I have been dabbling in the "low carb" world in recent months.
It all started with Intermittent Fasting and the Jason Fung theory that insulin resistance is the key factor that leads to obesity.
I'm not 100% convinced that he is correct - but at the same time I have been losing weight with Intermittent Fasting and reducing carbohydrate (especially refined and sugar).
Next question was - can you perform athletically without carbs?
This is where life gets difficult.
Evangelists like Timothy Noakes emphatically say "yes". There are athletes like Zack Bitter who are running Ultramarathons on High-Fat-Low-Carb diets.
Finally I found a very good summary in this article.
Adapting to Fat over Carbs
This was interesting to me, because it implies you can train your body to prefer fat longer.
Maximal fat oxidation has been reported to occur between 47 and 75% of VO2max, and varies between trained and untrained men and women [1, 5, 6]. Nonetheless, MFO has been observed to range from 0.17–1.27 g/min , where ketogenic adapted individuals can exceed ≥1.5 g/min
This is from the article, it shows the concept:
So that MFO point is not static - you can move the point to the right (ie. to higher intensities) with fat adaption.
... But There Is a Limit
Back to the article:
However, during sustained high intensity exercise (>70% VO2max) which is common during competition, CHO is the primary substrate relied upon despite short and long term fat acclimation
In other words - fat adapt all you want, if intensity gets too high, you're going to need carbs (CHO).
The Bottom Line
It seems entirely possible to perform at medium intensity on fat - but once the intensity ratchets up, you need carbs.
How I'm Applying This
Day to day, I'm using Intermittent Fasting, avoiding processed carbohydrate, and generally eating a higher fat lower carbohydrate diet. It has resulted in weight loss and I generally have felt better.
In training, though - I tinkered with fasted runs and with not supplementing with carbs.
It works OK, but since re-introducing sugar to my high intensity workouts I've felt a whole lot better. It is like rocket fuel! When I race I will certain use carbs to enhance performance.
Is that the end of fasted runs/rides? Certainly not! I will continue to use the no-carb run/ride when intensity is low, to teach my body to use fat more efficiently and not rely on "rocket fuel". I have some very long endurance events planned this year, the only way they will go well is to become a lean, mean, fat-burning machine!
All things in moderation.