Is HIIT better than steady-state cardio?

Neither is universally better. Intervals develop high-end capacity efficiently in less time; steady-state builds the aerobic base with a lower fatigue cost and is easier to recover from. A sensible programme uses mostly easy work with a smaller amount of hard intervals.

What each does well

• Intervals: Raise VO2 max and the ability to tolerate and clear high-intensity efforts. Time-efficient, and useful when the calendar is the constraint.
• Steady-state: Builds mitochondrial density, capillary networks, and stroke volume. Low fatigue cost, so it can be repeated often.

The trap in the "more efficient" argument

Intervals produce more adaptation per minute, but you cannot do many of them per week without accumulating fatigue that degrades everything else you do. Twenty minutes of hard intervals three times a week is not equivalent to three hours of easy work, because the easy work can also be repeated tomorrow. Volume you can sustain matters as much as intensity you can survive.

A reasonable weekly mix

Roughly 80 percent easy and 20 percent hard is a widely used starting point. In practice that might be three easy sessions and one interval session, or four easy and two hard for someone with more training history.

Calorie burn and fat loss

Both burn calories, and the differences over a week are much smaller than the marketing suggests. The post-exercise elevation in metabolism after intervals is real but modest. Which one supports fat loss better is mostly a question of which one you will do consistently without impairing your recovery.

If you also lift

Keep the majority of cardio easy and low-impact, and separate hard intervals from heavy leg sessions. High volumes of hard, high-impact intervals alongside serious lifting is the combination most likely to cause interference.

Practical advice

If you have limited time and reasonable fitness, one or two interval sessions a week is a good use of it. If you are starting from a low base, build several weeks of easy work first; intervals on an underdeveloped base are unpleasant and injury-prone.