Kidney disease often develops quietly. A person can lose part of their kidney function without feeling ill, which means prevention may need to begin years before symptoms appear.
New research from Japan suggests that one useful sign of future kidney health could be how physically fit a person is today.
The study focused on cardiorespiratory fitness, which is the body’s ability to keep supplying oxygen during activities that last for several minutes or longer. It depends on the combined work of the heart, lungs, blood vessels and muscles. Higher fitness usually makes everyday activities easier and allows a person to exercise longer without becoming exhausted.
Researchers wanted to know whether this type of fitness could also predict how quickly the kidneys age. The question is increasingly important in Japan because the country has a rapidly aging population. Older age is associated with a gradual loss of kidney function and a greater chance of developing chronic kidney disease.
Healthy kidneys constantly clean the blood. They remove waste, control the amount of water and minerals in the body, help regulate blood pressure and produce hormones needed for several important functions. When kidney filtering ability falls, waste and fluid can begin to build up.
Chronic kidney disease, often called CKD, means the kidneys have been damaged or are not working as well as they should over an extended period. Diabetes and high blood pressure are major causes, but aging itself is also associated with declining kidney performance. Because early CKD may have few symptoms, blood and urine tests are often needed to detect it.
The Japanese research involved 198 adults with preserved kidney function who lived in or near Ibaraki Prefecture. Instead of simply asking participants whether they exercised, researchers measured their fitness with a cycling test. The test was performed below each person’s maximum effort, making it possible to estimate endurance capacity without requiring an all-out exercise session.
After measuring fitness, the researchers tracked participants’ kidney function for five years. They wanted to see whether people who started with greater endurance experienced a different pattern of kidney aging. The results suggested that they did.
People with higher cardiorespiratory fitness generally experienced a slower decline in kidney function over the following years. The researchers also examined the chance of losing at least 20% of kidney function during the study period. Fitter participants were less likely to reach this level of decline.
This relationship did not disappear when the researchers took several other health differences into account. Their analysis considered age, sex, body composition, smoking, existing illnesses and each participant’s kidney function at the beginning. This makes it less likely that one obvious factor alone explains the association.
The research was published in Medicine & Science in Sports & Exercise. It provides new evidence that physical fitness may be relevant to kidney health even before a person has clear kidney disease. Much previous research has focused on people who already have CKD, while this study examined adults whose kidney function was still relatively well preserved.
Why might fitness matter to the kidneys? One possibility is that fitter people tend to have healthier blood vessels and better circulation. The kidneys contain a large network of tiny blood vessels, so anything that protects the cardiovascular system could potentially help preserve kidney function.
Physical activity can also improve several major risk factors for kidney disease. It can help lower blood pressure, improve the body’s handling of blood sugar and support healthier body weight. These effects could reduce long-term strain on the kidneys.
There may also be indirect explanations. People with high fitness may eat differently, sleep better, spend less time sitting or follow medical advice more closely. Researchers can adjust their calculations for some of these differences, but no observational study can measure every possible influence.
That is the main reason the results cannot prove that higher fitness caused the slower kidney decline. Participants were not randomly assigned to become more or less fit. The study shows that two things were connected over time, but it cannot establish with certainty that one produced the other.
The small size of the study is another limitation. Only 198 adults were included, and they came from one region of Japan. A much larger study might find a weaker or stronger relationship, and results could differ in populations with different lifestyles, ethnic backgrounds or rates of diabetes and high blood pressure.
Even so, the five-year follow-up is a meaningful strength. Rather than comparing kidney function and fitness at only one moment, the researchers were able to watch how kidney health changed after fitness had been measured. This gives the study a better view of the direction of the relationship.
The findings also highlight the difference between exercise and fitness. Someone can report being active but still have relatively low endurance, while another person may have good fitness because of years of regular activity. Directly measuring fitness gives researchers a more objective picture of the body’s physical capacity.
For most adults, cardiorespiratory fitness can be improved through regular aerobic activity. Walking at a brisk pace, cycling, swimming and similar activities challenge the heart and lungs and can gradually increase endurance. People who have been inactive or have medical problems may need to begin slowly and discuss safe exercise with a health professional.
The study does not suggest that fitness can replace established ways of protecting the kidneys. Controlling diabetes and high blood pressure, avoiding smoking, using medicines appropriately and receiving recommended health checks remain important. Kidney disease has many causes, so no single lifestyle factor can eliminate the risk.
The most interesting implication is that kidney protection may begin while the kidneys still appear healthy.
If future intervention studies show that deliberately improving cardiorespiratory fitness slows kidney decline, exercise programs could become an even more important part of preventing CKD in aging populations. That would turn fitness from a useful marker into a possible tool for protecting kidney health.
For now, the findings should be viewed as promising rather than conclusive. They show a strong enough relationship to justify larger studies, but they do not prove that exercise will prevent kidney disease in an individual person. Still, they add another reason to consider physical fitness an important part of healthy aging, with possible benefits reaching well beyond the heart.
Source: University of Tsukuba.
