There was an approximately 30 percent increase in basal energy expenditure from pre-pregnancy to late pregnancy whether expressed as basal VO2 ml/min or kcal/day. The increase in energy expenditure was approximately 14 percent in the NGT and 21 percent in the GDM subjects. These differences did not reach statistical significance (p=0.3 to 0.5) because of the great inter-individual variability.  

There was a significant inverse correlation between the changes in basal endogenous glucose production and fat oxidation from time P to E. Although there was generally an increase in basal endogenous glucose production, half the subjects had increasing and the other half had decreasing fat oxidation. The researchers anticipated an increase in fat oxidation to be associated with an increase in basal endogenous glucose production, but serum concentrations were unchanged over the same time period.  

In obese subjects there was no change in either basal carbohydrate oxidation or non-oxidative carbohydrate metabolism over time. However, there was a significant 50-80 percent increase in basal fat oxidation. The increase was particularly evident from time E to L, when there was increased accretion of adipose tissue and decreasing insulin sensitivity. These data support the hypothesis that increased lipid rather than carbohydrate oxidation may represent an adaptive mechanism for the prevention of additional weight (adipose tissue) in situations where there was preexisting obesity and decreasing insulin sensitivity. 

The metabolic changes during insulin infusion in late gestation in obese women are similar to what was observed in lean women. There are progressive decreases in glucose and lipid insulin sensitivity, carbohydrate oxidation and non-oxidative metabolism but a significant 5-fold increase in fat oxidation with advancing gestation. These data support the concept that during fasting, pregnancy is a state of accelerated starvation with increased maternal reliance on lipids rather than carbohydrates for meeting energy needs. Carbohydrates and amino acids are thus made available for feto-placental energy requirements and growth. 

Although the source of leptin is well documented, the role of the increased maternal leptin concentrations during gestation has remained elusive. In addition to maternal adipose tissue, the placenta produces leptin and leptin concentrations fall within 48 hours of delivery.

Conclusions

The researchers found that during pregnancy of the obese women in this study, there are significant alterations in body composition and energy expenditure among individuals, but no difference between women with NGT and GDM. There are significant increases in fat mass, basal metabolic rate and an increased reliance on lipid metabolism both in the basal state and during insulin infusion. The increased reliance on fat metabolism is accompanied by a concomitant decrease in carbohydrate metabolism under conditions of hyperinsulinemia but not in the basal state.

The authors speculate that the pre-pregnancy metabolic status of the individual (and not whether or not the individual develops GDM) that provides the baseline upon which the alterations in pregnancy metabolism are mediated through placental hormones, cytokines and growth factors. 

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