Body fat 'linked to Alzheimer's'

Having a “pot belly” in middle age raises the risk of Alzheimer’s disease and dementia later in life, according to the Daily Mail.

The news is based on research into whether total brain volume was associated with measurements such as body mass index (BMI), waist size, fat under the skin and fat around the organs. As part of the study, several hundred middle-aged participants had their body fat and brains scanned. The results suggested that a larger waist and more fat surrounding the organs were both associated with decreased brain volume. However, this early research did not examine whether any participants went on to develop Alzheimer’s or dementia.

This was preliminary research and the implications of these findings are currently unclear, although they should not be seen as evidence that body fat causes Alzheimer’s disease. Further research is needed to further investigate how body fat may affect the brain with age.

Where did the story come from?

The study was carried out by researchers from Boston University School of Medicine and was funded by a number of US government health institutes: the National Heart, Lung and Blood Institute, National Institute of Neurological Disorders and Stroke, and National Institute of Aging.

The study was published in the peer-reviewed medical journal Annals of Neurology.

The newspapers reported that this research had found a direct link to Alzheimer’s disease. However, the research looked at brain volumes rather than any clinical outcomes, such as Alzheimer’s disease or dementia. Therefore, it is not possible to say that there is an increased risk based on this study. The newspapers also highlighted that “middle-age spread” or carrying excess weight in middle age increased the risk. However, as body fat and brain volume measurements were both taken at a single point in time, it cannot be said whether one caused the other. Equally, even if the two factors are linked, this study cannot tell us why this may be.

What kind of research was this?

It has been suggested that global body mass and obesity, particularly during middle age, are associated with an increased risk of dementia and Alzheimer’s disease. The authors of this study wanted to see if there was an association between BMI and obesity and changes to brain volume.

This cohort study included participants from a larger cohort study, called the Framingham Offspring Cohort.

What did the research involve?

The study included 5,124 participants who were examined approximately every four years. A total of 4,379 were alive at the time of the seventh cycle, which took place between 1998 and 2001. Of these, 3,539 (average age 60 years) attended an examination at which the researchers calculated a number of bodily measurements: BMI, waist circumference, hip circumference and waist-to-hip ratio.

Between 2002 and 2005 as part of a secondary study, 1,418 participants had CT scans to measure their levels of subcutaneous fat (the fat just under the skin) and visceral fat (the fat between the internal organs and the muscles of the torso). The average age of the participants when they had a CT scan was 64 years.

The participants were invited to undergo a brain MRI scan, which was performed on 1,399 patients. The average age of the participants when they had a brain scan was 67. In total, 733 participants had both an interpretable abdominal CT scan of their body fat and a usable MRI scan of their brain.

The researchers also measured other factors that could contribute to the likelihood of dementia or changes to the brain. These were risk of stroke, how physically active the participants were and the responsiveness of their insulin system (a marker for diabetes).

What were the basic results?

The researchers observed that old age, diabetes and high blood pressure were associated with increased BMI, waist circumference, waist-to-hip ratio and the amounts of both subcutaneous and visceral fat. Levels of visceral fat and subcutaneous fat were also associated with each other.

Higher levels of all of the bodily measurements (BMI, waist ratio etc.) and both types of fat were associated with a smaller total brain volume. This association remained after statistical adjustments were made to account for the influence of blood pressure, smoking, diabetes, history of heart disease and the amount of exercise performed. Both types of fat were associated with decreased brain volume, but visceral fat appeared to have a stronger association than subcutaneous fat. However, after adjusting the values to take into account a marker of diabetes, the association between the fat measurements and brain volume was weakened and no longer significant.

Using MRI brain scans, the researchers also measured the volume of the fluid-filled spaces (ventricles) of the brain. These ventricles increase in size as brain volume decreases. They looked at a particular region of the ventricles called the temporal horn. This lies next to a brain structure called the hippocampus which is associated with short term memory. The researchers said that temporal horn volume can be used as a surrogate marker of the volume of the hippocampus and a larger temporal horn volume corresponds to a smaller hippocampus volume. Only waist-to-hip ratio was associated with enlargement of the temporal horn.

How did the researchers interpret the results?

The researchers said that both greater body size markers and greater CT-measured abdominal fat were associated with lower total brain volume in their middle-aged community of participants. The most prominent of these associations was with visceral fat.

Conclusion

This study showed that decreased brain volume was associated with higher BMI, waist-to-hip ratio, visceral fat level and subcutaneous fat level in participants aged on average 60 to 67 years. Although this study followed a relatively large group of participants, which is a strength, there are a few limitations that should be taken into account.

• As this was a cross-sectional analysis, it looked at participants at only one point in time, rather than following them over time. As brain volume and body measurements were measured at the same time, the study cannot show whether one caused the other or how any relationship between them might work. It is possible that there is a natural variation in brain volume over time, which cannot be captured by this single measurement.
• The study cannot tell us whether the body or brain volume measurements have any link to the development of dementia or Alzheimer’s in particular, as the study did not follow up any of the participants to assess whether they went on to develop cognitive impairment. Further research is needed to see if fat-related changes to brain volume are associated with risk of developing dementia.
• Participants with stroke and current dementia were excluded from the study. Those that were included may not be representative of the general middle-aged population as they may have fewer risk factors for dementia than people in these excluded groups.

The researchers say that this work was “exploratory”, and this study does warrant further research.

Regardless of a theoretical link to Alzheimer’s disease, there is a clear, known association between high BMI and fat (particularly fat around the stomach) and a greater risk of diabetes and heart disease. This was early, speculative research and it seems sensible for people to adopt a healthy diet and lifestyle to reduce these known risks, rather than being overly concerned with any possible link to dementia.