Both. End of debate?
This isn’t going to be an in-depth statistical analysis of both types of training and different studies that have shown results to connect these types of cardiovascular exercise to the process of burning subcutaneous body fat. Instead, this article can summarise the information out there to provide a definite answer, if generalised, but which is true.
LISSten and learn
‘Low Intensity Steady State (LISS) cardiovascular training for fat loss’ normally refers to an exercise that is performed for between 30-60 minutes at a continuous pace that maintains a heart rate at between 50-70% of the person’s maximum heart rate (MHR). Bodybuilders have used form of training this for many years with the aim of burning fat whilst maintaining as much muscle as possible.
The basic science behind this method says that when the human body is at rest (sleeping, sitting still watching TV, standing to wait for the bus) it wants to use nutritional fat and stored body fat as its go-to sources to create the energy it needs. Carbohydrates that have been recently ingested and are stored in the blood, or that have already been digested and converted to stored energy in the muscles, can provide nearly 0% of the fuel needed to perform habitual functions. This is why sleep is said to be an incredibly necessary and efficient process for fat loss. Just think for one second that sitting still is actually a highly efficient method for fat loss. What’s the catch?
The catch is provided for us with the issue that you can burn less calories (stored fuel within the body) when you sit still or sleep compared to when you move about. The more intensely that you move about (from walking to running to sprinting) the more your body will use carbohydrates (stored in the blood, the liver and the muscles after digestion) as an energy source over fat. So what LISS training looks to take advantage of is a balance between ensuring that fat is still the major energy source used to fuel the body during the activity, but that your overall calorie expenditure for the duration is raised higher than if you were sitting still.
Due to the low impact that this type of exercise, the body doesn’t look to recruit fuel desperately from any source it can find. The more intense the exercise, the more the body struggles to run the metabolic systems that process oxygen and stored fuel from food to facilitate movement, and the more it seeks any source it can to supply the energy for the movement that the brain is initiating. For us, the ‘any source possible’ that we are most concerned about is the breakdown of muscle for fuel – muscle breakdown is counterintuitive to most people’s goals i.e. to preserve, build or define our muscles and to lose fat. Thus, this type of activity limits muscle breakdown and maximises the calories burned from fat stores. This is why bodybuilders are known for taking long power walks to burn fat rather than running a fast 10km race.
As we can see in Figure 1, although 99% of the calories burned from sitting still are from fat, the 50-70% of the calories burned from fat by jogging total nearly four times the amount for the same time period. So as long as you have some stored carbohydrate available or recently consumed protein, a gentle jog can be a very efficient way of burning fat and preserving muscle.
Take the HIIT
High Intensity Interval Training (HITT) is the opposite of the above. You are looking to work at your maximum effort for between 10-45 seconds before resting for a given period of time (normally between 30 seconds and 3 minutes) before repeating until you cannot perform the exercise at an intensity high enough to call it your maximum. This normally occurs after 10-20 minutes of interval training.
Some of you may have cottoned on to an interesting dilemma based on what I presented earlier i.e. the higher the intensity of an exercise, the more you use carbohydrate as your preferred energy system. Thus, when you are performing HIIT, you are actually looking to use your carbohydrate stores (and creatine phosphate stores for those interested) over your fat stores as it is the quickest and most efficient form of energy to use for fuelling this type of exercise. So, you ask, why has this form of training become synonymous with ‘torching fat’ when these facts say that the form of exercise mainly uses stored carbohydrate sources?
Firstly, when performing HIIT, the full 100% quota of energy used isn’t carbohydrates. When sprinting for the 10-45 seconds of maximum intensity, the percentage of carbohydrate usage will be between 70-90% leaving between 30-10% of the energy being converted from fat and other sources. But, when you stop sprinting and start the rest period, the energy balance falls from being mainly carbohydrate to being more inclusive of fat stores to fuel bodily functions (from around 50% and increasing as the heart rate drops and recovers). So whereas LISS training keeps a steady drip of energy from the various energy stores, HIIT chops and changes throughout the workout.
LISS is more?
20 minutes of HIIT can burn between 300-400 calories. Over the duration of the session, let’s say an average of 30% of the energy used comes from fat stores. This is half the percentage of fat store usage compared to LISS training and equates to 150-200 calories of fat. From the 1 hour jogging session, we can see from Figure 1 that 250-350 calories burned came from fat stores. If these statistics alone were used to determine the answer to the question about whether HIIT or LISS was a better choice for fat burning, then LISS would win.
The after effects of HIIT
However, what HIIT provides that LISS doesn’t is a more substantial increase in the body’s metabolic rate over the next 24-48 hours. When you have finished a gentle 60 minute jog, your muscles recover my shuttling glycogen back into the semi-depleted stores from the food that you eat post-workout but the muscles would not normally need to be repaired to any great extent because of the low intensity of the exercise. When you perform HIIT, you are creating micro-trauma in the muscles similar to when you lift weights and feel that ‘burning’ sensation. This occurs when you push your muscles to the edge of their current ability which creates small trauma (or tears) in your muscles. These micro-trauma are healthy and natural but once the body has time to rest and recover, it needs to repair these micro-trauma to become stronger than it was before to ensure that next time it is put under such strain it has adapted to be able to cope better. In the long term, this will allow you to perform exercise to a higher level of intensity before experiencing these trauma – it is this cycle of exercise, rest and repair that is the basis of how the body gets stronger and fitter. However, for the purposes of this article, it is the micro-trauma that allow us to burn more calories from fat over the next 24-48 hours.
When the body repairs itself, it needs more calories from its various energy sources to repair efficiently whilst also being able to continue with regular cognition and functionality. Thus, after having completed HIIT and needing to repair the micro-traunma, your metabolic rate – your calorie burning furnace – is higher than normal meaning that you will burn more calories over the 24 hour period than if you had sat still for the same period of rest without the previous HIIT session. This increase in energy output allows you to create a calorie deficit (consuming less calories in a day than you expend) more easily which is a primary rule for creating fat loss. If you consume under your daily calorie maintenance level for your body with an elevated calorie expenditure, due to the active HIIT session and the increased metabolic rate after the HIIT session, your body will use more of its stored sources of energy (such as body fat) to facilitate the daily functions and the micro-trauma repair.
As long as you perform HIIT with a good supply of carbohydrates stored in your muscles, and allow your muscles to rest and recover, this type of training will not seek to use too much of the hard earned muscle that you have accrued – in fact, it has been shown that HIIT can actually support or accentuate muscle development – and it will actively promote the reduction of stored body fat over time.
Reverting to my first sentence. Both LISS and HIIT are good methods of reducing body fat when performed in conjunction with a calorie controlled diet to enable a daily calorie deficit. Thus, it is then up to your own preference (whether you like or need to work hard for a short period or whether you prefer to keep your training gentle for a longer period of time) and work/life/play schedule as to which option you chose to burn any excess body fat that you have in store.