The Best and Worst Cardio for Natural Bodybuilding

Metabolic Conditioning for Natties

If you’re into natural bodybuilding, the worst thing you can do is train like a drugged bodybuilder. That includes cardio. Here’s why.

Natural Bodybuilding and Cardio: The Facts

Figuring out the best way to train for muscle growth and strength is already complex. And it gets even more complicated when you add cardio to the mix. If you’re into natural bodybuilding, you’ve probably heard statements like these:

  • “Fasted cardio will eat into muscles!”
  • “Low-intensity cardio will make you look like a marathon runner!”
  • “Cardio after lifting will kill your gains!”

We’ve reached a point where we’re almost scared of doing cardio. We’re afraid of losing our hard-earned muscle.

Well, don’t look to “enhanced” bodybuilders for advice. Their use of anabolic drugs counteracts cardio’s drawbacks on muscle mass. But don’t throw the baby out with the bathwater either. Cardio remains a tool that can help us get leaner and healthier. Let’s look at the best and worst forms of cardio for the natural lifter.

It All Comes Down to Cortisol

Cortisol has a significant correlation with muscle mass, albeit an inverse one: the higher someone’s cortisol production, the harder time they will have building muscle and gaining strength.

In fact, a higher cortisol level is correlated with a drop in strength in older individuals (1). And while producing it is necessary for healthy human function, too much can kill your gains. It may do so in a few different ways:

1. Cortisol Can Increase Muscle Catabolism

Cortisol leads to tissue breakdown to make amino acids that can then be transformed into glucose to be used for energy (gluconeogenesis). That’s one of the main functions of cortisol. It’s not a self-destruct mechanism but a function designed to give you fuel when you need it.

2. Cortisol May Inhibit the Immune System

This is important because the immune system drives muscle damage repair (important for growth). By inhibiting the immune system, cortisol can slow down muscle repair and impair growth.

3. Cortisol Can Increase Levels of Myostatin

Myostatin plays a big role in how much muscle your body will allow you to build. The more myostatin you have, the harder it is to build new muscle. Cortisol can diminish muscle growth by increasing myostatin.

An enhanced lifter can handle a lot more cortisol with less negative impact on growth. The exceedingly high levels of anabolic hormones in his body can counteract (to an extent) excess cortisol production, at least when it comes to hypertrophy.

But a natural lifter wants to avoid producing too much cortisol from his cardio work. As such, the best cardio for the natural lifter is the one that can deliver results with the least amount of cortisol being released.

What Cortisol Does During Training

Cortisol has many purposes during a workout. The most important are:

1. Cortisol Mobilizes Energy

It will free up stored glycogen and fatty acids, and even break down muscle tissue to have energy available to fuel muscle contractions. The more energy you need to mobilize, the more cortisol you’ll release. Since the goal of cardio is often to lose fat, you do need to force the body to mobilize some fuel. But too much might offset the fat-loss benefits.

2. Cortisol Gets Us Amped Up

It increases adrenaline levels by increasing the conversion of noradrenaline into adrenaline. If you need to get amped up, motivated, or are facing something that’s perceived as stressful, you’ll produce cortisol. Doing a high volume of cardio leads to high cortisol levels and so will pushing to an extremely high-intensity level.

3. Cortisol Helps Us Maintain Blood Sugar Levels

If blood sugar levels are too low, cortisol and glucagon are released to bring it back up. Why’s that important? Because if you exercise in a fasted state, you’ll increase cortisol to a greater extent, especially if you need to mobilize a lot of fuel.

“Yeah, but I train fasted and it energizes me!”

Of course it does. Cortisol increases adrenaline levels. Feeling energized is the byproduct of jacking up adrenaline. And while it might make you feel good in the moment, it can do more harm than good if you stay like that for too long or produce it too often.

So we have three elements that raise cortisol:

  • Volume or burning a boatload of energy
  • Pushing yourself to the limit
  • Training fasted

The Worst Types of Cardio for Natural Lifters

The worst types are those that combine a high level of one element or a combination of elements, increasing cortisol above what’s necessary for fat mobilization.

1. Fasted Interval Training

Taking a 45-minute walk on an empty stomach in the morning is fine, but doing intense intervals fasted in the morning is not.

In the first case, sure, you’re fasted (which is one factor in releasing cortisol), but the intensity and energy expenditure is low. A 45-minute walk doesn’t require much glycogen mobilization and doesn’t have a big caloric expenditure. While it’s fasted, the total cortisol output will not be too bad.

But if you do intervals – especially intense intervals – while fasted, now we have a problem.

You’re fasted (first cortisol-promoting element) and you’re pushing yourself hard (second cortisol-promoting element). And depending on the number of intervals you do, you could have a large caloric expenditure. But more importantly, intervals rely more on glucose for fuel than walking. So, the risk of increasing cortisol is much higher.

2. Long High-Intensity Intervals

But what if you’re not fasted? If you’re doing intervals for 8-12 minutes, that’s fine. But doing intervals for 20-30 minutes is a no-no for natural lifters. Now you’re combining high intensity with long duration – two factors that increase cortisol levels.

