Intermyocellular lipid content and insulin sensitivity are increased following a short-term low-glycemic index diet and exercise intervention

Journal Title (Medline/Pubmed accepted abbreviation): Am. J. Physiol. Endocrinol. Metab.
Year: 2011
Volume: 301
Page number: E511-E516
doi: 10.1152/ajpendo.00221.2011

Summary of background and research design:

Background: Excess fat consumption and lack of exercise leads to an unhealthy accumulation of intramyocellular lipids (IMCL), or lipids in muscle cells, that may also contribute to insulin resistance and diabetes.
                A low glycemic index (GI) diet is a diet that is composed of foods that deliver glucose to the blood stream at a slow, steady rate (ex. whole wheat bread) as opposed to spiking blood sugar (ex. instant mash potatoes).  There is preliminary evidence that a low GI diet can also help reduce IMCL stores and additively contribute to improved insulin sensitivity.

Hypothesis:When involved in an exercise intervention, those on a low GI diet will have a greater reduction in IMCL stores that those on a high GI diet.

Subjects: Fifteen male and female  sedentary adults, age 66 ± 1 yrs old (n=7 low GI, n=8 high GI).

Experimental design: randomized, independent groups

Treatments: Low GI diet or high GI diet

Protocol :First, body composition, fitness parameters (ex.VO2max), and insulin sensitivity assessments were performed.  Proton magnetic resonance spectroscopy (MRS) was used to quantify IMCL in the soleus muscle (slow-twitch muscle in the calf).  The participants were randomly assigned to one of the treatment groups for 7 consecutive days.  For the duration of the study, the participants walked on a treadmill at about 80-85% of their VO2max for 60 min/day everyday.  Body composition, fitness parameters, and IMCL were assessed again at the end of the intervention.

Summary of research findings:
  • The participants experienced small but statistically significant weight loss (-1.7 ± 0.6 kg in the low GI group, -0.7 ± 0.2 kg in the high GI group).  There was no statistical difference between groups.
  • An increase in insulin sensitivity was experienced by both groups (without differences between groups).
  • Both groups experience a similar degree of increase in IMCL.  This suggests that lipid mobilization is changing to meet new energy demands.  This increase may not be permanent.
  • EMCL was reduced to a greater extent in the low GI diet group.
  • There was a significant time x trial interaction for the IMCL/EMCL ratio, with the ratio being higher for the low GI vs. high GI diet.  This ratio change appeared to be due mainly to a greater decrease in EMCL in the low GI group.

Key practice applications:

Even after only 7 days of a diet and exercise intervention, these participants experienced weight loss, insulin sensitivity, and mobilization of intracellular lipids.  From this short intervention, it did not appear that the low GI diet provided any additional benefit, but the authors still believe that, in a longer intervention, the low GI diet will lead to a more healthful distribution of lipid and other health parameters.

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