Gender-based differences in substrate-use during exercise at a self-selected pace

Journal Title (Medline/Pubmed accepted abbreviation): J. Strength Cond. Res
Year: 2011
Volume: 29
Issue: 9
Page number: 2544-2551

Summary of background and research design:

Background: During exercise, carbohydrates and fat are the most utilized energy sources.  As exercise intensity increases, it has been shown that carbohydrates are preferentially burned compared to fat.  There has been some evidence that females tend to burn a higher ratio of fat:carbohydrates during exercise than males.  Also, it is thought that the self-selected pace of an individual is the approximate pace that burns the highest proportion of fat.

Hypothesis: Men and women will self-select a pace that burns the highest fat:carbohydrate ratio and, because women tend to exhibit higher fat:carbohydrate oxidation ratios, they will have the highest fat:carbohydrate oxidation rates.

Subjects: There were 34 participants in total, 17 men and 17 women, age about 21-27 yrs old.  The subjects were classified as sedentary to moderately trained with mean body mass index (BMI) of 22-23 kg/m2.

Experimental design: ex post facto research design (research questions were posed based on data that were already gathered)

Protocol : Participants first were assessed for height, weight, body fat percentage (skinfold), and were familiarized with the treadmill.  At their second visit to the laboratory, they underwent a maximal exercise test and their VO2max was established.  Two days later, they performed the experimental trial.  They walked on the treadmill for 20 min at a self-selected pace.  They were able to adjust their pace at 5, 10, and 15 min, if desired.  Respiratory gases were measured throughout the trial, from which the respiratory exchange ratio (RER) was calculated.  (The RER is used to estimate the ratio of fat:carbohydrate oxidation).  Total energy expenditure was estimated from VO2 during the experimental trail and RER.

Summary of research findings:
  • There were no observed differences between men and women for maximal fat oxidation rate during the maximal exercise test.
  • Men required a lower energy intensity than women to reach their maximal fat oxidation rate.
  • Men and women self-selected similar paces during the 20 min exercise session, equal to about 37-40% of their VO2max .  However, VO2max was higher for men (similar % of VO2max during exercise).
  • Carbohydrate oxidation rates were greater in men than women throughout the whole 20 min trial but fat oxidation rates were similar. 
  • The contribution of fat oxidation to energy expenditure was higher in women than men.

Key practice applications:

Women and men both preferentially chose paces to achieve a percent of their VO2max that corresponds to their maximal rate of fat oxidation (about 28-45% VO2max for women, about 33-56% VO2max for men).  Therefore, a self-selected (i.e. comfortable) exercise pace should be effective for fat burning for both men and women.
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