Over the last few years high intensity interval training has become hugely popular due the effect know as EPOC (excess post exercise oxygen consumption). In essence all this means is increased calorie burning after the exercise has ended.
Traditional cardio went on the back burner and became less popular as a modality, except in the body building world. In recent times the debate between cardio and HIIT has come to the forefront again, with people debating the efficacy and efficiency of each and their role in fat loss specifically.
This can be incredibly confusing for those looking to lose weight or optimise weight loss. What’s the truth? What does the science say vs internet fitness gurus?
Fasting has also become immensely popular with 16:8, OMAD and all sort of other variations coming to the forefront of the weight loss world. Should HIIT or cardio be done fasted, as many people claim, to improve results?
HIIT vs Cardio — Which burns more fat
When we examine whether one form or another is better at getting results, in this case said to be fat burning, we need to look at it from multiple perspectives. Do you care about the short term results, say less than 24 hours, or is it more important which one actually yields better results over a longer period of time. Weight loss will obviously not happen in 1 day, so the long term results are more important for those looking to optimise.
Fat Burning During Cardio
A large percentage of the energy used during low intensity steady state training, more commonly known as cardio, is fat. At low intensities our bodies are primed for fat burning, which is exactly what we see during lab test. The other noticeable thing is that increased fat burning basically stops once exercise has stopped. This means the increased fat burning only happens during exercise.
Fat Burning During HIIT
Fat burning during HIIT is really the other way around. Because the intensity is so high our bodies have to use stored glycogen (carbs stored in the muscles and liver) as the predominant fuel source. Fat burning during these workouts is very low compared to cardio. Once the exercise ceases however, fat burning increases.
In other words, as exercise intensity climbs fat burning drops, as seen in cardio. Yet when the intensity is high fat burning increases post exercise, while it’s practically non-existent post training.
24 Hour In
Once both fat burning during exercise, as in cardio, and fat burning post exercise, as in HIIT, have been measured over a duration of an entire day one thing becomes very clear. Both, have increased fat burning, although at different times and the overall fat burning is practically identical. So far, you could do either HIIT or cardio and it would get you the same results over a 24 hour period.
Long Term Results
Long term things get a little more tricky as this is when fat loss gets serious. Successful fat loss happens over the long term, this could be multiple weeks or months. To lose fat you have to be calorie deficit, meaning absorbing less energy than you are expending. This is pretty basic and we won’t delve into it anymore in this article, just understand that it’s a requirement for energy (fat) to be lost.
Weight loss cannot happen without a calorie deficit and as such this is a pre-requisite for these things to have an impact on someone looking to lose weight. Assuming a calorie deficit has been achieved either via diet, exercise or both we can progress on to the long term findings.
One thing really stands out when looking at the long term impact and this is the maintenance of muscle tissue. Muscle tissue is one of the keys that keeps your calorie requirements higher and in a calorie deficit, if muscle isn’t stimulated appropriately, it can be broken down and used as a fuel source in addition to body fat.
HIIT does a much better job at maintaining muscle tissue than cardio does. This isn’t really surprising when you consider the intensity difference. Muscle that is required to work hard is regarded by the body as necessary and less likely to be broken down. This is one of the reasons marathon runners are thin. They body will adapt to its environment and the stressors it’s exposed to.
Lie in bed all day and you will lose a lot of muscle tissue, spend time stimulating the muscle and build/maintain it.
Fasted Or Fed
In a fasted state, such as when we sleep, the body breaks down fat as a fuel source. The logical conclusion here would be that the body would burn more fat during exercise if already in fasted state. One of the reasons people like to do morning fasted cardio is the idea that it will promote increased fat burning.
The same idea holds true for HIIT. With the rise of fasting modalities training in fasted state for fat loss has become more popular. But hold your horses, the science on this is extremely interesting. In reality, the body burns more fat post exercise when in a fed state. Yes, you read that right. When fed milk and glucose (sugar) pre exercise those performing the exercise burnt more fat over all post exercise than the fasted group.
This has a few important, often ignored, benefits. The first being that in HIIT and cardio are often performed for fat loss, so we will be in a calorie deficit. A calorie deficit also means recovery is slower. This is obviously down to the fact that we are consuming less energy. Someone at weight maintenance or in a calorie surplus can recover faster than someone in a deficit. Feeding pre exercise may not only result in increased fat burning but may also result in increased recovery, which we want to optimise during fat burning.
The next thing worth considering is performance. While losing weight performance can drop, especially when body starts getting low. Being fed pre exercise may increase exercise performance for some people as it allows for the replenishment of glycogen (the carbs stored in muscle). This is especially true for exercise at high intensities, as this is when glycogen will be used as fuel.
What does this mean for you?
Ultimately, what this means for someone losing weight is the following. If you want to optimise things, according to current findings, perform fed HIIT to increase fat burning as opposed to cardio. More importantly though, if you prefer to do fasted exercise or perform cardio then do that. This may sound counterproductive, but the key thing that impacts successful long term weight loss the most is adherence. This means taking part in exercise regimes and having nutritional habits that are as enjoyable as possible. Someone who performs fasted cardio long term will have better results than someone who performs fed HIIT for a month, even though one might be more optimal than another.
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