An Approach to Quantifying the Conversion of Carbohydrate to Fat in Humans

By: Jean-Marc Schwarz Ph.D. and Nathalie Bergeron, RD, Ph.D., ISOTEC® Stable Isotopes 2008 - 2010 Catalog

Stable Isotopes: An Approach to Quantifying the Conversion of Carbohydrate to Fat in Humans

Jean-Marc Schwarz Ph.D.
and
Nathalie Bergeron, RD, Ph.D.

Touro University,
1310 Johnson Lane,
Mare Island, Vallejo, CA 94592
and
University of California,
San Francisco, CA 94143

The recent completion of the Human Genome Project and the progress in proteomics have contributed to our current understanding of signal transduction pathways that control and modulate metabolism and, ultimately, determine human health. Transgenic animal model studies have shown, however, that the metabolic consequences of a gene alteration are often unpredictable, because they often fail to take into account the complex intertwine of metabolic redundancies in intact living organisms. These limitations have emphasized the need to develop new methodologies, able to monitor and quantify the dynamics of metabolic fluxes in vivo: dynamic metabolomics. Ultimately, the quantification of metabolic controls in whole body systems will dictate therapeutic intervention, based on a better understanding of the roots of metabolic abnormalities.

In this context, a new generation of tracer methodologies using stable isotopes has opened infinite options to study biosynthetic processes in vivo1. One application of this technique is to quantify the hepatic conversion of carbohydrate (CHO) to fat, de novo lipogenesis (DNL). Carbohydrates breakdown to produce 2-carbon units (monomers) used to synthesize fatty acid polymers. For example, eight monomers are assembled to make the 16-carbon, most common fatty acid, palmitate. With this approach, the monomer pool is first labeled with stable isotopes. Quantification, by gas chromatography/mass spectrometry (GC/MS), of the labeled monomer material incorporated into the polymers then allows the calculation of the proportion of newly synthesized fatty acids. In humans, the contribution of endogenous CHO to de novo fat synthesis was, until recently, always considered insignificant. However, over the past decade new evidence, based upon the use of stable isotopes to measure dynamic metabolic fluxes, suggest that hepatic DNL is not always trivial.

Using stable isotopes to study CHO and fat metabolism, we and others have found that the interplay between hepatic DNL and glucose production varies considerably depending on the diet and the health status of the subjects studied. The realization that hepatic DNL may be quantitatively significant under various nutritional and metabolic conditions is not only a shift from previous paradigms, but also points to a potential mechanism linking DNL to VLDL-triglyceride levels and liver fat content. Our ability to quantify DNL in humans may provide mechanistic insight regarding the contribution of DNL to such conditions as hypertriglyceridemia and non-alcoholic steatohepatitis, which are becoming increasingly prevalent in overweight and obese populations2.

Effect of dietary carbohydrates on hepatic DNL in healthy volunteers

Based on studies using MIDA (mass isotopomer distribution analysis) to quantify hepatic DNL, it is now generally accepted that in healthy individuals eating a Western high-fat diet, hepatic DNL is minimal in
the fasting state but increases multiple-fold postprandially because of the rise in lipogenic precursors that occurs after the consumption of a meal3. In contrast, DNL is present even in the fasting state, and becomes quantitatively significant with high CHO feeding4, and after overfeeding with simple CHO5 or fructose6. Hence, the use of stable isotope methodologies has allowed researchers to establish that the consumption of a diet rich in total or simple CHO is highly lipogenic, producing elevations in DNL that persist even after an overnight fast. In addition to increasing hepatic DNL, such diets concomitantly increase TG (triacylglycerol) levels, supporting the contribution of DNL to hypertriglyceridemia (Schwarz et al. 2003). It is noteworthy, however, that CHO-induced DNL appears to be specific to simple CHO. Indeed when simple sugars are restricted and a high-complex CHO diet is fed, DNL is trivial after an overnight fast (Parks et al., 1999).

Effect of hyperinsulinemia on hepatic DNL

We have also examined DNL in normo- and hyperinsulinemic obese volunteers and found that when fed a Western high-fat diet, hyperinsulinemic individuals have a higher rate of DNL compared to weight-matched normoinsulinemic obese subjects4, suggesting that in the absence of high CHO feeding, insulin resistance alone is sufficient to stimulate DNL. Similarly, hyperinsulinemic viscerally obese HIV-positive patients, as well as critically ill insulin resistant patients all have higher hepatic DNL compared to healthy subjects, irrespective of dietary CHO intake7-8. Taken together, these findings in hyperinsulinemic obese, critically ill, and HIV-infected subjects suggest that hyperinsulinemia per se stimulates DNL, independently from the macronutrient composition of the diet (Figure 1).

