A Novel Way to Follow Triglyceride Metabolism using 13C MRS

1
A Novel Way to Follow Triglyceride Metabolism
using 13C MRS

• D. K. Deelchand1, J. E. M. Snaar1, B. Ravikumar2,
  
• R. Taylor2 and P. G. Morris1

1Sir Peter Mansfield Magnetic Resonance Centre,
School of Physics and Astronomy, University of
Nottingham, Nottingham, U.K. 2Department of
Diabetes and Metabolism, University of
Newcastle-Upon-Tyne, Newcastle-Upon-Tyne, U.K.
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Introduction

• Triglycerides are the most common form of lipids
  in the human body
• They provide
• Insulation and protection for internal organs
• Important as energy sources
• Conventional methods to measure their
  concentration
• Morphometry
• Chemical analysis of tissue extracts
• Indirect calorimetry

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Introduction (2)

• Until recently in vivo magnetic resonance
  spectroscopy (MRS) has been applied
• To date, no technique to investigate postprandial
  trafficking of dietary fatty acids in humans
• Important in understanding e.g. type 2 diabetes
  where recent evidence suggests that their high
  lipid stores may contribute to insulin resistance
• Mechanism poorly understood

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Aim

• To follow the metabolic turnover of lipid stores
  in liver and calf muscle over a period of 24
  hours following ingestion of a labelled lipid
  using in vivo 13C NMR spectroscopy

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Methods

• 13C MRS measurements conducted on a 3T system
• Two homemade half-volume RF probes were used
  (denoted as leg and liver coils) each consisted
  of
• a circular 13C surface coil (7 cm diameter) and
• quadrature proton coils (leg probe 12 cm
  diameter and liver probe 13 cm diameter)
• Formate used as concentration reference at centre
  of 13C coil
• Subject lay supine inside magnet for each
  measurement
• Vacuum pillows used to ensure accurate
  positioning for subsequent measurements

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Parameters

• Simple 13C pulse acquire sequence with WALTZ-8
  decoupling (766 Hz bandwidth)
• CYCLOPS phase cycling
• TR 720 ms such that
• SAR for each coil was within required safety
  limits and
• allows enough time for the recovery of
  longitudinal magnetisation due to T1 relaxation
• Spectra collected in block of 500 scans with
  spectral resolution of 6 minutes (1,500
  acquisitions in total)

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Volunteers
Body mass index Waist-hip
ratio
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Labelled Lipid

• 98 U-13C algal lipid mixture consisting of
• Palmitic acid (45-55)
• Palmitoleic acid (10-15)
• Oleic acid (20-30)
• Linoleic acid (10-15)

- C C -
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Protocol
MR Spectroscopy
Calf Muscle
Liver
0 2 4 5
6 8 24
Time (hours)
Breakfast 3g Labelled Lipid
Meal
141.8g carbohydrate, 24.6g fat, 15.4g protein,
1232 kcal
101.5g carbohydrate, 21.1g fat, 30.5g protein,
733kcal
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Analysis

• Spectroscopic analysis with MRUI package in
  MATLAB
• AMARES algorithm used for peak fitting
• Prior knowledge (e.g. line-width, peak position
  constraints) applied if/when needed
• Estimate zero- and first- order phases for best
  fit

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Quantification

• Two phantoms, similar in shape to liver and calf
  muscle and containing 5.33 mmol/l of labelled
  lipid solution (dissolved in deuterated
  chloroform)
• Incorporated labelled lipid concentration found
  based on the increase in the ratio of the lipid
  peak to the formate peak (??) from baseline
• Where Rphantom is the ratio of the lipid to
  formate peaks in the phantom

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Liver Spectra

• Unsaturated Carbons Saturated
  Carbons

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13C Lipid Turnover in Liver
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13C Lipid Turnover in Muscle
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Lipoprotein Fractional Enrichment
From dietary fat
Synthesised by liver
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Exhaled Air 13CO2 Fractional Enrichment
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Summary

• Rapid incorporation of 13C enriched fatty acids
  into liver (maximum at 6 hours plt0.01). Label
  quickly displaced by subsequent unlabelled meals
• Consistent with presence of high level of 13C
  label in VLDL triglyceride at 8 hours
• Substantial 13C level of VLDL throughout the 24
  hours due to recycling of fatty acids
• Modest uptake of labelled lipid in skeletal muscle

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Conclusion

• We have developed a novel 13C MRS protocol to
  study the distribution and postprandial
  metabolism of lipids in humans
• Methodology will be applied to investigate the
  possible role of lipids in mediating insulin
  resistance in type 2 diabetes and non-alcoholic
  steatohepatitis

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Acknowledgements

• Peter Morris
• Roy Taylor
• Angelien Snaar
• Balasubramanian Ravikumar

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Blood Glucose Insulin Levels