A daily workout habit is hard to build from scratch. A randomized trial from Hong Kong tested a smaller starting point: one high-intensity interval training (HIIT) day a week, compared with three shorter HIIT days and with no exercise program.[1]
What the trial did
The trial enrolled 315 adults with overweight and central obesity (extra weight around the waist). About 65% were women, and the average age was 48. People were randomly assigned to three groups of 105: HIIT once a week, HIIT three times a week, or a control group that attended health education classes every other week. Both HIIT groups did 75 minutes of supervised treadmill HIIT per week for 16 weeks, so the weekly total was the same and only the split differed. Participants were ethnic Chinese adults with a BMI of 23 or more who had not exercised regularly in the past three months; intensity was set from a maximal fitness test, and they built up over four weeks before the full 75 minutes.[1]
One HIIT bout lasted 25 minutes: four hard 4-minute intervals at 85 to 95% of peak heart rate, with 3-minute easy recoveries between them. The thrice-weekly group did one bout per visit. The once-weekly group did all three bouts in a single visit, with 15- to 30-minute breaks between them, so it was not one continuous 75-minute workout.[1]
What it found
At week 16, compared with the control group, total fat mass was 0.8 kg lower in the once-weekly group (95% CI 0.2 to 1.4 kg) and 1.0 kg lower in the thrice-weekly group (95% CI 0.5 to 1.6 kg). Those are modest changes. Both HIIT groups also had lower body fat percentage and waist circumference and better cardiorespiratory fitness than the control group at week 16, and no study-related adverse events were reported.[1]
The effects did not all last. At week 32, about four months after the program ended, both HIIT groups still had smaller waists than the control group, but the differences in total body fat mass were no longer statistically significant. Only the thrice-weekly group still had a fitness gain at week 32. Blood pressure, fasting blood sugar, and cholesterol did not change. The authors suggested this may be because participants started in a fairly healthy range for those measures.[1]
What this does and does not tell you
This was a single-center trial of adults with overweight and central obesity, so the results may not apply to everyone. The sessions were supervised and heart-rate monitored. The authors also note that it is unknown whether a modest drop in fat mass leads to long-term health benefits.[1]
Who should check with a doctor first
HIIT is vigorous exercise. The American College of Sports Medicine’s screening model for starting exercise looks at three things: how active you are now, whether you have signs or symptoms or known heart, metabolic, or kidney disease, and how hard you plan to exercise.[2] If you are not active now and have symptoms such as chest discomfort or unusual shortness of breath, or a known heart, metabolic, or kidney condition, talk with your doctor before starting interval training. Chest discomfort or unusual breathlessness warrants a doctor visit whether or not you are currently active. If you have not been exercising, the usual advice is to start with light-to-moderate activity and build up; interval training at the trial’s intensity is not a starting point.
The takeaway
If a daily workout habit has never stuck, this trial shows a smaller plan can still help: a supervised, once-weekly HIIT day was linked to a modest drop in fat mass after 16 weeks compared with no exercise program. Expect a change of about 1 kg, not a transformation, and know that the waist change lasted longer than the fat-mass change. Pick a schedule you can keep, one day or three.
References
↑︎ Siu PM, Leung CK, Bernal JDK, et al. Once and thrice weekly interval training in adults with central obesity: a randomized controlled trial. Nat Commun. 2026;17(1):1410. DOI: https://doi.org/10.1038/s41467-025-68149-7
↑︎ Riebe D, Franklin BA, Thompson PD, et al. Updating ACSM’s recommendations for exercise preparticipation health screening. Med Sci Sports Exerc. 2015;47(11):2473-2479. DOI: https://doi.org/10.1249/mss.0000000000000664
Kurt Greiner
Kurt is a digital strategist and IT professional blending emerging technology with practical application to help businesses and individuals streamline their digital presence. His current work focuses on the intersection of intentional living and technological resilience, exploring how individuals can leverage modern tools to navigate the second half of life with purpose.
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