Weight management: can intense intervals make you lean?

Definitive answers from new research

To try and get some definitive answers on the effectiveness of HIIT for improving body composition, an international team of sports scientists has just published a new meta-analysis on this topic⁽¹⁵⁾. Appearing in the Journal of Clinical Medicine, this comprehensive analysis looked not just at the effects of HIIT on fat mass, body fat percentage, and fat-free (ie lean muscle) mass, it also sought to compare the efficacy of three different types of HIIT: cycling intervals, outdoor running intervals and treadmill running intervals.

The researchers started by scouring the sports science and medical databases for previous studies that could be combined and analysed as a group (the pooled data when analyzed enables scientists to generate more powerful and robust conclusions). To meet the criteria for inclusion, these studies had to:

·        Have data from a randomized control trial on adults aged 18 and over.

·        Have an HIIT exercise intervention involving either cycling intervals, outdoors running intervals, or treadmill running intervals.

·        Have a comparison between different HIIT modes which also included non-exercise control group.

·        Measure the trial outcomes in terms of body fat percentage, fat mass, and fat-free mass.

Out of 3282 publications initially identified, 46 studies were assessed for eligibility, out of which 36 were deemed to meet the strict criteria and which were subsequently analyzed. These 36 randomized controlled trials had intervention lengths of 3-15 weeks and in total, provided data from 1,130 individuals, 551 of whom were in a HIIT exercise intervention group, and 579 serving as controls. A minority of studies (seven) included both male and female participants, whereas 18 studies were exclusively performed with males and 11 studies exclusively with females. There was a broad mix of study participants, with some being highly trained or active, other being inactive but healthy with normal weight and others being inactive and overweight. A full list of the studies included and the participant characteristics can be found here.

What they found

There were a number of findings that emerged from the analysis. The key ones were as follows:

·        Performing a program of HIIT was most definitely effective for fat loss; the analysis showed a loss of 1.86kgs of fat when averaged out across the three types of HIIT, the many differing intervention groups and the differing intervention periods.

·        The method used to perform HIIT (cycling, outdoors running or treadmill running intervals) had a big impact on fat loss; outdoors running intervals produced an average fat loss of 4.25kg, cycling intervals a loss of 1.72kgs and treadmill intervals a loss of only 1.10kgs (not a large enough fat loss to be considered statistically significant) – see figure 1.

·        When it came to bodyfat % (the ratio of fat mass to lean mass), all three forms of HIIT produced a significant decrease, with a 1.53% drop averaged across all three HIIT methods. However, it was outdoors running that produced the biggest change in bodyfat % with a drop of 2.8%.

·        All three modes of HIIT induced gains in fat-free (lean muscle) mass; however it was only the cycling intervals that produced significant gains, with an average gain of 0.63kg - see figure 2.

·        Further analysis showed that the best gains from HIIT training occurred when participants trained for more than eight weeks, with at least three sessions per week using work intervals of less than 60 seconds duration ,and separated by up to but no more than 90 seconds of active recovery (see this article on active recovery).

Figure 1: fat loss and mode of HIIT

Horizontal scale at bottom shows fat loss in kilos. The horizontal diamonds show the spread of results with the middle apexes showing the average result for each mode of HIIT. Overground running showed the greatest fat loss (4.5kg) with cycling showing losses of 1.72kgs. While treadmill running showed some fat loss too, the large spread of results meant that there was too much uncertainty for the result to be significant.

Figure 2: lean muscle gain and mode of HIIT

Horizontal scale at bottom shows lean mass gain or loss in kilos. The horizontal diamonds show the spread of results with the middle apexes showing the average result for each mode of HIIT. Only the cycling HIIT produced gains in fat-free mass losses of 1.72kgs. Both running modes resulted in non-significant changes, although there was a trend toward lean muscle mass loss in the treadmill running studies.

Practical implications

This is the first ever study to bring together all the data on HIIT and body composition changes. So what conclusions can athletes draw from this new research? Firstly, it should be pointed out that there are some limitations in this study – principally that some of the studies in this analysis were studies on adults who were either obese, or inactive (or both). This means that we must be a somewhat cautious about drawing hard and fast conclusions for trained athletes. Also, the studies were rather varied in their protocols, which is less than ideal when looking for definitive results.

Despite these limiting factors, robust evidence shows that there are favorable body composition outcomes following HIIT, including overall reductions in bodyfat %, fat mass, and gains in lean muscle mass. Of course, endurance athletes will generally deploy interval training that reflects their sport (ie runners perform running intervals, cyclists cycling intervals etc) in order to maximize fitness. However, where weight management is the main goal, it’s recommended that outdoors running intervals are performed as these are likely to produce greater fat loss and reductions in weight. By contrast, where the main goal is reduced body fat combined with an increase in lean muscle mass (ie improved body composition overall), cycling proved to be the only effective mode of HIIT. If you’re a triathlete or biathlete or any other kind of multisport athlete where you’ll likely perform both running and cycling HIIT, you should factor this in when deciding which interval mode to perform when.

Regardless of the mode of interval training performed, the results clearly show that best results are achieved with programs of eight or more weeks, containing three interval sessions per week. Importantly, relatively short active rest periods of 90 seconds were definitely favored. In active rest periods, some activity (eg light jogging or low-intensity cycling) is maintained between the interval efforts rather than resting passively. By maintaining gentle activity, heart rates remain elevated to a greater degree, which promotes more aerobic metabolism and greater total calorie burn. Note however, where maximum fitness gains rather than body composition improvement is the main goal, passive rest periods are preferred – see this article.

Finally, why wasn’t the treadmill HIIT as effective as the outdoors running HIIT? This is difficult to answer. The researchers suggested a possible reason is that overground running, particularly with terrain variations (ie diverse surfaces and inclines/declines), may activate larger and more numerous muscle groups compared to treadmill running, which in turn more effectively increases metabolic rate. However, no testing was done to confirm this. The results on treadmill running were also further complicated by a relatively small number of studies. It’s likely that treadmill intervals do have some value for improving body composition, but the evidence suggests outdoors intervals are significantly better!

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