To many, drinking and smoking go together like peanut butter and jelly — one hand helping the other.

But aside from causing comorbid illnesses, the two behaviors in combination may hasten a much speedier mental decline with age, according to researchers from University College London. Smoking and heavy drinking was associated in a large cohort study with a 36 percent faster mental decline, which was exacerbated by increasing consumption of alcoholic beverages.

"Current advice is that smokers should stop or cut down, and people should avoid heavy alcohol drinking," study leader Gareth Hagger-Johnson, told reporters. "Our study suggests that people should also be advised not to combine these two unhealthy behaviors — particularly from midlife onwards. Healthy behaviors in midlife may prevent cognitive [mental] decline into early old age."

Investigators followed nearly 6,500 adults ages 45-69 for 10 years, inquiring about tobacco and alcohol consumption and assessing brain function. Three times during the study, participants were tested on verbal and math reasoning, verbal fluency, and short-term verbal memory.

Those in the study who smoked and drank heavily aged the equivalent of 12 years, 20 percent faster than others.

"When we looked at people who were heavy-drinking smokers, we found that for every 10 years that they aged, their brains aged the equivalent of 12 years," Hagger-Johnson said. "From a public health perspective, the increasing burden associated with cognitive [mental] aging could be reduced if lifestyle factors can be modified, and we believe that people should not drink alcohol more heavily in the belief that alcohol is a protective factor against cognitive decline."

However, the study failed to prove a causal relationship beyond an association between smoking and heavy drinking with accelerated mental aging.

Source: Hagger-Johnson, Gareth, Sabia, Severine, Brunner, Eric John, Shipley, Martin, Marmot, Michael, et. al. Combined Impact Of Smoking And Heavy Alcohol Use On Cognitive Decline In Early Old Age: Whitehall II Prospective Cohort Study. British Journal Of Psychiatry. 2013.