Physical activity, fitness and cancer

Background

Physical activity (PA) influences various biological mechanisms related to cancer development, and there is reason to believe that PA inhibits cancer development through these mechanisms. However, study results show relatively weak associations between PA and cancer risk. As of today (2023), there is strong evidence that PA reduces the risk of breast, uterine, and colorectal cancer. For other types of cancer, there is limited or no evidence. Challenges related to measuring physical activity and misclassification likely explain the weak associations.

When measuring physical fitness in an objective test, the test result reflects aerobic physical activity over time. In this project, we investigate whether physical fitness, assessed through repeated measurements, impacts cancer development (incidence/mortality) and cancer survival. We also explore whether associations between physical fitness and cancer outcomes differ from associations between self-reported physical activity and cancer outcomes.

Oslo Ischemia Study

The Oslo Ischemia Study includes around 2000 apparently healthy, working-age men aged 40-59 years, who participated in extensive cardiovascular health examinations in the 1970s and 80s. The first examination was conducted in 1973/74, with subsequent examinations every seven years. These health assessments included blood panels, heart and lung measurements, height and weight measurements, and a maximal exercise test on a bicycle (physical fitness). Additionally, participants answered a questionnaire on smoking habits and physical activity at work and during leisure time. Participants have been followed up for approximately 40 years regarding cancer incidence, mortality, and prognosis. The unique aspect of this study is that it provides information on measured physical fitness, as well as self-reported physical activity, complete cancer and death data, and a long follow-up period. All sub-projects are conducted within this male cohort.

Findings

Sub-project I: Many have studied the connection between cholesterol levels and prostate cancer risk, but without accounting for confounding factors and competing events. We found that low cholesterol levels, in addition to high physical fitness and low body mass index (BMI), might be associated with increased prostate cancer risk. These findings conflict with current recommendations for prostate cancer prevention.

Sub-project II: We have studied associations between physical fitness and cancer incidence, mortality, and prognosis. For all cancer types combined, we found a 15% lower incidence and a 32% lower cancer mortality for those with the best physical fitness, compared to those with the poorest physical fitness. Among those who developed cancer, good physical fitness was associated with a 26% better survival rate. We found no clear association between self-reported PA and cancer. Work-related PA was associated with increased cancer risk and mortality. These results highlight the challenges associated with self-reported physical activity.

Sub-project III: We found that men in good physical fitness have significantly reduced risk for cancer in the proximal colon (70%), lung (60%), bladder (60%), and pancreas (70%) compared to those in the lowest category of physical fitness. These associations are stronger than those previously shown in studies on PA. We also found that those with good physical fitness had more than double the risk of melanoma skin cancer, likely due to increased UV radiation from the sun. This assumption was confirmed in a collaborative study based on the Women and Cancer cohort (NOWAC), led by Prof. Marit B. Veierød (UIO). This finding underscores the importance of proper sun protection during outdoor activities.

Sub-project IV: Using health examination data (seven-year intervals), we examined whether changes in physical fitness, BMI, and smoking affect cancer risk and mortality. Men who maintained or improved their physical fitness had a reduced risk of cancer and cancer-related death. These results emphasize the significant benefits of mid-life lifestyle changes as cancer-preventive measures: maintaining or improving physical fitness (it's never too late to start being physically active, and one should never quit), preventing weight gain, and, most importantly, quitting smoking.

Sub-project V: We investigated associations between serum potassium and sodium levels and cancer risk. Increasing potassium levels were linearly associated with increasing cancer risk. Men with hyperkalemia had a 40% higher cancer risk than men with normal levels. Potassium might play a role in cell proliferation or differentiation, and individuals with high potassium levels could be a target group for cancer-preventive strategies. There was no association with sodium. These results are being followed up in a larger Swedish cohort (Malmö Primary Prevention cohort).