Yet another study showing low carb doesn't impair performance +
and by some metrics, at least in this study, might even improve it.

Ketoadaptation enhances exercise performance and body composition responses to training in endurance athletes (McSwiney et al., 2017)

Advantage of this study over previous ones: 12 weeks. I believe the choice to opt for self-selection over randomization was to improve adherence (which was pretty good for this 12 week-long study). Downside is, well, it's not randomized. Crossover RCT is best but it's always a trade-off: sample size, duration, tools, etc., everything has a price. Literally.

The study: we aren't told much about the diets, just high carb vs. ketogenic. And keto group was advised to drink broths for salts, mins, electrolytes, etc.* 

*I don't think this qualifies as cheating in this #context.

Before and after the 12-week dietary intervention, a battery of tests were performed: a six second all-out bicycle sprint (SS), immediately followed by a 100 km time trial (TT), immediately followed by a 3-minute sprint (CPT).

These were well-trained, healthy individuals who continued their training throughout the study. This & duration are two important nuances of this study (more on this below).

The biggest finding ...*drumroll* ... significantly greater fat loss in the keto group and this wasn't even a weight loss study. They also jacked up protein intake so they didn't lose muscle mass. Protein declined in the high carb group, but they were able to maintain muscle because carbs increased.



Whether they knew it or not, this study was designed to test peak power output before (SS) and after (CPT) exhaustively draining the tank (TT). The theory is that ketoadaptation: 1) spares glycogen so there's some juice left in the tank for the second peak power test, although racing 100 km is pretty tough so there couldn't have been much juice left in either group; and 2) ketoadaptation relies more on fatty acids at every level of output, as evidenced by the RER figure (below). Fuel usage comes close at high levels of output (both groups rely more heavily on glucose), but ketoadapted is always a little lower (eg, see the right-most point in the figure below). And fat stores are basically limitless whereas glycogen is not. This may or may not have been a factor here.


I don't know why the authors reported peak power relative to body weight. I could understand lean mass, maybe, but keto lost a lot of weight via body fat. If peak power remained the same (as has previously been shown), it would [falsely] appear to increase in this study.

They didn't have to do that. This is why: peak power output is gonna plummet after a 100 km time trial, in ANY group on ANY diet. In the #context of competition, absolute peak performance is what wins the race. That said, peak performance efficiency, eg, per kg lean mass, might have been interesting.

Time trial didn't change in either group, before or after the intervention. I don't care. The point of this part of the experiment, imo, was the impact of "draining the tank" on peak performance.

In the following maths, I'm comparing peak power output before the time trial (SS) vs. after (CPT). Obviously it goes down before and after the diet intervention in both groups, because a 100 km bike race is just gonna do that to you. Expectation: it stays the same in the high carb group because they didn't really make any changes in their diet/training. If ketoadaptation reduces the ability to perform at high intensity, then their decline in peak power output after the time trial should be greater than before.

In absolute terms, peak power output after the time trial in the high carb group declined a bit more at the end of the study (-34% at baseline vs. -39% at week 12). This alone is not very meaningful. 

However, interestingly, and in agreement with the abovementioned theory about ketoadaptation, the drop in peak power output was actually less after the diet intervention (-39% vs. -33%). This means their tank was a little larger, imo, due to increased quality &/or quantity of mitochondria and fat oxidation capacity. 

The effect sizes were medium-to-large although I have no idea if they reached statistical significance. But it was pretty cool amirite guys

I'm not advising trainers to put their athletes on ketogenic diets. I mean, they can, but the athlete needs to know their training is going to suffer for about 3 weeks and most studies show no difference in performance after ketoadaptation (here's the best proof of this to date. Seriously, read this. It's not the best study design, but the only one to address it directly.).

I would be happy to modify that statement if this study had been randomized. And maybe a little tighter control over protein intake (and crossover after an appropriate washout period sorrynotsorry). 

But the main conclusion: this study addressed something very important; something many trainers have been saying for a long time. Ketoadaptation doesn't diminish performance at high intensity even after "draining the tank."

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