I want to maintain or even grow muscle while burning fat (doesn't everyone – I know).

Assume a moderate daily deficit of 300 calories to achieve fat loss sustainably. Also assume a continuation of hypertrophy training. The variables are cardio and diet.

I want to know whether it'd make a difference if the deficit comes from cardio or eating less (or even a mixture of both).

My guess is that eating less would cause muscle reduction (BAD), whereas eating the same while doing cardio would not because the muscles would still get all the nutrients they need to grow or at least maintain (GOOD).

True or false?

Your body doesn’t really know the difference. Keep resistance training and maintain a modest deficit, and you’ve done everything you can to attenuate lean tissue loss. Therefore, given a choice between a deficit created by eating less and a deficit created by cardio, the plethora of other health benefits of aerobic training make the choice clear.

"Your body doesn’t really know the difference." It couldn't tell between more vs fewer nutrients/food?

Not really. At the end of the day, it doesn’t really matter how you got into a deficit. The things that matter for minimizing muscle loss are the size of the deficit, and whether or not you are resistance training.

If I achieved daily maintenance eating nothing but spoonfuls of olive oil my muscles wouldn't shrink?

Sure, but do you really think I needed to qualify my answer with “as long as you are eating the sort of diet that wont lead to death by anemia”? Just eat a well balanced diet with a decent amount of protein.

My olive-oil example is extreme but it shows that it's not all about calories in vs calories out. Macros matter; the body can tell the difference, at least in terms of macros. You've since edited your answer to include a recommended daily protein intake; if cardio allows me to consume more protein while keeping the same daily calorie deficit, then by your own logic, my body does know the difference. Or am I missing something?

I’m not sure what you’re getting at any more.

I'm saying you didn't need to qualify your answer with "as long as you are eating the sort of diet that wont lead to death by anemia", but your original claim that there's no difference whether a calorie deficit comes from reduced calorie intake or increased calorie output sounds wrong to me, for the reasons I've explained.

A calorie deficit from cardio is better because of the numerous, well documented benefits of cardio.
And it doesn’t affect muscle loss if you’re eating enough protein and resistance training.

A calorie deficit from cardio is better because of the numerous, well documented benefits of cardio.

yes agreed

And it doesn’t affect muscle loss if you’re eating enough protein and resistance training.

So you agree that my body can tell the difference?

No. Low protein, no resistance training, you will lose lean mass whether or not the deficit came from cardio.

High protein, resistance training, you will attenuate the loss of lean mass whether or not the deficit came from cardio.

It doesn’t matter how you got into a deficit, full stop. The way you attenuate muscle loss is the same: high protein, resistance train, and keep the deficit modest.

But I can eat more protein at the same deficit if I do cardio. Won't that added protein help build muscle or at least attenuate muscle loss?

The benefit of more protein drops off pretty quickly after around 1.3g/kg. You’ll find the range of 1.6 - 2.2 g/kg referenced as the optimal range for gaining lean mass, but the difference between 1.6 and 1.3 is pretty minuscule. However, when we go under 1.3 g/kg, we typically see outcomes drop off pretty quickly. 1.3 seems to be the best lower target, with higher intakes providing minimal additional benefits.

Gotcha. So, to conclude, it matters until 1.3g/kg. If someone needs to increase their protein intake to get to that level but would risk going into a calorie surplus (all else being equal) then they can use cardio to offset the added calories. Not to mention that cardio has other benefits, as you mentioned.

Cheers