Editing Exercise intensity (section)

==Fuel Used==
The body uses different amounts of energy substrates ([[carbohydrates]] or [[fat]]s) depending on the intensity of the exercise and the [[VO2 max|VO2 Max]] of the exerciser. [[Protein]] is a third energy substrate, but it contributes minimally (around 3% of the total energy expenditure during exercise<ref>Clauss, M., & Jensen, J. (2025). Effect of exercise intensity, duration, and volume on protein oxidation during endurance exercise in humans: A systematic review with meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 35(4): e70038. https://doi.org/10.1111/sms.70038</ref>) and is therefore discounted in the percent contribution graphs reflecting different intensities of exercise. The fuel provided by the body dictates an individual's capacity to increase the intensity level of a given activity. In other words, the intensity level of an activity determines the order of fuel recruitment. Specifically, exercise physiology dictates that low intensity, long duration exercise provides a larger percentage of fat contribution in the calories burned because the body does not need to quickly and efficiently produce energy (i.e., [[adenosine triphosphate]]) to maintain the activity. On the other hand, high intensity activity utilizes a larger percentage of carbohydrates in the calories expended because its quick production of energy makes it the preferred energy substrate for high intensity exercise. High intensity activity also yields a higher total caloric expenditure.<ref name="vehrs"/>

VO2 max acts as a key determinant of fuel usage during exercise. Higher [[VO2 max|VO2 Max]] individuals can sustain higher intensities in the "fat-burning zone" before shifting to carbohydrates, enhancing their endurance and efficiency.

This table outlines the estimated distribution of energy consumption at different percentages of [[VO2 max|VO2 Max]].<ref>{{Cite web |date=2019-10-29 |title=Calories Burned Running Calculator |url=https://caloriesburnedhq.com/calories-burned-running/ |access-date=2024-01-20 |language=en-US}}</ref>

{| class="wikitable" style="text-align: center"
! Intensity (% of VO<sub>2</sub> Max)
! % Fat
! % Carbohydrate
!Fuel Usage
|-
| 25
| 85
| 15
|Most energy from fatty acids.
|-
| 65
| 50
| 50
|Equal contribution from fatty acids, and carbohydrates.
|-
| 85
| 40
| 60
|Decreased fatty acid usage, high reliance on carbohydrates.
|}

These estimates are valid only when glycogen reserves are able to cover the energy needs. If a person depletes their glycogen reserves after a long workout (a phenomenon known as "[[hitting the wall]]"), the body will use mostly fat for energy (known as "[[second wind]]"). [[Ketone bodies|Ketones]], produced by the liver, will slowly buildup in concentration in the blood, the longer that the person's glycogen reserves have been depleted, typically due to starvation or a low carb diet (βHB 3 - 5 mM). Prolonged aerobic exercise, where individuals "hit the wall" can create post-exercise ketosis; however, the level of ketones produced are smaller (βHB 0.3 - 2 mM).<ref>{{Cite journal |last1=Koeslag |first1=J. H. |last2=Noakes |first2=T. D. |last3=Sloan |first3=A. W. |date=April 1980 |title=Post-exercise ketosis |journal=The Journal of Physiology |volume=301 |pages=79–90 |doi=10.1113/jphysiol.1980.sp013190 |issn=0022-3751 |pmc=1279383 |pmid=6997456}}</ref><ref>{{Cite journal |last1=Evans |first1=Mark |last2=Cogan |first2=Karl E. |last3=Egan |first3=Brendan |date=2017-05-01 |title=Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation |journal=The Journal of Physiology |volume=595 |issue=9 |pages=2857–2871 |doi=10.1113/JP273185 |issn=1469-7793 |pmc=5407977 |pmid=27861911}}</ref>
{| class="wikitable"
|+'''Exercise intensity (%W'''<sub>max</sub>''') and substrate use in skeletal muscle during aerobic activity (cycling)'''<ref>{{Cite journal |last1=van Loon |first1=L. J. |last2=Greenhaff |first2=P. L. |last3=Constantin-Teodosiu |first3=D. |last4=Saris |first4=W. H. |last5=Wagenmakers |first5=A. J. |date=2001-10-01 |title=The effects of increasing exercise intensity on muscle fuel utilisation in humans |journal=The Journal of Physiology |volume=536 |issue=Pt 1 |pages=295–304 |doi=10.1111/j.1469-7793.2001.00295.x |issn=0022-3751 |pmc=2278845 |pmid=11579177}}</ref>
| colspan="2" rowspan="2" |
! colspan="4" |'''Exercise intensity (W'''<sub>Max</sub>''')'''
|-
!'''At rest'''
!'''40%W'''<sub>max</sub>
Very low-intensity
!'''55%W'''<sub>max</sub>
Low-intensity
!'''75%W'''<sub>max</sub>
Moderate-intensity
|-
! rowspan="5" |'''Percent of substrate''' 
'''contribution to total energy expenditure'''
!'''Plasma glucose'''
|44%
|10%
|13%
|18%
|-
!'''Muscle glycogen'''
| -
|35%
|38%
|58%
|-
!'''Plasma free fatty acids'''
|56%
|31%
|25%
|15%
|-
!'''Other fat sources'''
'''(intramuscular and''' '''lipoprotein-derived triglycerides)'''
| -
|24%
|24%
|9%
|-
!'''Total'''
|100%
|100%
|100%
|100%
|-
! colspan="2" |'''Total energy expenditure (kJ min'''<sup>−1</sup>''')'''
|10
|50
|65
|85
|}