Keto alters the fuel use of your body changing it from a primarily carb burning machine (glucose) to a fat burning one. It will change the way you process energy and hamper explosive performance. Energy supply is a chemical process and it takes time to convert stored energy into ATP (adenosine triphosphate) for muscle contraction. Carbs are much closer to being ATP than most fats or proteins. The body will struggle to produce glycogen using gluconeogenesis at the rate you would need to be competitive at a real level long term. Naturally your body chooses carbs over all else to create energy. Glucose is the main respiratory substrate, which is immediately available from carbohydrates. More carbs > more glycogen (broken down to glucose) > more strength and energy for explosive/anaerobic training. For athletes and people who work out intensely for hours at a time daily fast-burning glucose as well as fructose is not only hugely recommended but even necessary to fill their fuel tank. This applies to recovery as well. Your body converts carbohydrates into glycogen which it then stores in the liver and muscles. Exercise burns up the glycogen stores in your muscles and liver and these stores will normally be replenished within 24 hours after exercise (keto adapted folks can restore their glycogen during that time period as well, but from other sources, take gluconeogenesis for example), however many times you won’t have 24 hours in between rounds of exercise, that’s when it comes in handy to eat carbohydrates to restore your glycogen level fast for your next exercise period. As outlined in this study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3905295/) because glycogen helps muscles recover and avoid cannibalizing themselves for fuel after high intensity exercise, replenishing glycogen can preserve muscles and accelerate recovery. Topping off glycogen stores will help you get ready for another run sooner. According to this paper (http://europepmc.org/abstract/med/9662687) chronically low glycogen stores in athletes can cause fatigue and even induce a catabolic state involving muscle breakdown by requiring the body to rely on proteins and amino acids for fuel. With low carb diets and serious training, as nutrition coach Robert Santana says, ‘’Things start off prety well, but quickly go to hell.’’ That’s because the first week you might still have plenty of stored glycogen in your muscles to get you through those workouts. High intensity anaerobic/explosive training is even more glycogen depleting than weight lifting and it’s crucial to ensure your muscle cells are fully stocked with glycogen that can be turned to ATP in an intense bout, just two 30 second sprints can deplete your glycogen stores by nearly half.
Now the common claims that performance will improve when you become ‘’fat-adapted’’. The tests that were done on Phinney’s cyclists showed that they were indeed thoroughly fat-adapted. So that wasn’t the issue. But their sprinting ability still suffered; in fact, the cyclists who were the most keto adapted were the ones whose sprinting performance deteriorated the most. No matter how fat adapted you get, anaerobic activities fueled by fat don’t have as much power as those fueled by carbs. Yes, you can run a long time, at a moderate pace, without refueling, but when you need to sprint, you’re going to feel absolutely empty. “Top Fuel” drag racers are the fastest accelerating racing vehicles in the world, but they don’t fuel up at the local 7-11. That’s because the gasoline you put in your Honda is like eating fat, and drag race fuel is like carbs.
It has to do with both burn rate and energy density. Gasoline has more available energy per liter than Top Fuel, just as fat has more energy than carbs (9 calories per gram vs. 4). More importantly, both gasoline and fat burn at a much slower rate than Top Fuel and carbohydrates do.
“The amount of energy the body can extract from carbs is much greater per unit of time than the energy it can extract from fat,” nutrition expert Alan Aragon, co-author of The Lean Muscle Diet, says. “When carbs are oxidized, they yield two-to-five times more ATP (energy) than fat.”
Aragon explained that when using fat for fuel, “You can’t access that energy as quickly. With fat, you have a bigger pool of energy, but you can only drain it with a straw. With carbs, the pool is smaller but you can drain it with a firehose.”
