The global incidence of dementia is rising, and exploring new ways of preventing the disease has become a scientific priority. Lifestyle and dietary factors regularly come under the microscope, but concrete evidence on what we should or shouldn’t be doing to lower our risk is often difficult to pinpoint.
A new cohort study from the University of Leeds recently hit the headlines, reporting that eating 25g of processed meat a day may increase dementia risk by 44%. On closer inspection however, these results are far less clear cut.
We spoke to Professor Janet Cade from the University of Leeds’ Nutritional Epidemiology Group, and Huifeng Zhang, the lead researcher behind the study, to find out more, and get to the root of why there is so much conflicting advice out there.
The media seems to be constantly telling us what we should and shouldn’t be eating to keep our brains healthy, could you explain the link between diet and brain health?
Janet: Throughout our life-course what we eat is important for our brain development. During pregnancy and infancy, the brain is growing and developing so there are well-known nutrients, such as the vitamin folate, that are linked to the development of our neural plate and its functioning. This is why it’s really important for women in pregnancy to eat well for the baby’s general growth and development, as well as brain health.
Then obviously as we get older we need to think about the dietary patterns that will protect our brain over time. There’s a range of nutrients associated with that – such as iron and protein. It’s important to remember, however, that a lot of this information is from animal studies, so how transferable that is to human development is important to consider.
Huifeng: Foods like vegetables, fruits and high protein foods are all important for our brain health. But this is based on epidemiology studies rather than experiments, so we’re really looking at associations rather than causality. But for some nutrients, like vitamin D, there are experiments and evidence that they are important for brain health.
Janet: Some of the more dieting related things – such as intermittent fasting or calorie reduction could also actually be beneficial to brain health through adapting stress responses.
However, for older people perhaps requiring care, they may well be undernourished so we wouldn’t be wanting to suggest that. Whereas maybe a later middle-aged person, who may be a bit overweight – that might be beneficial, it’s all about what’s appropriate to the person and their current nutritional status.
Your research is the first of its kind to look at links between red and processed meat and dementia risk, what were your motivations and what did you find?
Huifeng: We were interested in dementia because of its increasing prevalence. The causes of dementia remain unclear, and we wanted to understand the best way to prevent or halt its development, so it’s important to consider multivariable risk factors like diet.
We were also interested in meat consumption because in recent years it’s been related to a high increase in colorectal (colon and rectum) cancer, and some dietary guidelines suggest reducing our meat consumption. We wanted to ask if it’s true that meat consumption is detrimental to our health, and investigate the relationship between meat consumption and dementia risk
We found processed meats were associated with an increased risk of dementia, but unprocessed red meat was related to a reduced risk of dementia, so we are assuming the subtype of meat is important.
Processed meat may be high in some bad compounds and salt. Also, during processing some nutrients will be lost and detrimental compounds will be produced, which may be bad for our brain.
Janet: If we think about unprocessed red meat, it is high in protein and iron. Some animal studies show that higher protein intakes lead to lower amyloid-beta in the brain which could reduce the risk of dementia. Iron is associated with the myelin sheath in the nerves, which support the transmission of impulses to the brain more quickly. However, neither of those things might be actually what’s going on, but there could be an association.
What is amyloid-beta?
Dementia causes the cells in the brain that are responsible for receiving, processing and storing information to die.
While we don’t yet know exactly why this happens, some researchers believe a microscopic brain protein called amyloid-beta may be the cause.
Amyloid-beta is a ‘sticky’ protein that supports neural growth and repair. However as we get older, we can develop too much amyloid-beta, which can clump together and form ‘plaques’.
These plaques are thought to block communication between our brain cells, and can trigger an immune response that destroys the affected cells.
What is a myelin sheath?
The myelin sheath is the fatty tissue that covers and protects our nerves – think of it like insulation around an electrical wire.
A healthy myelin sheath ensures messages travel quickly between the brain and the body. As we get older, the myelin sheath can become more prone to damage, which can cause communication between the brain and body to slow down.
The standout statement of the study is that 25g a day of processed meat could increase dementia risk by 44%, is this more of a hypothesis, rather than a warning for us all to stop eating processed meats such as bacon?
Janet: We need to be careful when interpreting this result because we don’t want to be stigmatising people who eat in a certain way. We don’t want to make them feel guilty, or that because they ate in a certain way in the past this has led to them getting dementia, we are not saying that at all.
My husband is a GP, so he speaks one-to-one with patients, whereas Huifeng and I are epidemiologists so we look at whole populations. Something that’s true for the whole population isn’t necessarily true for an individual person.
What we’re trying to say is that this is a first look. There isn’t much data that has been explored, so that 44% increased risk is a relative risk, not an absolute risk.
