Diane M. O'Brien

Use of stable isotopes to examine how dietary restriction extends Drosophila lifespan.

Summary The ability of dietary restriction to increase animal life span is often thought to arise from differential allocation of resources between somatic investment and reproduction [1–4]. In this theory, reproduction is repressed upon dietary restriction to make scarce nutrients available to somatic functions that increase survival. Here, we label nitrogen and carbon in the dietary yeast of Drosophila melanogaster with stable isotopes to determine whether resources are invested to somatic tissues at the expense of reproduction. We find that females on a full diet acquire and allocate more dietary carbon, nitrogen and essential amino acids (EAA) to eggs than females on a restricted diet. Full-diet females also invest more carbon, nitrogen and EAA into somatic tissue than those on a restricted diet. Thus, the longer lifespan of flies on a restricted diet relative to those on a full diet cannot be explained by greater absolute somatic investment, and high somatic investment does not ensure longevity. We find, however, that resource allocation to somatic tissue relative to investment to eggs is greatest in females on a restricted diet. To account for these patterns we propose that dietary restriction in Drosophila may extend lifespan through somatic investment relative to damage incurred from reproduction [5].

Associations of obesity with triglycerides and C-reactive protein are attenuated in adults with high red blood cell eicosapentaenoic and docosahexaenoic acids.

Background:N-3 fatty acids are associated with favorable, and obesity with unfavorable, concentrations of chronic disease risk biomarkers.Objective:We examined whether high eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid intakes, measured as percentages of total red blood cell (RBC) fatty acids, modify associations of obesity with chronic disease risk biomarkers.Methods:In a cross-sectional study of 330 Yup’ik Eskimos, generalized additive models (GAM) and linear and quadratic regression models were used to examine associations of BMI with biomarkers across RBC EPA and DHA categories.Results:Median (5th–95th percentile) RBC EPA and DHA were 2.6% (0.5–5.9%) and 7.3% (3.3–8.9%), respectively. In regression models, associations of BMI with triglycerides, glucose, insulin, C-reactive protein (CRP) and leptin differed significantly by RBC EPA and DHA. The GAM confirmed regression results for triglycerides and CRP: at low RBC EPA and RBC DHA, the predicted increases in triglycerides and CRP concentrations associated with a BMI increase from 25 to 35 were 99.5±45.3 mg/dl (106%) and 137.8±71.0 mg/dl (156%), respectively, for triglycerides and 1.2±0.7 mg/l (61%) and 0.8±1.0 mg/l (35%), respectively, for CRP. At high RBC EPA and RBC DHA, these predicted increases were 13.9±8.1 mg/dl (23%) and 12.0±12.3 mg/dl (18%), respectively, for triglycerides and 0.5±0.5 mg/l (50%) and −0.5±0.6 mg/l (−34%), respectively, for CRP.Conclusions:In this population, high RBC EPA and DHA were associated with attenuated dyslipidemia and low-grade systemic inflammation among overweight and obese persons. This may help inform recommendations for n-3 fatty acid intakes in the reduction of obesity-related disease risk.

A test of alternative models for increased tissue nitrogen isotope ratios during fasting in hibernating arctic ground squirrels

SUMMARY We describe two models explaining the increase in tissue nitrogen isotope ratios (δ15N) that occurs during fasting in animals. The catabolic model posits that protein breakdown selectively removes the lighter isotope of nitrogen (14N) from catabolized tissues, causing an increase in the proportion of heavy nitrogen isotope (15N). The anabolic model posits that protein synthesis during fasting results in elevated δ15N values, as the unreplaced loss of 14N to urea results in a higher proportion of 15N in plasma amino acids used for protein synthesis. We effected a range of lean mass loss in arctic ground squirrels (Urocitellus parryii) fasting during hibernation and then collected organ and muscle tissues for analysis of δ15N values. The catabolic model predicts increased δ15N values in both liver and muscle, as these tissues undergo significant catabolism during hibernation. The anabolic model predicts no change in muscle, but an increase in δ15N values in liver, which has high levels of protein synthesis during euthermic phases of hibernation. We found a significant increase in liver δ15N values and no change in muscle δ15N values with lean mass loss, which supports the anabolic model. Heart, small intestine and brown adipose tissue also showed an increase in δ15N values, indicating protein synthesis in these organ tissues during hibernation. Urine was 3.8% lighter than plasma, and both urine and plasma increased in δ15N values with lean mass loss. This study helps clarify the mechanisms causing δ15N change during nutritional stress, thus increasing its utility for physiological research and reconciling previously contradictory results.

