Holmer, training low on carbs doubled fat burning, cost 7% power

Brady Holmer says restricting carbohydrate after a glycogen-depleting workout nearly doubled peak fat oxidation (how much fat you burn per minute) during a workout the next day.

It also shifted Fatmax, the intensity at which fat burning peaks, from 41% to 56% of VO2 peak. In plain terms, the body kept leaning on fat at harder efforts instead of switching over to carbs as early.

Then comes the bill. Holmer says the same low-carb approach brought "a ~7% drop in maximal power output at a higher perceived effort." So the work felt harder and the top end was lower.

Brady Holmer

@Brady_H

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a ~7% drop in maximal power output at a higher perceived effort

The part that undercuts the theory

The usual argument for training low is that it forces adaptations inside the muscle, more mitochondria, better machinery, more efficient energy production. Holmer says that did not show up. Low-glycogen training did not enhance markers of mitochondrial efficiency or energy production.

That is the whole story in one line. You get a bigger fat-burning number and you pay for it with power, and the adaptation that was supposed to justify the trade did not appear in the markers.

What it actually means

Burning more fat per minute is a change in fuel mix, not a change in speed. A runner who shifts Fatmax from 41% to 56% of VO2 peak has not become faster, they have become someone whose body reaches for a different fuel at a given effort. Marathon pace is still run on carbs, and race day is still fuelled with carbs.

If you are already doing an easy run before breakfast now and then, nothing here says stop. What this does push back on is the harder version of the idea, deliberately keeping carbs low after a depleting session so the next day's work lands on empty tanks. In Holmer's summary that version cost about 7% of maximal power, felt worse, and gave nothing back in the efficiency markers.

Holmer has been posting a run of fuelling takes lately, including his line that a carb load is a ratio change, not more food.