Optimal attentional allocation in the presence of capacity constraints in uncued and cued visual search.

The vision sciences literature contains a large diversity of experimental and theoretical approaches to the study of visual attention. We argue that this diversity arises, at least in part, from the field's inability to unify differing theoretical perspectives. In particular, the field has been hindered by a lack of a principled formal framework for simultaneously thinking about both optimal attentional processing and capacity-limited attentional processing, where capacity is limited in a general, task-independent manner. Here, we supply such a framework based on rate-distortion theory (RDT) and optimal lossy compression. Our approach defines Bayes-optimal performance when an upper limit on information processing rate is imposed. In this article, we compare Bayesian and RDT accounts in both uncued and cued visual search tasks. We start by highlighting a typical shortcoming of unlimited-capacity Bayesian models that is not shared by RDT models, namely, that they often overestimate task performance when information-processing demands are increased. Next, we reexamine data from two cued-search experiments that have previously been modeled as the result of unlimited-capacity Bayesian inference and demonstrate that they can just as easily be explained as the result of optimal lossy compression. To model cued visual search, we introduce the concept of a "conditional communication channel." This simple extension generalizes the lossy-compression framework such that it can, in principle, predict optimal attentional-shift behavior in any kind of perceptual task, even when inputs to the model are raw sensory data such as image pixels. To demonstrate this idea's viability, we compare our idealized model of cued search, which operates on a simplified abstraction of the stimulus, to a deep neural network version that performs approximately optimal lossy compression on the real (pixel-level) experimental stimuli.

Efficient data compression in perception and perceptual memory.

Efficient data compression is essential for capacity-limited systems, such as biological perception and perceptual memory. We hypothesize that the need for efficient compression shapes biological systems in many of the same ways that it shapes engineered systems. If true, then the tools that engineers use to analyze and design systems, namely rate-distortion theory (RDT), can profitably be used to understand human perception and memory. The first portion of this article discusses how three general principles for efficient data compression provide accounts for many important behavioral phenomena and experimental results. We also discuss how these principles are embodied in RDT. The second portion notes that exact RDT methods are computationally feasible only in low-dimensional stimulus spaces. To date, researchers have used deep neural networks to approximately implement RDT in high-dimensional spaces, but these implementations have been limited to tasks in which the sole goal is compression with respect to reconstruction error. Here, we introduce a new deep neural network architecture that approximately implements RDT. An important property of our architecture is that it is trained "end-to-end," operating on raw perceptual input (e.g., pixel values) rather than intermediate levels of abstraction, as is the case with most psychological models. The article's final portion conjectures on how efficient compression can occur in memory over time, thereby providing motivations for multiple memory systems operating at different time scales, and on how efficient compression may explain some attentional phenomena such as RTs in visual search. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

The importance of constraints on constraints.

The "resource-rational" approach is ambitious and worthwhile. A shortcoming of the proposed approach is that it fails to constrain what counts as a constraint. As a result, constraints used in different cognitive domains often have nothing in common. We describe an alternative framework that satisfies many of the desiderata of the resource-rational approach, but in a more disciplined manner.

Modeling human intuitions about liquid flow with particle-based simulation.

Humans can easily describe, imagine, and, crucially, predict a wide variety of behaviors of liquids-splashing, squirting, gushing, sloshing, soaking, dripping, draining, trickling, pooling, and pouring-despite tremendous variability in their material and dynamical properties. Here we propose and test a computational model of how people perceive and predict these liquid dynamics, based on coarse approximate simulations of fluids as collections of interacting particles. Our model is analogous to a "game engine in the head", drawing on techniques for interactive simulations (as in video games) that optimize for efficiency and natural appearance rather than physical accuracy. In two behavioral experiments, we found that the model accurately captured people's predictions about how liquids flow among complex solid obstacles, and was significantly better than several alternatives based on simple heuristics and deep neural networks. Our model was also able to explain how people's predictions varied as a function of the liquids' properties (e.g., viscosity and stickiness). Together, the model and empirical results extend the recent proposal that human physical scene understanding for the dynamics of rigid, solid objects can be supported by approximate probabilistic simulation, to the more complex and unexplored domain of fluid dynamics.

