As exercise intensity increases, the proportion of fat utilized as an energy substrate decreases, while the proportion of carbohydrates utilized increases (5). The rate of fatty acid mobilization from adipose tissue also declines with increasing exercise intensity (5). This had led to the common recommendation that low- to moderate-intensity, long duration endurance exercise is the most beneficial for fat loss (15). However, this belief does not take into consideration what happens during the post-exercise recovery period; total daily energy expenditure is more important for fat loss than the predominant fuel utilized during exercise (5). This is supported by research showing no significant difference in body fat loss between high-intensity and low-intensity submaximal, continuous exercise when total energy expenditure per exercise session is equated (2,7,9). Research by Hickson et al (11) further supports the notion that the predominant fuel substrate used during exercise does not play a role in fat loss; rats engaged in a high-intensity sprint training protocol achieved significant reductions in body fat, despite the fact that sprint training relies almost completely on carbohydrates as a fuel source.
Some research suggests that high-intensity exercise is more beneficial for fat loss than low- and moderate-intensity exercise (3,18,23,24). Pacheco-Sanchez et al (18) found a more pronounced fat loss in rats that exercised at a high intensity as compared to rats that exercised at a low intensity, despite both groups performing an equivalent amount of work. Bryner et al (3) found a significant loss in body fat in a group that exercised at a high intensity of 80-90% of maximum heart rate, while no significant change in body fat was found in the lower intensity group which exercised at 60-70% of maximum heart rate; no significant difference in total work existed between groups. An epidemiological study (24) found that individuals who regularly engaged in high-intensity exercise had lower skinfold thicknesses and waist-to-hip ratios (WHRs) than individuals who participated in exercise of lower intensities. After a covariance analysis was performed to remove the effect of total energy expenditure on skinfolds and WHRs, a significant difference remained between people who performed high-intensity exercise and people who performed lower-intensity exercise.
Tremblay et al (23) performed the most notable study which demonstrates that high-intensity exercise, specifically intermittent, supramaximal exercise, is the most optimal for fat loss. Subjects engaged in either an endurance training (ET) program for 20 weeks or a high-intensity intermittent-training (HIIT) program for 15 weeks. The mean estimated energy cost of the ET protocol was 120.4 MJ, while the mean estimated energy cost of the HIIT protocol was 57.9 MJ. The decrease in six subcutaneous skinfolds tended to be greater in the HIIT group than the ET group, despite the dramatically lower energy cost of training. When expressed on a per MJ basis, the HIIT group's reduction in skinfolds was nine times greater than the ET group.
A number of explanations exist for the greater amounts of fat loss achieved by HIIT. First, a large body of evidence shows that high-intensity protocols, notably intermittent protocols, result in significantly greater post-exercise energy expenditure and fat utilization than low- or moderate-intensity protocols (1,4,8,14,19,21,25). Other research has found significantly elevated blood free-fatty-acid (FFA) concentrations or increased utilization of fat during recovery from resistance training (which is a form of HIIT) (16,17). Rasmussen et al (20) found higher exercise intensity resulted in greater acetyl-CoA carboxylase (ACC) inactivation, which would result in greater FFA oxidation after exercise since ACC is an inhibitor of FFA oxidation. Tremblay et al (23) found HIIT to significantly increase muscle 3-hydroxyacyl coenzyme A dehydrogenase activity (a marker of the activity of b oxidation) over ET. Finally, a number of studies have found high-intensity exercise to suppress appetite more than lower intensities (6,12,13,22) and reduce saturated fat intake (3).
Overall, the evidence suggests that HIIT is the most efficient method for achieving fat loss. However, HIIT carries a greater risk of injury and is physically and psychologically demanding (10), making low- and moderate-intensity, continuous exercise the best choice for individuals that are unmotivated or contraindicated for high-intensity exercise.
1. Bahr, R., and O.M. Sejersted. Effect of intensity of exercise on excess postexercise O2 consumption. Metabolism. 40:836-841, 1991.
2. Ballor, D.L., J.P. McCarthy, and E.J. Wilterdink. Exercise intensity does not affect the composition of diet- and exercise-induced body mass loss. Am. J. Clin. Nutr. 51:142-146, 1990.
3. Bryner, R.W., R.C. Toffle, I.H. Ullrish, and R.A. Yeater. The effects of exercise intensity on body composition, weight loss, and dietary composition in women. J. Am. Col. Nutr. 16:68-73, 1997.
4. Burleson, Jr, M.A., H.S. O'Bryant, M.H. Stone, M.A. Collins, and T. Triplett-McBride. Effect of weight training exercise and treadmill exercise on post-exercise oxygen consumption. Med. Sci. Sports Exerc. 30:518-522, 1998.
