Lipid Kinetics

Metabolic Solutions helps researchers study lipid kinetics using stable isotope methods. The process of triglyceride breakdown or lipolysis results in the release of fatty acids and glycerol. Fatty acids can serve as energy substrates while glycerol can act as a gluconeogenic precursor. Isotopic tracers (1-13C-palmitate and D5-glycerol) can be used to quantify the rate of appearance of fatty acids and glycerol into the blood stream. De novo lipogenesis rates can be studied with deuterium labeling methods.

List of Lipid Kinetic Services

  • Rates of lipolysis
  • Fatty acid turnover
  • Fatty acid oxidation
  • Glycerol kinetics
  • Rates of De Novo Lipogenesis

Fatty Acid Turnover

A stable isotope labeled fatty acid, typically 13C-palmitate, is continuously infused intravenously in tracer amounts. The rate of appearance of endogenous unlabeled fatty acids into the bloodstream can be determined by calculating the dilution of infused isotope. Upon reaching steady-state, the rate of appearance equals the rate of disappearance or uptake. Therefore, the rate of appearance is equal to the flux or turnover rate of the substrate.

Glycerol Turnover

The rate of appearance of glycerol is a direct index of lipolysis. Measurement of glycerol appearance is useful since fatty acid flux underestimates the rate of lipolysis because of reesterification. Fatty acids can become reesterified within adipocytes, which prevent release of fatty acids into the bloodstream despite active lipolysis. However, glycerol cannot be reincorporated into triglycerides because glycerol kinase is absent within adipocytes.

A stable isotope tracer of glycerol (typically, D5-glycerol) is continuously infused. A priming dose of tracer is used to achieve steady-state levels quickly.  Stable isotope analysis of D5-glycerol at steady-state is performed.

Rates of Fatty Acid Futile Cycle

Lypolysis and subsequent reesterification of released free fatty acid represent a futile cycle. This futile cycle allows the adipocyte to rapidly adjust free fatty acid levels in meeting energy demands. Simultaneous isotopic infusions of labeled fatty acid, example 1-13C-palmitate and D5-glycerol, provides an index of the relative rate of fatty acid reesterification.

Fatty Acid Oxidation

The rate of fatty acid oxidation can be estimated by infusing a 13C-fatty acid and measuring the rate of excretion of expired 13CO2 in the breath. The procedure requires a steady-state level of 13C-fatty acid in the bloodstream and measurement of expired 13C-labeled carbon dioxide with stable isotope analysis. Using priming doses of 13C-sodium bicarbonate before the continuous infusion of tracers will allow isotopic equilibrium by 60 minutes.

New Research Methods for Measuring Lipid Kinetics

The obesity epidemic has focused much research attention to fat metabolism studies. Fat metabolism can be traced with isotope-labeled fatty acids. Votruba et al. have validated a method using deuterated palmitate to measure dietary fat oxidation. The method involves administration of 20 mg/kg D31-palmitate in a meal. As palmitate is oxidized, each deuterium atom is lost to water. Urine or plasma can be sampled to measure the labeling in total body water. Palmitate oxidation is often measured with 13C- or 14C-palmitate to labeled CO2 but the calculations of oxidation require an acetate recovery factor.

The advantage of the deuterium label method is that no recovery factor is needed. Westerterp et al. has verified the results of Votruba and found a mean dietary fat oxidation of 16 ± 6%. This compares similarly to other published studies. Westerterp found that dietary fat oxidation was negatively correlated with body mass index. The obese subjects had lower fat oxidation while the lean subjects had higher fat oxidation. It was hypothesized that dietary fat oxidation may play a role in human obesity.

Metabolic Solutions can also determine lipid kinetics using a deuterium oxide labeling method and stable isotope analysis of palmitate or cholesterol. Oral doses of deuterium oxide are administered to subjects. The hydrogen of lipids such as palmitate or cholesterol are labeled during synthesis. This method quantitates de novo lipogenesis in humans or animals.

More information about deuterium labeling of lipids is found in our section on Deuteromics.

Applications

The following examples offer protocols useful for studying various aspects of glucose kinetics.

ApplicationReference
Fatty Acid OxidationImpaired fatty acid oxidation in type 2 diabetics
Fatty Acid OxidationIncrease in fat oxidation on high fat diet
Fatty Acid TurnoverAcute IL-6 treatment increases fatty acid turnover
Fatty Acid TurnoverEndurance training increases fatty acid turnover
Glycerol TurnoverGlycerol turnover with growth hormone receptor deficiency

If you need protocol information on how to conduct lipid kinetic tracer studies, the following technical paper is available: Fatty acid metabolism measured with stable isotope tracers.

Examples of Published Lipid Kinetic Studies with Stable Isotope Analysis By Metabolic Solutions

1. Verbruggen SC, Coss-BU J, Wu M, Schierbeek H, Joosten KF, Dhar A, van Goudoever JB, and Castillo L. Current recommended parenteral protein intakes do not support protein synthesis in critically ill septic, insulin-resistant adolescents with tight glucose control. Crit Care Med. 39(11):2518-25, 2011.

“Conducted a primed, constant, 7-hr stable isotope tracer infusion with [1,1,2,3,3-2H5]glycerol in combination with a hyperinsulinemic euglycemic clamp.”

2. Van Pelt RE, Gozansky WS, Hickner RC, Schwartz RS, and Kohrt WM. Acute modulation of adipose tissue lipolysis by intravenous estrogens. Obesity 14(12):2163-72, 2006.

“Assessed whole-body lipolysis by 2H5-glycerol rate of appearance and abdominal and femoral lipolysis by subcutaneous microdialysis.”

Applications