Hepatic DNL measured after an overnight fast in hyperinsulinemic patients

Figure 1: Hepatic DNL measured after an overnight fast in hyperinsulinemic patients. DNL in the fasting state during a Western diet is significantly higher when compared to normoinsulinemic lean or obese subjects (see ref. in text).

The application of stable isotope techniques has also allowed researchers to investigate the relationship between DNL and hepatic steatosis in non-alcoholic fatty liver disease (NAFLD). Interestingly, hyperinsulinemic subjects with NAFLD have been found to have elevated hepatic DNL in the fasting state, and analysis of their liver biopsies show that 26% of their liver fat is derived from CHO9. These findings support the hypothesis that hepatic DNL may also be an important contributor to liver fat accumulation. Ultimately, these observations may provide a therapeutic target, focused on modulating DNL by therapeutic lifestyle changes or pharmaceutical intervention, to improve metabolic outcomes in these patients.

Characterizing a phenotype and distinguishing normal physiology from pathophysiology require tools able to assess kinetic metabolic fluxes within whole biological system. We believe that stable isotope techniques represent a powerful non-invasive way to explore and monitor metabolism. These approaches are highly sensitive and hold promise for use as routine clinical diagnostic tools because of their ability to foresee metabolic derailment and because of their capacity to unravel the impact of genetic and environmental factors on the phenotype of human disease.

 

Please visit the Metabolic Research webpage for a listing of related products.

 

Materials List

     

References

  1. Hellerstein MK, Neese RA. Mass isotopomer distribution analysis at eight years: theoretical, analytic, and experimental considerations. Am J Physiol. 1999; 276 (6): E1146-70.
  2. Szczepaniak LS, Nurenberg P, Leonard D, Browning JD, Reingold JS, Grundy S, Hobbs HH, Dobbins RL. Magnetic resonance spectroscopy to measure hepatic triglyceride content: prevalence of hepatic steatosis in the general population. Am J Physiol 2005; 288 (2): E462-8.
  3. Timlin MT, Parks EJ. Temporal pattern of de novo lipogenesis in the postprandial state in healthy men. Am J Clin Nutr. 2005; 81 (1): 35-42.
  4. Schwarz JM, Linfoot P, Dare D, Aghajanian K. Hepatic de novo lipogenesis in normoinsulinemic and hyperinsulinemic subjects consuming high-fat, lowcarbohydrate and low-fat, high-carbohydrate isoenergetic diets. Am J Clin Nutr. 2003; 77 (1): 43-50.
  5. Schwarz JM, Neese RA, Turner S, Dare D, Hellerstein MK. Short-term alterations in carbohydrate energy intake in humans. Striking effects on hepatic glucose production, de novo lipogenesis, lipolysis, and whole-body fuel selection. J Clin Invest. 1995; 96 (6): 2735-43.
  6. Faeh D, Minehira K, Schwarz JM, Periasamy R, Park S, Tappy L. Effect of fructose overfeeding and fish oil administration on hepatic de novo lipogenesis and insulin sensitivity in healthy men. Diabetes. 2005; 54 (7): 1907-13.
  7. Schwarz JM, Chiolero R, Revelly JP, Cayeux C, Schneiter P, Jequier E, Chen T, Tappy L. Effects of enteral carbohydrates on de novo lipogenesis in critically ill patients. Am J Clin Nutr. 2000; 72 (4): 940-5.
  8. Schwarz JM, Mulligan K, Lee J, Lo JC, Wen M, Noor MA, Grunfeld C, Schambelan M. Effects of recombinant human growth hormone on hepatic lipid and carbohydrate metabolism in HIV-infected patients with fat accumulation. J Clin Endocrinol Metab. 2002; 87 (2): 942-4.
  9. Donnelly KL, Smith CI, Schwarzenberg SJ, Jessurun J, Boldt MD, Parks EJ. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease. J Clin Invest. 2005; 115 (5): 1343-51.

 

Related Links