Principal Study: A systematic review and meta-analysis of interval training versus moderate-intensity continuous training on body adiposity

Rarely do Study Deep Dives cover a study that does not involve a nutrition or supplementation component. However, there are exceptions for studies that provide valuable information on related topics. The study under review is one such exception because it addresses whether interval training is more effective than steady-state cardio training for fat loss.

Exercise combined with diet modifications has been shown to be more effective than either alone for promoting weight loss. Moreover, as seen in Figure 1, exercise has been identified as a key player in long-term weight loss maintenance. Therefore, establishing exercise habits during the weight loss phase can help prevent weight regain and yo-yo dieting down the road. Yet, roughly 80% of adults in the U.S. do not currently meet the recommended physical activity guidelines from the CDC, which are the same recommendations made by the American College of Sports Medicine (ACSM). One of the most common excuses for lack of exercise is a lack of time.

The ACSM recommends that most adults engage in moderate-intensity cardiorespiratory exercise training for a minimum of 150 minutes per week, preferably through performing at least 30 minutes per day on at least five days per week. However, they state an alternative means to obtaining sufficient physical activity is to engage in vigorous-intensity cardiorespiratory exercise training for at least 75 minutes per week, preferably through performing at least 20 minutes per day on at least three days per week. Interval endurance training satisfies the vigorous-intensity requirement and has therefore been suggested to be a time-efficient alternative to regular moderate-intensity steady-state cardio.

Numerous studies investigating the health effects of interval and steady-state cardiorespiratory training have been published, with some directly comparing the two. Previous meta-analyses have suggested that interval training might produce superior improvements in cardiorespiratory fitness compared to steady-state cardio, and one large systematic review suggested that higher training intensities, as would be observed with interval training, have a more favorable effect on body composition than lower training intensities common to steady-state. Yet, despite the time efficiency of interval training for promoting cardiorespiratory fitness, it remains unclear whether it is more efficient for fat loss.

The purpose of the study under review was to pool data from studies that have directly compared interval training to steady-state cardio training and use a meta-analysis to determine whether one method resulted in greater fat loss than the other.

About 80% of the U.S. population does not meet physical activity recommendations, with many people citing a perceived lack of time. However, exercise not only benefits changes in body composition when combined with a weight loss diet, but also plays a key role in long-term weight loss maintenance. Interval training has been proposed to be a time-efficient alternative to steady-state cardio for improving health and fitness. The study under review is a meta-analysis of studies directly comparing the two modalities for their ability to reduce fat mass.

Who and what was studied?

The study under review is a meta-analysis of controlled trials directly comparing high-intensity interval training (HIIT) or sprint interval training (SIT) with moderate-intensity continuous training (MICT) regarding their effect on changes in body fat percentage and total body fat mass (the two primary outcomes). Aside from providing data on these outcomes, the only other inclusion criteria were that studies had an intervention duration of at least four weeks and utilized an exercise protocol that fits the following definitions, outlined in Figure 2:

SIT was defined as “all-out” (greater than 100% of VO2max) bursts of activity lasting eight to 30 seconds interspersed with less intense recovery periods. HIIT was defined as submaximal (80-100% of VO2max) bursts of activity lasting 60-240 seconds interspersed with less intense recovery periods. MICT was defined as traditional steady-state cardio performed continuously for 20-60 minutes at a moderate intensity of 40-59% of VO2max or 55-69% of max heart rate.

After screening 402 articles, 28 studies providing 35 comparisons were included in the meta-analyses—25 for body fat percentage and 10 for total body fat mass. The studies contributed a combined sample size of 837 men and women, with only four studies involving more than 20 participants. Most studies were in healthy young adults (n=12) or overweight / obese adults (n=10), and the remainder involved children and adolescents (n=3), adults with metabolic syndrome (n=1), colorectal cancer survivors (n=1), and hypertensive women (n=1). The average age of the participants from each study ranged from 10.4 to 65 years.

The studies were roughly split between using HIIT (n=16) and SIT (n=12), with stationary bicycling (n=14) and running (n=11) being the most common modes of training. Studies lasted four to 16 weeks, averaging 12 weeks. Fat mass was assessed with DXA (n=15), densitometry (n=3), bioelectrical impedance (BIA; n=5), calipers (skinfolds; n=5), MRI (n=1), and tomography (n=1). Only seven studies monitored and reported the diet of the participants and only 11 monitored and reported physical activity outside of the intervention.

Study quality was assessed using a 21-point scale that judged numerous aspects of methodology and data reporting. Scores ranged from nine to 20, with an average of 13.7 points. All studies specified their primary outcomes and adequately reported their statistics. However, blinding of the researchers was present in only six studies and only 13 studies attempted to quantify the energy expenditure of the interventions.

