A Swedish study published in Mental Health and Physical Activity has shed light on the well-known link between exercise and mental health. The researchers found that, when accounting for sedentary behavior, cardiorespiratory fitness does not appear to improve anxiety and depressive symptoms, while frequency of exercise does.

The link between exercise and mental health has been well-documented, and yet findings are limited when it comes to the type of physical activity that is most beneficial. The literature has yet to establish the relative importance of the frequency, intensity, and duration of exercise. As study authors Mats Hallgren and his team say, it is also unclear how cardiorespiratory fitness (CRF) relates to mental health.

CRF refers to the strength of the body’s circulatory and respiratory response to physical activity, and it tends to improve with regular exercise. In their study, Hallgren and his team set out to disentangle the relative importance of CRF and exercise frequency in the prevention of mental health symptoms. As the researchers say, filling in these gaps in research can help improve the design of exercise-based prevention strategies for mental health.

The researchers analyzed data from a general health assessment that was administered to a large number of Swedish employees. The sample was made up of 36,595 middle-aged men and women with an average age of 41. The questionnaires asked respondents how many times a week they had exercised in the past 30 days and how often they had experienced “worry, depressed mood or anxiety.” They also completed a test of cardiorespiratory fitness on a stationary bicycle and were then classified as either low, medium or high CRF.

First, the researchers found evidence that more frequent exercise was linked to improved mental health. Respondents who reported exercising at least 1-2 times a week were less likely to report symptoms of depression and anxiety, even after accounting for sedentary behavior — which was measured as the amount of leisure time participants reported spending sitting still.

As the researchers explain, sedentary behavior appears to play an important role in the link between exercise and mental health. Another study led by Hallgren found evidence that passive sedentary behaviors, such as TV-watching, increase depressive symptoms while mentally-active sedentary behaviors may actually prevent them.

Interestingly, cardiorespiratory fitness appeared to be less important when it came to the prevention of mental health symptoms. While respondents in the medium and high CRF groups reported fewer symptoms of depression and anxiety, this effect disappeared when sedentary behavior was taken into account.

“Taken together,” Hallgren and colleagues report, “this suggests that high CRF may not be necessary to prevent common mental health symptoms. Instead, regular participation in a preferred form of structured exercise may be of greater relative importance.”

The researchers discuss the possibility that the impact of cardiorespiratory fitness may only be relevant when comparing people with very low CRF levels to those with higher levels. Previous research suggests that CRF appears to be most beneficial to somatic health when looking at improvements among those with low fitness levels. It could be that the current study’s sample, which was made up exclusively of employed persons, did not include enough people with low fitness levels to capture such an effect.

A substantial limitation was that symptoms of depression and anxiety were measured using a single item, and the researchers stress that this measure does not reflect a diagnosis of depression or anxiety. Moreover, symptoms of anxiety and symptoms of depression are distinct and would be better assessed separately in future studies.

The researchers conclude that regular exercise may be enough to lower anxiety and depressive symptoms among most people, while it may be useful to encourage exercises that boost CRF among people with particularly low levels of fitness.

The study, “Associations of exercise frequency and cardiorespiratory fitness with symptoms of depression and anxiety – a cross-sectional study of 36,595 adults”, was authored by Mats Hallgren, Aaron Kandola, Brendon Stubbs, Thi-Thuy-Dung Nguyen, Peter Wallin, Gunnar Andersson, and Elin Ekblom-Bak.