Research: Does the “fat burning zone” allow for maximal fat oxidation?
Individuals are often advised to stay within particular heart rate zones, in an attempt to achieve maximal fat oxidation (Fatmax), commonly referred to as the “fat burning zone”. The “target heart rate” graph found frequently throughout gyms, places the “fat burning zone” between 50-65% of max heart rate (HR) (HMPC, 2013). However, conclusive research in this area is sparse. Steffan, Elliott et al (1999) found total fat oxidation was greater during low- (LI) compared to high-intensity exercise (HI). Though, the study states that this cannot be used to predict substrate utilization as the data was from a maximal graded exercise test.
Wolfe (1998) states that during LI exercise, glycolysis is not markedly stimulated, so the increased availability of fatty acids allows their oxidation to serve as the predominant energy source. However, it is argued that although LI does derive the greatest percentage of calories from fat, total calorie expenditure is significantly less than HI intensity, meaning HI utilizes a greater amount of fat substrate (DuVall, 2013).
When confronted with an obesity epidemic with approximately one billion adults being overweight and 475 million obese (Obesity International Obesity Taskforce, 2010), the demand for reliable data is significant. Conclusive evidence will allow for more efficient fat loss programs to be constructed. The aim of this study was to determine if low intensity is optimal for maximal fat utilization, if not, what intensity is.
(H0): Low intensity exercise is not optimal for maximal fat substrate utilization.
(H1): Low intensity exercise is optimal for maximal fat substrate utilization.
23 Sport and Exercise Science students (16 male, 7 female) participated in this study (aged 18.6 ± 0.4years). Mean stature 177.91cm ± 7.17 and mean mass 72.21kg ± 6.55. Participants were selected by convenience sampling. This study abides by the University of Exeter’s ethical guidelines, written informed consent was obtained from all participants prior to testing.
Participants completed three different cycle intensities in chronological order (LI-HI). Participant performed all testing within 2 hours, completing three intensities. Testing was completed within a week.
Stature (stadiometer) and mass (scales) were taken and the polar heart rate strap (Polar Electro, Finland) was placed on the participant prior to testing. The Hans-Rudolph Douglas bag system (Hans-Rudolph Inc., USA) was vacuumed before testing using Harvard dry gas meter (Harvard Apparatus, UK).
Participants cycled using a Monark 827e friction-braked cycle ergometer (Monark Exercise AB, Vansbro, Sweden) for 4 minutes, maintaining 80 revs per minute, weight placed on ergometer determined the intensity for participant. Throughout the last minute of each set of cycling, participants wore a nose clip and a mouthpiece, connected to Hans-Rudolph Douglas bag system (Hans-Rudolph Inc., USA). Once complete, participants HR was recorded, Douglas bag was closed, and participant rested for 5 minutes before the next intensity. Douglas bag was connected to Servomex 570A oxygen analyser (Servomex, UK). Expired air, O2 and CO2 were recorded when values stable. The Douglas bag was opened and vacuumed before the procedure was repeated for all intensities.
Data Analysis Techniques
VO2 and VCO2 (Appendix 1,2 and 3) were calculated by the volume of air in the Douglas bag after 60 seconds (VE), as well as the percentage of CO2 (FECO2) and O2 (FEO2) in the Douglas bag conveyed as a fraction. These VO2 and VCO2 values were then used to calculate respiratory exchange ratio (Appendix 4). Finally, the respiratory exchange ratio table (Lusk, 1924) was used to find the percentage of energy derived from fat.
IBM SPSS Statistics 20 was used to perform the following statistical analysis – bivariate correlation, scatter graph, histogram and means. Significance level was set at p < 0.5.
Fat substrate utilization and kilojoule expenditure
Mean fat use (g.min-1) was greatest at moderate intensity (MI) (.485g.min-1 ± .275, 72% of max HR), smallest at LI (.429g.min-1 ± = .395, 55% of max HR), HI was .432g.min-1 ± = .503, 88% of max HR. LI exercise utilized the highest fat percentage of kilojoules (kJ) (42.15%), MI (26.94%) and HI (21.54%). As intensity increased kJ.min-1 increased (see fig. 1)
Figure 1: Relationship between mean kJ.min-1 and exercise intensities
Bivariate analysis showed no significant correlation between HR and g.min-1 (LI p < -.172, MI p < -.319 and HI p < -.198) or HR and kJ.min-1 (LI p < .098, MI p < .052 and HI p < .762). Fig.2 demonstrates little systematic tendency.
Figure 2: Correlation between maximal heart rate percentages and percentage fat substrate utilization
The aim of the study was to find out whether the “fat burning zone” allows for Fatmax. Results found LI did utilize the highest % fat use. However, LI exercise actually utilized the lowest g.min-1, compared to HI, and MI which achieved Fatmax. Therefore, LI kJ.min-1 is significantly lower than MI and HI. Therefore, the null hypothesis can be rejected.
Literature to support the results that Fatmax occurs at around MI is present (Valizadeh et al, 2011) (Hansen, 2005); Anchten et al (2002) found maximal fat oxidation occurred at 74 ± 3%HRmax. Although, supporting studies had a similar set of participants (young, fit and healthy), Steffan (1999) found there was no difference in substrate use between sedentary obese and normal-weight woman, consequently making this study applicable to a wide range of individuals. This study highlights the importance of g.min-1 in contrast to focusing on the % fat use (Wolfe, 1998). This is important as fat use % does not necessarily lead to Fatmax, the sum-total of fat oxidation should be the priority when designing future studies.
These findings could guide individuals towards MI, which also requires more kJ.min-1 than LI. This could lead to a greater calorie deficit, and therefore further weight loss (Jennings, 2012). However, total kJ.min-1 expenditure could be questioned as LI exercise could be maintained for a longer, which could also lead to further fat oxidation. Achten (2004) states that the mode of exercise can also affect fat oxidation, with fat oxidation being higher during running than cycling.
In conclusion, the aim of this study was to determine if LI is optimal for Fatmax, it was found not to be, with MI achieving Fatmax. Individuals looking for Fatmax could be guided towards MI instead of LI. However, more research needs to be done in this area. Future studies should look into the use of different exercise activities on Fatmax as well as the combination of intensity and duration of exercise on Fatmax.
- VO2 = (VESTPD x (0.2093 – FEO2)
- VESTPD = VEATPS x Barometric pressure mmHg – H2O vapour pressure.273
760.(273 + ToC)
- VCO2 = (VESTPD x (FECO2 – 0.0003)
- Volume of carbon dioxide produced
Volume of oxygen consumed
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