| Abstract |
Carnitine occupies a central role in transferring long-chain fatty acids into mitochondrial matrix for oxidation, and under specific conditions, to provide energy to the organism. Lack of carnitine is manifested as a disorder of intermediate metabolism with clinical symptoms of variable severity. Both gestation and diabetes mellitus, are characterized by secondary deficiency and reduction of carnitine levels. However, the impact of the simultaneous interaction of these two conditions is not clear yet. The present Doctoral Thesis, was designed to address specific questions concerning carnitine changes occurring in both these conditions and to investigate the parameters of intermediate metabolism, by employing gestational diabetes (GD) as an experimental model. For the specific needs of the study, 119 women were recruited and were further divided into three groups: 40 (33.6%) normal yang non-pregnant women (NNP), 46 (38.7%) normal pregnant women (NP) with uncomplicated pregnancy and 33 (27.7%) pregnant women with gestational diabetes (GD). Variable parameters, total carnitine and its fractions (free and acyl-carnitine), β-hydroxy-butyrate, lactic acid and free fatty acids, were studied in blood samples derived between 30th and 33rd week of gestation. Blood samples from each woman were obtained following overnight fasting and 1h after meal. Analysis of fasting total carnitine (TC) levels among the three groups demonstrated that the GDM group did not exhibit any further reduction in carnitine levels, compared to normal pregnant, while both groups of pregnant women displayed significantly lower carnitine levels compared to the NNP group. Following a carbohydrate meal, the total carnitine levels did not show any major changes both in normal pregnant women and in the gestational diabetes group. Thus, these data document for the first time, that gestational diabetes does not exert any additional detrimental effect on the total carnitine levels, either in fasting or in refeeding. No measurements of total carnitine levels following a carbohydrate meal were performed in the NNP group, since several studies previously have established no significant effects between the two conditions. During fasting period, the GDM group displayed higher free carnitine (FC) and lower acyl-carnitine (AC) levels compared to the NP group (P<0.001 and P<0.010, respectively). Furthermore, the NNP group demonstrated higher fasting FC levels compared to the NP and GDM groups (P<0.001 in both cases), while the fraction of AC was found to be at similar levels to the NP group (P=0.661), but significantly higher to the GDM group (P<0.001). The NP group seems to exhibit a similar pattern with the NNP group in both stages of the experimental design. The lower levels of FC in normal pregnant, were not only due to the increased AC levels occurring during fasting, but also to the fact that total plasma carnitine levels are reduced during gestation. AC levels displayed no statistically significant difference between the two groups (P=0.661) for two reasons; first, because the AC fraction is not affected in great deal during gestation and second, because the response of pregnant women during fasting, along with the already established trend of metabolism towards lipid oxidation during the last trimester, result in fast mobilization and increased oxidation of free fatty acids (FFA), which eventually leads in similar high AC levels in both groups. Besides, this was corroborated by the fasting levels of β-hydroxy-butyrate. Following a comparison of these levels between the two groups, i.e. NP vs. NNP, there was a marked but not statistically significant difference (P=0.63) with slightly higher levels occurring in the NP group, due to the aberrant metabolism. The variable degree of ketosis occurring during gestation is more evident in pregnant women with gestational diabetes. In our study, although significantly higher β-hydroxy-butyrate plasma levels were encountered in the GDM group (P=0.011), the same group simultaneously exhibited lower AC and higher FC levels, compared to normal pregnant women. Additionally, although in the GDM group there were significantly higher levels of β-hydroxy-butyrate compared to the NP group, there was no statistical difference in FFA levels between the two groups (P=0.130). This leads to the conclusion, that in pregnant women with gestational diabetes, the mechanism of ketogenesis is additionally enhanced though other biochemical pathways, beside β-oxidation. The most plausible explanation for this phenomenon, seems to be the increase in ketogenesis by utilizing either oxaloacetic acid or lactic acid, as a substrate (Coris cycle). Thus, while the increased rate of β-oxidation and production of ketone bodies, is associated with a parallel increase of AC levels, ketone body production via glycolysis, does not directly affect carnitine fractions, and this explains the finding of the lack of further increase of the AC fragment in GDM. Following carbohydrate meal, the AC fraction both in NP and GDM groups did not show any significant change (P=0.986 and P=0.779, respectively). The FC fraction however, showed no change in the NP group (P=0.113), while exhibited a statistically significant increase in the GDM group (P=0.040). This small increase, reflects the increase in total carnitine levels reported above, and as was discussed, lies within normal limits. Postprandial levels of β-hydroxy-butyrate exhibit the anticipated fall in all groups; the same pattern is also observed with the FFA. These manifestations are expected, and are due to the secretion of insulin, following food intake. The increased resistance of insulin exhibited by women during pregnancy, is reflected in β-hydroxy-butyrate and FFA levels. Both before and after meal, the highest levels of β-hydroxy-butyrate are exhibited sequentially by the GDM, NP and NNP groups. For the FFA levels, we notice that, while there is no difference between the NP and GDM groups, postprandial, FFA levels are higher in the GDM group (P=0.004), suggesting an underlying increase of insulin resistance, which is higher in the GDM group. Regarding the lactate levels in both stages of the experiment, there were maintained within normal limits. Following the carbohydrate meal, an increase in lactate levels was noticed, which was within normal limits (≤15mg/dl). The differences between GDM and NNP groups (P=0.036) and the small difference between NP and NNP groups (P=0.046) probably are due to the increase of insulin resistance, developing in both groups of pregnant women. In conclusion, carnitine in pregnant women does not seem to be additionally effected by the co-existence of diabetes and gestation. Intermediate metabolism seems to be modified regarding ketogenesis in the presence of gestational diabetes, with the activation of an alternative ketogenesis pathway, utilizing oxaloacetic acid or lactate as a substrate, which seems to operate independently from the enzymic system of carnitine.
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Ο μεταβολισμός της καρνιτίνης και οι μεταβολές στον ενδιάμεσο μεταβολισμό των διαβητικών έγκυων γυναικών
