Risk Factors and Etiology of Obesity and Type 2 Diabetes

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Decreased insulin action, impaired insulin secretion and increased adiposity are important risk factors for development of type 2 diabetes. These risk factors are present even when individuals have both normal fasting and two-hour glucose concentrations indicating a very early role for decreased insulin secretion in the development of type 2 diabetes. We have also demonstrated that reduced insulin clearance based on calculations from our euglycemic-hyperinsulinemic clamp measures increased risk for diabetes independent of insulin action and secretion. We have also found that the one-hour glucose concentration, which correlates well with insulin secretion and insulin action, is just as good a predictor of the development of diabetes as the 2-hour glucose concentration and predicts diabetic retinopathy. Previous predictors of weight gain based on this study have included, higher respiratory quotient, higher insulin mediated glucose uptake, lower free T3, and relatively lower energy expenditure. Variability in energy expenditure is a mediator of weight change, and we have continued to evaluate factors related to metabolic rate. We had previously confirmed that lower energy expenditure relative to body size predicted weight and fat mass gain in a larger cohort with longer follow-up. Using resting energy expenditure measured at 5 and 10 years of age, we demonstrated that relatively lower resting energy expenditure at age 10 (but not at age 5) predicted greater increase in body mass index indicating an effect of metabolic rate on weight in late childhood. Using data from our whole room indirect calorimeters, we have recently demonstrated that spontaneous physical activity or fidgeting behavior declined from 1985 to 2005 indicating an effect of increasing sedentary lifestyle even on unprompted movement. The thermic effect of food and the energy cost of arousal are overlapping and difficult to measure components of energy expenditure. By examining time point data from our metabolic chambers, we were able to estimate these components (which we termed awake fed thermogenesis (AFT)) which accounted for approximately 10% of total energy expenditure. We found that lower AFT predicted weight gain, but only in individuals with BMI 29 kg/m. This indicates that increased insulation associated with adiposity leads to lower cost of metabolizing macronutrients. As individuals gain weight, their energy expenditure increases more than would be expected based on their increased weight. We have found that increased fasting plasma glucose which may be a marker for increase hepatic glucose production explains part of the large than expected increased EE, and predicted less weight gain. Given that energy expenditure and substrate oxidation continue to play an important role in maintaining energy balance, we have continued to investigate factors which explain the inter-individual variance in these measurements. On a cellular level, in adipocytes we found that higher in vitro lipolysis is associated with higher whole-body lipid oxidation and less weight gain. We have also found that sphingolipid (which are cell membrane components important in cellular signaling) and endocannabinoid concentrations in skeletal muscle are associated with lower energy expenditure and weight gain. In mediation analysis, sphingomeylins effect on energy expenditure was mediated by the endocannabinoid anandamide. Genetic factors underlie adiposity and its risk factors. Mutations in the melanocortin 4 receptor gene are associated with increased body mass index and lower 24-hour energy expenditure in humans. Individuals with MC4R mutations have accelerated weight gain in childhood but not in adulthood indicating a more potent effect of this mutation in early life. Furthermore, we found that presence of MC4R mutations predicted development of diabetes in childhood independent of body weight. The mechanism by which MC4R leads to hyperphagia and weight gain is not clear. One possible mediator is brain derived neurotrophic factor (BDNF) a downstream effector of MC4R signaling which has been implicated in childhood hyperphagia and weight gain. However, serum BDNF did not differ between individuals with and without MC4R mutations. Recently a more common single nucleotide polymorphism (SNP) in MC4R has been identified that is associated with increased BMI in Native Americans of southwestern heritage. This common SNP is near the promoter region and is also associated with lower energy expenditure and increased ad libitum food intake. Using whole genome sequencing, we also found that a stop codon in the gene D-amino acid oxidase, an enzyme involved in dopamine synthesis is associated with lower 24h and sleeping EE.