Intralipid

The FPGA analyzer was also tested against a software analysis scheme in phantoms. Phantoms with a range of viscosities (and therefore effective diffusion constants D_B) were produced utilizing glycerol, 20% Intralipid (Baxter Healthcare, Deerfield, IL), and water, following recipes from Cortese [31 (link)] to maintain a constant ${\mu }_s^{\prime }$ . We produced four phantoms with four different flow levels. We measured the flow levels using the DCS system with the FPGA analyzer and an independent DCS system with offline DCS analysis in a semi-infinite geometry for ~5 minutes for each phantom. BFI of the lowest viscosity phantom (no glycerol) was set as baseline. BFI is normalized to baseline using equation 5 as the relative BFI or rBFI. The phantom test was conducted in the room temperature of 25 °C. The rBFI of each phantom was measured by both DCS systems. Bland Altman plot was used to compare the consistency of the BFI measurement with 50 pairs of measurements for each phantom.

The tissue optical phantoms used for characterization (Supplementary Information: Tissue phantoms), contained fluorescein (20 μM) (Sigma) which brought up in 1% Intralipid (10% Solution, Baxter Healthcare) in PBS with varying concentrations of India ink following a well established protocol^{37 (link)}. The corresponding scattering coefficient μ's was equal to 11 cm^-1, a value typically considered for mouse tissue phantoms^{37 (link)}. Optical densities of ink concentrations in PBS were determined by measuring the absorbance spectrum at 910 nm. Fluorescent images of the solution were taken at 10-micron intervals through the depth of the phantoms.

Participants underwent saline-control or lipid/heparin infusions (Liposyn II or Intralipid; 20% lipid emulsion at 45 cc/hour, heparin at 0.4 U/kg/min), in the presence or absence of a HEC, conducted at 4 separate visits, for 6 hours commencing at approximately 6AM. The study was initiated with Liposyn II (Abbott Laboratories, North Chicago, IL; 10% safflower oil, 10% soybean oil, 1.2% egg phosphatides and 2.5% glycerin; major component fatty acids: approximately 65.8% linoleic, 17.7% oleic, 8.8% palmitic, 3.4% stearic, and 4.2% linolenic acid), but due to a product recall, some participants received Intralipid (Baxter Healthcare Corporation, Deerfield, IL; 20% Soybean oil, 1.2%egg yolk phospholipids, 2.25% glycerin; major component fatty acids: linoleic (44-62%), oleic (19-30%), palmitic (7-14%), linolenic (4-11%) and stearic (1.4-5.5%) instead. Liposyn and Intralipid are reported to induce similar degrees of insulin resistance (20 (link), 21 (link)) and this was confirmed within this study (see results).

Participants were studied on 2 occasions in random order, 2 weeks apart. On each occasion, participants were admitted to the clinical research unit at 1700 h on the day before the study. At 1800 h, they consumed a standard 10 kcal/kg meal (55% carbohydrate, 30% fat, 15% protein) followed by an overnight fast. At 0630 (−210 min) the following morning, a forearm vein was cannulated for infusions. In addition, a cannula was inserted retrogradely into a vein of the contralateral dorsum of the hand, which was placed in a heated Plexiglas box maintained at 55°C to allow sampling of arterialized venous blood. On one occasion, at 0700 (−180 min), an infusion of Intralipid (20%, 0.011 mL/kg/min; Baxter Healthcare, Deerfield, IL) and heparin (200 units prime, 0.2 units/kg/min continuous) was commenced to raise FFA concentrations and create an insulin resistant state.^{17 (link)} This was continued till the end of the study. At time 0 (1000 h), participants ingested a glucose drink (1 g/kg body weight).
The Glycerol study day differed from the Intralipid study day in that at 0700, glycerol was infused at 5 μmol/kg/min to match the amount of glycerol present during Intralipid infusion. Blood samples were obtained at periodic intervals for hormone and substrate measurement over the course of the experiment.