At low and moderate intensities, this doesn’t matter much. If you’re always at cruising speed and the straw provides enough fuel, then that’s okay. Second, while fat-adaption has some advantages for aerobic exercise, it also comes with trade-offs that negate these benefits, as well explained by pro cycling coaches Chris Carmichael and Jim Rutberg: “The HFLC [High Fat Low Carb] strategy has been shown to increase the utilization of fat for energy, especially in long-term (20 months) fat-adapted athletes (Volek, 2015). However, the oxygen cost of locomotion increases while exercising on a HFLC strategy (Burke, 2016). It takes approximately 20% more oxygen to liberate energy from fat compared to carbohydrate, which means relying primarily on fat reduces economy. This isn’t necessarily a problem, since you have a large supply of energy to burn, but these findings don’t indicate an IMPROVEMENT in endurance performance.”
Finally, it can take several months to become fat adapted, and during that time you will only make progress in becoming fat adapted and not in getting faster/stronger. You have to be able to weather that “pause” in your progress. I’m not an elite athlete by any means, but the idea that I’d have to experience several months, sometimes even up to half a year or more, of poor performance to become fat adapted is a hard pill to swallow. It’s a big commitment — both in time and in how strict you have to be in your diet, indefinitely, to stay in ketosis — with an unclear payoff. Following a LCHF diet will indeed increase your ability to utilize fat at any given exercise intensity. However, this also comes with a decreased ability to utilize carbohydrates during exercise, leaving an athlete with a reduced capacity to perform higher-intensity exercise, as Don Heatrick says as well, you are just becoming a more effective fat burning machine but at the expense of being able to utilize carbohydrates as effectively as well.
Carbohydrates obviously have other benefits besides for anaerobic exercise, including muscle recovery, testosterone production, and the fact that the healthiest foods on the planet generally contain carbs, but that’s another topic. Ketogenic diets can be great if you are sedentary and looking to lose some weight in the short term, but it is not an optimal diet for healthy people seeking optimal athletic performance and muscle gain, and problably also not healthy in the long term. Certain letogenic diet advocates themselves push a ”Targeted Keto Diet” when it comes to sports performance, where you consume carbohydrates in targeted amounts before high intensity exercise which you then burn up.
High-Quality Carbohydrates and Physical Performance : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5794245/
Carbs, not fats, boost half-marathon race performance : https://www.sciencedaily.com/releases/2015/12/151215094542.htm
Effects of acute carbohydrate ingestion on anaerobic exercise performance : https://jissn.biomedcentral.com/articles/10.1186/s12970-016-0152-9
Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance : https://www.ncbi.nlm.nih.gov/pubmed/16141377
Enhanced Endurance Performance by Periodization of Carbohydrate Intake : https://www.ncbi.nlm.nih.gov/pubmed/26741119
Keto advocate Steve Phinney in his study ketogenic diets and physical performance : “sprint capability … remained constrained during the period of carbohydrate restriction.” : https://nutritionandmetabolism.biomedcentral.com/articles/10.1186/1743-7075-1-2
Journal of Strength and Conditioning Research, 2003: “it may be advisable for athletes who are performing high-volume resistance training to ingest carbohydrate supplements before, during, and immediately after resistance training.”
International Journal of Sport Nutrition and Exercise Metabolism, 2006: Study compared high protein vs. high carb on cycling performance and asserted that for high protein: “Performance was significantly impaired.”
Journal of Sports Sciences, 2011: “During sustained high-intensity sports lasting ~1 h, small amounts of carbohydrate, including even mouth-rinsing, enhance performance via central nervous system effects.”
Applied Physiology, Nutrition and Metabolism, 2013: “a low-CHO (carbohydrate) diet reduces both performance and total aerobic energy provision during supramaximal (very high intensity) exercise.”
Sports Medicine, 2013: “small amounts of carbohydrate ingestion during exercise may also enhance the performance of shorter (45-60 min), more intense (>75 % peak oxygen uptake; VO(₂peak)) exercise bouts, despite the fact that endogenous carbohydrate stores are unlikely to be limiting. The mechanism(s) responsible for such ergogenic properties of carbohydrate ingestion during short, more intense exercise bouts has been suggested to reside in the central nervous system.” This means that simply tasting carbs is enough to get your brain to open the already existing energy stores in the bloodstream, enhancing performance.
Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men: a randomized-sequence crossover trial : https://www.ncbi.nlm.nih.gov/pubmed/29619799