Can amino acid carbon isotope ratios distinguish primary producers in a mangrove ecosystem

RATIONALE The relative contribution of carbon from terrestrial vs. marine primary producers to mangrove-based food webs can be challenging to resolve with bulk carbon isotope ratios (δ(13)C). In this study we explore whether patterns of δ(13)C values among amino acids (AAs) can provide an additional tool for resolving terrestrial and marine origins of carbon. METHODS Amino acid carbon isotope ratios (δ(13)C(AA)) were measured for several terrestrial and marine primary producers in a mangrove ecosystem at Spanish Lookout Caye (SLC), Belize, using gas chromatography-combustion-isotope ratio mass spectrometry. The δ(13)C values of essential amino acids (δ(13)C(EAA)) were measured to determine whether they could be used to differentiate terrestrial and marine producers using linear discriminant analysis. RESULTS Marine and terrestrial producers had distinct patterns of δ(13)C(EAA) values in addition to their differences in bulk δ(13)C values. Microbial mat samples and consumers (Crassostrea rhizophorae, Aratus pisonii, Littoraria sp., Lutjanus griseus) were most similar to marine producers. Patterns of δ(13)C(EAA) values for terrestrial producers were very similar to those described for other terrestrial plants. CONCLUSIONS The findings suggest that δ(13)C(EAA) values may provide another tool for estimating the contribution of terrestrial and marine sources to detrital foodwebs. Preliminary analyses of consumers indicate significant use of aquatic resources, consistent with other studies of mangrove foodwebs.

Isotopic estimates of sugar intake are related to chronic disease risk factors but not obesity in an Alaska native (Yup'ik) study population.

Background/Objectives:Sugar intake may be causally associated with chronic disease risk, either directly or by contributing to obesity. However, evidence from observational studies is mixed, in part due to the error and bias inherent in self-reported measures of sugar intake. Objective biomarkers may clarify the relationship between sugar intake and chronic disease risk. We have recently validated a biomarker of sugar intake in an Alaska Native (Yup’ik) study population that incorporates red blood cell carbon and nitrogen isotope ratios in a predictive model. This study tested associations of isotopic estimates of sugar intake with body mass index (BMI), waist circumference (WC) and a broad array of other physiological and biochemical measures of chronic disease risk in Yup’ik people.Subjects/Methods:In a cross-sectional sample of 1076 Yup’ik people, multiple linear regression was used to examine associations of sugar intake with BMI, WC and other chronic disease risk factors.Results:Isotopic estimates of sugar intake were not associated with BMI (P=0.50) or WC (P=0.85). They were positively associated with blood pressure, triglycerides (TG) and leptin, and are inversely associated with total-, high-density lipoprotein- and low-density lipoprotein-cholesterol and adiponectin.Conclusions:Isotopic estimates of sugar intake were not associated with obesity, but were adversely associated with other chronic disease risk factors in this Yup’ik study population. This first use of stable isotope markers of sugar intake may influence recommendations for sugar intake by Yup’ik people; however, longitudinal studies are required to understand associations with chronic disease incidence.

CDKAL1 and HHEX are associated with type-2 diabetes-related traits among Yup’ik people

Genome‐wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) associated with type 2 diabetes (T2D), mainly among individuals of European ancestry. In the present study, we examined the frequency of these SNPs and their association with T2D‐related traits in an Alaska Native study population with a historically low prevalence of T2D. We also investigated whether dietary characteristics that may protect against T2D, such as n‐3 polyunsaturated fatty acid (PUFA) intake, modify these associations.

Estimating lean mass over a wide range of body composition: a calibration of deuterium dilution in the arctic ground squirrel

Calculating body water through isotope dilution has become a useful way to nondestructively estimate body composition in many species. The most accurate estimates using this method require calibration against proximate chemical analysis of body composition for individual species, but no studies to our knowledge have calibrated this method on a hibernating mammal that seasonally undergoes dramatic changes in body composition. We use deuterium oxide to estimate total body water in captive arctic ground squirrels, Urocitellus parryii, and compare two approaches of calculating lean mass from total body water, both calibrated against lean mass based on proximate analysis. The first method uses a single tissue hydration constant to calculate lean mass from total body water; the second method uses a predictive equation to calculate lean mass from total body water and body mass. We found that the predictive equation performs better over the large range of body composition common to this species. Distillation of blood samples did not affect lean mass estimates from either calculation method. These findings indicate that isotope dilution using a predictive equation should work well as an alternative to destructive methods in other small mammals that undergo radical changes in body composition across their annual cycle.

