How "efficient" can someone be at doing cardio?

Question

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.}

Answer 1

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.