High-intensity interval training aka HIIT became popular among people who want to lose weight — if your heart is racing, lungs rasping, and you throw up in your mouth a little, you are doing great, you will lose weight. That’s what people think at least.

Turns out, HIIT is no better than low/moderate-intensity steady-state cardio (LISS) for weight loss (duh, bodybuilders knew it all along).

A new meta-analysis by Steele et al. (2021) showed that doing hard-core HIIT workouts will not make you leaner than someone chilling on the treadmill:

The study analyzed 54 studies that compared the effects of IT and MICT on measures of body composition and concluded: "The patterns of intensity of effort and duration during endurance exercise has minimal influence on longitudinal changes in fat mass and fat-free mass. From a practical standpoint, this implies that individuals can choose the intensity of effort and duration combination (i.e., IT or MICT) that best suits their needs and lifestyle."

An earlier meta-analysis of 31 studies by Keating et al. reported the same — both HIIT and LISS lead to similar fat loss:

A systematic review and meta-analysis of interval training versus moderate-intensity continuous training on body adiposity study concluded that: "High-intensity interval training appears to provide similar benefits to moderate-intensity continuous training for body fat reduction, although not necessarily in a more time-efficient manner."

Now, I know what you are thinking — “But Egis, I do HIIT sooo hard that due to the greater excess post-exercise oxygen consumption that HIIT mediates, my body burns more calories throughout the day. And I lose more weight.”

Er, no, those extra calories that post-exercise oxygen consumption aka EPOC aka the Afterburn Effect burns are minuscule. Almost meaningless from a fat loss standpoint. And yet some people think that when they do HIIT, calories just pour out of them later during the day, like gravel from a dump truck.

This study found that EPOC in the hours following the high intensity workout was negligible:

EPOC after high-intensity circuit training was almost meaningless in this study too:

This study found the same — ass-busting workout intensity does bugger-all in terms of EPOC:

EPOC 37 kcal using 60-second rest intervals and 57 kcal 20-second rest intervals

So no matter how you look at it, HIIT does not confer superior weight loss benefits compared to LISS apart from being more time-efficient. So let’s not glorify HIIT just because some fitness chick with shredded abs on Instagram said it’s cool.

Application

The evidence does not support the claim that HIIT offers an advantage over LISS for fat loss. HIIT is more time-efficient though. However, there is an efficiency/effort tradeoff — HIIT requires less time but more effort than LISS.
The majority of my clients do not need to do any HIIT or even LISS to lose weight. It’s entirely possible to create a calorie deficit and lose weight with dietary changes and resistance training alone (source).
Given that exercise adherence is key, whether you do HIIT or LISS should come down to your personal preference.
If you like HIIT, keep in mind that, unlike LISS, HIIT is hard to recover from. Recovery ability is already hindered due to you being in a calorie deficit and adding HIIT may hinder recovery from resistance training.
The primary fuel source for HIIT is glucose which is also the primary fuel for resistance training. Meaning, HIIT robs fuel from resistance training which sucks because resistance training is what maintains muscle mass when dieting (here’s the kick-ass article on maintaining muscle mass when dieting).
If you prefer HIIT to LISS, use it very sparingly and cap the number of HIIT sessions at two per week. If you don’t give a flying fuck about HIIT, eliminate it completely and just get enough daily steps or incorporate a few LISS sessions aka Netflix on treadmill sessions.