Metabolic response to starvation. I. Relative effects of pregnancy and sex steroid administration in the rat

Mechanisms of hepatic fatty acid oxidation and ketogenesis during fasting

Fasting is part of many weight management and health-boosting regimens. Fasting causes substantial metabolic adaptations in the liver that include the stimulation of fatty acid oxidation and ketogenesis. The induction of fatty acid oxidation and ketogenesis during fasting is mainly driven by interrelated changes in plasma levels of various hormones and an increase in plasma nonesterified fatty acid (NEFA) levels and is mediated transcriptionally by the peroxisome proliferator-activated receptor (PPAR)α, supported by CREB3L3 (cyclic AMP-responsive element-binding protein 3 like 3). Compared with men, women exhibit higher ketone levels during fasting, likely due to higher NEFA availability, suggesting that the metabolic response to fasting shows sexual dimorphism. Here, we synthesize the current molecular knowledge on the impact of fasting on hepatic fatty acid oxidation and ketogenesis.

Sex differences in fuel use and metabolism during development in fasting juvenile northern elephant seals

Many polygynous, capital breeders exhibit sexual dimorphism with respect to body size and composition. Sexual dimorphism is often facilitated by sex differences in foraging behavior, growth rates and patterns of nutrient deposition during development. In species that undergo extended fasts during development, metabolic strategies for fuel use have the potential to influence future reproductive success by directly impacting somatic growth and acquisition of traits required for successful breeding. We investigated sexual dimorphism associated with metabolic strategies for fasting in developing northern elephant seals. Thirty-one juvenile seals of both sexes were sampled over extended fasts during annual autumn haul-outs. Field metabolic rate (FMR) and the contribution of protein catabolism to energy expenditure were estimated from changes in mass and body composition over 23±5 days of fasting (mean ± s.d.). Protein catabolism was assessed directly in a subset of animals based on urea flux at the beginning and end of the fast. Regulatory hormones and blood metabolites measured included growth hormone, cortisol, thyroxine, triiodothyronine, insulin, glucagon, testosterone, estradiol, glucose, urea and β-hydroxybutyrate. Males exhibited higher rates of energy expenditure during the fast but spared body protein stores more effectively than females. Rates of protein catabolism and energy expenditure were significantly impacted by hormone levels, which varied between the sexes. These data suggest that sex differences in fuel metabolism and energy expenditure during fasting arise early in juvenile development and may play an important role in the development of adult traits associated with reproductive success.

Metabolic interactions of diabetes and pregnancy

Many of the embryonic and fetal abnormalities that occur in pregnancies complicated by maternal diabetes are the result of development in a metabolically abnormal environment. Diabetic embryopathy (birth defects and spontaneous abortions) results from maternal metabolic abnormalities during the first 6-7 weeks of gestation. The embryopathy appears to be multifactorial in origin, and the resulting defects remain important causes of morbidity and mortality in diabetic pregnancies. Diabetic fetopathy (predominantly macrosomia and neonatal hypoglycemia) results from fetal overnutrition and hyperinsulinemia during the second and third trimesters. Fetopathy may cause significant morbidity not only in the perinatal period, but also in later life as overweight infants grow up to be overweight children and young adults. Careful regulation of maternal metabolism from the preconceptional period onward can reduce greatly or even eliminate the excess risks that have been incurred by infants of diabetic mothers in the past. Successful management of maternal diabetes requires knowledge of the alterations in intermediary metabolism that normally occur during pregnancy, as discussed in this chapter.

Hypocaloric diets and ketogenesis in the management of obese gestational diabetic women

The extent to which given levels of caloric restriction will improve glycemic status but increase plasma ketone bodies in gestational diabetic women has received little attention. After reviewing the underlying physiology, we present data on two feeding studies investigating the question. In the first, a weight-maintaining approximately 2400-kcal/day diet was fed on a metabolic ward to 12 gestational diabetic women for 1 week. In the second week, subjects were randomized to a continuation of the 2400-kcal/day diet or to a 1200-kcal/day diet. Twenty-four-hour mean glucose levels remained unchanged in the control group but declined in the calorie-restricted group (6.7 mM or 121 mg/dl in week 1 vs 5.4 mM or 97.3 mg/dl in week 2) (p less than 0.01). Nine-hour overnight fasting plasma insulin also declined but oral glucose tolerance did not improve with caloric restriction. Fasting plasma beta-hydroxybutyrate rose in the calorie-restricted group, along with an increase in ketonuria, but not in the control group. A second study compared the impact of a 33% calorie-restricted diet or insulin to a full-calorie diet in a similar 2-week experimental design and measured hepatic glucose output and insulin sensitivity with dideuterated glucose before and during an insulin clamp. Diet in three subjects improved fasting and 24-hr mean glucose by 22 and 10%, respectively, whereas prophylactic insulin in three subjects produced 0 and 4% reductions, respectively. On average, ketonuria after a 9-hr fast declined to an equivalent degree with both treatments. Hepatic glucose output and insulin sensitivity were not statistically significantly altered by gestational diabetes or the therapeutic interventions compared to nondiabetic normal weight or obese pregnant controls. In conclusion, 50% caloric restriction improves glycemic status in obese women with gestational diabetes but is associated with an increase in ketonuria, which is of uncertain significance. An intermediate 33% level of caloric restriction (to 1600-1800 kcal daily) may be more appropriate in dietary management of obese woman with gestational diabetes mellitus and more effective than prophylactic insulin. Further studies are required to confirm these findings.

