Determining the Responsiveness of Intestinal Lipoprotein Production to an Elevation of Plasma Free Fatty Acids
Condition: Insulin Resistance
MeSH terms: Insulin Resistance
Study type: Interventional

Summary: 
      Lipoproteins are large complexes of molecules that transport lipids (primarily triglycerides
      and cholesterol) through the blood. The intestine has traditionally been viewed as a
      'passive' organ with respect to lipoprotein production, with intestinal lipoprotein
      production rates responding mainly to fat ingestion and absorption. The investigators have
      recently demonstrated in animal models that there is an overproduction of intestinal
      lipoproteins in both the fasted and the fed state. The investigators have also recently
      demonstrated that an elevation of plasma free fatty acids (FFAs) stimulates intestinal
      lipoprotein in hamsters. It is not known whether intestinal lipoprotein production can be
      acutely stimulated by an elevation of plasma FFAs in humans.

      Hypothesis: Intestinal lipoprotein particle production in humans can be stimulated by an
      acute elevation of plasma free fatty acids.
    

Detailed description: 
      This study proposes to use a published stable isotope method to study the kinetics of apoB48
      and apoB100 in the constant fed state in healthy subjects. These studies will be performed in
      10 healthy, lean men and women aged 18 to 65 years of age. Each subject will serve as his/her
      own control and will undergo 3 separate lipoprotein turnover studies, in random order, 4 to 6
      weeks apart. Since the infusion of intralipid (a synthetic triglyceride emulsion that
      provides a source of fatty acids) and heparin (to activate lipoprotein lipase) raises both
      FFAs and glycerol, two control studies will be performed, one with saline and one with
      glycerol infusion. ApoB48-containing lipoprotein particle production will be determined as
      outlined above but for this study in the fasted state, in response to the following
      interventions:

        1. Intralipid (20% solution @ 40 ml/hr) and heparin (250 u/hr) infusion to achieve an
           ~2-fold elevation of plasma FFAs (as previously shown), starting 90 minutes prior to and
           continuing throughout the lipoprotein turnover experiment,

        2. Saline infusion control study, and

        3. Glycerol control study in which glycerol will be infused at a rate of 2.25 g/hr to
           simulate the infusion of glycerol contained in Intralipid.

      Stable isotope infusion protocol. Following a 14-h overnight fast, an IV will be inserted
      into a superficial vein in each forearm, one for infusion and one for sampling. At 7 am, a
      fasting blood sample will be drawn and the subject will begin to ingest the first of 15
      identical small hourly meals, each equivalent to 1/15th of their daily food intake. This will
      be achieved by giving the patient the drink BOOST (Mead Johnson Nutritionals, Ottawa, On)
      and, using the Harris Benedict Equation (HBE) to determine the number of total energy
      requirements. This is based on height, weight, age and activity factors. The subject will
      have nothing else to eat until the end of the study. At the same time an IV infusion with
      either heparin plus intralipid or saline or glycerol as indicated above will be started. At
      10 am (3 hours after starting the ingestion of hourly feeds), a primed-constant infusion of
      deuterium-labeled leucine ([D3]L-leucine 98%, Cambridge Isotope Laboratories, MA) will be
      started, as previously described (0.6 mg/kg as an initial injection and then 0.6 mg/kg/hr
      thereafter). In addition, an IV bolus of d5-glycerol (100 mmol/kg) will be administered.
      These are standard techniques used for the assessment of lipoprotein metabolism in humans.
      Leucine, an amino acid and glycerol, an intermediary metabolite, are used by cells in the
      body as building blocks for proteins, fats and for energy. The form of leucine that will be
      administered is deuterated leucine (chemical formula L-[5,5,5-2H3]) and the form of glycerol
      is d5-glycerol, which has been enriched with the naturally occurring isotope (chemical
      variant) of hydrogen (2H). Deuterated leucine and glycerol are stable isotopes that occur
      naturally in the environment and in the body, and there are no health/safety issues regarding
      the infusion of the amounts of deuterated leucine and glycerol indicated above. The solutions
      are prepared using sterile techniques and are monitored for contamination prior to
      administration. Blood samples will be collected prior to and at the following time points
      after administration of the stable isotopes: 1hr, 2hr, 3hr, 5hr, 7hr, 9hr, 10hr, 11hr and
      12hr. A total of 340 ml of blood will be drawn.
    

Intervention name(s): Intravenous Intralipid; Intravenous leucine, glycerol
Intervention type(s): Drug; Drug
Intervention description(s): intravenous infusion; intravenous infusion
Intervention MeSH terms: Soybean oil, phospholipid emulsion; Glycerol
Allocation: Randomized
Intervention model: Single Group Assignment
Masking: None (Open Label)
Primary purpose: Diagnostic
Keywords: intestinal lipoprotein; very low density lipoprotein; free fatty acids; Apo B48 and Apo B100; triglycerides; Intestinal lipoprotein production