Human liver glycogen metabolism assessed with a 13C-enriched diet and a 13CO2 breath test

(13)C-Breath testing in animals: theory, applications, and future directions

The carbon isotope values in the exhaled breath of an animal mirror the carbon isotope values of the metabolic fuels being oxidized. The measurement of stable carbon isotopes in carbon dioxide is called (13)C-breath testing and offers a minimally invasive method to study substrate oxidation in vivo. (13)C-breath testing has been broadly used to study human exercise, nutrition, and pathologies since the 1970s. Owing to reduced use of radioactive isotopes and the increased convenience and affordability of (13)C-analyzers, the past decade has witnessed a sharp increase in the use of breath testing throughout comparative physiology--especially to answer questions about how and when animals oxidize particular nutrients. Here, we review the practical aspects of (13)C-breath testing and identify the strengths and weaknesses of different methodological approaches including the use of natural abundance versus artificially-enriched (13)C tracers. We critically compare the information that can be obtained using different experimental protocols such as diet-switching versus fuel-switching. We also discuss several factors that should be considered when designing breath testing experiments including extrinsic versus intrinsic (13)C-labelling and different approaches to model nutrient oxidation. We use case studies to highlight the myriad applications of (13)C-breath testing in basic and clinical human studies as well as comparative studies of fuel use, energetics, and carbon turnover in multiple vertebrate and invertebrate groups. Lastly, we call for increased and rigorous use of (13)C-breath testing to explore a variety of new research areas and potentially answer long standing questions related to thermobiology, locomotion, and nutrition.

Current and future uses of breath analysis as a diagnostic tool

The analysis of exhaled breath is a potentially useful method for application in veterinary diagnostics. Breath samples can be easily collected from animals by means of a face mask or collection chamber with minimal disturbance to the animal. After the administration of a 13C-labelled compound the recovery of 13C in breath can be used to investigate gastrointestinal and digestive functions. Exhaled hydrogen can be used to assess orocaecal transit time and malabsorption, and exhaled nitric oxide, carbon monoxide and pentane can be used to assess oxidative stress and inflammation. The analysis of compounds dissolved in the aqueous phase of breath (the exhaled breath condensate) can be used to assess airway inflammation. This review summarises the current status of breath analysis in veterinary medicine, and analyses its potential for assessing animal health and disease.

The 13CO2 breath test for liver glycogen oxidation after 3-day labeling of the liver with a naturally 13C-enriched diet

OBJECTIVE

When naturally (13)C-enriched carbohydrate is used to label hepatic glycogen, (13)C-liver glycogen oxidation can be monitored subsequently by measuring the (13)C enrichment of breath CO(2) during a sedentary fast. In our previous breath test studies, we used a 1-d labeling protocol to enrich liver glycogen. Others found that after 3 d of labeling the liver glycogen (13)C enrichment is identical to the dietary carbohydrate (13)C enrichment.

METHODS

We compared a diet protocol in which naturally (13)C-enriched carbohydrate was given for 3 d before the breath test with our previously applied 1-d labeling design. The (13)CO(2) breath test was combined with indirect calorimetry. The results were compared with those from our previous studies. In addition, we compared liver glycogen oxidation rates with those from our present technique and different techniques as used in other published studies.

RESULTS

Six healthy volunteers were included in this study. The (13)C enrichment of breath CO(2) at plateau excretion level did not differ after 1 or 3 d on a labeling diet. However, the end of plateau time tended to be later after the 3-d diet, 14.3 h versus 12.5 to 13.5 h postprandially in the 1-d labeling studies. Also, the return to baseline time was later in the 3-d study, at 25.8 h versus 19.0 to 23.2 h postprandially after 1 d of labeling. The liver glycogen oxidation rate was similar in both techniques until 17 h postprandially. After this time the 3-d labeling protocol showed a higher level of liver glycogen oxidation.

CONCLUSION

The results indicated that the labeling of liver glycogen is slightly less complete after 1 d on a (13)C-enriched diet as compared with 3-d labeling. Our (13)C breath test results compared rather well with studies from the literature using the (13)C-NMR technique, the D(2)O technique, or the (13)CO(2) breath method to measure liver glycogen oxidation.