5. Coyle, E.H. Fat Metabolism During Exercise. [Online] Gatorade Sports Science Institute. http://www.gssiweb.com/references/s...020000006d.html [1999, Mar 25]
6. Dickson-Parnell, B.E., and A. Zeichner. Effects of a short-term exercise program on caloric consumption. Health Psychol. 4:437-448, 1985.
7. Gaesser, G.A., and R.G. Rich. Effects of high- and low-intensity exercise training on aerobic capacity and blood lipids. Med. Sci. Sports Exerc. 16:269-274, 1984.
8. Gillette, C.A., R.C. Bullough, and C.L. Melby. Postexercise energy expenditure in response to acute aerobic or resistive exercise. Int. J. Sports Nutr. 4:347-360, 1994.
9. Grediagin, M.A., M. Cody, J. Rupp, D. Benardot, and R. Shern. Exercise intensity does not effect body composition change in untrained, moderately overfat women. J. Am. Diet Assoc. 95:661-665, 1995.
10. Grubbs, L. The critical role of exercise in weight control. Nurse Pract. 18(4):20,22,25-26,29, 1993.
11. Hickson, R.C., W.W. Heusner, W.D. Van Huss, D.E. Jackson, D.A. Anderson, D.A. Jones, and A.T. Psaledas. Effects of Dianabol and high-intensity sprint training on body composition of rats. Med. Sci. Sports. 8:191-195, 1976.
12. Imbeault, P., S. Saint-Pierre, N. AlmÃ©ras, and A. Tremblay. Acute effects of exercise on energy intake and feeding behaviour. Br. J. Nutr. 77:511-521, 1997.
13. Katch, F.I., R. Martin, and J. Martin. Effects of exercise intensity on food consumption in the male rat. Am J. Clin. Nutr. 32:1401-1407, 1979.
14. Laforgia, J. R.T. Withers, N.J. Shipp, and C.J. Gore. Comparison of energy expenditure elevations after submaximal and supramaximal running. J. Appl. Physiol. 82:661-666, 1997.
15. Mahler, D.A., V.F. Froelicher, N.H. Miller, and T.D. York. ACSM's Guidelines for Exercise Testing and Prescription, edited by W.L. Kenney, R.H. Humphrey, and C.X. Bryant. Media, PA: Williams and Wilkins, 1995, chapt. 10, p. 218-219.
16. McMillan, J.L., M.H. Stone, J. Sartin, R. Keith, D. Marple, Lt. C. Brown, and R.D. Lewis. 20-hour physiological responses to a single weight-training session. J. Strength Cond. Res. 7(3):9-21, 1993.
17. Melby, C., C. Scholl, G. Edwards, and R. Bullough. Effect of acute resistance exercise on postexercise energy expenditure and resting metabolic rate. J. Appl. Physiol. 75:1847-1853, 1993.
18. Pacheco-Sanchez, M., and K.K Grunewald. Body fat deposition: effects of dietary fat and two exercise protocols. J. Am. Col. Nutr. 13:601-607, 1994.
19. Phelain, J.F., E. Reinke, M.A. Harris, and C.L. Melby. Postexercise energy expenditure and substrate oxidation in young women resulting from exercise bouts of different intensity. J. Am. Col. Nutr. 16:140-146, 1997.
20. Rasmussen, B.B., and W.W. Winder. Effect of exercise intensity on skeletal muscle malonyl-CoA and acetyl-CoA carboxylase. J. Appl. Physiol. 83:1104-1109, 1997.
21. Smith, J., and L. McNaughton. The effects of intensity of exercise on excess postexercise oxygen consumption and energy expenditure in moderately trained men and women. Eur. J. Appl. Physiol. 67:420-425, 1993.
22. Thompson, D.A., L.A. Wolfe, and R. Eikelboom. Acute effects of exercise intensity on appetite in young men. Med. Sci. Sports Exerc. 20:222-227, 1988.
23. Tremblay, A., J. Simoneau, and C. Bouchard. Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism. 43:814-818, 1994.
24. Tremblay, A., J. DesprÃ©s, C. Leblanc, C.L. Craig, B. Ferris, T. Stephens, and C. Bouchard. Effect of intensity of physical activity on body fatness and fat distribution. Am J. Clin. Nutr. 51:153-157, 1990.
Treuth, M.S., G.R. Hunter, and M. Williams. Effects of exercise
intensity on 24-h energy expenditure and substrate oxidation. Med. Sci.
Sports Exerc. 28:1138-1143, 1996.