Folks who embark on a ketogenic diet often wonder how their new low-carb way of eating will affect their ability to exercise. Will they be able to maintain their fitness (strength, endurance, speed, performance) without the carbohydrates that previously fueled much of their workout efforts? Is it advisable to try? What if they often want to lose weight—will combining keto with cardio exercise unlock new levels of fat loss, or will it be counterproductive?
The confusion and apprehension are understandable. In most people’s minds, exercise and carbs go hand in hand. Anyone who has played organized sports or trained for any kind of endurance event is surely familiar with the practice of carb loading or “carb-ups”—consuming large quantities of spaghetti, bagels, ice cream, and the like before a big workout or race. Even casual exercisers know they need carbs to get through a cardio session. Right?
Not necessarily. Cells indeed use glucose for energy, and that demand is proportionate to the intensity and duration of a workout. But it’s also true that you can become more efficient at burning fat and ketones during exercise when you commit to a ketogenic diet. Keto and cardio exercise can coexist happily, no extra carbs required… often.
In today’s post, we’re going to talk about what it takes to ensure that your cardio workouts don’t suffer when going keto.
This is a common concern and not entirely unfounded. Many people do report feeling sluggish when they first go keto. You might feel like your performance in the gym or on the road 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). We call the first three to six weeks of a keto diet the adaptation period for a reason. Your body is adjusting to its new lower-glucose, higher-fat and ketogenic milieu. This is a learning process for your cells, in a way.
The more glycolytic (glucose-demanding) your workouts, the more you will notice this. Very intense or prolonged, difficult workouts that fall into the category of chronic cardio or “black hole” sessions are especially likely to suffer.
Likewise, if you were eating a high-carb, low-fat diet before going keto, the metabolic shift will be greater than if you were already eating a lowish-carb, moderate-fat diet more typical of a Primal eating pattern. In the latter case, you should already be fat-adapted to a degree, so it won’t be such a profound metabolic overhaul.
For the first three to six weeks, you’ll want to dial back the intensity and/or frequency of your cardio workouts. Trade your more intense cardio (and strength) sessions for walks, yoga or Pilates, easy bike rides, or other gentle forms of movement. Keep all your cardio workouts in the aerobic, predominantly fat-fueled zone by maintaining a heart rate below 180 minus your age in beats per minute. This roughly equates to a heart where you can breathe through your nose or converse comfortably.
Other important things to keep in mind:
As you become more keto-adapted, pay attention to your energy levels and adjust workouts accordingly. Make sure you’re eating enough to support your activity level. Tracking your food for at least a few days here and there is a good idea. Keto is a very satiating diet, so it’s easy to undereat. And continue to be careful with intermittent fasting for the same reason, especially if you are an active premenopausal woman.
During the first few weeks of starting a keto diet, 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 at least three weeks of strict keto—six or more is 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-duration endurance training.) At this point, you have some options:
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 percent full,1 and perhaps on par with non-ketogenic folks if you have been keto for a long time.2 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.
If you feel underpowered during exercise, another option is adding more protein and/or fat instead of adding back carbs. Some people report good success with “protein ups” timed around heavier workout days. Your sluggishness may simply be due to eating too little overall.
Lastly, you might experiment with exogenous ketone supplements. This is 100 percent optional. They aren’t cheap, and they aren’t necessary. But for anyone looking to add some extra oomph, just know this option is available. You might also enjoy some cognitive and mood-boosting benefits as a side effect.
Instead of adding simple carbs before workouts, you could instead add back nutrient-dense carbs after exercise, 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. Opt for Primal-friendly carb sources like sweet potatoes, plantains, or even potatoes or legumes.
In either case—adding carbs before or after exercise—the amount you add should be proportionate to the difficulty (intensity) of the workout. You don’t need to carb up for your yin yoga class.
Another question we frequently get is, “Will adding keto to my cardio routine (or vice versa) help me lose weight?” Which really means, help me reduce body fat?
Maybe. The evidence is clear: food intake plays a bigger role in fat loss than 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 and craving sweets. Ketones have known appetite-suppressing effects, so a ketogenic diet might help counteract any increased hunger that comes with exercise and a caloric deficit.3
That said, I think the root of this question is the fact that ketosis is a fat-burning state. So, the logic goes, since you are metabolizing fat for energy, you will automatically shrink your body fat stores. Moreover, if you combine keto and cardio, 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.
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,4 and of course, a lot of people simply enjoy their cardio. Keep doing cardio, but don’t put 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.
“Cardio” can mean a whole range of things to different people—everything from power walking your local mall to hour-long kickboxing classes to playing soccer to conducting grueling HIIT workouts.
The types of cardio exercise that are most compatible with a strict low-carb keto diet will favor fat-burning over glucose-burning. In other words, aerobic workouts that keep you out of black hole territory. But I’ll reiterate once more that you still need to incorporate resistance training and occasional high-intensity sessions, plus plenty of low-level everyday movement, to build well-rounded fitness—a la the Primal Blueprint Fitness recommendations.
If you’re committed to highly glycolytic exercise modalities like CrossFit, for example, or high-volume endurance training, you’ll need to be extremely intentional about eating enough calories, supplementing electrolytes, and prioritizing rest and recovery between workouts.
You probably already know Mark’s stance on cardio: avoid chronic cardio exercise patterns. That’s true no matter how you eat. And remember, while “cardio” is important, all types of exercise deliver a cardiovascular health benefit.
That said, this is what you can do to transition smoothly to keto and to combine keto and cardio in the safest, most effective, most enjoyable way:
Always listen to your body. Excessive hunger, carb cravings, fatigue, or lack of motivation to exercise are signs. Not signs of failure, but signs of a mismatch between the type or amount of exercise you’re doing and what or how much you’re eating. In the longer term, watch for indications that you’re pushing too hard. These include hair loss, unexplained weight gain, especially in your belly area, and sleep issues. Remember, you’re doing this to be healthy. Both keto and cardio are tools you can leverage to build better health today and greater healthspan in the future, but they must be wielded appropriately.
Thanks, everyone. Questions, comments? Share them below, and have a good week.
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