Accelerated turnover of MHC class II molecules in nonobese diabetic mice is developmentally and environmentally regulated in vivo and dispensable for autoimmunity.

The H2-A(g7) (A(g7)) MHC class II (MHCII) allele is required for type 1 diabetes (T1D) in NOD mice. A(g7) not only has a unique peptide-binding profile, it was reported to exhibit biochemical defects, including accelerated protein turnover. Such defects were proposed to impair Ag presentation and, thus, self-tolerance.

Bayesian hierarchical methods to interpret the (13)C-octanoic acid breath test for gastric emptying.

Background: The (13)C-octanoic acid breath test is a convenient method for assessing gastric emptying (GE). Success depends on obtaining a well-characterized time profile of the excretion of label in breath, which may not be the case if GE is delayed.

Aims: To use Bayesian techniques in conjunction with hierarchical modelling as a method to increase the success of the modelling process.

How much human milk do infants consume? Data from 12 countries using a standardized stable isotope methodology.

The WHO has developed new growth curves based on breast-fed infants. Recommendations for energy intake have been adopted based on measurements of total energy expenditure. Data on human milk (HM) intake are needed to estimate the energy intake from this food source.

High lactation index is associated with insulin sensitivity.

The aim of the study was to evaluate the contribution of lactation to insulin sensitivity in women 12 to 18 month postpartum using an oral glucose tolerance test (OGTT). Mean lactation index (LI), a scoring system that considers the establishment and maintenance of the lactation was used. Lactation index was calculated according to the number of months of breast-feeding per child with a maximum of 72 points.

Recent progress in stable isotope methods for assessing vitamin metabolism.

Purpose Of Review: Recent developments in mass spectrometric methodology, in particular the widespread adoption of liquid chromatography/mass spectrometry, have presented investigators with new opportunities to investigate micronutrient absorption and metabolism. This review focuses on recent reports of the use of stable isotope techniques to facilitate research into vitamin uptake and utilization in humans.

Recent Findings: Stable isotopes are used principally in two ways, as analytical standards in isotope dilution assays and as tracers in studies of physiology.

Effects of growth hormone and free fatty acids on insulin sensitivity in patients with type 1 diabetes.

Context: Because GH stimulates lipolysis, an increase in circulating free fatty acid levels, as opposed to a direct effect of high GH levels, could underlie the development of insulin resistance in type 1 diabetes (T1D). Our aim was to explore the relative contributions of GH and free fatty acids to the development of insulin resistance in patients with T1D.

Patients: Seven (four females, three males) nonobese patients with T1D aged 21-30 yr were studied on four occasions in random order.

A novel stable isotope approach for determining the impact of thickening agents on water absorption.

Research on the bioavailability of water from thickened fluids has recently been published and it concluded that the addition of certain thickening agents (namely, modified maize starch, guar gum, and xanthan gum) does not significantly alter the absorption of water from the healthy, mature human gut. Using xanthan gum as an example, our "proof of concept" study describes a simple, accurate, and noninvasive alternative to the methodology used in that first study, and involves the measurement and comparison of the dilution space ratios of the isotopes (2)H and (18)O and subsequent calculation of total body water. Our method involves the ingestion of a thickening agent labeled with (2)H 1 day after ingestion of (18)O.

A novel derivative for the assessment of urinary and salivary nitrate using gas chromatography/mass spectrometry.

Previous gas chromatography/mass spectrometry (GC/MS) methods for determining nitrate in biological samples involve either hazardous chemicals or produce multiple isomers that can be difficult to quantitate. Modification of these methods, by the nitration of mesitylene instead of benzene and in the presence of trifluoroacetic anhydride rather than sulphuric acid, should enable simple isotopic quantitation for use in tracer studies, for example, in the measurement of nitric oxide production. Desiccated urine and saliva samples, in addition to aqueous labelled and unlabelled nitrate standards, were treated with trifluoroacetic anhydride and mesitylene at 70 degrees C for 1 h, cooled, then sequentially washed with deionised water and aqueous sodium bicarbonate.

