Investigation of hepatic gluconeogenesis pathway in non-diabetic Asian Indians with non-alcoholic fatty liver disease using in vivo (³¹P) phosphorus magnetic resonance spectroscopy

Abstract

Objective

To study hepatic gluconeogenesis pathway in non-diabetic Asian Indian males having non-alcoholic fatty liver disease (NAFLD) using in vivo (³¹P) phosphorous magnetic resonance spectroscopy (MRS) and correlate these data with anthropometry and insulin resistance.

Research design and methods

Forty non-diabetic patients with NAFLD and 20 healthy controls were divided into (i) obese with NAFLD (group I, n = 20), (ii) non-obese with NAFLD (group II, n = 20) and (iii) non-obese without NAFLD (group III, n = 20). Anthropometric and biochemical profiles, short insulin tolerance test (SITT), liver ultrasound, and ³¹P MRS (to determine hepatic gluconeogenesis metabolite; phosphomonoesters (PMEs), inorganic phosphate (Pi) and their ratios with respect to ATP) were done.

Results

Insulin resistance (Kitt value) was highest in group I (p < 0.05; compared to other two groups), but was also higher in group II as compared to group III (p = ns). The values of PME/Pi, PME/γATP, PME/βATP, PME/tATP ratios were higher (p < 0.05) in group I compared to other two groups. Interestingly, non-obese subjects with NAFLD also showed more derangements of hepatic gluconeogenesis metabolites than non-obese subjects without NAFLD. Positive correlation was observed between PME and other ratios in relation to body mass index, waist circumference, body fat percentage and fasting serum insulin levels in all the three groups.

Conclusions

Derangements in hepatic gluconeogenesis as assessed non-invasively using ³¹P MRS, was observed in obese and non-obese, non-diabetic Asian Indians with NAFLD. Further research is warranted whether this investigation in NAFLD subjects could be developed as a non-invasive tool to assess those predisposed to develop hyperglycemia.

Introduction

Normal liver contains approximately 5 g of lipids per 100 g of wet weight. Hepatic steatosis (fatty liver) is the term used when lipids, predominantly triglycerides, in liver are more than 5% of liver weight [1], [2]. NAFLD is defined as presence of fat in liver with/without presence of inflammation or fibrosis in a person taking less than 20 g of alcohol per day [3]. Data from developed countries indicate prevalence of hepatic steatosis to be 20–25% [4]. It is increasingly becoming apparent that NAFLD is another feature of the metabolic syndrome, with insulin resistance being the common factor. The strong association of NAFLD with other features of the metabolic syndrome (abdominal adiposity, glucose intolerance, dyslipidemia, hypertension, and atherosclerotic cardiovascular disease) supports this observation. Insulin resistance is closely linked to NAFLD even in lean subjects [5], [6].

The prevalence of type 2 diabetes mellitus and coronary artery disease (CAD) in migrant and native Asian Indians is rising [7]. Insulin resistance and the metabolic syndrome are present in a high percentage Asian Indians and could explain in part this increased prevalence [8], [9], [10]. Recent data suggest that 1/4th–1/3rd of the adults and children in urban areas of India are affected by insulin resistance and the metabolic syndrome [9], [10], [11], [12], [13], [14].

Derangements of hepatic pathway of carbohydrate metabolism can lead to increase in hepatic gluconeogenesis and hyperglycemia [15]. At present, these metabolic disorders have been investigated in vivo using plasma glucose and insulin assays, however, these tests may not accurately reflect dynamic changes in hepatic carbohydrate metabolism. Hepatic gluconeogenesis pathway could be assessed by studying the levels and activity of intermediate substrates and enzymes using magnetic resonance spectroscopy (MRS). It is a safe, non-invasive and versatile tool for in vivo metabolic studies and provides direct simultaneous visualization of metabolites in tissues at serial time points. This technique also allows measurement of phosphorylated compounds, including phosphorylated intermediates of gluconeogenesis. At clinical magnetic field strengths, some of resonances obtained with use of phosphorous (³¹P) MRS in human liver are multi-component, which include phosphomonoester (PME), inorganic phosphate (Pi), and three nucleotide triphosphate (NTP) resonances [16]. PME resonance includes phosphocholine, phosphoethanolamine, adenosine monophosphate (AMP), glucose-6-phosphate and 3-phosphoglycerate and other intermediates of gluconeogenesis. Only a limited number of studies have been done since the ³¹P MRS machine is costly and is not available in most centers. The study done by Ikehira et al. [17] have shown that MRS performed in the human liver using 1.5 Tesla MR equipment is a valuable method for studying dynamic hepatic carbohydrate metabolism. Further, Leij-Halfwerk et al. [16] have shown that patients with lung cancer have increased glucose flux and hepatic gluconeogenesis. Finally, alterations of gluconeogenesis have been shown in cirrhosis of liver. The baseline PME values were elevated by 35% (p < 0.05) in the compensated cirrhosis group and by 57% (p < 0.01) in the decompensated cirrhosis group [18]. Importantly, hepatic gluconeogenesis pathway in NAFLD using MRS has not been studied so far.

We hypothesized that abnormalities in hepatic gluconeogenesis pathway occur in non-diabetic Asian Indians with NAFLD regardless of overall obesity. For this purpose we investigated non-diabetic males non-invasively using in vivo³¹PMRS to study carbohydrate metabolism and also correlated these data with insulin resistance and parameters of body composition.