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It's a common belief that doing cardio for weightloss loses its efficacy over time because that person becomes "efficient" at doing the cardio. As in, the cardio you do burns less over time so you have to do more cardio to make up the deficit. This a really common belief in the bodybuilding world such that a lot of bodybuilders won't do cardio in their off-season because they don't want to become efficient at doing cardio. Then they'll often start their season doing 90 minutes a week doing cardio, and by the end of it they're doing 7+ hours per week1.

So that begs the question, how "efficient" can someone become doing cardio? Let's say a person is capable of running a 5K in 30 minutes and will burn 300 calories doing so. If they run this 5K at the same pace every other day for six months, they'll become very good at it. It may even seem effortless. After the 6 months, they do their run. Will they burn 250? 200? Even less? It clearly would never reach 0. Any sort of movement is going to yield some kind of energy output. What would be the limit?

EDIT: In the hypothetical, the person remains the same weight.

1: I personally don't believe efficiency is the reason this is necessary, but that's not the focus of this question.

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    expert marathon runners/cyclist consume between 4% and 6% less calories from those activities when compared to untrained people, at least this is what I remember from an old book I read when I was younger, the science must have found something new or maybe it's just the hype... the fitness industry is famous for it's random beliefs.
    – Drien RPG
    Jan 22 at 20:44
  • "This a really common belief in the bodybuilding world such that a lot of bodybuilders won't do cardio in their off-season because they don't want to become efficient at doing cardio." Are you sure about this? I have literally never heard any bodybuilder making any claim anything like this before. Jan 22 at 21:48
  • @David Scarlett Well I mean it's a claim I've seen come and go on various bodybuilding forums throughout the years. It's a belief my wife and her IFBB Wellness coach seem to believe which is what inspired this question. There's also a couple coaches on YouTube, one IFBB and one WBFF pro, that have made similar claims. Greg Doucette has also made videos addressing it, but he believes it's "stupid" because also you have to do is run faster. Maybe "really common" is a bit of an exaggeration, but it is a belief that some people have that affects their training and planning for competition.
    – DeeV
    Jan 23 at 4:45

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I'd strongly question the idea that there is a common belief within bodybuilding that cardio loses its efficacy as a weight loss tool due to increases in running efficiency meaning that bouts of running don't burn as much energy as the trainee becomes more practiced. I think the only reason why bodybuilders have traditionally increased their running volume during contest prep (and in more modern times, probably do the same with step count instead of cardio) is that late into prep they're experiencing such strong metabolic adaptation that they need a lot of physical activity energy expenditure in order to be able to allow them to both continue losing weight and eat enough to avoid malnutrition.

But as for the question of changes in running economy with experience, yes, experienced runners have better running economy than novices1, and are better at maintaining running technique in the face of fatigue2. There doesn't appear to be data on how long this increase in economy takes to appear, but six weeks isn't enough to see a change.3 The Ruiter et al study1 mentioned above only saw a 20% reduction in energy needs from novice to experienced runners, which should give you an idea of what kind of improvements in economy are possible, however within the context of burning energy for weight loss, it must be noted that an increase in running economy does not mean that you'll burn less energy on a run or will need to run for a longer period of time in order to burn the same amount of energy. It just means that you'll be running faster at the same heart rate or perceived level of exertion. So it most likely actually makes no difference at all to your ability to burn energy.

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  • The last two points was what I always figured would offset the difference, but I never knew how significant the drop could be. 20% is actually pretty steep.
    – DeeV
    Jan 23 at 4:50

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