Modeling 13C breath curves to determine site and extent of starch digestion and fermentation in infants

BACKGROUND

The colon salvages energy from starch, especially when the capacity of the small intestine to digest it is limited. The aim of this study was to determine the site and relative extent of starch digestion and fermentation in infants.

METHODS

Thirteen infants (10 male and 3 female infants), median age 11.8 months (range, 7.6-22.7 months), were fed a starchy breakfast containing 13C-labeled wheat flour after an overnight fast. Duplicate breath samples were obtained before breakfast and every 30 minutes for 12 hours. Breath 13CO2 enrichment was measured using isotope ratio mass spectrometry, and results were expressed as percentage dose recovered (PDR) for each 30 minutes. The PDR data were analyzed and mathematically modeled assuming either a constant estimate of CO2 production rate or adjusted for physical activity.

RESULTS

Mean +/- SD cumulative 13C PDR (cPDR) at 12 hours was 21.3% +/- 8.4% for unadjusted data and 26.5% +/- 11.6% for adjusted data. A composite model of two curves fit significantly better than a single curve. Modeling allowed estimation of cPDRs of small intestine (17.5% +/- 6.5% and 22.7% +/- 9.3% for unadjusted and adjusted data, respectively) and colon (4.6% +/- 2.9% and 6.3% +/- 5.4%).

CONCLUSIONS

Modeling of 13CO2 enrichment curves after ingestion of 13C-enriched wheat flour is an attractive means to estimate the contribution of the upper and lower gut to starch digestion and fermentation.

Muscle glycogen does not interfere with a 13CO2 breath test to monitor liver glycogen oxidation

Naturally 13C-enriched carbohydrate has been used to label the liver glycogen pool for metabolic studies. The utilization of this glycogen was then monitored by the appearance of 13CO2 in breath. Using this method, it is assumed that during sedentary fasting the contribution of muscle glycogen towards oxidation is negligible. We investigated the influence of a different level of 13C enrichment of muscle glycogen on the 13C enrichment of breath CO2 while the breath test was carried out. In six healthy volunteers, the muscle glycogen stores were grossly depleted by a cycling exercise prior to consumption of the 13C-enriched diet which was given over a 10 h period. The oxidation of liver glycogen was measured during an 18 h sedentary fast. The results were compared with a control group who had not depleted their muscle glycogen before labelling. A higher 13C enrichment of muscle glycogen did not interfere with two parameters of liver glycogen oxidation, i.e. the duration of the plateau phase of 13CO2 and the return to baseline time. It was also shown that the 13C-labelled muscle glycogen was still available after the 18 h fast because a strenuous exercise led to a rapid 13CO2 enrichment. It is concluded that muscle glycogen 13C enrichment does not invalidate a 13CO2 breath test to measure liver glycogen oxidation during a sedentary fast.

Influence of the 13C-enrichment of the habitual diet on a 13CO2 breath test used as an index of liver glycogen oxidation: a validation study in western Europe and Africa

A diet containing naturally 13C-enriched carbohydrate combined with a 13CO2 breath-test analysis can be used to monitor liver glycogen oxidation in persons used to a diet low in 13C, e.g., the Western European diet. In this study, we evaluated this test principle further by changing the way we label the glycogen pool. The 13C enrichment of exhaled CO2 was studied in two groups, one in Europe and one in Africa. The European group (n = 12) was accustomed to a diet low in 13C, and they went on a 13C-enriched study diet to identify liver glycogen. The African group (n = 6) was accustomed to a diet naturally high in 13C, and they went on a diet low in 13C. The basal 13C abundance in exhaled CO2 was higher in the African group (1.0879 At%; atmospheric 1.1 atom percent) than in the European group (1.0821 At%). During the study period, the parameters for liver glycogen oxidation--the 13CO2 enrichment plateau, the plateau duration, and the return to baseline time--did not differ between groups. The abundance of 13CO2 in exhaled CO2 over time in the two groups was similar but inverse. This study confirms the use of a 13CO2 breath test to monitor liver glycogen oxidation and demonstrates how to use such a test in persons accustomed to a diet high in 13C.