Retooling is the word this research group uses for what a muscle does when the diet changes. Not a change in which fuel is available, but a change in the enzymes and transporters the fibre has built, which is what is being tested here, because the practical question is not how fast the tooling goes in but how fast it comes back out.
Drawn from the paper's discussion.
The group had already shown that three to four weeks of a ketogenic diet impaired real performance in elite athletes, and the obvious next move, the one athletes were already making in the field, was to keep the fat adaptation and add the carbohydrate back just before the race. So the study asked two things at once. How briefly can the adaptation be induced, and does acutely restoring carbohydrate restore carbohydrate oxidation and performance with it.
Drawn from the paper's introduction.
Thirteen world class male race walkers completed economy testing, a 25 kilometre training session and a 10,000 metre race at baseline with high carbohydrate availability, then repeated all of it after five to six days on either a ketogenic diet, seven athletes, under 50 grams of carbohydrate a day and 80 percent of energy as fat, or a high carbohydrate diet, six athletes, at 9.7 grams per kilogram per day.
Five to six days was enough. Fat oxidation during exercise rose by more than 200 percent in the ketogenic arm, reaching a mean of about 1.43 grams per minute, which the authors note matches the rates seen in athletes adapted for three to four weeks and for more than a year.
It cost oxygen. Relative oxygen uptake was higher by about 8 percent and 5 percent at the speeds corresponding to the 50 kilometre and 20 kilometre events, at P less than 0.0001.
Everyone then ate 24 hours of high carbohydrate plus a 2 gram per kilogram pre race meal. During the race warm up, fat oxidation fell and carbohydrate oxidation rose compared with the previous day, at P less than 0.001, but neither returned to baseline. Carbohydrate oxidation reached only 61 percent and 78 percent of the baseline values at those same two speeds.
Performance moved differently in the two groups, at P equals 0.009. Every athlete in the high carbohydrate arm improved, by 5.7 percent with a standard deviation of 5.6. Six of the seven in the ketogenic arm were slower, by 2.2 percent with a standard deviation of 3.4.
Five to six days of a high carbohydrate diet afterwards returned substrate use to baseline values. The tooling came out, it just took longer than a day.
| Adaptation period | 5 to 6 days |
| Ketogenic arm | under 50 g/day carbohydrate, 80 percent of energy as fat, n equals 7 |
| High carbohydrate arm | 9.7 g/kg/day carbohydrate, n equals 6 |
| Increase in exercise fat oxidation, ketogenic arm | more than 200 percent, mean about 1.43 g/min |
| Increase in relative oxygen uptake at 50 km and 20 km speeds | about 8 and 5 percent, P less than 0.0001 |
| Carbohydrate oxidation after 24 hours of restoration, as a share of baseline | 61 percent at the 50 km speed, 78 percent at the 20 km speed |
| Performance change, high carbohydrate arm | 5.7 percent faster, standard deviation 5.6, all six athletes improved |
| Performance change, ketogenic arm | 2.2 percent slower, standard deviation 3.4, six of seven athletes slower |
| Between group difference in performance change | P equals 0.009 |
| Time to return substrate use to baseline | 5 to 6 days of high carbohydrate eating |
Proposed by the authors This is the explanation the authors offer in their discussion. This study did not test it.
The authors say plainly that studies of ketogenic diets have not directly investigated the mechanism behind the increased fat use, and that it is likely to be the same up regulation of fat availability, mobilisation, transport and mitochondrial uptake documented in the earlier biopsy work on non ketogenic high fat diets.
For the half that matters practically, their own summary is that the retooling is maintained in the face of acute increases in carbohydrate availability, and is reversed by five days of a chronic high carbohydrate diet. The tooling is not sensitive to the last meal. They connect the blunted carbohydrate oxidation to the earlier finding of lower pyruvate dehydrogenase activity, and read the failure to restore performance as limited by that blunting rather than by a shortage of fuel.
Drawn from Discussion, PMC7891450.
The timescale is now asymmetric in a way that matters. Adaptation is fast, under a week, and de adaptation is slower, five to six days, which means the popular strategy of adapting to fat and topping up carbohydrate the day before a race is working against the clock rather than with it. This confirms in a ketogenic diet what the earlier non ketogenic work had already found.
Thirteen world class male race walkers over five to six days of diet and a 10,000 metre race, with 24 hours of carbohydrate restoration.
The same design with restoration periods of two, three and five days, which would locate the point at which carbohydrate oxidation actually returns.
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