“Obesity ‘bad for brain’ by hastening cognitive decline”, reports the BBC, with most of the mainstream national media covering the same story.
The news is based on the results of a large, long-term UK study. The study assessed whether people were overweight, obese, or had other metabolic problems and then tested their ability in cognitive function tests at intervals over a long period of time. The research found that cognitive decline over a 10-year period was similar in all participants, although participants who were both obese and had metabolic abnormalities showed the greatest decline.
It is already well-established that obesity and metabolic abnormalities are risk factors for a number of diseases. This study suggests that these conditions may also affect your brain function, though we do not know how much impact the small differences in cognitive function scores (such as a 0.7 score difference between obese and normal weight people with metabolic abnormalities) would have actually made to the person’s daily life and functioning. It is also important to be aware that the cognitive tests weren’t a diagnosis of dementia or Alzheimer’s disease.
Nevertheless, the study adds to the evidence for maintaining a healthy weight to optimise your overall health.
Where did the story come from?
The study was carried out by researchers from the French research centre INSERM, University College London and a number of French hospitals and other research institutes. It was funded by the National Institutes for Health, the Academy of Finland, the BUPA Foundation, UK, and the Medical Research Council, UK. The study was published in the peer-reviewed journal, Neurology.
The results of this study were widely reported. The majority of the coverage was accurate, although it should be emphasised that the researchers looked at cognitive function, and not the development of dementia. The Daily Telegraph’s headline that ‘Obese lose memory sooner’ misses the point that the research looked at a range of cognitive function tests, and that the results do not support this claim about memory alone.
What kind of research was this?
This was a prospective cohort study that followed participants for a decade and aimed to examine the association of body mass index (BMI) and metabolic status in midlife with cognitive function and decline.
A cohort study is the appropriate study design to investigate whether BMI or metabolic status affect cognitive decline. However, cohort studies cannot prove that BMI or metabolic status directly caused any differences in cognitive function, as there may be other unmeasured factors that are having an effect.
What did the research involve?
The researchers analysed data from 6,401 British civil servants (71.2% male), who were aged between 39 and 63 years old at the start of the analysis in 1991-1993. The researchers collected data on height and weight so that BMI could be calculated at this point. The participants were categorised into normal, overweight and obese using World Health Organisation classifications. At this point, the researchers also collected data on a number of metabolic factors, and participants were defined as having ‘metabolic abnormalities’ if they had two or more of the following:
- high levels of triglycerides (a type of fat) in their blood (greater than or equal to 1.69mmol/l), or if they were taking lipid (fat) lowering drugs
- high blood pressure (systolic blood pressure greater than or equal to 130mm Hg, diastolic blood pressure greater than or equal to 85mm Hg), or if they were taking antihypertensive drugs to lower their blood pressure
- high blood sugar (glucose greater than or equal to 5.6mmol/l), or if they were taking medication for diabetes
- low levels of high density lipoprotein (HDL) cholesterol, often considered to be ‘good cholesterol’ (HDL cholesterol less than 1.04mmol/l for men and less than 1.29mmol/l for women)
The researchers combined the data on BMI and metabolic status to define six categories:
- normal weight and metabolically normal
- normal weight and metabolically abnormal
- overweight and metabolically normal
- overweight and metabolically abnormal
- obese and metabolically normal
- obese and metabolically abnormal
The researchers assessed participants’ cognitive function at three time points:
They did this using tests of memory, verbal and mathematical reasoning, and verbal fluency (sematic and phonemic fluency). Scores on these tests were combined to give a global cognitive score.
The researchers looked at the association of BMI and metabolic status at study start, and cognitive function and decline over the 10-year period that was assessed. They adjusted for age, gender and education in their analyses, although they found no difference with gender and therefore presented the results for men and women together.
What were the basic results?
At the start of the study (baseline), 52.7% of participants were normal weight, 38.2% were overweight and 9.1% were obese. A metabolic abnormality was identified in 31.0% of people (18.3% of normal weight participants, 41.7% of overweight participants and 60.1% of obese participants).
Compared to participants with normal weight who were metabolically normal, all other categories had poorer cognition scores when they were first tested five years later. Among participants who were metabolically normal, cognitive scores decreased with increasing BMI (this means participants who were obese but metabolically normal had lower scores). However, there was no difference in cognitive scores with increasing BMI in participants who had a metabolic abnormality. There was a significant difference in scores between metabolically normal and metabolically abnormal participants in the normal-weight and overweight categories, but not for participants who were obese.
Over the 10-year follow-up, the cognitive scores in all groups decreased. The rate of decline was similar in all groups, suggesting that differences between groups did not change over time. However, there was a trend for increasing BMI in the metabolically abnormal group to be associated with faster decline (this means that people who were obese and metabolically abnormal had faster decline in cognition scores than normal weight and metabolically abnormal people). However, there was no significant difference in decline between obese metabolically normal and metabolically abnormal participants.
How did the researchers interpret the results?
The researchers concluded that “in these analyses the fastest cognitive decline was observed in those with both obesity and metabolic abnormality”.
So, does being obese put you at greater risk of dementia, as the Independent claimed? The results of this study do not prove this, but they do give us a better understanding of this complex area.
The research underpinning the headlines was a long-term cohort study measuring BMI and metabolic abnormalities in middle-aged participants, recruited from the British civil service. First testing cognitive function five years later, it found that there was some association between these risk factors and cognitive decline over the following 10 years. Cognitive decline over the 10-years of follow-up was similar in all participants, although participants who were obese and metabolically abnormal had the greatest decline. However, the metabolically normal obese group did not have significantly better cognition than the metabolically abnormal obese group at the end of the study.
It is already well-established that obesity, having abnormal amounts of fat or cholesterol in your blood, high blood pressure, and diabetes or abnormal blood sugar levels are risk factors for a number of diseases. This study suggests that these conditions may also be bad for your brain function, though we do not know how much impact the small differences in cognitive function scores would have actually made to the person’s daily life and functioning.
It is also important to be aware that the cognitive tests weren’t a diagnosis of dementia or Alzheimer’s. When interpreting these results it is important to note that the study was performed using participants from the civil service, the majority of which were male, and we don’t know how well the results can be applied to other population groups.
Nevertheless, this study adds to the evidence for maintaining a healthy weight to optimise your overall health.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.