There already is considerable evidence that exercise in general can help to stave off and treat depression. A large-scale 2016 review that involved more than a million people, for instance, concluded that being physically fit substantially reduces the risk that someone will develop clinical depression.
Other studies and reviews have found that exercise also can reduce symptoms of depression in people who have been diagnosed. But most of these past studies and reviews have focused on aerobic exercise, such as walking or jogging.
One analysis of past research last year found strength training can help people feel less anxious and nervous. But anxiety is not depression.
For the new study, which was recently in JAMA Psychiatry, the same researchers turned their attention to depression.
The researchers began by gathering all of the best past studies related to resistance exercise and depression. They were interested only in randomised experiments with a control group, meaning that some people had been assigned to start exercising while others had not. The experiments also had to include testing for depression before and after the training.
The researchers ultimately found 33 experiments of weight training and depression that met their criteria.
The studies involved almost 2000 men and women of various ages, some of whom had been diagnosed with depression, while others had not. The researchers aggregated the results and then began digging through the data.
What they found was that resistance training consistently reduced the symptoms of depression, whether someone was formally depressed at the start of the study or not. In other words, if people began the study with depression, they usually felt better after taking up weight training. And if they started out with normal mental health, they ended the experiment with less chance of having become morose and sad than people who did not train.
Perhaps most interesting, the amount of weight training did not seem to matter.
The benefits essentially were the same, whether people went to the gym twice a week or five times a week and whether they were completing lots of repetitions or only a few.
The mental health impacts were similar, too, for men and women and for younger lifters (often college students) and people who were middle-aged or elderly. This kind of review cannot tell us, though, how strength training might be influencing mental health.
The exercise probably has both physiological and psychological consequences, said Brett Gordon, a graduate student at the University of Limerick, Ireland, who led the new review. The weight training could be changing aspects of the brain, including the levels of various neurochemicals that influence moods, he said.