We know obesity can increase our risk of physical health problems.  Might being obese affect our brain as well as our body?

The physical health risks of obesity are well known. In England, for instance, obese adults are five times more likely to be diagnosed with diabetes than adults of a healthy weight.

But might obesity be bad for our brains as well as our bodies?

Can obesity increase the risk of cognitive problems as we grow older?

Throughout our lives obesity seems to be associated with decreases in:

• Our ability to focus/pay attention;
• The speed with which our brain can send messages to achieve small movements of our hands, wrists, fingers, feet, toes, lips, and tongue;
• How quickly we can carry out a simple or automatic mental task.

This is what researchers describe as a decrease in attention, fine motor speed and processing speed.

Obesity also seems to be associated with some reduction in executive function as we get older. This is a set of mental skills that helps us get things done. For example executive function helps us manage our time, plan and organise, remember details and change our focus.

That’s according to an American study, based on research with 732 people aged 18 – 88, with different BMI levels.

Does obesity plus metabolic risk factors accelerate mental decline?

Metabolic risk factors increase your risk of heart attack, stroke and diabetes. They include high levels of blood sugar and high blood pressure.

Being obese and having metabolic risk factors could result in a faster decline in the way your brain works. That’s according to a study of 6,401 civil servants in the UK, published in 2012. 

A brain scanning study published in 2016 also suggests that being overweight or obese is associated with greater age-related brain degeneration. The effects were largest in the middle-aged, where obesity-related changes were estimated to ‘age’ the brain by ten years. The 527 participants were cognitively healthy adults recruited from the local Cambridgeshire community over a period of 5 years.  

Does obesity in mid-life increase the risk of developing dementia? 

The age at which you become obese may be important. If you become obese in your thirties this may triple your risk of developing dementia later in life. However if you become obese in your 70’s there is no increased risk of dementia compared to others of your age with a healthy weight. That's according to a study by researchers at Oxford University published in 2014. 

Unfortunately only 19 of the 451,232 people in the study were obese in their thirties, so we can’t place too much weight on this finding. However, it is in line with previous studies which report an increased risk of dementia in people who are obese under the age of 60 but a reduced risk in older obese people. This was most recently investigated in a systematic review published in 2016.  

Another study published in 2017, analysed data from 1.3 million adults living in the United States and Europe. The researchers found that having a higher BMI in midlife suggested an increase in the risk of developing dementia. However, weight loss that occurred in the 20 year period between weight measurement and onset of dementia may have skewed results. Of the 1.3 million participants, a total of 6,894 developed dementia during up to 38 years of follow up.  

Is obesity in healthy older adults linked to memory and thinking problems?

Having a higher BMI (body mass index) was found to be associated with a smaller hippocampus (a part of the brain that is important for memory). Those with a higher BMI at the start of the research experienced greater shrinkage of the hippocampus during the study.

That’s according to Australian research presented at Neuroscience 2014 in November 2014.

Dr Doug Brown, Director of Research and Development at the Alzheimer’s Society said: 'Although this study didn't look directly at dementia in these participants, the hippocampus is an important area of the brain and is often one of the first areas to be affected in dementia especially Alzheimer’s disease.’  

In addition, psychologists from the University of Arizona found that a greater BMI was associated with reduced mental function and greater levels of inflammatory protein. While these links are indirect, the evidence suggests that brain inflammation could be one mechanism by which differences in body mass influence the way our brains work. Their findings were based on data from more than 20,000 participants in the English Longitudinal Ageing Study, where measures of memory, BMI, and blood plasma levels of an inflammatory marker called C-reactive protein were collected every 2 years between 1998 and 2013. 

So what does the research suggest? 

Research into the effect of obesity on the brain is still in its infancy and the initial studies have often been small scale. So more research is clearly still needed. However the findings so far, from studies in Australia, the US and the UK all point in the same direction. The NHS also advises that obesity can lead to psychological problems, such as low self-esteem or depression.  

So it seems likely, subject to further, larger scale research, that obesity could be bad for our brain as well as our body – particularly if we also have metabolic risk factors.     

What can we do to avoid obesity or lose weight if we are obese?

1.Eat less and exercise more is the usual approach to weight management i.e. aim to achieve a balance between calories in and calories out. This is useful for achieving short term weight loss.

To avoid mindless eating don’t have buffet meals when eating out, use smaller plates at home and don’t snack while watching TV.

2. For longer term weight management we probably need to focus more on what we eat and drink. In particular:

• Drink water not fizzy drinks or fruit juice. Fizzy drinks often contain a lot of sugar (calories) but don’t make you feel full, so you end up eating as much as usual plus the calories from the fizzy drinks. And fruit juices don’t make you feel as full as actual fruit.
  
• Limit your alcohol intake – to avoid the hidden calories.
  
• Eat more vegetables, fruit, nuts and whole grains – and less ‘junk food.’ This provides more fibre, which is known to help with weight loss.  This is also likely to encourage diversity in your gut microbiota, which some researchers believe help keep you thinner.  

• 3. Exercise - This won’t necessarily help you lose much weight – but it seems to affect what is going on inside your body in a positive way and so has important health benefits. For example it can reduce the factors that increase your risk of heart attack, stroke and diabetes. As we’ve seen above, that may also help keep your brain healthy and slow the pace of cognitive decline.   

Reviewed and updated by Grace Blaiklock, June 2018. Next review date June 2022.