Accelerated starvation in late pregnancy: a comparison between obese women with and without gestational diabetes mellitus

We compared the glucose, insulin, free fatty acid, and 3-hydroxybutyrate responses to a briefly extended overnight fast during the third trimester of pregnancy between two groups: obese women with normal glucose tolerance (n = 10) and age- and weight-matched women with gestational diabetes mellitus (n = 10). After a 12-hour overnight fast, plasma glucose (95 +/- 4 vs. 78 +/- 2 mg/dl; p less than 0.01), insulin (32 +/- 5 vs. 17 +/- 2 microU/ml; p less than 0.02), and free fatty acid (860 +/- 63 vs. 639 +/- 79 mmol/L; p less than 0.05) levels were higher in the patients with gestational diabetes mellitus. 3-Hydroxybutyrate levels were similar in the two groups at that time (0.23 +/- 0.04 vs. 0.18 +/- 0.03 mmol/L; p greater than 0.3). When the fast was extended to 18 hours by having the patients skip breakfast, glucose levels fell more rapidly in the group with gestational diabetes mellitus but remained elevated compared with the nondiabetic women. Insulin levels declined at a similar rate in the two groups. Free fatty acid levels did not increase significantly in the group with gestational diabetes mellitus during the extended fast. In contrast, free fatty acid levels increased by 44% in the normal pregnant women, reaching the level observed in the group with gestational diabetes mellitus after 18 hours. 3-Hydroxybutyrate levels remained virtually identical in the two groups throughout the brief fast. Thus, compared with that of normal pregnant women, the response of obese women with gestational diabetes mellitus to brief caloric deprivation during late pregnancy was characterized by a greater fall in plasma glucose values without a greater propensity to ketosis. Our findings may have important implications for the dietary management of obese patients with gestational diabetes mellitus.

Metabolic effects of combined oral contraceptive preparations

Fasting intermediary metabolite concentrations were measured in comparable groups of women either in the follicular or luteal phase of the menstrual cycle or taking low-dose estrogen combined oral contraceptive preparations. Blood total ketone bodies concentrations were significantly elevated in oral contraceptive users (p less than 0.05 v follicular and luteal groups) whilst blood alanine levels were lower (p less than 0.01 v follicular).

Effect of fetal growth on maternal protein metabolism in postabsorptive rat

Rates of protein synthesis were measured in whole fetuses and maternal tissues at 17 and 20 days of gestation in postabsorptive rats using continuous infusion of L-[1-14C]leucine. Fetal protein degradation rates were derived from the fractional rates of synthesis and growth. Whole-body (plasma) leucine kinetics in the mother showed a significant reduction of the fraction of plasma leucine oxidized in the mothers bearing older fetuses, a slight increase in the plasma flux, with total leucine oxidation and incorporation into protein remaining similar at the two gestational ages. Estimates of fractional protein synthesis in maternal tissues revealed an increase in placental and hepatic rates at 20 days of gestation, whereas the fractional synthetic rate in muscle remained unchanged. A model for estimation of the redistribution of leucine between plasma and tissues is described in detail. This model revealed a more efficient utilization of leucine in fetal protein synthesis in comparison with other maternal tissues, a greater dependency of the fetus on plasma supply of leucine, and a significant increase (2-fold) in the release of leucine from maternal muscle as the fetal requirements increased proportionately with its size. The latter conclusion, supported by nitrogen analysis and the ratio of bound-to-free leucine in maternal tissues, confirms the importance of maternal stores in maintaining the homeostasis of essential amino acids during late pregnancy.

Metabolic effects of progesterone

Progesterone has important effects on carbohydrate, lipid and protein metabolism. This steroid induces hyperinsulinemia, possibly by direct action on pancreatic islets, while promoting glycogen storage in the liver. Paradoxically, it antagonizes the effects of insulin on glucose metabolism in adipose tissue and skeletal muscle. Progesterone stimulates deposition of body fat but had catabolic effects on protein metabolism. Provisional evidence is offered that the steroid may influence ketone body production by the liver as well. When these steroid actions are considered together, their most relevant expression appears to be the physiologic changes observed during normal pregnancy.