Postexercise Repletion of Muscle Energy Stores with Fructose or Glucose in Mixed Meals

A recent study published in the American Journal of Clinical Nutrition asserts that consumption of fructose “lowers whole-body glycogen synthesis and impairs subsequent exercise performance, presumably because of lower hepatic glycogen stores.” The study sought to compare “isocaloric mixed meals containing fat and protein with either pure fructose or pure glucose” on intramyocellular lipids (IMCL), muscle glycogen, and whole-body metabolism.

A total of eight, trained male cyclists completed the study. The participants [age 29 ± 2 y; weight 71.2 ± 1.9kg; BMI 22.6 ± 0.4] were recruited from triathalon and cycling clubs in Lausanne, Switzerland and were nonsmokers, took no medications, and had no family history of diabetes.

The double-blind, crossover-controlled, randomized study included a preliminary visit to measure peak oxygen consumption and then two treatments with a 2-4 week washout period between the two conditions.  Each experiment began with a 2.5 day controlled diet (day 1 until 1130 on day 3) which provided 1.5 times their calculated basal energy expenditure and was comprised of 55% carbohydrate, 30% fat, and 15% protein. On day 1, participants competed 1 hour of endurance exercise. On day 2, participants were asked to be sedentary. On day 3, the subjects participated in a prolonged exercise session (1230-1500). Following the prolonged exercise session, IMCL and muscle glycogen stores were measured. Participants were then provided with experimental isocaloric mixed diets which contained either pure fructose (FRU) or pure glucose (GLU) in 8 liquid meals ingested at 1800, 2000, 2200 on day 3 and at 0800, 1000, 1200, 1400, and 1600 on day 4. The beverages contained the same total energy and macronutrient distribution. On day 4, before the 0800 liquid meal serving, plasma metabolic markers and energy metabolism were measured. These were monitored over a 6 hour period (0800-1400). At 1700 on day 4, IMCL and muscle glycogen were measured. After this collection, participants were provided with sandwiches and asked to fast for the remainder of the day. On day 5, participants returned to the testing facility in a fasted state and had their metabolism measured during a 3 hour standardized exercise program.

Researchers noted the following results:

  • IMCL and muscle glycogen repletion were comparable between glucose and fructose.
  • On day 4 fasting glucose, fructose, insulin, glucagon, lactate, and triglyceride concentrations were not different between fructose and glucose groups. After liquids consumption (0800-1400) postprandial glucose and insulin responses were lower in the fructose group compared to the glucose group. After liquids consumption (0800-1400), analogous postprandial blood glucagon, lactate, and triglyceride were higher in the fructose group compared to the glucose group.
  • On day 4, fasting free fatty acid concentrations were lower in the fructose group compared to the glucose group (P=0.01). After liquids consumption (0800-1400) free fatty acid concentrations were suppressed under both fructose and glucose conditions.
  • On day 4, fasting metabolic rate, respiratory exchange ratio, and substrate oxidation were similar in the fructose and glucose groups. After liquids consumption (0800-1400) postprandial energy expenditure was higher in the fructose group compared to the glucose group, corresponding with a higher respiratory exchange ratio and an increased net carbohydrate oxidation. Lipid oxidation was lower in the fructose versus glucose group. Protein balance was similar between both groups.
  • 4 of the 8 subjects were unable to maintain the target power output after the FRU diet suggesting impaired endurance exercise capacity.
  • Compared with glucose, fructose consumption increased postprandial energy expenditure and net carbohydrate oxidation. Whole-body carbohydrate storage was 18g less in FRU compared to GLU.

Researchers concluded “this study indicates that pure fructose or glucose ingested together with Fat and protein in the 24 h period after a strenuous exercise session leads to similar energy storage in IMCLs and muscle glycogen. Muscle glycogen synthesis was likely fueled in FRU by glucose and lactate derived from fructose and released into the blood by splanchnic organs. Net whole-body glycogen storage was lower, and performance during subsequent exercise was decreased in FRU.”