Dear Mark: Keto Carb Timing, Fat-Burning Aerobic Zone, Stored vs Dietary Fat

Dear_Mark_Inline_PhotoFor today’s edition of Dear Mark, I’m answering three questions from readers. First, why do I recommend that keto dieters (and low-carbers in general) position their carbohydrate intake shortly after their workouts? If it’s to refill glycogen stores, aren’t they already depleted by virtue of us being low-carb? Second, do my current 180-age aerobic HR recommendations clash with the original PB “move frequently at a slow pace” recommendations? And finally, how does the body decide what to do with stored versus dietary fat?

Let’s go!

Peloncito wondered:

Here’s a question I’d love someone to answer: there’s a lot of discussion in the book about timing any carbs you consume for when ‘glycogen suitcases’ are open, i.e. post-exercise. This makes perfect sense with Primal Blueprint, but now I’m wondering, do you even HAVE glycogen stores on Keto? Wouldn’t the 20g or carbs you’re eating get burned right away for energy, never allowing you to make any? Surely the body wouldn’t activate gluconeogenesis just to make a little spare glycogen for an emergency? Following that logic, why would it matter when you eat carbs, if you do – surely they’re going straight into virtually empty ‘suitcases’ any time of day?

Glycogen stores get smaller and emptier on keto. It’s true.

But they’re still there. And you generally have some glycogen on keto. Muscle glycogen is only used locally. You won’t pull biceps glycogen to provide power for your quadriceps. Biceps glycogen can only power the biceps. Furthermore, glycogen is primarily used to enable intense, protracted movements. The average person won’t burn much muscle glycogen walking to work, lifting shopping bags, and performing other common everyday movements. Becoming keto- and fat-adapted makes you even less likely to use glycogen for everyday movements. You’ll reserve that precious glycogen for the times you truly need it—sprinting, lifting heavy, running hard and long.

What does hard training do? Why is the training window so crucial for keto dieters looking to eat some carbs?

It expands glycogen stores. The more you train, the more glycogen you can store.

It depletes glycogen. The harder you train, the more glycogen you use, and the more carbs you need to replenish it.

It increases glycogen synthesis. Without exercise, dietary carbs aren’t as likely to become glycogen. Training upregulates your conversion of carbs into glycogen. You turn more carbs into glycogen after training than after sitting around.

Another reason taking it post-workout is so important? This enhanced glycogen synthesis doesn’t last long. After just 2 hours post-workout it slows by 45%, even in the presence of high levels of glucose and insulin. So, while you can synthesize glycogen at any time of the day, it gets a whole lot more efficient and effective after a workout session.

Derek asked:

Recently finished the book and sensed this clarifying blog post was its thesis.. I do still have a question from the keto reset text.

A proposed part of the keto plan is fat burning exercise at an intensity target of 180-age (if I correctly recall).

This seems a rather high intensity compared to the “move frequently at a slow pace” (approx 55% of Max HR) prescribed in the PB. Cortisol spike alert ?

Am I misinterpreting the fat burning exercise suggestion in 21D KR?

No, that’s right.

In the earlier days, I recommended 55%-75% of max HR, not just 55%. 55% was the absolute low end that could still be considered exercise.

In Primal Endurance, I clarified my “cardio” recommendations to focus on the 180-age target. This isn’t actually all that different from what I recommended back in the day.

The updated message is that ANY movement contributes to aerobic fitness and the MAXIMUM heart rate to promote fat burning, increased aerobic capacity is 180-age in beats per minute. In many cases, this falls pretty close to 75% of max as it happens.

Pcskier asked:

Just finished the book. Still confused on one point. Once the carbs/excess glucose are gone, body switches to fat/ketones. But what differentiates our bodies from burning stored fat vs dietary fat? Every time I eat I wonder were this increased (but not by much…I was already very primal) fat is going to go.

Dietary fat takes precedence over stored fat. You burn it and store what’s left over. If you have more energy coming in than you’re expending, you’re going to store. If you have less energy coming in than you’re expending, you’re going to turn to your adipose stores.

This is why eating actual meals and limiting snacking or grazing is so helpful for fat loss. Without calories coming in, you have to turn to your stored body fat. It also explains why people have so much weight loss success with keto—becoming fat-adapted tamps down the hunger and makes skipping meals a breeze.

As far as ketones go, they aren’t “free.” Their presence actually reduces lipolysis (the liberation of free fatty acids from adipose tissue). This is completely normal, not pathological; as ketones are a form of energy or fuel, their availability abrogates the need for more energy from adipose tissue. Reducing lipolysis also keeps ketone levels from getting too high.

