Can Keto and Cardio Mix?

February 27, 2019

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We get lots of questions about how a ketogenic diet works in the context of exercise: Is it possible to maintain one’s fitness (strength, endurance, performance) and also drop one’s carb intake to ketogenic levels? Is it advisable? Will it help me lose weight faster?

Mark already addressed some of these topics, but it’s clear that many people still feel uncertain about how to pair a keto diet with their current workout routine.

Rather than write a single behemoth post, I’m going to tackle this in two parts. For today, let me talk keto and cardio, specifically how keto works for the average fitness enthusiast who thinks more in terms of general exercise. In a couple weeks I’ll follow up with a post on keto for runners and other endurance types who tend to focus on training programs and racing.

So, keto and cardio… This is for people who like to attend group fitness classes, or go out for jogs or spins on the bike, or do a mix of low heart rate exercise with occasional bouts of HIIT. (This is a problem with the term “cardio”—it can mean so many things.)

You probably already know Mark’s stance on cardio: avoid chronic cardio exercise patterns. The Primal Blueprint approach to exercise comprises lots of everyday movement, lifting heavy things, and occasionally going all out. If you simply must do cardio, most of these sessions should be conducted at an aerobic heart rate not higher than 180-age, as detailed in the Primal Endurance book. So, with the caveat that cardio exercise in the traditional sense of slogging away on an elliptical machine or treadmill doesn’t jibe with the Primal Blueprint approach, let’s get to some frequently asked questions.

Will My Workouts Suffer When I Go Keto?

This is a common concern because some people do report that they feel sluggish when they first go keto. And yes, you might feel like your performance in the gym (cardio, strength, HIIT—all of it) takes a hit in the first few weeks of keto. Rest assured that this is a temporary dip as your body becomes efficient at using fat and ketones for energy in the absence of incoming carbs (glucose). It’s a learning process for your body, so to speak.

The more glycolytic your workouts, the more you are going to notice this. Prolonged, difficult workouts that fall into the category of chronic cardio or “black hole” sessions are especially likely to suffer.

To help mitigate temporary performance decrements during the transition to keto:

Dial back the intensity and/or frequency of your workouts for a few weeks. Trade some of your more intense cardio (and strength) sessions for walks, yoga or Pilates, or other gentle forms of movement.
Mind your electrolytes. If you are feeling weak or lightheaded, if you get a headache, or you just feel “off,” this is likely due to electrolyte imbalance. Try adding ¼ – ½ teaspoon of salt to a glass of water with lemon juice and see if that helps. You want to make sure you are getting 4.5 grams of sodium, 300-400 mg of magnesium, and 1-2 grams of potassium each day on top of your normal food.
While your body is making the switch, give it plenty of fuel. Consume extra fat and eat plenty of calories. If fat loss is a goal, you can adjust your macros and calories as needed once you are feeling in the groove with keto.
Tough it out. Don’t cave and add carbs in the first few weeks (see the next point). Know that this is temporary, and you should be back to normal within three to six weeks.

Do I Need To Add Back Carbs To Fuel My Workouts?

During the first few weeks of starting keto, you should not add back carbs. It is important to create a low-glucose, low-insulin environment to promote ketogenesis and the adaptations that accompany a ketogenic state. If your workouts are too hard right now, the correct answer is to change your workouts, not to increase your carbs.

After you have done a dedicated period of a minimum three weeks of strict keto—six or more is even better—you should be feeling better during your workouts if you are not engaging in prolonged, chronic cardio activities. (It might take longer to adapt to longer endurance training, as we will discuss in the next installment.) At this point you have some options:

One, you can continue in strict ketosis (less than 50 grams of carb per day) as long as you are feeling good.

Two, you can start experimenting with eating carbs strategically before your workouts. This is known as a targeted keto approach. There are various ways of implementing this, but the basic formula is that you would ingest 25-30 grams of glucose or dextrose (not fructose) about half an hour before high-intensity workouts to replenish muscle glycogen.

There are a few caveats here. First, most sources of glucose/dextrose are not Primal (think hard candy, gels). Probably the closest is pure maple syrup, but that also delivers a hit of fructose. If you are a Primal purist, you will have to decide if this is a compromise you want to make. Second, people tend to overestimate the degree to which they are actually low on glycogen and how much it matters. It is a common misconception that once you go keto you have “no glycogen.” While muscle glycogen stores are reduced, your tanks are probably still at least 50% full, and perhaps on par with non-ketogenic folks if you have been keto for a long time. Furthermore, the average low-to-medium intensity cardio session isn’t truly depleting glycogen. Remember, the point of becoming fat- and keto-adapted is that you burn predominantly fat and ketones at these lower intensities, sparing glycogen. You have to go hard and/or long to really burn through your muscle glycogen stores. Thus, you should target pre-workout carbs only before truly high-intensity sessions.

Instead of adding simple carbs before workouts, another option if you feel like you need more carbs is to add back nutrient-dense carbs after workouts, when insulin sensitivity is increased. This might make sense if you feel like your ability to recover between workouts is lagging, or you want to recover quickly because you have back-to-back hard sessions planned. In either case—adding carbs before or after exercise—the amount you add should be proportional to the difficulty (intensity) of the workout. You don’t need to carb up for your yin yoga class, for example.

Lastly, if you are feeling underpowered during exercise, instead of adding back carbs you can experiment with adding more protein and/or fat. Some people report good success with “protein ups” timed around heavier workout days.

Will Adding Keto to My Cardio Routine Help Me Lose Weight?

Maybe. It’s a common refrain that “abs are built in the kitchen,” meaning that your food plays a bigger role in fat loss than does your exercise. This isn’t to say exercise is unimportant; it does matter. A caloric deficit is necessary to lose body fat, and exercise is one way to create a caloric deficit. However, this can also backfire if your exercise routine leaves you hungrier, so you unintentionally overeat calories due to increased hunger and cravings. Ketones have known appetite suppressing effects, so a ketogenic diet might help counteract any increased hunger that comes with exercise.

That said, I think the root of this question is the fact that ketosis is a fat-burning state, and so the logic goes that if you are metabolizing fat for energy, you will automatically shrink your body fat stores. Moreover, if you add keto and cardio together, especially if you are exercising in the so-called “fat-burning zone,” you will lose more fat than either alone. Right? Not necessarily. The fat you burn can come from your adipose tissue or from your plate. If you are eating an excess of fat calories relative to your daily caloric needs, you still won’t lose body fat.

We know that for body recomposition, the best bang for your buck comes from a combo of resistance training and HIIT. Cardio exercise still has many benefits for physical and mental health, and of course a lot of people simply enjoy their cardio; but you shouldn’t be putting all your eggs in the cardio basket if fat loss is your goal. All else being equal, though, it certainly can’t hurt to upregulate your body’s ability to use fat for energy.

Summary Recommendations:

When first starting out with keto, follow the recommendations laid out in The Keto Reset Diet, and be strict for at least three weeks.
If you are struggling in your cardio workouts during this period, don’t add back carbs! Dial back your workouts, add calories (via fat or protein), or both.
Once you believe you are keto-adapted, then you can start to experiment with targeted carbs and/or carb ups if you so choose.
No matter your diet, avoid chronic cardio exercise patterns that increase stress and your body’s demand for glucose.
Check out this post for additional tips for exercising while keto.