Adaptive allocation of human visual working memory capacity during statistical and categorical learning.

Human brains are finite, and thus have bounded capacity. An efficient strategy for a capacity-limited agent is to continuously adapt by dynamically reallocating capacity in a task-dependent manner. Here we study this strategy in the context of visual working memory (VWM). People use their VWM stores to remember visual information over seconds or minutes. However, their memory performances are often error-prone, presumably due to VWM capacity limits. We hypothesize that people attempt to be flexible and robust by strategically reallocating their limited VWM capacity based on two factors: (a) the statistical regularities (e.g., stimulus feature means and variances) of the to-be-remembered items, and (b) the requirements of the task that they are attempting to perform. The latter specifies, for example, which types of errors are costly versus irrelevant for task performance. These hypotheses are formalized within a normative computational modeling framework based on rate-distortion theory, an extension of conventional Bayesian approaches that uses information theory to study rate-limited (or capacity-limited) processes. Using images of plants that are naturalistic and precisely controlled, we carried out two sets of experiments. Experiment 1 found that when a stimulus dimension (the widths of plants' leaves) was assigned a distribution, subjects adapted their VWM performances based on this distribution. Experiment 2 found that when one stimulus dimension (e.g., leaf width) was relevant for distinguishing plant categories but another dimension (leaf angle) was irrelevant, subjects' responses in a memory task became relatively more sensitive to the relevant stimulus dimension. Together, these results illustrate the task-dependent robustness of VWM, thereby highlighting the dependence of memory on learning.

Living alongside more affluent neighbors predicts greater involvement in antisocial behavior among low-income boys.

BACKGROUND: The creation of economically mixed communities has been proposed as one way to improve the life outcomes of children growing up in poverty. However, whether low-income children benefit from living alongside more affluent neighbors is unknown. METHOD: Prospectively gathered data on over 1,600 children from the Environmental Risk (E-Risk) Longitudinal Twin Study living in urban environments is used to test whether living alongside more affluent neighbors (measured via high-resolution geo-spatial indices) predicts low-income children's antisocial behavior (reported by mothers and teachers at the ages of 5, 7, 10, and 12). RESULTS: Results indicated that low-income boys (but not girls) surrounded by more affluent neighbors had higher levels of antisocial behavior than their peers embedded in concentrated poverty. The negative effect of growing up alongside more affluent neighbors on low-income boys' antisocial behavior held across childhood and after controlling for key neighborhood and family-level factors. CONCLUSIONS: Findings suggest that efforts to create more economically mixed communities for children, if not properly supported, may have iatrogenic effects on boys' antisocial behavior.

Understanding oligonucleotide-templated nanocrystals: growth mechanisms and surface properties.

We describe studies of nanoparticle synthesis using oligonucleotides as capping ligands. The oligonucleotides nucleate, grow, and stabilize near-infrared fluorescent, approximately uniform PbS nanocrystals in an aqueous environment. The properties of the resulting particles strongly depend upon the sequences as well as synthesis conditions. Fourier Transform infrared measurements suggest that functional groups on the nucleobases such as carbonyl and amine moieties are responsible for surface passivation, while the phosphate backbone is strained to accommodate nucleobase bonding, preventing irreversible aggregation and thereby stabilizing the colloids. Our theoretical model indicates that oligonucleotide-mediated particle growth relies on the chemical reactivity of the oligonucleotide ligands that saturate dangling bonds of growing clusters, and favorable sequences are those that have the highest surface reactivity with growing particles. The oligonucleotide template approach is facile and versatile, offering a route to produce a range of material compositions for other chalcogenide semiconductor quantum dots and metal oxide nanoparticles.

Systematic social observation of children's neighborhoods using Google Street View: a reliable and cost-effective method.