That’s it for today, everyone. If you’ve got any more questions, send ’em along. If you have input on today’s questions, chime in down below.

Thanks for reading!

TAGS:  dear mark, Keto

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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54 thoughts on “Dear Mark: Keto Carb Timing, Fat-Burning Aerobic Zone, Stored vs Dietary Fat”

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  1. Wow, this is way more in-depth than I care to get regarding the workings of keto or low carb. For me, the beauty of the Paleo/Primal lifestyle is that you don’t have to get into all this stuff. You just do it and watch health improve as the excess weight falls away. How and why it happens occupies zero percent of my thoughts.

    1. That is part of the beauty of it, but some people, myself included, prefer to understand the full mechanism behind why it works. Both approaches are valid. If you feel good and are happy with the results, that’s awesome! I like to dig down to the cellular respiration level because of my biochemistry background, but that’s definitely not for everyone.

    2. That’s alright, but understanding nuance means understanding how to apply the dietary mechanism to your current circumstance instead of simply following the advice without understanding the logic behind it.

      In other words, if you understand how it works, you understand how to make it work for you.

    3. I am totally right there with you on that belief also Shary!! Counting macros ain’t happening for me!

      1. Stefan, having a good handle on food/nutritional properties works better for me. I don’t bother with counting carbs or anything else, but I do know the difference between starchy and nonstarchy foods, which foods will pack on the weight, and which ones are low-glycemic enough to be considered “free” carbs. Being fat-adapted is good, but I know that too much dietary fat (which varies) can slow or even halt weight loss, that too much protein can do likewise, and that there has to be nutritional balance for optimal health. I read labels and understand ingredients and therefore know exactly whether something is worth eating or simply Paleo junk food.

        None of this is hard to learn and you only have to learn it once, versus doing a lot of measuring, counting, and plotting based on what we can only assume is happening at our personal cellular level. It’s an effective approach that requires a lot less effort.

  2. Re heart rate: barefoot 5-mile hill hike this weekend with an 80-pound pack on a 170-pound keto-adapted body averaged 122bpm. Fat loss occurred. I’m 41, not 58, so there seems to be variation in the 180-minus-age metric.

    The metric that seems to work best for me is to move as intensely as possible while still breathing through flared nose. Breathing through mouth implies accelerating VO2 requirement implies glycolysis. That would have happened had I maintained 139bpm. Q.v. Apache spirit-runner training.

    1. 180-age is the MAXIMUM aerobic heartrate. Anything below that would also be fat burning, and according to Brad Kearns on Primal Endurance anything below the 180-age level is fat burning and beneficial, even walking to the mailbox.

      1. Consider whether 180-age might result in a HR too high to be a true maximum in a trained individual. It probably is not far off for the majority of the population, but we should consider whether there are more accurate, more paleo metrics such as breathing patterns.

        1. MAF allows you 5 extra bpm after 2 years of specific training. This has been proven effective for performance and training longevity many times. If you want more info just Google Dr Phil Maffetone

    2. Dude, you are such the alpha primal male, that’s awesome!

    3. Please tell me you carry the sledgehammer and wear the kilt when you hike. That would be total badassery.

  3. “Without exercise, dietary carbs aren’t as likely to become glycogen.”

    What ? Proofs for that saying ?

    So when you carb-load 3 days before a marathon, and don’t do much exercice, where does it go ?

    I really doubt that it is not stored as glycogen. Sure after exercie the cells are more sensible, but they sure also get very well filled up if you don’t exercice right before ingesting carbs.

    1. Mathieu, carb-loading is so 1970… and the theory has been disproven and discredited by Prof Tim Noakes and others.

      That carb-loaded carb turns into fat, mainly. Look around at the starting line of your next marathon. Carb loading is not working so well weightwise for many people,

      1. For performance it is still the key. No elite athlete, up to the marathon distance, is low carb. That’s a fact.
        I carb-load, and am always in perfect condition for D-Day. Never seen the “wall”, always running equal or negativ split and finishing strong.
        The key is to TRAIN with limited carbs (smart carbs => train low, compete high). But for the marathon itself, you have to carb-load if you want to perform at the maximum of your possibilities.
        I’ve six-packs all year long so I don’t agree with your comment regarding carbs turning into fat 😉 Carbs from good sources, without excess, won’t turn into fat.

        1. Mark and his partner Brad Kearns address this issue in their book Primal Endurance. They talk about getting fat-adapted so that when you use up glycogen, your body simply starts burning fat, a much more efficient fuel. They point to numerous elite athletes who are doing this now, and not carb loading before events at all.

