Short, hard and genuinely effective, but not a replacement for everything else. The evidence on VO2max, time efficiency, and the point where more intensity starts working against you.
High-intensity interval training is the rare fitness trend that survived contact with the research. It does what it says. What it does not do is replace the rest of your training, and there is a ceiling on it that most enthusiasts sail straight past.
What counts as HIIT
Repeated bouts of hard effort separated by recovery. “Hard” means genuinely hard, roughly 85 to 95 per cent of maximum heart rate, or an effort you could not hold for more than a couple of minutes. Formats range from the classic four-by-four (four minutes hard, three minutes easy, four times) to thirty-second sprint intervals. A circuit class where your heart rate never really peaks is not HIIT, whatever the timetable says.
The VO2max result
VO2max, the maximum rate at which you can use oxygen, is among the strongest predictors of all-cause mortality in the exercise literature. Improving it matters.
A 2015 meta-analysis by Milanovic and colleagues in Sports Medicine pooled trials comparing HIIT against moderate-intensity continuous training in healthy adults. HIIT produced larger improvements in VO2max, with the pooled advantage in the region of 5 per cent over comparable continuous training. That is a meaningful edge for a modality that takes less total time.
The time-efficiency argument
Gibala and colleagues at McMaster spent years demonstrating that very small volumes of very hard work can produce adaptations comparable to far larger volumes of moderate work. Some of their protocols involved a total of a few minutes of actual hard effort per session. For anyone whose real constraint is time rather than motivation, that is the strongest case for intervals.
The ceiling nobody mentions
This is the part that gets left out of the enthusiastic version. In 2021, Flockhart and colleagues published a study in Cell Metabolism that progressively increased HIIT volume in healthy volunteers over several weeks. Adaptations improved through the moderate phase, as expected. Then, in the highest-volume phase, things reversed: mitochondrial function declined, and glucose tolerance measured by continuous monitoring got worse. Performance fell off too.
Intensity is a stimulus, and like any stimulus it has a dose-response curve with a downslope. The people most likely to find that downslope are exactly the ones most attracted to HIIT: driven, time-poor, inclined to believe that if some is good, more is better. It is not.
Intensity is a stimulus, and like any stimulus it has a dose-response curve with a downslope.
Where it fits with everything else
The standard endurance-training structure is roughly eighty per cent easy work and twenty per cent hard. That ratio exists because the easy volume builds the aerobic base and the hard work sharpens the top end, and reversing the proportions produces athletes who are permanently tired and no faster. For a general population that means zone 2 sessions and walking as the bulk, with one to three interval sessions layered on top, plus strength work which HIIT does not replace.
How to use this
- One to three sessions a week, maximum. Two is a good default for most people.
- Keep the easy days genuinely easy. The most common failure is turning every session into a medium-hard slog, which gives you the fatigue of hard training and the adaptations of neither.
- Pick a format you will repeat. Four-by-four on a bike or a hill is as good as anything and easier to pace than sprints.
- Watch for the downslope. Sleep getting worse, resting heart rate creeping up, motivation gone: that is the signal to cut volume, not to push through.
This article is for educational purposes and is not medical advice. Talk to your doctor before starting any new protocol, especially if you have a medical condition, take medication, or are pregnant. Daylight is a food supplement and nothing here is a claim about what it does.
