A new study has dashed hopes that people may be able to protect themselves from dementia through medicine, diet or exercise.

“To put it simply, all evidence indicates that there is no magic bullet,” Dr. Eric Larson wrote in the Annals of Internal Medicine.

The study outlined in the medical journal looked at four types of intervention to try to prevent dementia — prescription drugs, exercise, cognitive training, and nonprescription vitamins and supplements.

Researchers found none worked.

The Lancet, a British medical journal, ​reported earlier this year that about one-third of dementia cases could be linked to such conditions as cigarette smoking, high blood pressure, obesity, a lack of exercise and depression.

While Larson said there was no simple answer to the prevention of dementia, he highly recommended a commonsense, healthful lifestyle that may help delay the disease. It would involve exercising regularly, refraining from smoking, eating a healthful diet and taking part in activities that stimulate the brain.