BACKGROUND: Children growing up in poor versus affluent neighborhoods are more likely to spend time in prison, develop health problems and die at an early age. The question of how neighborhood conditions influence our behavior and health has attracted the attention of public health officials and scholars for generations. Online tools are now providing new opportunities to measure neighborhood features and may provide a cost effective way to advance our understanding of neighborhood effects on child health. METHOD: A virtual systematic social observation (SSO) study was conducted to test whether Google Street View could be used to reliably capture the neighborhood conditions of families participating in the Environmental-Risk (E-Risk) Longitudinal Twin Study. Multiple raters coded a subsample of 120 neighborhoods and convergent and discriminant validity was evaluated on the full sample of over 1,000 neighborhoods by linking virtual SSO measures to: (a) consumer based geo-demographic classifications of deprivation and health, (b) local resident surveys of disorder and safety, and (c) parent and teacher assessments of children's antisocial behavior, prosocial behavior, and body mass index. RESULTS: High levels of observed agreement were documented for signs of physical disorder, physical decay, dangerousness and street safety. Inter-rater agreement estimates fell within the moderate to substantial range for all of the scales (ICCs ranged from .48 to .91). Negative neighborhood features, including SSO-rated disorder and decay and dangerousness corresponded with local resident reports, demonstrated a graded relationship with census-defined indices of socioeconomic status, and predicted higher levels of antisocial behavior among local children. In addition, positive neighborhood features, including SSO-rated street safety and the percentage of green space, were associated with higher prosocial behavior and healthy weight status among children. CONCLUSIONS: Our results support the use of Google Street View as a reliable and cost effective tool for measuring both negative and positive features of local neighborhoods.

Spatial classification of youth physical activity patterns.

BACKGROUND: Physical activity is an essential element in reducing the prevalence of obesity, but much is unknown about the intensity and location of physical activity among youth-this is important because adolescent health behaviors are predictive of behaviors in adults. PURPOSE: This study aims to identify the locations where youth moderate-to-vigorous physical activity (MVPA) occurs, and to examine how MVPA varies according to urbanicity (urban, suburban, rural). METHODS: Participants included adolescent students (N=380, aged 12-16 years) from Halifax, Nova Scotia. Locations of MVPA were measured using accelerometers and GPS data loggers for up to 7 days. Specialized software was developed to integrate and process the data. Frequencies of MVPA by location were determined, and differences in MVPA were assessed for association with urbanicity. RESULTS: Active commuting accounted for the largest proportion of time in MVPA among urban and suburban students. Rural students achieved most MVPA at school. Other residential locations, shopping centers, and green spaces accounted for a majority of the remaining MVPA. Minutes in MVPA varied significantly overall (196.6 ± 163.8, 84.9 ± 103.2, 81.7 ± 98.2); at school (45.7 ± 45.2, 18.6 ± 28.0, 29.8 ± 39.7); while commuting (110.3 ± 107.1, 31.5 ± 55.2, 19.5 ± 39.7); and at other activity locations (19.7 ± 27.1, 14.8 ± 26.8, 12.0 ± 22.1) and by urbanicity. CONCLUSIONS: Findings reveal that the journeys between locations are as important as home and school settings in contributing to greater MVPA in adolescent youth. The relative importance of context as a contributor to MVPA varies with urbanicity. Combining actimetry and GPS data provides a precise link between physical activity measurements and contexts of the built environment.

Redox-modulatory vitamins and minerals that prospectively predict mortality in older British people: the National Diet and Nutrition Survey of people aged 65 years and over.

