Oral hydroxycitrate supplementation enhances glycogen synthesis in exercised human skeletal muscle

Journal Title (Medline/Pubmed accepted abbreviation): Br. J. Nutr.
Year: 2012
Volume: 107
doi: 10.1017/S0007114511003862
Summary of background and research design

Background: Citrate is a compound found in the body that regulates the activity of many enzymes involved in carbohydrate and fat metabolism.  It is thought that hydroxycitrate (HCA) can inhibit some of these metabolism enzymes, thereby favoring fat catabolism (burning) and sparing carbohydrate catabolism.

Hypothesis: Supplementation with HCA will increase the rate of glycogen synthesis when taken after exercise.

Subjects: Eight healthy males, age 22.0 ± 0.3 y

Experimental design: randomized, placebo-controlled, cross-over trial

Treatments: A meal with 70% carbohydrates (2 g CHO/kg body weight) with a 500 mL beverage with or without 500 mg HCA.

Protocol: Before the exercise protocol, muscle biopsies were taken from the vastus lateralis (thigh) muscle and VO2max was determined.  On the days of the trial, the participants arrived at the laboratory after an overnight fast.  After a 5 min warm-up, they cycled at 75% their previously determined VO2max for 60 min.  After exercise, they consumed the meal.  Muscle biopsies were acquired immediately after exercise and 3 hrs post-exercise for analysis of muscle glycogen, GLUT4, and FAT/CD36 expression.  Blood samples were collected throughout recovery for analysis of blood concentrations of glucose, insulin, non-esterified fatty acids, and glycerol.  At least 1 wk later, the participants repeated the protocol with the other treatment.

Summary of research findings
  • Three hours into recovery, the average respiratory quotient was significantly lower with the supplement than with the placebo (about 0.84 with the supplement vs. about 0.89 with the placebo), suggesting that there was a larger percentage of energy coming from fat vs. carbohydrates.
  • Blood insulin concentration was significantly lower with HCA at every point during recovery, yet glucose concentrations were the same.  This implies that HCA may improve insulin sensitivity.
  • Glycogen synthesis rates were greater with HCA supplementation compared to the placebo.
  • After HCA supplementation, compared to the placebo, expression of GLUT4, the cell membrane-associated transport protein that brings glucose into the cell, was significantly lower on muscle cells after exercise whereas FAT/CD36 mRNA was greater (CD36 protein brings fat into the cells for oxidation).  It should be noted, however, that while the mRNA expression for FAT/CD36 was elevated by hydroxycitrate, the actual expression of the FAT/CD36 protein was not significantly elevated.

Key practice applications
As a post-exercise supplement, HCA appeared to allow for faster recovery of glycogen stores.  These data suggest that HCA may promote fat vs. glucose oxidation, which would tend to preserve glycogen.  It would be interesting to see if HCA could improve performance at a subsequent exercise session.
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