These are the findings of a new study published online in the journal Neurology on Tuesday.
Investigators from the the French research institute INSERM in Paris used data on 6,401 people who took part in the Whitehall II study of British civil servants.
The participants, 71% of whom were men, were of average age 50 in 1991-1993 which the investigators define as the start of their study period. At this point, they also gathered data on the participants' metabolic status and body mass index, BMI, a well-known measure of obesity used in research that equals the person's weight in kg divided by the square of their height in metres.
And then, three times over the following ten years, in 1997-1999, 2002-2004, and 2007-2009, the participants also underwent tests of cognitive skills such as memory, reasoning, and verbal fluency.
When they analyzed the data, the researchers defined normal BMI as in the range 18.5-24.9 kg/m2, overweight as 25-29.9 kg/m2; and obese as 30 kg/m2 or more.
And they defined metabolic abnormality as having two or more of the following risk factors:
- High blood pressure, or taking medication for it,
- Low HDL or "good" cholesterol,
- High blood sugar or taking medication for diabetes,
- High blood levels of triglycerides (blood fats) or taking medication to lower them.
People who are overweight may experience a decline in memory and thinking skills, according to experts.
350 of the 582 participants in the obese category also met the criteria for metabolic abnormality.
When they analyzed the data over the 10 years of the study, the researchers found participants who were both obese and metabolically abnormal showed a 22.5% faster decline in memory and thinking skills than participants of normal weight and no metabolic abnormality.
The researchers conclude:
"In these analyses the fastest cognitive decline was observed in those with both obesity and metabolic abnormality."
However, being obese but metabolically normal was not good news either, because participants in this category still experienced more rapid decline than normal weight, metabolically normal participants.
Lead author Archana Singh-Manoux, is with INSERM in Paris and University College London in the UK.
She told the press the study results do not support the idea of "metabolically healthy obesity" that suggests obese people without metabolic risk factors are less likely to suffer from heart problems and cognitive decline than obese people with metabolic risk factors.
"More research is needed to look at the effects of genetic factors and also to take into account how long people have been obese and how long they have had these metabolic risk factors," said Singh-Manoux, adding there was also a need to:
".. look at cognitive test scores spanning adulthood to give us a better understanding of the link between obesity and cognitive function, such as thinking, reasoning and memory."
Written by Catharine Paddock PhD