The predictive power, for total, vascular, cancer and respiratory mortality, of selected redox-modulatory (vitamin and mineral nutrient) indices measured at baseline, was studied in the British National Diet and Nutrition Survey (community-living subset) of people aged 65 years and over. Mortality status and its primary and underlying causes were recorded for 1054 (mean age 76·6 (sd 7·4) years and 49·0 % female) participants, from the baseline survey in 1994­5 until September 2008. During this interval, 74 % of the male and 62 % of the female participants died. Total mortality was significantly predicted by baseline plasma concentrations (per sd) of vitamin C (hazard ratio (HR) 0·81; 95 % CI 0·74, 0·88), α-carotene (HR 0·90; 95 % CI 0·81, 0·99), Se (HR 0·76; 95 % CI 0·69, 0·84), Zn (HR 0·79; 95 % CI 0·72, 0·87), Cu (HR 1·27; 95 % CI 1·14, 1·42) and Fe (HR 0·81; 95 % CI 0·74, 0·89). Total mortality was also significantly predicted by baseline dietary intakes (per sd) of food energy (HR 0·86; 95 % CI 0·79, 0·94), vitamin C (HR 0·88; 95 % CI 0·80, 0·94), carotenoids (HR 0·89; 95 % CI 0·83, 0·96), Zn (HR 0·89; 95 % CI 0·82, 0·96) and Cu (HR 0·91; 95 % CI 0·84, 1·00). Prediction patterns and significance for primary vascular, cancer and respiratory mortality differed in certain respects, but not fundamentally. Model adjustment for known disease or mortality risk predictors resulted in loss of significance for some of the indices; however, plasma Se and Zn, and food energy remained significant predictors. We conclude that total and primary vascular, cancer and respiratory mortality in older British people of both sexes is predicted by several biochemical indices of redox-modulatory nutrients, some of which may reflect the respondents' acute-phase status at baseline, whereas others may reflect the healthiness of their lifestyle.

Depression, physical function, and risk of mortality: National Diet and Nutrition Survey in adults older than 65 years.

OBJECTIVE: the authors used an objective assessment of physical function and a range of sociodemographic, dietary, and health behaviors to explore the possible factors that could explain the association between depression and mortality in community-dwelling elderly participants aged 65 years and older. DESIGN: prospective follow-up of the National Diet and Nutrition Survey in older adults. SETTING: community sample. PARTICIPANTS: a total of 1,007 participants (522 men, 485 women; mean age: 76.4 ± 7.3 years). MEASUREMENTS: : Depression was assessed from the 15 item Geriatric Depression Scale (GDS) and physical function using hand grip strength. Participants were followed up for death over an average of 9.2 years. RESULTS: : At baseline, 20.9% of participants demonstrated depression (GDS-15 score ≥ 5). Depressed participants were at a higher relative risk of all cause mortality during follow-up (age- and sex-adjusted hazard ratio = 1.24, 95% confidence interval: 1.04-1.49). Other risk factors for depression also related to mortality included smoking, physical inactivity, and low grip strength. These factors collectively explained an estimated 54% of the association between depression and mortality. Low-grade inflammation and low plasma vitamin C were also independently associated with depression and mortality but did not explain any of the association between depression and mortality. CONCLUSION: late-life depression is associated with a higher risk of mortality. Physical inactivity and physical dysfunction might partly mediate this association, although further longitudinal studies are required to fully elucidate these mechanisms.

Biochemical risk indices, including plasma homocysteine, that prospectively predict mortality in older British people: the National Diet and Nutrition Survey of People Aged 65 Years and Over.

Predictive power, for total and vascular mortality, of selected indices measured at baseline in the British National Diet and Nutrition Survey (community-living subset) of People Aged 65 Years and Over was tested. Mortality status and its primary and underlying causes were recorded for 1100 (mean age 76.7 (sd 7.5) years, 50.2% females) respondents from the baseline survey in 1994-5 until September 2008. Follow-up data analyses focussed especially on known predictors of vascular disease risk, together with intakes and status indices of selected nutrients known to affect, or to be affected by, these predictors. Total mortality was significantly predicted by hazard ratios of baseline plasma concentrations (per sd) of total homocysteine (tHcy) (95% CI) 1.19 (1.11, 1.27), pyridoxal phosphate 0.90 (0.81, 1.00), pyridoxic acid 1.10 (1.03, 1.19), alpha1-antichymotrypsin 1.21 (1.13, 1.29), fibrinogen 1.14 (1.05, 1.23), creatinine 1.20 (1.10, 1.31) and glycosylated Hb 1.23 (1.14, 1.32), and by dietary intakes of energy 0.87 (0.80, 0.96) and protein 0.86 (0.77, 0.97). Prediction patterns and significance were similar for primary-cause vascular mortality. The traditional risk predictors plasma total and HDL cholesterol were not significant mortality predictors in this age group, nor were the known tHcy-regulating nutrients, folate and vitamin B12 (intakes and status indices). Model adjustment for known risk predictors resulted in the loss of significance for some of the afore-mentioned indices; however, tHcy 1.34 (1.04, 1.73) remained a significant predictor for vascular mortality. Thus, total and primary vascular mortality is predicted by energy and protein intakes, and by biochemical indices including tHcy, independent of serum folate or vitamin B12.

