13. FATS AND CARBS UTILIZATION AT DIFFERENT EXERCISE INTENSITY

Photo by Pruthvi Sagar A R on Unsplash

In a previous post we saw that, while even in lean people fats are a virtually unlimited resource, carbs are not, and that durability (i.e. fatigue resistance) is linked in in part to muscle glycogen (carbs) sparing.

So, it can be important to understand how different fuels (carbs vs fats) are used at different exercise intensity.

How much fats and carbohydrates contribute to energy production at different exercise intensity?

An old but gold ‘pure physiology’ study published by Romijn and Colleagues on American Journal of Physiology Endocrinology and Metabolism in 1993 tried to answer this question. (1)

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WHAT DID THEY DO?

• Five well-trained male cyclists (~ 24 years, ~ 67 VO2max) were recruited.

• On different days, after 10-12hrs fasting, they performed:

  • 2 hrs at 25% of VO2max (recovery intensity, read zone 1)

  • 2 hrs at 65% of VO2max (read high zone 2, Endurance Intensity in a 5/7zones model)

  • 30 min at 85% of VO2max (read around the second threshold/critical power/FTP, zone 4 in a 5/7 zones model)

• Use of carbohydrates (muscle glycogen and plasma glucose) and fats (plasma free fatty acids and intramuscular triglycerides) were calculated.

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WHAT DID THEY FOUND?