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
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
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
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
- 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
- While your body is making the switch, give it plenty of fuel.
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
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
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
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
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.
- When first starting out with keto, follow the recommendations
laid out in The Keto Reset Diet, and be strict for at least
- 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
- 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.
Thanks, everyone. Questions, comments? Share them below, and
have a good week.
Koeslag T, Noakes T, Sloan A.
Post-exercise ketosis. J Physiol
Malhotra A, Noakes T, Phinney S.
It is time to bust the myth of physical inactivity and obesity:
you cannot outrun a bad diet. Br J Sports Med
Matoulek M, Svobodova
S, Vetrovska R, Stranska Z, Svacina S.
Post-exercise changes of beta hydroxybutyrate as a predictor of
weight changes. Physiol Res. 2014;63 Suppl
Newman JC, Verdin E. ?-hydroxybutyrate:
much more than a metabolite. Diabetes Res Clin Pract.
Sleiman SF, Henry J, Al-Haddad R, et al. Exercise
promotes the expression of brain derived neurotrophic factor (BDNF)
through the action of the ketone body ?-hydroxybutyrate.