Substrate utilization is influenced by acute dietary carbohydrate intake in active, healthy females

Journal Title (Medline/Pubmed accepted abbreviation): J. Sports Sci. Med.
Year: 2011
Volume: 10
Page numbers:59-65

Summary of Background and Research Design

Background:Low carbohydrate diets have been promoted in both athletes and non-athletes for weight loss. The composition of a meal determines the blood levels of glucose, fat, and other nutrients and metabolites immediately after the meal as well as the metabolism of stored carbohydrates and stored fats during post-meal exercise.

Hypothesis:1) A low carbohydrate (LC) vs. a low fat (LF) meal will affect postprandial blood insulin, glucose, triglyceride, and free fatty acid concentrations in female adults. 2) A single bout of moderate-intensity exercise following a LC or LF meal will alter the utilization of different macronutrients (fat vs. carbohydrates).

Subjects:Eight females age 33.0 ± 6.3 who regularly exercise at least 3x/wk. Subjects were evaluated during the follicular stage of their menstrual cycles (days 5-13) and had been on hormonal contraceptives for at least 6 mos.

Experimental design:randomized, cross-over

Treatments and protocol:All subjects reported to the laboratory in a fasted state. On day 1, body composition was measured and VO2 max was determined. On testing day 2 (at least 3 days after day 1), subjects arrived to the laboratory after an overnight fast. Gas exchange (O¬2 and CO2) was measured while the subjects rested, and then the subjects consumed either a LF or LC meal, each about 400 kcal. Gas exchange measurements were acquired for 55 min after the meal (averaged for first 25 min and final 30 min). After a 5 min break, subjects exercised on a treadmill at 60-65% of their VO2max for 30 min. Blood was drawn before the meal and before and after exercise and was analyzed for insulin, glucose, free fatty acids, and triglycerides.

Summary of research findings:
  • Oxygen consumption during exercise was significantly greater after the LC meal compared to the LF meal (2.1 ± 0.4 L/min for LC vs. 1.9 ± 0.3 L/min for LF, P < 0.05).
  • Energy expenditure during exercise was significantly greater following the LC meal compared to the LF meal (305 ± 54 kcal for LC vs. 281 ± 50 kcal for LF, P < 0.05).
  • Not surprisingly, energy for exercise derived from fat was significantly lower after the LF meal and energy derived from carbohydrates was significantly lower after the LC meal.
  • The respiratory exchange ratio (RER) was significantly lower following the LC meal for the entire postprandial period measured, including during exercise, corroborating the observation that more fat is being burned for fuel compared to carbohydrate.
  • Blood insulin levels were significantly attenuated following the LC diet compared to the LF diet.

Interpretation of findings/Key practice applications:

Low carbohydrate meals promote the mobilization and utilization of fat for energy, therefore promoting a lean body composition and reducing the risk of chronic diseases such as diabetes and cardiovascular disease. This phenomenon is valid after just a single meal and does not require habitual diet strategies.


It would be interesting if the authors could distinguish between blood lipids that were recently absorbed from the meal vs. any stored lipids that were broken down for energy expenditure. There was no indication in this article of how the altered substrate utilization affected exercise performance or gastrointestinal tolerance during or after exercise.
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