High intensity interval training involves short bursts of intense aerobic activity within a stretch of more moderate exercise: intermittently sprinting for 30 seconds, for example, in the middle of a moderate-pace jog.
For the National Institutes of Health-funded study, Nair and his colleagues enlisted the help of both men and women from two age groups: The “young” volunteers ranged in age from 18 to 30; “older” volunteers ranged in age between 65 and 80. Next, the researchers divided these participants into three mixed-age groups and assigned each a different supervised exercise training program lasting three months.
The high-intensity interval training training group did three days a week of cycling, with high-intensity bouts sandwiched between low-intensity pedaling, and two days a week of moderately difficult treadmill walking. The strength training group performed repetitions targeting both lower and upper body muscles just two days each week. Finally, the combined training group cycled (less strenuously than the first group) and lifted weights (fewer repetitions than the second group) for a total of five days a week.
There were clear differences, then, in the amount of time different participants spent in the gym.
Before and after each training session, the researchers assessed various aspects of each volunteer’s physiology, including body mass index, quantity of lean muscle mass and insulin sensitivity, one indication of diabetes. The researchers also did routine biopsies of each volunteer’s thigh muscles and performed a biochemical analysis in order to establish a comprehensive fingerprint of the muscle.
Analyzing the gathered data, Nair and his colleagues found that all forms of exercise improved overall fitness, as measured by cardiorespiration, and increased insulin sensitivity, which translates into a lower likelihood of developing diabetes. Although all exercise helped with musculature, strength training was most effective for building muscle mass and for improving strength, which typically declines with age.
Meanwhile, at the cellular level, high-intensity interval training yielded the biggest benefits.
Specifically, in the HIIT group, younger participants saw a 49% increase in mitochondrial capacity, while older participants saw a 69% increase. Most cells in our bodies contain infrastructure known as mitochondria. These “organelles” — a mini-version of an organ within a cell — perform as tiny batteries do, producing much-needed energy.
Interval training also improved volunteers’ insulin sensitivity more than other forms of exercise. Drilling down deeper, Nair and his colleagues compared the protein-level data gathered from participants to understand why exercise provided these benefits.
Enhancing your cellular machinery
If we think of the cell as a corporate hierarchy, genes (DNA) are the executives issuing orders to their middle managers: messenger RNA. Tasked with transcribing this order, the RNA turns to ribosomes, which perform a supervisory role by linking amino acids in order to assemble protein molecules. Finally, the proteins, cellular work horses, carry out the task originally dictated by the gene.
“Proteins sustain environmental damage and the damaged proteins have to be … replaced with newly synthesized (produced) proteins,” explained Nair in an email. “With aging in sedentary people, production of many protein molecules decline. … Gradually the quantity of these protein molecules decrease causing functional decline.”
Analyzing the muscle biopsies, the researchers discovered that exercise boosts cellular production of mitochondrial proteins and the proteins responsible for muscle growth.
“Exercise training, especially high intensity interval training, enhanced the machinery (ribosomes) to produce proteins, increased the production of proteins and enhanced protein abundance in muscle,” Nair said. He said the results also showed that “the substantial increase in mitochondrial function that occurred, especially in the older people, is due to increase in protein abundance of muscle.”
In some cases, the high-intensity regimen actually seemed to reverse the age-related decline in both mitochondrial function and muscle-building proteins.
Exercise’s ability to transform mitochondria could explain why it benefits our health in so many different ways, according to the authors. Muscle cells, like brain and heart cells, are unusual in that they divide only rarely compared with most cells in the body. Because muscle, brain and heart cells do wear out yet are not easily replaced, the function of all three of these tissues are known to decline with age, noted Nair.
If exercise restores or prevents deterioration of mitochondria and ribosomes in muscle cells, exercise possibly performs the same magic in other tissues, too. And, although it is important simply to understand how exercise impacts the mechanics of cells, these insights may also allow researchers “to develop targeted drugs to achieve some of the benefits that we derive from the exercise in people who cannot exercise,” Nair said.
‘Almost a medicine’
According to Jennifer Trilk
, an assistant professor of physiology and exercise science at University of South Carolina School of Medicine Greenville
, the new study is comprehensive and supports previous research, combining it all into one paper.
“We cannot have enough studies surrounding this information because of how impactful it is for health,” said Trilk, who was not involved in the research. She explained that if younger people boost mitochondrial function when they’re young, they would be preventing disease, while for an older population, they would also be preventing disease while maintaining skeletal muscle, which wanes in older age.
“Mitochondrial function is important to almost every cell in the human body,” Trilk said. “So when you don’t have mitochondrial function or when you have mitochondrial dysfunction, you have dysfunction of cells, so from a molecular standpoint, you start seeing cellular dysfunction years before you start seeing the global effect, which ends up coming out as symptoms of diseases: diabetes, cancers and cardiovascular disease.”
Juleen Zierath, a professor of integrative physiology at Karolinska Institutet in Stockholm, Sweden
, finds the study to be “a really comprehensive and thorough analysis of human skeletal muscle before and after” adapting to different exercise regimens. Zierath, who did not participate as a researcher in the current study, also appreciated the fact that the authors comprehensively examined the effects on both younger and older participants.
“A strength is that they studied males and females,” Zierath said, though she noted that the number of participants in each group was “quite small.” Still, this is a minor flaw.
“It teases out some of the training regimes that might be leading to greater effects on what they call mitochondrial fitness,” she said. Compared with the other two exercise programs, interval training “really had a more robust effect” on the machinery of cells, she said.
“It boosted the proteins that are important for mitochondrial function — the oxygen powerhouse of the cells,” Zierath said. “It reversed many of what we call age-related differences in mitochondrial function and oxidative metabolism.”
“Part of what happens with HIIT is, you disturb homeostasis, you exercise at a really high level, and the body needs to cope with that,” she explained.
Even though one program had superior effects, “every single exercise protocol they tested had positive effects,” said Zierath, who is looking forward to future research in this vein.
“We need to understand even more about how the human body adapts to different exercise regimes and how this can be important for mitigating what we see as sort of aging-related changes that occur in the functionality of muscle and the ability of the muscle to metabolize fuel, sugar and fat,” she said.
“Exercise is almost a medicine in some respects,” Zierath said. “It’s never too late to start exercising.”