Folate, related B vitamins, and homocysteine in childhood and adolescence: potential implications for disease risk in later life.

OBJECTIVES: Folate and the metabolically related B vitamins are an important priority throughout life, but few studies have examined their status through childhood and adolescence. The aims of the current study were to investigate age, gender, and lifestyle factors as determinants of folate, related B-vitamin status, and homocysteine concentrations among British children and adolescents and to propose age-specific reference ranges for these biomarkers, which, at present, are unavailable. PARTICIPANTS AND METHODS: Data from the National Dietary and Nutritional Survey of 2127 young people aged 4 to 18 years were accessed to provide a representative sample of British children. All of the subjects who provided a blood sample for homocysteine concentrations were included in the current study (n = 840). Of these, laboratory biomarkers of folate (serum and red cell folate: n = 832 and 774, respectively), vitamin B(12) (n = 828), vitamin B(6) (n = 770), and riboflavin (n = 839) were also examined. RESULTS: The biomarker status of all 4 of the relevant B vitamins decreased significantly with age. Correspondingly, homocysteine concentrations progressively increased, with median values of 5.6, 6.3, and 7.9 mumol/L for children aged 4 to 10 years, 11 to 14 years, and 15 to 18 years, respectively, and were higher in boys compared with girls (15-18 years only). Independent of age and gender, fortified breakfast cereal intake (consumed by 89% of the sample) was associated with significantly higher B-vitamin status and lower homocysteine concentrations. CONCLUSIONS: It is not generally appreciated that the well-established progressive increase in homocysteine from 4 to 18 years reflects decreases in the biomarker status of all 4 metabolically related B vitamins. We suggest age-specific laboratory reference ranges for homocysteine and related B-vitamin concentrations for potential use within a pediatric setting.

The RARE model of rapid HIV risk assessment.

The impact of the HIV/AIDS epidemic on minority communities called for interventions to stem the increase in new HIV infections and identify HIV-positive individuals for referral to care and treatment services. The Rapid Assessment, Response and Evaluation (RARE) project was designed to provide highly affected communities with a tool that would quickly identify conditions that fuel new infections and serve as barriers to HIV-positive individuals getting HIV testing, care, and treatment. RARE brought indigenous community health outreach workers and key community-level stakeholders together to advocate for the transfer of findings into programmatic and policy responses in places where high risk behaviors were practiced. This article describes RARE's qualitative methods that captured the voice of those most affected by the HIV/AIDS threat and identified critical insights and dynamics about factors that lead to HIV infections and those that can move positive individuals into care and treatment.

Dependence of blood indices of selenium and mercury on estimated fish intake in a national survey of British adults.

