Yes, though I'm relatively new to running and have never done a full marathon. Here's what I've found:

For decades, the belief was always that nobody can ever possibly oxidize more than 1g/min. of fat - at the very extreme highest that would be completely unrealistic for nearly everyone anyway... so usually much lower. For this reason, it was believed that endurance athletes must constantly pound carbs in order to have access to energy needed to keep going. A whole industry of sports drinks, energy bars, Gu packs, etc. was built around this idea.

In 2009, a study was released where an entirely different dynamic was reviewed. Up until that point, nobody had ever measured "fat adapted" athletes. To do so first requires subjects to, ya know, become fat adapted. You can't measure someone who just ate low carb for the last day or 2 and get the same results. It takes weeks to become fat adapted. When using fat adapted athletes as a sample group, fat oxidation was measured as high as 1.8g/min., but median came in around 1.4g/min. of fat oxidation (IIRC - need to go back and find that study).

In short: It is possible, but you can't just do it tomorrow. It takes time to become fat adapted through sufficiently low carb intake for weeks. Don't try what some do - eat very low carb during the week and then load up on carbs over the weekend.

There are other things related to low carb intake that are important, particularly electrolytes. This is even more true for active low carbers. Electrolyte imbalance is often referred to as "keto flu." Cures are pickle juice and getting enough sodium, magnesium, and potassium (to a lesser extent, as sufficient sodium and magnesium will prevent potassium leach). You may find that you need to add a lot of salt to foods or take supplements. If you are not a member of the Low Carber Daily group here, that is a good resource for low carb related things such as keto flu issues, food ideas, etc.

I have done this successfully. There are really important things to keep in mind. First off, if you are already low carb, then you don't really need time to adjust. If you are just getting into low carb, you will need a gradual adjustment period. Now here is the tricky part. Your body needs glycogen. If you run out, you will hit the wall and your run will be over. Therefore, you need to make sure you are still getting enough carbs. Fortunately, a low carb lifestyle typically will get your enough carbs in most cases. Make sure your fuel for your long runs have extra carbs though. If you are shooting for low-to-no-carb, then this is dangerous and you want to work with a trained nutritionist, as there is a lot of timing to it all.

Also, keep in mind that with low carb you will be eating away A LOT of muscle. Make sure you are doing strength training and getting a good amount of protein. About 1-2 weeks before your race, usually just 1 week, make sure you are consuming a little more carbs than usual everyday. You don't need to carb load, but you need to make sure your glycogen will be maxed for your run. And because you were on low carb before, it will take a little bit to acclimate your system back to those carbs.

On a really good note though, this makes your body extremely efficient at glycogen use.

silverfiend wrote: »

daj150 wrote: »

.... Your body needs glycogen. If you run out, you will hit the wall and your run will be over. Therefore, you need to make sure you are still getting enough carbs. Fortunately, a low carb lifestyle typically will get your enough carbs in most cases. Make sure your fuel for your long runs have extra carbs though.

This is half true. If your body is still in the carb burning mode, you will run out of fuel and hit the proverbial wall. Once you have been low carb, especially if you are keto, long enough then your body becomes fat-adapted and converting fat to sugar is it's natural process. You don't have a "wall" to hit here. Your body can make the conversion just fine while you run. The very small amount of actual blood-sugar that your body truly needs, it is more than capable of creating on its own from the protein in your diet.

daj150 wrote: »

If you are shooting for low-to-no-carb, then this is dangerous and you want to work with a trained nutritionist, as there is a lot of timing to it all.

Absolutely false! first, there is no such thing as a zero carb diet. even if all you ate was meat, eggs, cream and cheese you would still be getting some carbs. A LCHF diet call for low or very low (never zero) carbs, high fat, and medium protein. There is zero danger to exercising, lifts, swimming, cycling or running on a LCHF diet.

daj150 wrote: »

Also, keep in mind that with low carb you will be eating away A LOT of muscle.

Another complete falsehood. LCHF has a higher percentage of protein and any "standard" diet. You can always modify to low carb / medium fat / higher protein when you are doing a lot of lifting and other muscle building routines.

daj150 wrote: »

Make sure you are doing strength training and getting a good amount of protein.

very good advice!

daj150 wrote: »

About 1-2 weeks before your race, usually just 1 week, make sure you are consuming a little more carbs than usual everyday. You don't need to carb load, but you need to make sure your glycogen will be maxed for your run. And because you were on low carb before, it will take a little bit to acclimate your system back to those carbs.

no, no, no. Changing your diet back to a carb-centric before a race is just going to cause trouble. why would you upset the fat-burning mode you worked to put your body into? you don't need glycogen stores, this is just another name for "carb loaded"

I agree with all of your responses. That isn't to say that a person couldn't eat low carb while training, then return to SAD 1-2 week pre-race, but it would be more harmful than helpful.

