Creating a sustained increase in the quantity, and use, of cellular machinery for burning fat as fuel during exercise, a process known as fat adaptation, is as promising as any in sport science. Being able to shift your body’s metabolism to burning fat (which it stores more of) instead of relying on the intake of carbs could fundamentally change how we train and compete…if it works. Let’s take a look at some of the science on fat adaptation so far.
Fat adaptation happens and happens quickly. It happens based on the magnitude you decrease carb consumption, not because you increase fat consumption. You can eat a high-fat diet and train like a boss, but if you are still consuming loads of carbs, you will not cause one iota of fat adaptation.
“Anti-carb” adaptation happens, and maybe even faster. The requirement of reduced carbs to cause fat adaptation also causes reduced enzymes for burning carbs during exercise at the same or greater magnitude and rate that it causes increased fat utilization. As much as you become better at burning fat, you become worse at burning carbs.
Performance at or above lactate threshold is hurt most. Discussion of fat adaptation should be entertained when race intensity is well below lactate threshold entirely—i.e. races lasting longer than one hour that are “steady state,” preferably much longer, without many hills and no sprinting.
Getting fat-adapted requires lower training intensities. Consuming a low-carb diet requires training intensity to reduce, or your body will reduce it for you. Even at lower absolute intensity, fatigue mounts quickly, both acutely and chronically, because you are functioning at higher relative percentages of maximum ability with a low-carb diet.
When you carb up again, fat-adaptation is quickly lost. Fat adaptation is highly transient. When you start “carbing up” again, you lose the adaptation, regardless of training volume and intensity.
The combo of these points means that you either have to be training with reduced intensity and reduced carb-burning machinery in the race prep, or you have to erase your fat adaptation to reap the full benefits of higher carb feeding leading into your race.
The biggest point: Fat-adapted athletes do not perform better than chronic high-carb athletes when there are not glaring confounding variables! This is the only point a non-scientist should consider when examining the mountains of research.
Do fat-adapted athletes race faster or perform better in endurance sport when other factors are held equal? Unfortunately, no. There are no studies where fat-adapted athletes raced faster than was readily explained by weight loss that also happened to occur in their subject group. In studies where both groups (high-carb vs. fat-adapted/low-carb) lose the same amount of weight, there have been no performance differences between groups. Except, of course, when high-carb groups perform slightly better, occasionally in longer-distance tests, and virtually always in shorter-distance tests.
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When fat-adapted athletes race chronic high-carb athletes in carb-restricted race scenarios, they do occasionally perform better, especially in cases of extreme carb-restriction. If you plan to do a race where you will encounter prolonged periods of carb-restriction, then fat-adaptation may be useful in preparation for that. Remember, preparing for that scenario comes with the tradeoff of training with lower quality in the lead-up to the race to ensure that you’re still carrying any fat-adaptation with you by the time you hit the start line. An example of this scenario would be any event where you simply can not feed yourself because the race is long and you have no access to nutrition, or it’s so hectic that you can’t possibly consume anything. Further, to choose a fat-adaptation approach, the trade-offs of prepping for that scenario by a fat-adaptation diet and the associated reduced training intensity must be outweighed by the potential benefit of being able to stave off hypoglycemia in that race.
There are no sporting scenarios where fat-adaptation is better than chronic high-carb diet. None. When would I recommend fat adaptation as a portion of the training/nutrition process? (While still using high-carb fueling on race day.) A perfectly flat 70.3 or 140.6 where there may be such intense crosswind that you cannot safely drink or eat or gain access to more carbs for more than 60 minutes at a time, or perhaps some very long yet mild obstacle course race where it’s for some reason highly impractical to carry carbs or impossible to consume them due to the constant challenging terrain that requires use of hands and prohibits wearing a pack with hose to your mouth. Read: I can not think of anything resembling reality!
As a sport scientist, triathlon coach, and nutrition consultant, I still hope to find out that I’m wrong and that there is more opportunity here, because if there is something I am not doing to make my athletes faster, I want to know about it ASAP. Plus, having done a bit of fat-adaptation training just to see what it feels like, I can say it does make you feel tough as nails.
Dr. Alex Harrison, a certified USA Triathlon coach, holds a PhD in Sport Physiology and Performance. He is the author of The RP Diet for Endurance and more than a dozen articles. When he isn’t pumping out training and nutrition plans in his RV-garage-turned-mobile-office, he can be found on his bike, clinging for dear life to his wife’s wheel.