Both!  Carrying out both HIIT (high intensity interval training) and LISS (low intensity steady state) concurrently may be most beneficial to fat loss.

Glycogen is the preferred fuel source during LISS, and fats are the preferred fuel source during HIIT. HIIT will release the hormones adrenaline and noradrenaline which translates to the removal of fats (triglycerides) from their cells, whereas LISS will release noradrenaline in the main, so isn’t as efficient as HIIT at removing triglycerides from fat cells.

HIIT is known for the EPOC effect (Excess post oxygen consumption) meaning that your body will continue to burn calories several hours after training. However, the EPOC effect is this dependent on two factors; i) the duration of the HIIT session, and ii) the intensity of the HIIT training session, as EPOC increases linearly the greater the duration of the training session and exponentially with greater intensity.

By contrast, LISS is better for fat oxidation (fat burning) and insulin sensitivity (determining where calories go – skeletal muscle or fat cells). Although it is well supported that excessive LISS training whilst the prime training objective is to gain muscle, results in dietary protein being used to in order produce and increase the density of mitochondria instead of (building) muscle, and muscle fibres actually shrink in order to use oxygen more efficiently, similarly, excessive HIIT training impedes preservation of lean tissue through the activation of the AMPK molecule, which directly interferes with the mTor (mammalian target of rapomyacin) pathway. The mTor molecule is critical to muscle preservation and growth.

Consequently, for fat loss, it is perhaps most efficient  to carry out HIIT and LISS training concurrently:

• HIIT is better than LISS training at removing fat cells owing to Adrenaline and Noradrenaline release.

• LISS training is better than HIIT training at oxidizing fat so with fat released from HIIT training, it makes good sense to oxidize fat through LISS training immediately following HIIT.

In order to achieve efficient fat loss, following a 5 minute warmup, try 8 x 20sec HIIT (with 10sec low intensity intervals), and then succeed it with the 20min LISS training. 20min LISS training at 130 bpm 2-3 times per week. Remember that HIIT has a threshold start point at 70% of the maximum heart rate. Anything less and it is effectively LISS training.