3. Fasted, Moderately Intense, Steady-State Cardio

Let’s go back to fasted training. If you decide to do fasted jogging for 45-90 minutes, you’re also putting your muscle at risk. Now you have a large caloric expenditure and you’re fasted – two cortisol-promoting elements. The same applies to fairly intense bicycling or other cardio.

4. Intense Post-Lifting Cardio

If it requires a significant amount of energy, endurance work will increase AMPK levels. This is great for losing fat, slowing down aging, and decreasing the risk of metabolic disorders. But it can hurt muscle growth by inhibiting mTOR.

When you’re lifting, the mTOR activation from the workout contributes to gaining muscle. But if you release AMPK right afterward, you can decrease the anabolic response to the workout.

So if you’re a natural lifter, it’s best to avoid cardio that’ll mobilize a lot of energy right after your workout. While taking a 20-30 minute slow walk on the treadmill is fine, doing intense intervals or long-duration endurance training after your lifting workout is best avoided if you cherish your gains.

The Best Cardio for Natural Lifters

A natural bodybuilder should either go easy or go all-out, but not for long. Let’s first look at the types of cardio, then we’ll get into timing and conditions.

1. Easy, Slow Cardio

It can be used a lot by natural lifters. When I got ready for my last two photoshoots, I would walk 45 minutes in the morning (empty stomach) and another 45 minutes in the afternoon (walking the dogs). Both of these sessions were very low intensity. Even the fasted one didn’t lead to overproduction of cortisol.

Why do it then? After all, walking doesn’t burn a lot of calories. Well, no, it doesn’t. But it’s still enough to contribute to a couple pounds lost over my 10-week prep.

I suspect fasted walking can even help reduce cortisol by reducing stress. It also increases the enzymes responsible for fatty acid mobilization. TC Luoma wrote an article showing that fasted cardio can increase the enzymes responsible for mobilizing and using fat. So while it might not make a huge difference right now, it could make your body more efficient at burning fat for fuel in the future.

2. Hard As Hell Cardio

Lactic acid could decrease myostatin by increasing myogenin and follistatin – binding proteins that inhibit myostatin. Remember, the more myostatin we have, the less muscle we can build. Lactate training could help you build muscle by inhibiting myostatin. Lactate is maximized during intense efforts lasting 30-60 seconds.

So go hard as hell for 30-60 seconds, then take ample time to recover between sets (2 or even 3 minutes). The longer rest intervals minimize adrenaline (and thus cortisol) and also lead to better performance, leading to a stronger physiological effect.

Some options: Conventional sprints, hill sprints, sled pushing sprints, Assault Bike sprints, stationary bike sprints, rowing ergometer sprints, etc.

Doing 4-8 of these improves cardiovascular function and stimulates fat loss. You can vary the duration of the sprints (30, 45, and 60 seconds are all good), the apparatus you’re using, and the number of reps you do to keep things fresh and interesting. You can even combine more than one modality in a workout. For example:

6 sets of…

  • Rowing ergometer, 30 seconds on, no rest, then…
  • Sled pushing, 30 seconds on, rest 2-3 minutes, repeat

The goal is to produce lactic acid and then recover almost completely between sets. I’ve seen it work wonders for many clients.

For more on this type of exercise, I recommend the book “Sprint 8 (on Amazon)” by Phil Campbell. That’s what I use with my body transformation clients with great success.

What’s the Best Time to Do Cardio?

For fat loss, it probably doesn’t matter much when you do cardio. But when it comes to gains, you’d ideally want to separate it from your lifting. The best option is to do weight training and cardio on separate days.

The second best option is to do them at different times in the day, like cardio in the morning and lifting in the afternoon.

The third best would be to do the cardio work at the beginning of your workout and then lift. I recommend you train your upper body when you use that approach.

If you walk, it’s actually fine to do it fasted. But the lactate work needs to be done in a fed state. This means to eat and digest at least one meal or go with one scoop of Metabolic Drive (on Amazon) if you don’t have time to digest a meal.

To Recap

If you’re a natural lifter who wants to build more muscle, you should avoid the following types of cardio:

  • Fasted Intervals
  • Fasted Moderately Intense Steady-State Cardio
  • Long or High-Intensity Intervals
  • Intense Post-Lifting Cardio

If you’re a natural lifter who wants to build more muscle, these two types of cardio are your best bet:

  • Walking (the more, the better)
  • Lactate Sprints with Long Rests

A Reminder

Just because a competitive bodybuilder does fasted, intense cardio doesn’t mean that you should too. I know it’s tempting because we tend to look at the pre-contest regimen of bodybuilders for fat-loss strategies.

But the fact that they’re using steroids, which counter the drawbacks of excess cortisol, gives them a different physiology than yours. You can’t do exactly what they’re doing.

And sure, you can do all the bad types of cardio without witling away into nothingness, but that doesn’t mean it’s optimal. Building muscle is already difficult without doing suboptimal things.




  1. Peeters GMEE et al. The relationship between cortisol, muscle mass and muscle strength in older persons and the role of genetic variations in the glucocorticoid receptor. Clin Endocrinol (Oxf). 2008 Oct;69(4):673-82. PubMed.

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