A subgroup analysis was conducted to assess how energy expenditure influenced the results. Studies were grouped according to whether the HIIT or SIT intervention was matched to the MICT intervention for total energy expenditure. Additionally, subgroup analyses were performed according to the type of comparison group (HIIT vs. SIT), intervention duration (shorter than 12 vs. longer than 12 weeks), participant age (younger than 30 vs. older than 30 years), sex, and BMI. Finally, a sensitivity analysis was performed to investigate how study quality affected the results.

This was a meta-analysis of 28 studies comparing the effect of HIIT or SIT against that of MICT on changes in body fat percentage and total body fat mass. Most studies were conducted in healthy young or overweight and obese adults and used cycling and running as their mode of exercise. The average intervention duration was 12 weeks and few studies monitored participant diet and physical activity outside of the intervention.

What were the findings?

There were no significant differences between HIIT or SIT and MICT for changes in body fat percentage (-1.26 vs. -1.48 %, respectively) or total body fat mass (-1.38 vs. -0.91 kilograms, respectively). Heterogeneity was virtually non-existent for both outcomes. Subgroup analyses suggested that MICT might (p=0.09) be more effective than HIIT or SIT for reducing total body fat when HIIT or SIT resulted in a lower exercise-induced energy expenditure. However, when energy expenditure was matched between MICT and HIIT or SIT, there were no significant differences.

Other subgroup analyses showed no significant differences in body fat percentage or total body fat mass when comparing HIIT vs. SIT, men vs. women, studies lasting more vs. less than 12 weeks, normal-weight vs. overweight vs. obese participants, people younger vs. older than 30 years, or studies rated as low vs. moderate vs. high quality. There was no evidence of publication bias.

HIIT and SIT result in similar reductions in body fat percentage and total body fat mass as MICT, especially when matched for energy expenditure. However, MICT might be more effective for reducing total fat mass than HIIT or SIT, if it results in a greater exercise-induced energy expenditure.

What does this study really tell us?

The study under review tells us that when it comes to losing your gut, the intensity of exercise doesn’t matter so much. There was no significant difference between intervals and steady-state cardio for reductions in fat mass or body fat percentage, with both modalities resulting in marginal reductions. Rather, the caloric expenditure of the exercise session might be a more important variable. After all, the subgroup analysis found that steady-state cardio tends to produce greater body fat reductions than interval training if it burns more calories.

Overall, the study under review supports the notion that performing interval training for a shorter total workout time can be an effective alternative to regular steady-state cardio, provided caloric expenditure is matched. So, interval training might be a way to get more people to exercise, since a lack of time is cited as a primary barrier to exercising. Moreover, subgroup analyses found no difference between HIIT and SIT for fat loss, suggesting that personal preference, enjoyment, and physical capabilities should dictate what type of endurance training is used for fat loss.

Or rather, these should dictate what type of endurance training is used to complement a calorie-restricted diet, since this meta-analysis found marginal reductions in fat loss from exercise alone. Still, with 12 of the 28 studies using normal-weight participants, it’s reasonable to speculate that the reductions in fat mass were attenuated by the participants’ lean phenotype and that greater fat loss would be observed in people with overweight or obesity. However, subgroup analysis found no difference in fat loss between normal-weight, overweight, and obese people.

Another explanation for the marginal fat loss could be the lack of control over the participant’s habitual diet and physical activity levels, which can be seen in most of the included studies. One short-term study in people with prediabetes reported that a 10-day supervised HIIT intervention resulted in significantly greater levels of leisure-time physical activity during an unsupervised one-month follow-up, in addition to maintenance of the HIIT routine, compared to the same 10-day supervised intervention using MICT. Compensatory mechanisms regarding food intake are another issue. Some people might lose their appetite while others think they “earned” that slice of chocolate cake, thus erasing any exercise-induced energy deficit that would have resulted in fat loss.

The marginal fat loss reported in this meta-analysis helps emphasize why this meta-analysis is an important contribution to the topic investigated. Since only four of the included studies involved more than 20 people, and since the difference between interval and steady-state cardio was so small, it is possible that most studies out there may have lacked the statistical power to detect between-group differences. By pooling the data from these studies, this meta-analysis largely overcame this limitation of the individual studies and found no evidence to support the superiority of one exercise modality over the other regarding fat loss. The low heterogeneity of the meta-analyses further supports this notion, since it indicates that the studies are similar and that pooling them together is appropriate.

There was no evidence of publication bias, and subgroup analyses suggested that neither study quality, participant sex, nor age influenced the results. There are likely to be individual differences in fat loss from jogging or sprinting, perhaps not because of the exercise session per se, but because of the other lifestyle factors it affects, such as food intake and leisure-time physical activity. Studies are necessary to investigate how distinct types of exercise influence these variables and whether certain biochemical, psychological, or physiological parameters can predict someone’s response to the type of exercise in question.

