin-like growth factors I and II were measured in chicks fed three deficient levels of Met. Control (0.5%) and Met-deficient diets (0.4%, 0.3%, 0.2%) were fed to triplicate groups of 7 male broiler chicks each from 8 to 22 days of age. To account for the effects of differences in feed intake on measurements taken, additional groups of control chicks were pair-fed with the Met-deficient chicks by matching the daily feed intake of chicks fed the control diet with each of the deficient pens. Chicks receiving the marginal level of 0.4% Met increased their feed intake by 10%, but there was no significant change in body weight. Based on earlier studies, the extra energy consumed is converted to fat which replaces water and protein. The more severe Met deficiencies of 0.3% and 0.2% caused marked and graded reductions in feed intake and weight gain. However, corresponding pair-fed control chicks were significantly heavier. These changes suggest more marked changes in metabolic processes with 0.3 and 0.2% Met than observed with 0.4% Met. Liver weights (mg/gm body weight) were heavier in chicks fed 0.3% or 0.2% Met but not 0.4%. Plasma T3 levels were higher in all deficient chicks compared to control, but this was significant only with 0.3% Met. However, with both 0.3% and 0.2% Met plasma T3 was significantly elevated compared to their pair-fed controls. Plasma thyroxine (T4) was lower in all deficient groups, but this was significant only with 0.2% Met while no significant differences occurred between deficient chicks and their pair-fed controls. Therefore, previous observations that plasma T3 is also elevated in protein-deficient chicks may be partially explained by low intakes of Met. Plasma IGF-I values were not significantly altered by the Met deficiency, but these values were consistently lower in deficient chicks and deserve further study. Plasma IGF-II levels were significantly less in deficient chicks fed 0.25% Met compared to their pair-fed controls suggesting that the Met deficiency can interfere with IGF-II metabolism. We conclude that a deficit of dietary Met alters both plasma T3 and IGF-II levels, but the effect is dependent on the degree of deficiency.