Obesity Does Not Accelerate Mental Decline in Older Age

CHICAGO, Sept. 20 -- Obesity in older age appears not to be the risk factor for cognitive decline that it is in middle age, researchers found.

Overweight and obese adults 65 and older lost cognitive function at about the same rate as those who were of normal weight, regardless of race (P=0.539 black and P=0.065 non-black), said Maureen T. Sturman, M.D., M.P.H., of Rush University Medical Center here, and colleagues online in the journal Neurology.

For older adults, "it may be that lifestyle guidelines such as increasing cognitive activity, cardiorespiratory fitness, and dietary recommendations are more important than those that focus on weight loss alone," they suggested.

Obesity has been linked to development of hypertension, diabetes, and coronary artery disease in older age, which could have an indirect detrimental effect on the brain, they noted.

Studies of obesity in middle age have also shown increased risk for later cognitive decline and dementia.

But, the association of weight loss with aging and dementia has confounded the role of weight in late life, and studies of the association in older age have not had diverse populations, the researchers noted.

So they analyzed longitudinal data from the Chicago Health and Aging Project, which was designed to look at risk factors for a range of chronic diseases among adults age 65 and older in a three-neighborhood area on the south side of the city.

In the study, 3,885 participants (61% women, 61% black) were given four cognitive tests at baseline and every three years.

Participants' mean body mass index (BMI) at baseline was 27.1 kg/m²; 24.6% were obese with a BMI of at least 30 kg/m²; 37.4% were overweight with a BMI from 25 to 29.9 kg/m²; and 2.0% were underweight with a BMI less than 18.5 kg/m².

At baseline, there was a significant curvilinear association between BMI and cognitive function scores in both black (P=0.001) and non-black participants (P=0.002), suggesting that both higher and lower BMI would predict worse cognitive function than normal weight.

But only underweight was significantly associated with worse global cognitive function at baseline when BMI was treated as a non-continuous variable (P=0.001 for both racial groups).

Cognitive function declined over the average 6.4 years of follow-up for both black and non-black participants (P=0.001 for both).

However, a higher BMI was associated with less decline than was seen in normal weight participants (P=0.009 in blacks and P=0.006 in others).

This potentially protective effect could be due to the effect of leptin on spatial memory and synaptic plasticity as seen in animal studies or insulin-like growth factor levels or enhanced cerebral blood flow, the researchers colleagues suggested.

The rate of cognitive decline was not associated with BMI for black or (P=0.539) or non-black participants (P=0.065).

There was a small but significant three-way interaction with age (P=0.001), "suggesting that at older ages higher BMI is associated with slightly less cognitive decline than at lower ages."

Adjusting for comorbid illness did not change the associations substantially, but excluding the 1,010 patients with preclinical or early dementia at baseline did.

Excluding those with a Mini-Mental State Examination score less than 24 attenuated the curvilinear association between BMI and baseline cognitive function such that it was no longer significant for black participants (P=0.227), although it remained significant for non-black participants (P=0.030).

Excluding these participants also reduced the effect of BMI on change in cognitive function by 43% in black subjects and 25% in white subjects and eliminated the significance of this association for both groups (P=0.415 and P=0.086, respectively).

"These results, therefore, suggest that obesity in old age does not have a substantial relation to cognitive decline in a cognitively healthy community population," the investigators concluded.