Doing short bursts of intense exercise for 12 weeks doesn't improve the maximum oxygen your body can use during exercise in people with excess body fat, even though they can exercise longer before getting tired.

Doing short bursts of intense exercise a few times a week for 12 weeks makes people with excess body fat able to exercise longer before getting tired, but doesn't significantly improve their maximum oxygen capacity.

Eating very few carbs and more fats for 12 weeks doesn't improve the maximum oxygen your body can use during exercise in people with excess body fat when measured in absolute terms.

Older men in their 60s and 70s didn’t get better at using oxygen during exercise after three weeks of intense sprinting, but younger men did—so aging might make it harder for your body to improve aerobic fitness from short, intense workouts.

Doing regular exercise for 12 weeks helped improve heart and lung fitness in obese black South African women, as measured by their ability to use oxygen during exercise. This finding is from the abstract summary - full study details were not available

When endurance athletes do high-intensity interval training at very hard effort (over 85% of their maximum oxygen capacity), they burn fat at extremely high rates - about 1.58 grams per minute on average, with some burning over 1.85 grams per minute. These are the highest fat-burning rates ever measured in people.