Resource allocation to testes in walnut flies and implications for reproductive strategy

Testes size often predicts the winner during episodes of sperm competition. However, little is known about the source of nutrients allocated to testes development, or testes plasticity under varying nutrient availability. Among many holometabolous insects, metabolic resources can derive from the larval or adult diet. Distinguishing the source of nutrients allocated to testes can shed light on life history factors (such as maternal influences) that shape the evolution of male reproductive strategies. Here we used an experimental approach to assess resource allocation to testes development in walnut flies (Rhagoletis juglandis) from differing nutritional backgrounds. We fed adult male walnut flies on sugar and yeast diets that contrasted with the larval diet in carbon and nitrogen stable isotope ratios. This design allowed us to assess the dietary source of testes carbon and nitrogen and its change over time. We found significant incorporation of adult dietary carbon into testes, implying that walnut flies are income breeders for carbon (relying more on adult resources). In contrast, we found little evidence that walnut flies incorporate adult dietary nitrogen into testes development. We discuss the implications of these allocation decisions for life history evolution in this species.

Bi-cultural dynamics for risk and protective factors for cardiometabolic health in an Alaska Native (Yup’ik) population

Alaska Native people experience disparities in mortality from heart disease and stroke. This work attempts to better understand the relationships between socioeconomic, behavioral, and cardiometabolic risk factors among Yup’ik people of southwestern Alaska, with a focus on the role of the socioeconomic, and cultural components. Using a cross-sectional sample of 486 Yup’ik adults, we fitted a Partial Least Squares Path Model (PLS-PM) to assess the associations between components, including demographic factors [age and gender], socioeconomic factors [education, economic status, Yup’ik culture, and Western culture], behavioral factors [diet, cigarette smoking and smokeless tobacco use, and physical activity], and cardiometabolic risk factors [adiposity, triglyceride-HDL and LDL lipids, glycemia, and blood pressure]. We found relatively mild associations of education and economic status with cardiometabolic risk factors, in contrast with studies in other populations. The socioeconomic factor and participation in Yup’ik culture had potentially protective associations with adiposity, triglyceride-HDL lipids, and blood pressure, whereas participation in Western culture had a protective association with blood pressure. We also found a moderating effect of participation in Western culture on the relationships between Yup’ik culture participation and both blood pressure and LDL lipids, indicating a potentially beneficial additional effect of bi-culturalism. Our results suggest that reinforcing protective effects of both Yup’ik and Western cultures could be useful for interventions aimed at reducing cardiometabolic health disparities.

Stable isotope analysis of CO2 in breath indicates metabolic fuel shifts in torpid arctic ground squirrels

Stable carbon isotope ratios (δ13C) in breath show promise as an indicator of immediate metabolic fuel utilization in animals because tissue lipids have a lower δ13C value than carbohydrates and proteins. Metabolic fuel consumption is often estimated using the respiratory exchange ratio (RER), which has lipid and carbohydrate boundaries, but does not differentiate between protein and mixed fuel catabolism at intermediate values. Because lipids have relatively low δ13C values, measurements of stable carbon isotopes in breath may help distinguish between catabolism of protein and mixed fuel that includes lipid. We measured breath δ13C and RER concurrently in arctic ground squirrels (Urocitellus parryii) during steady-state torpor at ambient temperatures from -2 to -26°C. As predicted, we found a correlation between RER and breath δ13C values; however, the range of RER in this study did not reach intermediate levels to allow further resolution of metabolic substrate use with the addition of breath δ13C measurements. These data suggest that breath δ13C values are 1.1‰ lower than lipid tissue during pure lipid metabolism. From RER, we determined that arctic ground squirrels rely on nonlipid fuel sources for a significant portion of energy during torpor (up to 37%). The shift toward nonlipid fuel sources may be influenced by adiposity of the animals in addition to thermal challenge.