Study: Once-weekly interval training cuts belly fat nearly as much as three sessions

Many adults find it hard to carve out time for exercise. Although health specialists often advise several workouts each week, work and family commitments frequently get in the way.

A new trial brings some reassuring evidence for people carrying extra belly fat.

Led by Professor Parco Siu at the University of Hong Kong (HKU), the study found that completing one longer interval training workout each week reduced body fat by almost the same amount as doing three shorter sessions across the week.

The changes were not large, but they indicate that regular training can still pay off even when free time is limited.

Belly fat increases health risks

Fat stored around the abdomen is considered more harmful than fat carried elsewhere, as it is linked with a higher likelihood of heart disease, diabetes, and premature death.

In a review covering more than 2.5 million people, each additional 10 cm around the waist was associated with an 11 percent increase in the risk of early death. That is why reducing belly fat can be worthwhile.

The difficulty is persuading people to exercise often enough to lose it, with lack of time most frequently cited as the main obstacle.

Workout schedules for belly fat

Researchers enrolled 315 adults who were overweight and had excess fat around the waist. Participants were, on average, 48 years old, and nearly two-thirds were women.

Eligibility required a waist circumference of at least 90 cm for men and at least 80 cm for women. Everyone had been inactive for months before joining the trial.

Participants were randomly allocated to one of three groups: a single weekly interval training session, three shorter interval sessions per week, or health education classes without exercise.

Those in the class-only group attended health talks every two weeks and did not complete any workouts.

Equal workout time for belly fat

Both exercise groups completed the same weekly treadmill workload: 75 minutes of interval training, alternating four minutes hard with three minutes easy.

Seventy-five minutes matches the weekly volume of vigorous physical activity recommended by the World Health Organization (WHO). The once-weekly group performed the full amount in one visit.

The research team also included Martin Gibala of McMaster University, who has studied brief, high-intensity training for many years.

The trial was registered on ClinicalTrials.gov before it started, and no exercise-related harms were reported.

What one session looked like

The single weekly workout was substantial rather than short. Participants completed three 25-minute interval blocks consecutively, with a 15 to 30 minute rest period between blocks.

Within each block, there were four 4-minute efforts at 85 to 95 percent of peak heart rate, separated by three minutes of easy walking. During the first four weeks, the programme ramped up gradually before participants moved to the full training load.

The main time advantage appeared outside the gym. When travel and the workout itself were combined, the once-weekly group spent about 4.5 hours per week on exercise-related time, compared with 8.2 hours for the three-sessions-per-week group.

One session, less body fat

At the end of 16 weeks, both exercise groups had reduced body fat, while the class-only group had not. Average fat loss was about 0.8 kg in the once-weekly group and about 1.0 kg in the three-times group.

The difference between the two exercise schedules was too small to be considered meaningful. Waist size fell by roughly 2.5 cm in both exercise groups, whereas there was no such change in the class group.

Expressed as percentages, body fat dropped by 2.6 percent in the once-weekly group and 3.5 percent in the three-times group. Adherence was high overall, with 94 percent completing the full 16-week period.

“While thrice-weekly interval training remains a commonly recommended approach for the therapeutic management of excess adiposity, our findings show that once-weekly interval training offers similar benefits and represents a practical exercise strategy,” said Professor Siu.

“For many adults with central obesity who struggle to balance work, study, family and other commitments, time constraints are a primary barrier to exercising multiple days per week.”

Belly fat workout improved fitness

Aerobic fitness also improved in both training groups, reflecting better ability to use oxygen during demanding exercise.

On average, fitness increased by around 6 percent in the once-weekly group and around 11 percent in the three-times group.

In this area, the schedules did not perform equally. After accounting for age, sex, and other factors, the three-times group showed a clear advantage in fitness.

Only the three-times group achieved a meaningful reduction in overall body weight and lowered its body-mass index. No group showed changes in blood pressure, blood sugar, or cholesterol.

Most participants began the trial with these health markers already within a healthy range, leaving limited scope for improvement.

Four months later

Training ended at 16 weeks, and researchers assessed participants again four months later.

By that stage, the difference in fat loss between the exercise groups and the class-only group had narrowed, although both exercise groups still had slightly smaller waists.

The study also leaves important issues unresolved. It remains unclear whether fat loss of this size translates into lower long-term risks of heart disease or premature death.

Participants were Chinese adults who were relatively healthy at the outset. The findings may not apply to people with diabetes or other conditions, and the study was not designed to prove that one weekly session truly matches three.

“Instead of relying solely on high frequency exercise prescriptions, once weekly interval training can be considered a feasible and effective alternative,” noted Professor Siu.

For people who cannot manage exercise on several days each week, one longer weekly session may be the option they are most able to maintain.