Carried out by researchers at Massachusetts General Hospital (MGH), the new study used a technique called Mendelian randomisation, which involves studying genetic variants to see if certain factors are associated with a higher or lower risk of disease, for example, showing higher or lower average levels of a factor such as physical activity because of inherited gene variants, which may be related to a risk of depression.

This type of analysis gives more reliable results, so if an association is found it is more likely to suggest a direct relationship.

For the analysis, the researchers identified gene variants from 377,234 participants who self-reported their levels of physical activity, and from 91,084 participants who had their physical activity measured by accelerometers, wrist-worn devices which measure movement.

The researchers also looked at data from 143,265 participants with depression.

The findings, published online in JAMA Psychiatry, suggest that higher levels of physical activity, as measured by the accelerometers, appear to protect against the risk of depression.

However, the same association was not found for self-reported physical activity.

The researchers suggest that this may be due to inaccurate reporting by participants, or due to the fact that the accelerometers measure everyday activities and not just workouts, for example walking to work, climbing the stairs, mowing the lawn, which the participants may not consider physical activity and therefore did not report.

The analysis did not show a causal relationship in the other direction, to suggest that depression reduces physical activity.

“Using genetic data, we found evidence that higher levels of physical activity may causally reduce risk for depression,” says lead author Karmel Choi, PhD. “Knowing whether an associated factor actually causes an outcome is important because we want to invest in preventive strategies that really work.”

“On average,” Choi says, “doing more physical activity appears to protect against developing depression. Any activity appears to be better than none; our rough calculations suggest that replacing sitting with 15 minutes of a heart-pumping activity like running, or with an hour of moderately vigorous activity, is enough to produce the average increase in accelerometer data that was linked to a lower depression risk.”