So you want to get lean and you think cardio is the answer, right? Ok, let’s talk about why you should (not) do cardio.
Cardio as a fat burner
You may do cardio because it burns fat. Many studies show pretty exciting numbers for fat oxidation and other metabolic changes, but they’re usually short term measurements and they’re relative numbers. In absolute terms, the actual amount of fat that is oxidized during a training session is minimal. You’re looking at grams of fat and the total amount of grams is generally closer to having one digit than three digits.
Moreover, as Alan Aragon showed in his Myths Under the Microscope series, performing your cardio fasted or in the ‘fat burning zone’ doesn’t help either. In fact, it’s often counterproductive. This is because at the end of the day weight loss is always determined by energy balance. Cardio cannot change the laws of thermodynamics. Regardless of how much cardio you do, if you consume more energy than you burn, you will not lose weight. Cardio burns calories and its effects on fat loss exist only insofar as they contribute to an energy deficit in your diet. In line with this reasoning, studies have found that it’s not more effective to burn extra calories with aerobic exercise than simply consuming less of them: the weight and fat loss is the same .
But wait a minute, doesn’t cardio, especially HIIT, stoke the metabolic fire? Yes, but contrary to some older research, the increase in metabolism after training called excess post-exercise oxygen consumption (EPOC) for even badass cardio protocols only lasts 3 to 24 hours and comprises just 6 to 15% of the net total oxygen cost of the exercise session.
Okay, so cardio, irrespective of modality and intensity, isn’t a miracle fat burner, but it still contributes to the energy deficit in the diet and over the long term this will amount to significant fat loss, even if it’s not some magical quick fix. What’s wrong with that?
The costs of cardio
The calories you burn with your cardio come at a cost and I’m not just talking about time and energy here. Charles Poliquin talked about the costs of excessive LISS in his article The (Many) Negatives of Aerobic Training, but even shorter cardio sessions can be detrimental to your physique and strength. Doing cardio alongside resistance training decreases the positive effects of both. Concurrent strength and endurance training decreases gains in cardiorespiratory fitness, explosiveness, strength and muscle mass [2, 3, 4]
This robust finding is called the concurrent training effect or the interference effect, because the negative interaction between cardio and strength training is a result of trying to make the body adapt in two opposite directions. When you put stress on your body, it adapts and this adaptation is specific to the demands placed on it. The demands you place on your body with cardio and strength training are mutually exclusive. These adaptations include local changes in muscle fiber type composition and speed of muscle activation, but also central changes in cellular pathway signaling, gene activation and enzyme concentrations [2, 3]
For example, protein kinase B/Akt (PKB) decreases protein breakdown and activates protein synthesis, making its presence very desirable for muscle gains. AMP-activated protein kinase (AMPK) increases mitochondrial protein, glucose transport, and a number of other factors that result in increased endurance and cardiorespiratory fitness. The problem is, AMPK and PKB block each other’s downstream signaling. That’s why doing cardio limits hypertrophy and strength gains not just in the legs or whatever body parts you use for your cardio, but in the whole body.
The intuition behind the interference effect is simple. You can’t be the world’s best sprinter and marathon runner at the same time. If you want your body to be exceptional at something, you will have to specialize in it.
It should be noted that some relatively short duration studies using untrained sedentary individuals have found no interference effect, because the strength of the interference effect depends on the discrepancy between the required adaptations of the training you do .
Beginners can get away with doing lots of different stuff, because their level of muscle mass and strength does not require much specialized adaptation in the first place. The more your body adapts in any direction, however, the more pronounced the interference effect becomes. It may take 2 to 3 months of consistent progress for endurance training to start interfering with strength training.
Can the interference effect be avoided? Not completely. As I said, the magnitude of the inference effect is a function of how dissimilar the required adaptations to the different training stresses are. The longer or more frequent you do cardio, the worse the interference. You could make the cardio undemanding, so that the body wouldn’t adapt to it, but that would make it burn even fewer calories and is far from guaranteed to work.
HIIT is much less affected by interference than LISS, because the required adaptations of HIIT and strength training are more similar, but as per the specificity principle it would only partially mitigate the interference. Besides, if you continue to increase the intensity and rest periods of the cardio, at one point you’re just doing more strength training with inferior exercises (see my 7 Principles of Exercise Selection for why sprints are inferior to squats for leg hypertrophy).
Your body cannot serve two masters. Concurrent strength and endurance training decreases your body’s adaptations to both and results in suboptimal strength and muscle gains. The only way to completely avoid the interference effect is to avoid cardio. Many people have gotten shredded without cardio by following a meticulous diet. Dieting isn’t cool, it doesn’t make you look badass and it doesn’t give you any rush of adrenalin or endorphins, but it does give you something much more valuable: results.
Written by: Menno Henselmans
Originally published on HumanEngine
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