More than 5 million Americans are living with Alzheimer's disease, and that number could quadruple by 2050.  

Research has suggested people can reduce their risk for dementia with regular exercise, maintaining a healthy weight, and not smoking.  But a new study finds that romantic relationships may also protect against Alzheimer's disease.  

Doctors found that people living alone at mid-life were twice as likely to develop dementia or Alzheimer's disease and those who had been widowed had even greater risk - three times higher than people who were cohabitating or married during middle age. 

Researchers note that couple-hood has several advantages: couples watch out for one another's health.  And the mental and social stimulation of having a partner may help keep the brain fit.