Mice raised in a complex environment providing social interactions, opportunities to learn and increased physical activity are less likely to get cancer, and better at fighting it when they do, a new study by researchers from the United States and New Zealand suggests.

A mild boost in stress hormones seems to be what keeps the cancer at bay by switching on a molecular pathway that restrains tumor growth.

After six weeks, mice who had been injected with melanoma cells and raised in an enriched environment — extra-large cages housing 20 individuals with running wheels and other toys — had tumors that were almost 80% smaller than those in mice raised in standard housing — five animals to a cage with no additional stimulation.

The “enriched” mice, the researchers found, had slightly raised levels of stress hormones, but the most striking physiological change was markedly reduced levels of the hormone leptin, known to regulate appetite. Blocking leptin abolished the effects of enrichment, suggesting that the hormone was key to the pathway that led to the anti-cancer effects.

Overexpressing BDNF in the hypothalamus, the researchers found, mimicked the protective effects of enrichment, suggesting BDNF, too, was a critical regulator of the protective pathway.