Finally, this study did not consider the level of participant adherence among the included studies. This is problematic because it is unreasonable to expect fat loss to occur when the participants don’t show up for their morning run. Furthermore, there may be differences in the sustainability of interval vs. steady-state exercise programs, especially for people with lower levels of physical fitness. Similar to appetite and physical activity, the long-term viability of an exercise intervention will come down to personal preference.

These results show that the type of cardio performed for fat loss (intervals vs. steady-state) probably doesn’t matter as much as the number of calories burned in the exercise session. Moreover, the overall amount of fat loss is small. Rather, a focus should be placed on how the exercise session impacts other areas of life, such as appetite, food intake, and leisure-time physical activity. A focus should also be placed on whether you can see yourself sticking with your chosen exercise modality for the long-term.

The big picture

So, endurance exercise isn’t very helpful for fat loss on its own. However, it’s important to look at the big picture. There is little doubt that fat loss is a central component of improving health among people who have fat to lose, but it isn’t the only component. Exercise has many health benefits outside of fat loss that make its inclusion into daily life a sensible idea.

Several researchers have proposed that a person can be “fat-but-fit” or “healthy obese” based on the seemingly obvious concept that health is a function of both fitness and fatness factors. This concept is summarized in Figure 3. Although more fatness generally causes increases in chronic inflammation and downstream effects on metabolism, improving fitness through exercise causes an increased responsiveness to anabolic hormones that might become dysregulated from chronic inflammation and energy excess (such as insulin signaling) and increased release of anti-inflammatory signaling molecules. For example, both strength and endurance training have been shown to increase insulin sensitivity in overweight and obese adults by promoting glucose uptake via insulin-independent pathways and by increasing skeletal muscle antioxidant capacity. A systematic review of randomized controlled trials in at-risk populations reported that exercise interventions have moderate to strong favorable effects on triglycerides, total cholesterol, LDL, HDL, glucose, and insulin.

Importantly, not all types of exercise have the same effects on health. A systematic review of 66 studies and 162 study groups compared the effects of diet, endurance training, strength training, or their combinations on various health markers in overweight and obese adults. For example, strength training was significantly more effective than endurance training for increasing fat-free mass and high-intensity exercise (strength training with at least 75% of 1-rep-max or interval endurance training) was more effective than lower intensities (including steady-state cardio) for reducing insulin, blood lipids, and inflammatory markers. Accordingly, there may be a health advantage to performing interval endurance training relative to the classic steady-state, even if it doesn’t lead to more fat loss.

Exercise encompasses a diversity of modalities that have different effects on health. They may affect some areas similarly, such as fat loss, but evidence shows that they also have different effects on hormones, inflammation, and cardiometabolic risk factors. These differences cannot be overlooked when the goal is improving health.

Exercise may not be all that for fat loss, but it certainly impacts fitness and health improvement. As such, all forms of exercise should be encouraged despite their relatively minimal contribution to fat loss. Strength training is especially important for developing lean body mass. High-intensity training such as interval endurance training appears to be more effective at reducing inflammation and increasing insulin sensitivity than lower-intensity training such as steady-state cardio.

Frequently Asked Questions

Q. How can we encourage more people to be physically active?

Increasing physical activity at the population levels depends on numerous factors, including the efficacy of the chosen interventions that in turn are influenced by their policies, the environment in which they are enacted, the target population, and other important contexts like scalability and culture. In other words, there is no clear-cut answer to this question.

A review of this topic identified several strategies that do have some evidence to support their use. Individual-level policies that educate on behavioral skills such as goal setting, building social support, and problem solving appear to be effective in adults, whereas workplace interventions do not. Similarly, classroom- and college-based interventions do not have sufficient evidence supporting their use in children, adolescents, and young adults, but enforcing compliance via school-based physical education classes is highly effective at increasing physical activity levels.

Interestingly, interventions that use family-based social support appear to be relatively ineffective while those that build or strengthen non-family social support (e.g., walking groups or buddy systems) have strong evidence for use in Western nations. Moreover, community-wide campaigns that target social support and health education should be utilized over mass media campaigns. Finally, “point-of-decision” prompts such as motivational signs placed near stairwells or the base of elevators to encourage stair use are effective, as is changing the local environment to be more physically active friendly (e.g., walking trails and fitness centers).

What should I know?

Most people do not exercise regularly. A common excuse for the lack of exercise is a lack of time. Accordingly, interval training has been suggested to be a time-efficient alternative to the more classic steady-state endurance training that continues to be the poster child of aerobic exercise. The meta-analysis under review showed that both training modalities resulted in similar levels of fat loss, which was minor.

However, other evidence has shown that exercise has additional benefits on health that warrant its inclusion in daily life, such as reducing inflammation and increasing insulin sensitivity. Moreover, high-intensity exercise appears to be more effective than lower intensity exercise at inducing these beneficial changes, which might be one reason to prefer interval training over steady-state even if fat loss would be similar. Ultimately, though, adherence is key. Thus, enjoyment and personal preferences when it comes to exercise are what’s most important.