"Both high and moderate intensities improved the composition of weight carried around the middle," Dr. Rose said.
Specifically, the study noted improvements in visceral adipose tissue among both the high- and moderate-intensity participants. Visceral fat is distinct from the subcutaneous fat found just under the skin; it is stored deep within the abdominal cavity, where it wraps around vital organs such as the liver, pancreas, and intestines.
Medical professionals often refer to visceral fat as "active fat" because it can influence how hormones function in the body. Excessive amounts of visceral fat are strongly linked to an increased risk of insulin resistance, type 2 diabetes, cardiovascular disease, and other chronic metabolic conditions. By effectively targeting this deep-seated fat, HIIT and moderate exercise provide benefits that go far beyond aesthetics, potentially lowering the risk of long-term illness.
However, the researchers were careful to manage expectations regarding the scale of these changes. While the improvements were statistically significant, they were relatively small. The team cautioned that the differences between the groups were not always "clinically meaningful" when compared directly to low-intensity exercise, especially when accounting for the inherent uncertainty in body composition measurements. Nevertheless, the trend suggested a clear hierarchy of benefit for those able to tolerate higher intensities.
Why HIIT May Help Aging Muscles Stay Strong
The physiological "magic" of HIIT likely stems from the specific type of stress it places on the body. Unlike traditional steady-state cardio, where the heart rate remains relatively constant, HIIT forces the body to adapt to rapid shifts in demand.
Associate Professor Mia Schaumberg, a physiologist at the University of the Sunshine Coast and co-author of the study, explained the mechanics of the training used in the trial. "High-intensity training in this study involved repeated short bursts, or intervals, of very hard exercise—where breathing is heavy and conversation is difficult—alternated with easier recovery periods," she said.
This "stop-and-go" nature of HIIT creates a unique stimulus for the muscular system. "HIIT likely works better because it puts more stress on the muscles, giving the body a stronger signal to keep muscle tissue rather than lose it," Schaumberg added.
From an evolutionary and biological perspective, the body is an efficient machine that seeks to conserve energy. If a person performs only light or moderate activity, the body may determine that it does not need to maintain expensive, energy-consuming muscle tissue during a period of fat loss. However, when the body is pushed to its limits during high-intensity intervals, it receives a signal that the muscle is essential for survival and performance, prompting the preservation of that tissue.
This preservation is vital for preventing sarcopenia. As muscle mass naturally declines with age, the threshold for disability becomes closer. Maintaining even a small amount of additional lean tissue can mean the difference between an active, independent lifestyle and one marked by frailty and a higher risk of falls.
More Recent Studies Reveal Additional Benefits of HIIT