OBJECTIVE: Contributions of fish and other foods to variance of selenium and mercury status were studied in British adults. SETTING AND DESIGN: Plasma and red-cell selenium and whole-blood mercury concentrations were measured during the National Diet and Nutrition Survey of Adults aged 19-64 years in mainland Britain, 2000-2001 (n = 1216). Food intake was weighed for seven consecutive days, and foods were combined in groups for data analysis. Four subsidiary groups characterised fish intakes: fried white fish, 'other' white fish, shellfish and oily fish. RESULTS: Geometric means and 5-95% ranges were: for whole-blood mercury, 5.61 (1.30-22.2) nmol l(-1); for plasma selenium, 1.09 (0.83-1.43) micromol l(-1); for red-cell selenium, 1.64 (1.14-2.40) micromol l(-1). Twenty-eight per cent had no fish intake recorded during 7 days; the remaining 72% had a median intake of 237 g over the 7-day period, 5-95% range 45-780 g. Total fish intake was strongly and directly correlated with blood mercury, and moderately with red-cell and plasma selenium. Thus, sqrt(total fish intake) was correlated with: loge(blood Hg), t = +19.7; loge(plasma Se), t = +9.8; and loge(red-cell Se), t = +9.6, all P < 0.0001. All three biochemical (mercury and selenium) indices were strongly correlated with oily fish intake, and moderately correlated with shellfish and 'other' ( = non-fried) white fish, but none was significantly correlated with fried white fish. Blood mercury was strongly and directly correlated with red-cell and plasma selenium, and both increased with age. CONCLUSIONS: Dietary fish, especially oily fish, is a strong predictor of blood mercury and selenium in British adults.

Plasma homocysteine, folate and vitamin B(12) compared between rural Gambian and UK adults.

The disease risk indicator plasma total homocysteine (tHcy) is influenced by genetic and environmental factors, including folate and vitamin B(12) status. Little is known about the determinants of tHcy in rural West Africa. We explored the hypothesis that tHcy in rural Gambian adults might vary between the sexes and physiological groups, and/or with folate and vitamin B(12) status. Comparisons were made with a British national survey. Non-pregnant Gambian women (n 158) had tHcy concentrations (geometric mean 9.0 micromol/l) similar to those of non-pregnant UK women (n 449; 9.4 micromol/l), whereas pregnant Gambian women (n 12) had significantly lower values (6.2 micromol/l). Gambian men (n 22) had significantly higher values (14.7 micromol/l) than British men (n 354; 10.8 micromol/l). Gambian lactating women and British men and women exhibited significant inverse relationships between log(e)(tHcy) and folate status; however, only the British subjects exhibited significant inverse relationships between loge(tHcy) and vitamin B(12) status. In the British sample, and in Gambian lactating women, folate and vitamin B(12) status variations together accounted for 20-25 % of the variation in log(e)(tHcy). Within the UK, black-skinned adults had folate and tHcy levels similar to those of their white-skinned counterparts, but significantly higher vitamin B(12) values. We conclude that, whereas folate and vitamin B(12) status are similar between British and rural Gambian populations, tHcy is higher in Gambian men and lower in pregnant Gambian women, and that serum vitamin B(12) values appear to be higher in black-skinned than white-skinned British subjects. Possible reasons are discussed.

Homocysteine, folate, and vitamins B6 and B12 blood levels in relation to cognitive performance: the Maine-Syracuse study.

OBJECTIVE: Our objective was to examine associations among plasma homocysteine concentrations (tHcy), the tHcy-cofactors (folate, vitamins B6 and B12), and multiple domains of cognitive performance, with statistical adjustment for possible confounds, including cardiovascular disease risk factors (CVD-RF) and cardiovascular disease (CVD). METHODS: Subjects were 812 participants (58% women) of the Maine-Syracuse study who were free of dementia and stroke. Employing a cross-sectional design and multiple regression analyses, fasting concentrations of tHcy and its vitamin cofactors (folate, B6, and B12) were related to multiple domains of cognitive performance. RESULTS: With adjustment for age, education, gender, ethnicity, and the vitamins, tHcy was inversely associated with visual-spatial organization, working memory, scanning-tracking, and abstract reasoning. The same results were found with adjustment for age, education, gender, ethnicity, CVD-RF, and CVD. Vitamin cofactors were positively related to cognitive performance, but with adjustment for CVD-RF and CVD, only vitamin B6 was related to multiple cognitive domains. CONCLUSIONS: The inverse association of tHcy with multiple domains of cognitive functioning is not necessarily dependent on vitamin levels, vitamin deficiency, prevalent CVD risk factors, and manifest CVD. Serum folate, serum B12, and plasma B6 vitamin concentrations are positively associated with cognitive performance. Investigation of other possible mechanisms (e.g., tHcy neurotoxicity) mediating tHcy associations with cognitive performance is important, as are clinical trials examining the efficacy of folate, vitamin B6, and vitamin B12 for maintenance of cognitive functioning.