          1. Fat will NEVER be more efficient that glycogen. It is an almost unlimited source of energy, and you can be more efficient to burn it, and that’s the goal with “train low compete high” and the low carb approach. But more efficient than glycogen for intense exercice ? Forget it.That’s delusional. Never been proven.

          2. Well, I am not a scientist, so I can’t argue the point as an expert, but that is the entire subject of the book Primal Endurance, which is full of studies and scientific support, and that is pretty much the message that Mark has been preaching for the last five years or so. His latest book covers this as well. If you are not inclined to trust those sources, it took me 2 minutes to find this study about endurance athletes performing as well on a VERY low carb diet, showing that their bodies become good at burning fat for fuel instead, and I am sure a search for “fat-adapted” “endurance” “keto”, etc, would lead to many more:

          3. Well, you are reading wrong again. Being more efficient at burning fat does not mean being more performant during that activity. It is still easier for the body to burn glycogen.
            For longer and less intense sport, yes it makes sens, that’s why it works better for ultra endurance. But like I wrote before, I am talking about shorter intense running activities, up to the marathon (not in five hours…). And there, carbs are the key to performance.

          4. The study indicated that they performed just as well as those burning primarily carbs, and this matches the studies mentioned by Mark and Brad in their book. In both Primal Endurance and his latest book, the Keto Reset, Mark discusses the benefits of spending the time getting “fat-adapted” and using fat as your primary fuel source even when running marathons, without the need to carb-load before. If you search “fat-adapted marathon” you will see a lot of the subject, some pro, some con, some in between. Primal Endurance argues that it is not only possible, it is preferable.

          5. There was a recent study done with endurance athletes. They were broken down into “fat burners” and “sugar burners” for short. What they found was that both athletes performed well. The diet didn’t make any difference in the race. The main difference was that the fat burners burned their glucose stores at about 1/3 the rate as the sugar burners. So they could go much longer without bonking. When they measured the remaining glucose, the fat burners could do two more races at their current glucose burn rate and the sugar burners only had enough for about 1/2 race before they ran out. At that point they’d need to switch to fat burning. But because they aren’t fat adapted, their efficiency at accessing that fat for energy would be lower. So you are both correct. But the fat adapted athlete has an advantage, not so much in general performance, but in having a little extra glycogen in their back pocket, for a longer period of time, should they need to put the pedal to the metal – so to speak.

          6. Like I said, for ULTRA. For shorter distances, no advantage, and even the opposite.
            Even elite ultramarathoneners admitted consuming lot of carbs prior and during races.
            You have names from elite runners, up to marathon distance?
            I’m talking about runners who have already optimised their capacitities without lchf. Switching to lchf doesn’t seem to help, and can even be deleterious.
            Lchf is good, makes sense for some usage, but don’t make it the panacea for intense exercice performance. The evidence is weak.

          7. You’re mixing your terms. Of course glycogen is easier to burn than fat for intense excercise. Like sprinting. That’s what it’s there for. But you’re talking about a marathon. Nobody is sprinting a marathon. For that marathon, efficiency does translate to performance to the extent that being able to access energy better improves performance. As in, a lot.
            As for your sixpack, are you sure that’s not just because you’re super skinny and unhealthy? I see a lot of scrawny skateboarders around with a six pack and they sure aren’t examples of fine musculature.

          8. “Nobody is sprinting a marathon.” I am afraid that for most marathoners, 4:40/mile (world record pace) is an all-out sprint.
            Much of the confusion concerning the marathon is the relative difference between distance and time. For a 2:02 marathoner, the race is primarily “glycolytic aerobic”, which is fueled primarily by carbs. For the 4+ hour marathoner, the effort is “lypolitic aerobic” primarily fueled by fat.
            I eat primarily LCHF because by far most of my training is aerobic AND I consider LCHF to be more healthful. I used to train HCLF, but found that it was ultimately unhealthful.

  4. Mark, over the last couple of months, and especially after reading the book, I have been doing some intermittent fasting and eating in the “Keto Zone”, but not strictly keto (around 50-60 carbs) and have had great results. I have lost 7 pounds, have more energy and feel great! But I am worried about one thing: am I getting enough calories? At 6’1″, 190 lbs, age 51, I am eating around 1400 calories at most, often less. I am just less hungry and tightening the eating window makes it easier to eat less. But am I messing up my metabolism? is my BMR getting out of whack?

    1. I was thinking that one through also. I assume, if you are truely fat adapted then your body just takes whatever energy it needs from stored fat to meet BMR requirements. So if you eat 1400 calories but your body needs 1600 the other 200 comes from stored fat.

      So I would think that as long as you meet your protein requirements and keep your carbs low the body can make up the difference with stored fat.