Insulin administration and rate of glucose appearance in people with type 1 diabetes.

Objective: To assess whether prandial insulin, in addition to basal insulin, has an effect on the rate of glucose appearance from a meal in people with type 1 diabetes.

Research Design And Methods: The rate of glucose appearance from a mixed meal (Ra(meal)) was investigated in six adult (aged 24 +/- 2 years), lean (BMI 23.6 +/- 1.

A single glucose derivative suitable for gas chromatography/mass spectrometry and gas chromatography/combustion/isotope ratio mass spectrometry.

The incorporation of stable isotopes improves the assessment of glucose metabolism and, with some researchers using two tracers, (2)H-glucose assessed by gas chromatography/mass spectrometry (GC/MS) and (13)C-glucose by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS), a common derivative for both is advantageous. The most commonly used derivatives for GC/MS are inappropriate for GC/C/IRMS as additional functional groups dilute the label. We therefore considered the suitability of six derivatives for both GC/MS and GC/C/IRMS.

The hydration ability of three commercially available sports drinks and water.

This paper compares the hydration ability of three commercially-available sports drinks with water under conditions of rest and exercise, using a deuterium dilution technique. For the rest group, 0.05g/kg of body weight of deuterium, contained in gelatine capsules, was ingested with one of the test solutions and saliva samples were collected every five minutes for an hour while the subject remained seated.

Analysis of isotope ratios in vitamin K1 (phylloquinone) from human plasma by gas chromatography/mass spectrometry.

Vitamin K(1) is a fat-soluble vitamin required for the gamma-carboxylation of vitamin K-dependent proteins. Recent work has suggested an important role for vitamin K(1) in bone health beyond its more established function in the control and regulation of blood coagulation. However, current UK recommended intakes do not reflect this recent evidence.

Measurement of gastric emptying by the 13C-octanoate breath test--rationalization with scintigraphy.

The (13)C-octanoate breath test has not achieved universal acceptance for the measurement of solid phase gastric emptying, largely because the results are not comparable with those from direct methods such as scintigraphy. To convert breath-test data to their scintigraphic equivalent scale corrections are applied which have been obtained from population studies, but there is no guarantee that these are applicable in all cases. We propose an alternative method applicable on an individual basis based upon a simple physiological model which does not require any change to the breath-test protocol.

A stable isotope minimal model protocol with oral glucose administration.

A protocol for investigating glucose metabolism whereby stable isotope tracer is given intravenously after an oral glucose challenge is described. Frequent sampling of plasma glucose and insulin allows the tracer disappearance to be interpreted on the basis of established minimal models. We have investigated the glucose effectiveness and insulin sensitivity parameters and their reproducibility in a group of six healthy adults, each studied twice.

13C- and 2H-labelled glucose compared for minimal model estimates of glucose metabolism in man.

In the present study, we have investigated the use of 1-[(13)C]glucose and GC/combustion/isotope-ratio MS as an alternative to 6,6-[(2)H(2)]glucose and GC/MS in the determination of parameters of glucose metabolism using the IVGTT (intravenous glucose tolerance test) interpreted by labelled (hot) minimal models. The study has been done in four populations, normoglycaemics (subdivided into lean and obese individuals), subjects with impaired glucose tolerance and those with diabetes mellitus. Although the use of carbon label may in some circumstances be compromised by substrate recycling, our hypothesis was that this would not be an issue under the condition of suppression of hepatic glucose production during the short timescale of an IVGTT.

Measurement of gastric emptying by octanoate metabolism.

Purpose Of Review: Since its introduction just over 10 years ago, there have been a number of studies that have used the octanoate breath test to assess gastric emptying. Although use of the method is on the increase (the number of gastric emptying studies published on PubMed using the octanoate breath test has doubled between the periods 1997-2000 and 2001-2004 compared with a drop of approximately 20% in the use of scintigraphy over the same periods), the methodology has not achieved universal acceptance, primarily because it can provide results comparable to established techniques only indirectly.