If we look at the absolute risk, it’s very small indeed and we’ve resisted calculating the absolute risk because the Biobank cohort itself is not fully representative of the UK population, so we can’t actually say ‘this is your risk’. But, in relation to the people in the study who were consuming that amount of processed meat, that was the risk, but for anybody out on the street, the risk to them is going to be tiny because their overall risk of dementia is very small.
Were there any associated risk factors for dementia that were common for those who ate large quantities of processed meats, and how this contributed to the overall results?
Janet: While Huifeng did her absolute best to try and take account of all known confounding factors, things that might be associated with the disease outcome, it’s still possible that our statistical models weren’t able to fully take those things into account, or we didn’t have good enough measures of them. Whether it was smoking, physical activity – these are the things that can go along with less healthy lifestyles in general, so the study is hypothesis-generating rather than trying to say much more.
Research suggests the average daily consumption of processed meat in the UK is currently around 25g, so whilst we don’t need to stop consuming processed meat in response to this study, is it more a case of cutting down for our general health?
Janet: That’s exactly it, and also for environmental indicators because we know that meat has a big contribution to greenhouse gas emissions and things like that so even if you weren’t worried about it for your health, maybe eating less is the way forwards. But yes, I haven’t cut out bacon and I’m still going to eat it.
A lot of questions have been raised on the health benefits of a vegetarian diet, and whether meat substitutes are actually healthy. Some reports have also suggested a vegetarian diet could lead to a higher risk of stroke. Is it getting more difficult for people to ensure they’re eating the right things?
Janet: Yes, there’s no easy way for them to really know that. I think the trend towards vegetarianism is probably a good one in terms of health for ourselves and the planet. However, a lot of these new products are very highly processed which might mean they have a lot of fat and salt in them. Buying the ready-made vegetarian option isn’t always the healthier option. As always, it’s about getting the right amount of calories, and the right balance of nutrients for your age and life stage.
Huifeng: I think the vegetarian diet can be contradictory too, because I also read some evidence saying that it can be related to a lower risk of stroke because cholesterol is very low in the vegetarian diet. I think the research you mentioned is from association studies as well, just like our findings. With the competing information out there, we should perhaps just follow dietary guidelines like the UK Eatwell plate, because that is a summary of the current available evidence, and they update their guidelines based on any new evidence coming out.
Janet: I think it would be helpful for those guidelines to be maybe more specific when you’re thinking about the elderly population because they’re developed for the general healthy adult population so clearly one size doesn’t fit all, but it’s a good basis.
For those living with forms of dementia, are there any particular diets, such as the Mediterranean diet that could be beneficial?
Janet: I think it’s tricky, the Mediterranean diet is thought to be beneficial because it contains lots of fruit and vegetables, red wine, and less meat – and it’s considered to be anti-inflammatory, but the evidence isn’t very strong. I think that there have been some studies perhaps hinting, but there’s no confirming evidence saying that Mediterranean diets definitely are beneficial to our brain. It’s too early to say.
I would suggest sticking with the guidance on the UK Eatwell plate too. I suspect that maybe some older people’s appetites aren’t very good, so it’s then more about making sure that they are getting the right amount of calories.
Also, from my previous research, we’ve developed a company called Dietary Assessment Limited, and a tool called Myfood 24, and have actually done a bit of work with care homes. I think some carers and relatives have been worried recently because they can’t visit relatives and monitor their calorie intake, or you know, maybe they’ve got a gut problem that means they need to eat more fibre. So we wanted to see if having a tool that helps monitor what foods and nutrients they’re eating would potentially be helpful with this group.
What do you hope your research will mean for the education and prevention of dementia?
Janet: This is a very early stage of epidemiological research into diet and dementia, so what we might hope is that more studies would explore this using better measures of both diet and dementia, to see whether or not there is something here.
Our colleagues in the laboratory could explore whether or not they could replicate the findings with processed and unprocessed meat, and then raise awareness in general with regard to dementia in the community.
While we don’t want to be raising alarm, we at least want to make people think that actually what we eat can affect us in all different ways. Although we’re not saying it causes anything here, it’s one extra piece in the jigsaw to say why we should eat more healthily.
Huifeng: I agree – we need more evidence, and also I think we need closer collaboration between the research group and policymakers, to produce more convincing evidence and guidelines, and also with social media to interpret those guidelines and findings properly, to give more accurate information to the public.
What would you like the key message from your research to be?
Janet: We’re not saying there isn’t anything, but we’re definitely saying we haven’t shown a causal effect, it’s more ‘let’s look a bit more deeply’. I suppose it’s a message to funding bodies too to say you should fund more and better observational epidemiology.
To carers, it’s more about supporting the person they’re caring for to eat in a way that’s enjoyable, eating healthily doesn’t have to be miserable, you can eat nice things and be healthy.
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