      1. That makes sense, but will it keep doing this indefinitely with a consistently lower calorie input? My concern is that my BMR will adjust down to the lower calorie expectation so that my weight loss will slow and, if I ever do eat more than that new lowered BMR, I will start gaining.

      2. REALLY good question to have clarification on. i.e. at what point is a deficit TOO MUCH of a deficit that impacts overall health, metabolism and energy levels. Hmmmm.

        1. i’d use the “how do I feel” metric. If you’re maintaining your muscle mass, you feel energetic, and you can still do all the things you want to do, then you are probably in the healthy zone still. Since you are still losing fat, that means your calorie uptake is still considerably higher than what you are eating ( meals+stored fat). Eventually that will level off and you’ll probably just naturally eat a little more to maintain health.

  5. When you are breaking a fast while in ketosis, so long as you dont eat over abundant carbs and protein (carbs around 25 g and protein around 0.7 g per lean mass lb) does it matter how much fat you do (or do not) eat?

    Another way to ask: if you are fat adapted , can stored fat be accounted for in a meal’s macros?

  6. ‘Dietary fat takes precedence over stored fat. You burn it and store what’s left over.’ My understanding is that dietary fat requires insulin to be stored. If we are in ketosis then there is little or no carbohydrate to promote insulin production and thus no fat storage. Still confused and wanting clarification.

    1. All food consumed generates an insulin response with carbs being the highest. Dr Jason Fung explains this real well in his books and site. So if you stuff your face with fat and go above what your calorie needs are then that surplus will get stored as fat.

      That is why many people who are trying to lose fat don’t because they are tracking macros which tell them to eat X amount of fat. Or they really do not know how much fat they are eating and wonder why they are not loosing weight. aka, Jimmy Moore.

      Most people over weight just need to stay very low carb, meet protein requirements with fat coming from what is in the food and from cooking. Putting a half stick of butter on the steak after it is cooked is not the way to drop the pounds.

      1. I think the point is, and what has worked for me in losing 55lbs so far this year, is to eat enough fat to feel satiated. Because if you don’t feel satiated it’s much more difficult to stick to your carb and protein goals and not overeat otherwise. I also generally squash my carb cravings with fat now too (although I don’t get many of those except on the days after a cheat).

  7. I would take some exception to the first answer. Much of the muscle glycogen is restored without dietary carbohydrates. Through gluconeogenesis and from the glycerol cleaved from triglycerides when used stored bodyfat for fuel (3 glycerol molecules make 1 glucose molecule). Volek and Phinney showed that in this study.

    “Rates of muscle glycogen synthesis in humans are highest when large amounts of carbohydrate are consumed immediately post-exercise (30), yet the LC athletes had similar rates of glycogen repletion compared to the HC athletes, despite receiving a negligible amount of carbohydrate after exercise (4 vs 43 g) and more fat (31 vs 14 g).”

  8. Is keto recommended for mid aged women not on hard core exercise routines but doing free hand exercise and walking?

    1. Yes its worked for many people in similar situations. Not absolutely everyone does well on it but the very large majority of people who give it a good try seem to.

    2. Taru, I’ve been what I call “borderline keto” for quite a while. I don’t track anything, but consume mostly fat, protein and non-starchy veggies. I was primal for probably at least 5 years as well. I am not into any type of hard core exercise…mostly body weight stuff at home and tons of walking. I’m 51 and feel amazing. Going “borderline keto” has definitely improved my energy, focus and muscle definition. Very happy with results, but everyone is a little different. Listen to your body!

  9. So I’m a liitle confused…, should we eat carbs within 2 hours after training or When Hunger Ensues Naturally (WHEN)?

  10. I finished the book last week and loved it! Sorry to digress, but I was hoping you could provide a bit of clarity on the carb count. The book mentions not counting some carbs such as those in leafy vegetables – do you have a list of items not to count – or is it just lettuce and kale? The book also advises to count all carbs and not calculate net carbs – but other info on primal blueprint mentions net carbs. I’m just looking for a bit of clarity on both of these points please.

    1. Personally I consider all, vegetables as free carbs. I think tracking carbs from kale, cabbage, brussels sprouts, tomatoes, etc would cause unnecessary stress. I think we often forget, that if you’re already ‘primal” you’ve already “won” in a sense. Any tweaking after that should be done for fun and curiosity purposes.