Recent Findings: Recent methods for overcoming this difficulty are reviewed, including modified methods for breath test interpretation and the application of the related saliva test.

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.

Use of isotopically labelled octanoic acid to assess the effect of meal size on gastric emptying.

It has been proposed that the (13)C-octanoic acid breath test (OBT) provides a safe, non-radioactive means of measuring gastric emptying. However, deuterated octanoic acid provides a better marker when compared with scintigraphy, as the kinetics are less complex than those of the (13)C label. The appearance of (2)H in saliva is modelled as a two-compartment body water system, using an asymmetric triangular gastric emptying function.

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.

Using a non-invasive stable isotope tracer to measure the absorption of water in humans.

The development of solutions that prevent dehydration or promote adequate re-hydration play a vital role in preventing fatigue during exercise, however, the methods commonly used to assess the hydration ability of such solutions are invasive and often assess the components of absorption separately. This paper describes using a non-invasive deuterium tracer technique that assesses gastric emptying and intestinal absorption simultaneously to evaluate the uptake of water during rest and exercise. The kinetics of absorption are further examined by mathematical modelling of the data generated.

Glycogenesis and glucose oxidation during an intravenous glucose tolerance test in man.

The quantity of deuterated glucose customarily given in labelled IVGTTs (intravenous glucose tolerance tests) changes the isotopic composition of the subject's body water enough to be detected by mass spectrometric techniques. Glucose undergoing direct glycogenesis does not contribute label to the body water pool, and isotope incorporated into it must have come from glucose that has either been oxidized or undergone indirect glycogenesis. By subtracting the amount of label found in body water from the total amount of glucose utilized, as calculated from the minimal model of glucose disappearance, it should be possible to study the partitioning of the dose given between direct glycogenesis in skeletal muscle and other metabolic pathways.

Quantitative analysis using gas chromatography/combustion/isotope ratio mass spectrometry and standard addition of intrinsically labelled standards (SAIL)--application to isoflavones in foods.

We have investigated a novel application of gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) for the quantitative analysis of the isoflavones in food matrices. Previous methods have been hampered by the lack of analytical standards to introduce early enough in the extraction procedure to allow compensations for losses at all stages of the procedure. In this work we have produced standard materials that can be added at the initial extraction, by intrinsically labelling soya plants by growing them in an atmosphere enriched in the stable isotope of carbon in CO(2).

Measurement of insulin sensitivity indices using 13C-glucose and gas chromatography/combustion/isotope ratio mass spectrometry.

Important aspects of glucose metabolism can be quantified by using the minimal model of glucose kinetics to interpret the results of intravenous glucose tolerance tests. The power of this methodology can be greatly increased by the addition of stable isotopically labelled tracer to the glucose bolus dose. This allows the separation of glucose disposal from endogenous glucose production and also increases the precision of the estimates of the physiological parameters measured.

(2)H- and (13)C-labelled tracers compared for kinetic studies of ascorbic acid metabolism in man: a factor analytical approach.

A recent report that a (13)C stable isotope method can be used to measure the kinetics of ascorbic acid uptake and distribution in man has raised some interesting questions with regard to the physiological interpretation of the data obtained, in particular the sizes of the ascorbate distribution spaces. In order to prove that this result is not a function of the label used we have compared the behaviour of two different tracers to see if they are likely to give comparable results for the kinetic parameters. Volunteers received an oral dose of ascorbic acid, half of which was labelled with (2)H, and half of which was labelled with (13)C.

Measurement of gastric emptying in man using deuterated octanoic acid.

Gamma scintigraphy is considered the gold standard for the measurement of gastric emptying in humans. Recently, it has been proposed that a [(13)C]octanoate breath test can be used as an alternative technique for measuring gastric emptying of the solid phase, but the results from the two methods are not directly equivalent since in the breath test the label is subject to post-absorptive processing and consequently the emptying functions cannot be observed directly. This work investigates an alternative stable isotope method using deuterated octanoate where the kinetics of redistribution between and elimination from the various body pools are much more easily modelled.