Keeping the heart and lungs fit may do more than make exercise easier.
A new study from researchers in Japan suggests that people with better cardiorespiratory fitness may experience a slower decline in kidney function as they grow older.
The findings raise the possibility that staying physically fit earlier in life could help protect the kidneys later on.
The kidneys perform several jobs that are essential for survival. They filter waste and extra water from the blood, help control blood pressure, balance important minerals and support the production of red blood cells. Kidney function naturally tends to decline with age, but the speed of that decline varies greatly from one person to another.
When kidney function falls too far, a person may develop chronic kidney disease, or CKD. Early CKD often causes no obvious symptoms, so people may not know anything is wrong until the condition has progressed. More advanced kidney disease can lead to tiredness, swelling, changes in urination and a higher risk of heart disease.
Japan has one of the world’s oldest populations, making healthy aging an especially important public health issue. As more people live into their 70s, 80s and beyond, conditions related to aging, including declining kidney function, are becoming increasingly common. Researchers are therefore interested in finding practical ways to slow this decline before serious disease develops.
One possible protective factor is cardiorespiratory fitness. This describes how well the heart, lungs, blood vessels and muscles work together to supply the body with oxygen during sustained physical activity. People with higher fitness can usually perform activities such as brisk walking, cycling or climbing stairs for longer before becoming tired.
The researchers studied 198 adults living in and around Ibaraki Prefecture who had preserved kidney function at the beginning of the research. Their fitness was measured using a cycling exercise test that did not require participants to exercise until complete exhaustion. The team then followed changes in their kidney function for five years.
The results showed a clear pattern. Participants with higher cardiorespiratory fitness tended to have a slower age-related decline in kidney function. This relationship remained even after the researchers considered other factors that could influence kidney health.
Those factors included age, sex, body composition, smoking, other medical conditions and kidney function at the start of the study. Adjusting for these differences is important because fitter people may also have healthier lifestyles in other ways. Even after these adjustments, better fitness remained linked with slower kidney decline.
The researchers also looked at whether participants experienced a substantial loss of kidney function during follow-up. People with higher cardiorespiratory fitness were less likely to experience a decline of 20% or more. A change of this size can be clinically important because it may signal meaningful worsening of kidney health.
The study was published in the journal Medicine & Science in Sports & Exercise. It adds to growing evidence that physical fitness is connected with health throughout the body, rather than benefiting only the heart and muscles. Previous research has linked better fitness with lower risks of cardiovascular disease, disability and early death.
There are several possible reasons fitness could be connected with healthier kidneys. Regular physical activity can help control blood pressure, blood sugar and body weight, all of which can affect kidney health. Exercise can also improve the function of blood vessels, potentially helping the kidneys receive a healthier supply of blood.
However, the researchers did not prove that becoming fitter directly prevents kidney decline. This was an observational study, meaning the team measured fitness and watched what happened over time rather than randomly assigning people to different exercise programs. Other differences between participants could still partly explain the results.
The study was also relatively small, with fewer than 200 participants from one area of Japan. Larger studies involving people from different countries, ages and backgrounds will be needed to determine how widely the findings apply. Clinical trials could also test whether improving fitness through exercise actually slows kidney decline.
Another important point is that cardiorespiratory fitness is not exactly the same as how much exercise someone reports doing. Fitness reflects the body’s actual ability to perform sustained activity and is influenced by exercise habits, age, genetics and health. Measuring it directly may therefore provide information that questionnaires about physical activity can miss.
The findings are encouraging because fitness can often be improved. Activities such as brisk walking, cycling, swimming and other forms of regular aerobic exercise can gradually strengthen the heart and lungs. The appropriate amount and intensity depend on a person’s age, health and physical ability.
For people who already have kidney disease or other medical conditions, exercise plans may need to be adjusted with professional advice. The study focused on adults whose kidney function was still preserved, so its results cannot automatically be applied to people with advanced CKD. That distinction is important when interpreting the findings.
Overall, the research offers a useful new clue about healthy aging. Its strongest feature is that kidney function was followed for five years after fitness was objectively measured, allowing researchers to examine changes over time rather than relying on a single snapshot. The association also remained after many common kidney risk factors were considered.
At the same time, the study should not be taken as proof that exercise alone can prevent chronic kidney disease. Kidney health is affected by many factors, including diabetes, high blood pressure, medicines, genetics and other illnesses. Still, maintaining good cardiorespiratory fitness may be one practical part of a broader strategy for protecting health as people age.
If future trials confirm a direct protective effect, encouraging people to build and maintain fitness before kidney problems appear could become an important prevention strategy. The findings suggest that looking after the heart and lungs may also help the kidneys age more slowly. That possibility is especially valuable in societies where people are living longer than ever before.
Source: University of Tsukuba.