The 'anomalous' absorption of labelled and unlabelled vitamin C in man.

Previous studies of vitamin C absorption in man using stable isotope probes have given results which cannot easily be reconciled with those obtained using non-isotope measurement. In order to investigate some of the apparent paradoxes we have conducted a study using two consecutive doses of vitamin C, one labelled and one unlabelled, given 90 min apart. Compatibility of the experimental results with two feasible models was investigated. In Model 1, ingested vitamin C enters a pre-existing pool before absorption, which occurs only when a threshold is exceeded; in Model 2, ingested vitamin C is exchanged with a pre-existing flux before absorption. The key difference between these two models lies in the predicted profile of labelled material in plasma. Model 1 predicts that the second unlabelled dose will produce a secondary release of labelled vitamin C which will not be observed on the basis of Model 2. In all subjects Model 1 failed to predict the observed plasma concentration profiles for labelled and unlabelled vitamin C, but Model 2 fitted the experimental observations. We speculate on possible physiological explanations for this behaviour, but from the limited information available cannot unequivocally confirm the model structure by identifying the source of the supposed flux.

Stable isotope-labelled vitamin C as a probe for vitamin C absorption by human subjects.

Factors affecting absorption of physiological doses of vitamin C in man have not been widely studied, partly because few suitable tools exist to distinguish recently absorbed vitamin C from endogenous vitamin. Stable isotope-labelled vitamin C provides such a tool. Fifteen healthy non-smoking subjects aged 26-59 years were studied. Each received 30 mg l-[1-(13)C]ascorbic acid orally on two occasions, 3-4 weeks apart. The ascorbate was given alone or with Fe (100 mg as ferrous fumarate) or with red grape juice, which is rich in polyphenols. Blood was collected at frequent intervals for 1 h, and then each hour for a further 3 h. Total concentration of vitamin C was measured fluorometrically and its (13)C-isotope enrichment was measured by GC-MS after conversion to volatile trimethylsilyl esters. Peak plasma enrichment occurred within 25-50 min. No kinetic variables were significantly altered by the iron fumarate supplement. Grape juice attenuated vitamin C absorption, reaching significance at the 20 min time point. There were weak correlations between isotope enrichment and body weight or endogenous ascorbate concentration. The increment in total plasma ascorbate was smaller if calculated from isotope enrichment than from vitamin C concentration increase. The dilution pool was much larger than the plasma ascorbate pool. Further studies are needed to resolve these paradoxes. Stable isotope-labelled ascorbate is potentially useful for measurement of vitamin C absorption by human subjects.

Plasma carotenoid and vitamin E concentrations in women living in a rural west African (Gambian) community.

Vitamin E and carotenoid pigments are important nutrients for the maintenance of health and protection of tissues against free radical damage. They also play a potential role in disease-risk-prediction and -protection, but little is known about their physiological and sociodemographic correlates and determinants, especially in a West African context. As part of a study of reproductive health in rural Gambian women, blood samples were obtained from 1286 women aged 15 to 54 years, living in the Farafenni area of The Gambia. Measurement of two forms of vitamin E and six carotenoids in plasma was performed by high performance liquid chromatography. All eight components, but especially the carotenoids: lycopene, alpha- and beta-carotene, exhibited a major seasonal variation, with maximum levels between May and July, corresponding to the end of the "mango" season and the beginning of the "rainy" season. Only the tocopherols varied significantly (increasing) with age. Several unexpected ethnic group differences were observed, and canthaxanthin was present at lower concentrations in women with manual occupations, compared to those with non-manual occupations. There were also significant differences associated with pregnancy and the postpartum period, especially for the tocopherols, but fewer differences associated with marital status. The observed patterns confirm that there are important seasonal, physiological, possibly genetic, and sociodemographic determinants of these nutrients in blood plasma, which may have significance for health and longevity.