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

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For 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!

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