I have been meaning to ask this question for a while, and I hope that it's within the scope of this site's FAQ. For about half a year or so, I've been going to the gym approximately 5 times a week, training back, chest, arms and legs. Lately, I've joined a HIIT class that I find very enjoyable. There are classes everyday, but I personally join three times a week, twice on my rest days and once on arm day.

My question is: Suppose that I am on a 10% calorie deficit with all the activity above (I am using this). Will HIIT training kill my gains?

Are there any studies that support/go against this idea that cardio kills gains?

Current stats: My height is 186cm (6ft 1in) and I weigh 77kg (169.756lbs). I consume at least 1g of protein per pound of bodyweight according to the good old broscience rule, and 5g of creatine everyday.

  • 1
    If you're on a calorie deficit, why are you lifting weights 5 times a week? This seems a bit excessive with regard to restitution.
    – Alec
    May 11 '15 at 9:46
  • @Alec A 10% calorific deficit is the recommended one on the site if you want to "gain mass and lose fat". A 10% calorie deficit means 10% under my TDEE (where energy expenditure includes all the cardio and weight lifting).
    – user13379
    May 11 '15 at 9:50
  • @Alec In addition, I have found the calculator to actually over calculate the amount of calories that I actually need. In the first two weeks, I kept track of all the calories I ate and still gained 2kg.
    – user13379
    May 11 '15 at 9:52
  • @BenLim - Unless you are obese, it is extremely hard to both gain mass and lose weight.
    – JohnP
    May 11 '15 at 15:29
  • 1
    @BenLim - I don't. When I want to figure calories, I generally pick 2 or 3 of the different formulas available and average the results. Mostly for personal I use the mirror and a scale as the gauge.
    – JohnP
    May 11 '15 at 17:45


Strictly speaking, anything other than walking, light stretching, resting, lifting heavy weights, and eating is cutting into your strength and muscle gains. Those attributes are built with specific stimulus and recovery. Cardio is not part of that specific stimulus or recovery and therefore is an interloper. But that's only the case if your single goal is pure strength and muscle, which is a fairly narrow goal.


Greg Nuckols asserts that cardio won’t hugely impact your gains in the short run, and may be beneficial for strength and size in the long run

1) You can still get bigger and stronger with doing strength training and cardio simultaneously.

2) In the short term, concurrent training (cardio and lifting together) is about 31% less effective for hypertrophy, and about 18% less effective for strength.

3) Frequency and duration of aerobic training affected strength and hypertrophy gains – more frequency and volume of aerobic training meant smaller strength and size improvements.

4) When looking at the data more closely, mode of exercise mattered. Running, but not cycling, negatively impacted strength and size gain

However, he also notes that the evidence suggests that low intensity cardio should be prioritized over interval training for the most part.


Being on a caloric deficit already puts strength and muscle gains on the chopping block. Adding cardio might work for your situation or it might nix the little strength gains you're getting while on a deficit. High-intensity interval training is also not specifically what Nuckols is discussing in these studies, and may both interfere more with your strength/hypertrophy training and not give you the benefits he discusses. Furthermore, it's not clear whether a class really qualifies as HIIT, since HIIT generally doesn't take class-sized portions of time. A half hour of "HIIT" is probably misnamed, since high intensity output requires shorter output duration.

  • Dave, thank you for this answer. First of all, let me mention about the calorie deficit thing. It was only "suggested" on Scooby's site that when using the calculator, if I want to "gain muscle and lose fat" at the same time, I should set the parameters of the calculator to be on a 10% deficit. I can change the parameters easily. Now I have always wondered how soccer players/ sprinters get jacked. Clearly they do a lot of cardio and gym work at the same time. Doesn't this contradict the view that cardio kills gains?
    – user13379
    May 11 '15 at 14:40
  • @BenLim I wouldn't say that's clear at all. Sprinters' gym plans and amount of running volume varies, and the soccer players I see aren't particularly muscular nor do they lift weights very much. But to your point: "cardio kills gains" is a useful mnemonic, but at its heart it's an enormous simplification of books worth of exercise science regarding the tremendous complexity and variability of the human athlete. Nuckols notes some boundaries on that simplification, which I describe. Lastly, about the deficit, it makes a difference whether you are trying to gain strength or muscle. May 11 '15 at 14:49
  • Thanks for your reply. I see where you're coming from. Indeed, as a mathematician I often find it extremely hard to be able to say definitively yes or no on any issue in bodybuilding science. Do you have any recommendations for a calorie calculator/how much of a calorie surplus do you recommend?
    – user13379
    May 11 '15 at 15:02
  • If you want to get your nerd on mathematics-style then you're going to have to learn the underlying science instead of relying on high-level abstractions like calorie calculators and bodybuilding proverbs. You'll also have to put in serious gym time with different approaches based in logical hypotheses, since bodybuilding is still an n=1 sport with tremendous individual variation and mental factors. May 11 '15 at 19:26


...but only in excess. Cardio used as warm-up exercises should not be detrimental to gains. But prolonged sessions of cardio will reduce size.

  • Can you elaborate, perhaps cite some references other than bro-science knowledge?
    – JohnP
    May 11 '15 at 15:29
  • If you're asking about scientific papers, I do not have any. I am answering from my experience with long distance runners
    – iGbanam
    May 11 '15 at 16:03
  • So you are taking a cohort that at the most will do low weight high rep activities along with their 50-80 mpw running and applying that to someone that lifts 5 days a week?
    – JohnP
    May 11 '15 at 16:46

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