    2. If you get all your carbs from veggies, as I do, I find my gross carb count for the day is near 50 grams, but the net is closer to 20–which as an experiment, I am trying to stay down near. But in the long term, I will go back to eating as many veggies as I like and not worrying about it. The book was a little vague/contradictory, but I believe Mark’s intent was to say, keep the carbs down low but don’t worry about above-ground veggies (leafy vegetables, brussell sprouts, asparagus, broccoli, cauliflower, etc)

  11. For me I wonder if there’s a ceiling on keto and fat intake. For instance, since I’ve dropped so much BF and gone the strength training and muscle gain route, all my macros went up slightly. The carbs are still under 40g but it’s appetite driven. Fat intake doubled from fat loss days. At 185 lbs and around 10-12%BF, my fat intake is up to 9oz a day (for muscle gain)
    I have to eat some very carefully composed highly nutrient dense meals to nail all these macros. Heavy training drives the appetite to do it, thankfully. I may be storing a small amount of BF beyond what I lost but to combat this, i do a full 24 no cal fast per week while maintaining a traditional eating window similar to Mark’s described in the book.
    This is also when I sprint. No point to me in sprinting when full.
    Seems to me that I’ve reached a ceiling of ability to consume too many more calories for my activity level.
    The positives about keto is that if I stop training as heavy as I am now, my appetite should follow and limit my caloric intake. That spells out effortless weight management.
    The little bit of BF I might gain back? Give me a month of IF and a few rounds of sprinting in the spring and I’ll see you at the beach the month after. Build the engine in the adaptation phase and never care to look back.

  12. Hi Mark, First time caller, long time listener. Just finished reading Keto reset. As a mom with two young kids I’m going to need to prepare a lot of stuff for the first 21 days in advance. Do you know how well the recipes in the book do if frozen after they are prepared?

  13. “Dietary fat takes precedence over stored fat. You burn it and store what’s left over. If you have more energy coming in than you’re expending, you’re going to store. If you have less energy coming in than you’re expending, you’re going to turn to your adipose stores.”

    Thanks for answering my question on “Ask Mark Monday” in the very first week! Good to know there really are people on the other end!

    So I guess what I’m hearing is…to burn body fat, we obviously still have to run a caloric deficit. But being Keto/fat adapted simply makes it easier for our bodies to quickly access our fat stores for energy, rather than looking for glucose to burn, or trying to make glucose from protein via gluconeogenesis…i.e. the dreaded “using muscle” for energy that is so feared by bodybuilders and those trying to build/maintain lean muscle tissue.

    So there is less risk of accessing lean tissue for energy when in a ketogenic state, since the body is ‘better’ at accessing fat stores and that becomes preferential to accessing muscle stores….?

    Sorry to beat this horse but I want to be able to understand and be able to explain this in an easily understandable matter…so it doesn’t sound suspiciously like bro-science!

  14. Ultra Endurance Race Day Nutrition. Hello, I had asked a similar question on another thread, but I didn’t phrase it very well. Based on one of your suggestions, I read Primal Endurance last week. Now I have a more specific question.

    I would like some opinions on what some of you are doing for race prep and race day nutrition for events beyond marathon distance. I’ve been without sugars or breads since July, and eating the Primal Diet for the past several months. Right now I’m trace to light Keto. It feels pretty good. I also did a VO2 Max study in July, so I know where my aerobic break point is for now.

    To celebrate my 60th next summer I’m signed for a spring ultra, mid-summer IM 70.3 and a September Ironman plus some shorter stuff mixed in. (I only mention age because I used to be able to get away with a lot more crap. Now I plan better.) I intend to do them all as a fat-burner, so any specifics about what I should mix in my water and/or carry with for nutrition would be a big help. I loved Tailwind as a carb-burner because of the carb/salt/electrolyte combo. Is there still a place for that during a race? My runs right now under 15 miles are usually done while I’m fasting, but I’ve also been experimenting with coconut butter/MCT oils.

    What would you suggest nutritionally for events up to 24 hours? What types of salts and electrolytes do you use if just burning fat and no sports mixes? I usually break it down by the hour when I do my race prep. Many thanks!

  15. Hi Mark,
    Wow! This makes so much sense and it is very helpful (re: carb intake after workout rather than before).

    I have to ask you a follow up question. Based on this concept, let’s say I run a 160k ultrarunning mountain race as a fat adapted athlete. There are unavoidable times were I will tap in my glycogen stores (going up hills). If carbs will be a small part of my nutrition during the race would it make sense for me to target these carb intake strategically (post climb for example)? I guess my question is will my body replenish my glycogen store when I go back to MAF fat burning zone or do I need to completely stop activity?


  16. Hi Mark,
    It seems everywhere I look people are saying for fat burning it’s 220 – age, but you recommend 180 – age. I have been doing my best to keep it at 124, (I am 56)…but it sure feels a lot slower than I am accustomed to. Will you pls confirm that 180 – age is the correct formula?
    Thank you!