Something that is important for every athlete (even recreational athletes) and everyone who is dieting: How different macros (specifically, carbs, protein, and fat) are processed, used, and stored in our body (including body protein / muscle and body fat), and when (timing); and how different energy is handled (glucose, glycogen, fat and muscle). As fat-adapted athletes, we need to understand these things even more so than those eating SAD.

No need to fuel mid-run. Your body already has protein and fat. Even if you did consume protein / fat mid-run, you wouldn't use most of it right away as does a non-fat adapted athlete when eating carbs and using the glucose within the next 10-80 min. post-consumption. Protein and fat take much longer to digest and convert into energy than carbs, and they do so differently. If you are going to do anything at all for timing, eat protein 90 min. - 4 hours before the run or fat 10-12 hours beforehand in order to give those foods time to become digested to where gluconeogenesis occurs. But you really don't need to do this because:

1. Gluconeogenesis occurs with your own protein (muscles) and fat just as easily as with consumed protein / fat, and that muscle and body fat can be replaced later with protein / fat anyway. This process creates glucose from protein / fat.
2. When you are fat adapted, you oxidize fat at a much faster rate than those who are not fat adapted. Those who are NOT fat adapted rely on GU and other carbs to provide glucose because they can't oxidize fat fast enough so they need a lot of glucose. Also, they can't convert protein / fat to glucose quickly enough to be able to provide fuel at the necessary rate and that is why they must go to outside sources (food like GU) for additional glucose. By becoming fat adapted, you decrease reliance on glucose for fuel and directly access fat as your primary fuel source. Between the 2 methods of accessing energy (from glucose or from fat), your body's preference for fat helps you to avoid the glucose shortage that comes from glucose use at a faster rate than glucose replenishment. If you are fat adapted, you shouldn't need to replenish glucose any faster than it can be created through gluconeogenesis and glycogen.

*The thing we haven't discussed is if you are exerting yourself above lactate threshold. In a marathon, you wouldn't typically be doing that anyway because it is a marathon and not a sprint. *Yes, I used that term literally, HA!

codename_steve wrote: »

Cool! Thanks for taking the time for such a detailed explanation; I think I get it! Would speed workouts be considered lactate threshold? I can't remember what constitutes lactate threshold. I currently alternate between intervals and tempos once a week, usually around 45 minutes.

Probably yes. The best estimate would come from using an HRM and doing some calculations, but then still an estimate. In short, lactate threshold is going to be at higher exertion. For longer runs like marathons, you shouldn't be at that speed. But for training runs, that could be.

ETA: The reason this is important for low carbers is the same as why it is important for those who eat SAD. At such high levels of exertion, glycogen is released at a high rate. As a type 1 diabetic who usually wears a continuous glucose monitor (CGM), I can usually see this because glycogen is released into the blood as glucose at a rate even faster than it is used. The result is a net increase in blood glucose. At some point, that glucose needs to either be burned within a fairly short time window or it will get converted back to glycogen and/or to fat. This is true regardless of whether you eat low carb or not.

If you don't believe me, ask any type 1 diabetic who works out. Usually they will tell you working out drops BG, potentially leading to hypoglycemia (less true for us low carbers who are not relying as much on glucose). Then ask if they have ever done a cardio activity at a high intensity and noticed a spike in BG. Most all of us have experienced that at some point or another. The glycogen release at a higher level than necessary is what causes that spike in BG during exercise.

codename_steve wrote: »

So what would you suggest is the take home message about lactate threshold? I know it can be good for improving speed, but can it adversely effect things while transitioning to being fat adapted?

First, let's be clear that this isn't something understood very well by the scientific community and there is not a lot of agreement.

Having said that, my opinion and experience is that it matters more during the process of becoming fat adapted than when you are there. Frankly, I don't suggest taking part in such high-intensity training until you get over that "hump" of adaptation.

Once you have gotten to where you are fat adapted, it isn't as much of an issue. Then again, I'm sure there are those who will disagree. If you decide to try speed work during the fat adaptation process, at least do yourself the favor of staying real close to where you can stop and rest. Don't stray far from home... just run around the block repeatedly, on the treadmill, or on a track where you keep looping back to your car in short distance. Be capable of stopping on short notice if needed.