As I pivot back hard from “coaches brain” to “science brain” during my dissertation writing push I want to share some of my favorite “historical” figures from my field looking at the relationships between exercise intensity & substrate (carb & fat) metabolism.
The first may be my favorite figure — if that’s a thing. Simply capturing the transition of exercise expenditure, peaking then decreasing of fat oxidation & increasing of carbohydrate oxidation with intensity. Really capturing the RELATIVE contribution of each.
The second shows similar things but was the same study design followed up in women.
There is a lot of miss information on here around what exercise is “fat blasting” or “fat burning zones”. The later reflecting to a greater % of fat being used at lower intensities — but when looking at it relatively absolute fat oxidation will increase & peak with intensity. We call this a fat max— falling ~65-70% of one’s VO2 max.
Along with the obsession of doing HIIT but restricting carb, when they contribution to energy metabolism with intensity drastically moves toward carbs!
This isn’t to say you shouldn’t do high intensity exercise for fat burning benefits — as total energy expenditure IS GREATER & there are adaptations in response to this that also increase our ability to burn fat.
If anything we should do both! 😉— & also most HIIT on here isn’t HIIT, or fat blasting. But that’s another rant for another day.
I will leave with this as well. While there are some studies with noted differences in menstrual cycle phase (& some not), women don’t appear to have a different fat max across cycles. And this difference tends to go away with 1) intensity or 2) pre exercise feeding. The emphasis for carbohydrates in cycling trainees is important! A lot of any individually experienced effects can possibly be reduced through diet to work with these possible hormonal impacts on metabolism. (I have other posts on this.)
Anyway. I felt like getting nerdy. If this is over your head what I’m saying is fat blasting HIIT is a scam, fat burning zone isn’t what they say it is — exercise at multiple intensities. And all people who do higher intensity work need carbs!