Appendix
Calculations
Chylomicron remnant-TG concentrations were calculated from unbound concentrations of [U-13C]palmitate-TG using a model(1) described in the appendix. The calculation assumes no fatty acid selectivity during remnant formation (2) and that all [U-13C]palmitate-TG in the unbound fraction represents chylomicron remnants:
chylomicron remnant palmitate in TG (µmol/l) = unbound [U-13C]palmitate in TG (µmol/l)
chylomicron TTR
chylomicron remnant TG (µmol/l) = chylomicron remnant palmitate in TG (µmol/l)
chylomicron palmitate
Where chylomicron TTR = [U-13C]16:0/16:0 TTR and chylomicron palmitate is the molar proportion of palmitate in the chylomicron TG at peak enrichment.
Calculations of the contribution of different sources of fatty acids to VLDL-TG were performed based on the principles described by Barrows and Parks (3) and Vedala et al(4) and were calculated at 360 min:
The percentage contribution of systemic plasma NEFA to VLDL-TG, where systemic plasma NEFA includes fatty acids from two sources (subcutaneous adipose tissue lipolysis and the ‘spillover’ dietary fatty acids from the hydrolysis of chylomicron-TG)
=TTR [2H2] VLDL-TG * 100(Equation 1)
TTR [2H2] NEFA
The percentage contribution to VLDL-TG of fatty acids from dietary spillover NEFA
= Equation 1 *TTR [U-13C] NEFA * 100(Equation 2)
TTR [U-13C] Sf >400
where [U-13C] Sf >400 is the highest isotopic enrichment in the Sf>400 fraction.
The percentage contribution of fatty acids from endogenous (those mainly derived from the lipolysis of subcutaneous adipose tissue) systemic NEFA to VLDL-TG
= Equation 1 - Equation 2 (Equation 3)
The percentage contribution of fatty acids to VLDL-TG from dietary remnants
=TTR [U-13C] VLDL-TG * 100 - Equation 2(Equation 4)
TTR [U-13C] Sf >400
The remaining contribution was fatty acids from ‘other’ sources which will include splanchnic lipolysis (mainly visceral adipose tissue) together with hepatic sources (such as DNL and stored hepatic TG). From here the ‘other’ sources will be referred to as splanchnic sources
= 100 – (the contribution from endogenous systemic NEFA + dietary spillover NEFA + dietary remnants)
To calculate the amount of fatty acids contributing to VLDL-TG from different sources
= (percentage contribution/100) * VLDL-TG concentration (µmol/l)
1. Fielding B, Hodson L, Roberts R, Bickerton A, Karpe F, Frayn K: Use of a stable isotope technique to estimate postprandial chylomicron triacylglycerol concentrations without physical separation of the lipoproteins Diabetologia - Abstracts of the 41st Annual Meeting of the EASD Athens, Greece 10-15 September 2005 48 Suppl 1:A 197, 2005
2. Summers LK, BarnesSC, Fielding BA, Beysen C, Ilic V, Humphreys SM, Frayn KN: Uptake of individual fatty acids into adipose tissue in relation to their presence in the diet. Am J Clin Nutr 71:1470-1477, 2000
3. Barrows BR, Parks EJ: Contributions of different fatty acid sources to very low-density lipoprotein-triacylglycerol in the fasted and fed states. J Clin Endocrinol Metab 91:1446-1452, 2006
4. Vedala A, Wang W, Neese RA, Christiansen MP, Hellerstein MK: Delayed secretory pathway contributions to VLDL-triglycerides from plasma NEFA, diet, and de novo lipogenesis in humans. J Lipid Res 47:2562-2574, 2006