Smoking and dementia risk
Smoking has been known for a long time to increase the risk for dementia. But how does smoking increase our risk for dementia? Is nicotine by itself increase our risk for dementia? How about vaping, does this also increase our risk for dementia?
Let’s find out.
Smoking and health consequences
It is no secret by now that smoking increases our risk for many diseases. Most prominent health conditions for smokers are cardiovascular diseases, such as heart attacks and strokes, followed by lung conditions, such as COPD (Chronic obstructive pulmonary disease), and last but not least a whole variety of cancer conditions. We all know by now that cigarette smoking is not good for us and that we know this is due to public health policy in the Western World, which has relentlessly made clear to people that smoking is detrimental to our general health and can shorten our life significantly. In a way, it seems quite surprising that these negative health effects were not known earlier, since a large proportion of the population started smoking from the 1920s onwards with the peak of people smoking in the 1960s to 1980s. Wouldn’t scientists have not picked up on this link before?
One reason why the negative health consequences of cigarette smoking were not known by the wider population was due to the powerful influence and ‘deep pockets’ of the tobacco industry. Specifically, once the first scientific evidence in the 1940s-1950s emerged that cigarette smoking might be unhealthy, the tobacco industry started discrediting scientists reporting these findings, and sponsoring other scientists to find more ‘positive’ scientific results for the benefits of cigarette smoking. They cleverly used advertising agencies to help them convey the message of the benefit of cigarette smoking and to discredit researchers to the public. These discrediting approaches were so successful that it took until the 1980s for health policy to change. (There are several great non-fiction books on this topic, of which I am giving the links at the end of the article, in case you are interested. It is an absolutely fascinating read, as the same techniques were later used by the oil industry for discrediting climate change science). It took therefore decades for health research to make the public aware of the dangers of cigarette smoking. Despite this being an interesting tidbit, it is important to understand how the tobacco industry actively discredited health research as it also influenced the research on the link between cigarette smoking and dementia risk.
Dementia risk
Most of us will already know that cardiovascular health is an important risk factor for the development of dementia, with poorer cardiovascular risk increasing our risk for dementia. This is mostly due to mini-strokes in our brain, which seem to accelerate the build of proteins – the key disease process for most dementias. Since smoking causes poorer cardiovascular risk, it therefore also indirectly increases our risk for dementia. This fact has been known for many decades, but again has been systematically discredited by the tobacco industry who funded their own dementia risk studies. It might come not as a surprise that the tobacco industry-sponsored dementia research found no increased risk for dementia in smokers. What was surprising was the fact that their sponsored studies found that cigarette smoking and nicotine were beneficial for the brain and in fact reduce the risk for dementia. Quite astonishing.
It was the classic obfuscation technique the tobacco industry had used so successfully over the decades to cast doubt on the negative health consequences of smoking. It basically ‘muddied the water’ of scientific evidence, with some studies showing an increased risk for dementia due to cigarette smoking while others (tobacco industry-funded ones) found no increased risk for dementia in smokers. It took until the 2000s that scientists decided to analyse the dementia risk data for cigarette smoking by separating the tobacco industry-funded studies from the remaining ones. And lo and behold, it was only the tobacco industry-funded studies that found no increased dementia risk, whereas all the other studies – funded in different countries and health systems – all found an increased risk for dementia in smokers. That put finally an end to that discussion and it is since accepted that cigarette smoking increases the risk for dementia – full stop.
So, how much does cigarette smoking increase our risk for dementia?
The scientific evidence is now clear that dementia risk increases in smokers by at least 30%. So, we have at least a 30% higher percentage chance of developing dementia if we are smoking compared to non-smoking. Now, I am saying at least 30%, since the scientific literature agrees that this is very likely an underestimate. The reason why this is an underestimate is that studies establishing this risk likely suffered from a ‘survivor bias’ effect.
What is a ‘survivor bias’?
Survivor bias
Scientific studies talk about a survivor bias if the people they recruit are unlikely to represent the population they want to investigate as they are too healthy. In other words, the people taking part in these studies are survivors of another condition which most other people with that condition will have died of beforehand. We know that smoking causes so many other health conditions and hence it is likely that smokers die of those conditions before they develop dementia. Smokers who develop dementia are therefore unusual survivors as they are likely the ‘healthiest smokers’, if you like they are smoking survivors. This means that dementia studies might be inherently biased towards healthy smokers who likely have a lower risk than the ‘real’ smokers. This is an important concept to understand as it should make clear that the risk of smoking for dementia is likely much higher, it is just the case that many smokers will not live long enough to actually develop dementia.
How about former smokers or people who stopped smoking? Do they have a similar risk of dementia as current smokers?
This is a very interesting topic that is still being investigated as it is not straightforward to distinguish between groups of former smokers. For example, previous occasional smokers (also sometimes referred to as social smokers) might be at quite different risk to previous chain-smokers. Still, existing data suggests that the risk for dementia in people who have stopped smoking is lower than current smokers. However, the risk for dementia for those former smokers is still higher than for non-smokers. As I said, one needs to be careful interpreting those findings as the ‘former smoker’ category might include a large variety of people from former ‘social’ smokers to chain smokers. Of course, the survivor bias applies to these studies as well and that those former smokers are actually ‘healthier’ than expected. In essence, the picture is a bit complicated.
How does smoking then increase our risk for dementia?
To understand this, we have to take a step back and explore how smoking affects our brain cells before we can understand how smoking might impact dementia disease processes. We already know that cardiovascular factors play a big role in our brain health and that smoking causes a decline in our cardiovascular health and hence brain health. But there is also one more specific process that has been shown to directly damages our brain cells because of smoking – oxidative stress.
What is oxidative stress?
Oxidative stress
In a nutshell, oxidative stress is an imbalance between so-called free radical and antioxidant molecules in our body. This might sound complicated but is actually fairly straightforward to understand. We need oxygen in our cells to work properly and therefore all our cells contain oxygen. However, oxygen is a highly reactive molecule and if not ‘controlled’ correctly, can cause quite a bit of havoc in our cells. Most molecules carrying oxygen make themselves, therefore, less reactive to the surroundings which allow them to transport the oxygen safely to the place where it is needed in the cell. But some oxygen-carrying molecules do not ‘control’ or transport their oxygen as safely – for yet unknown reasons. Those molecules are free radicals. The reason why they are called free radicals is that since they have less control over the oxygen, the oxygen can react with anything in the cell causing havoc or even damage to various cells structures, such as our DNA or proteins in the cell. Free radicals are true to their name!
The occurrence of free radicals is common in our cells and hence the body has other molecules (antioxidants) which are dampening the effect of free radicals on the cells. Basically, the antioxidants can control or contain the free radicals and therefore the free radicals cannot cause such havoc or damage in the cell. In oxidative stress, the balance of the free radical and antioxidants becomes uneven. As with any equation, this can go two ways, we can develop oxidative stress because we have increasing levels of free radicals or we can have decreasing levels of antioxidants. It is important to understand this relationship as we can modify the levels of free radicals and antioxidants and hence our level of oxidative stress via our lifestyle choices. For example, being exposed to air pollution (see also my article on this here), chemicals, radiation and certain pesticides & cleaners can increase our levels of free radicals significantly. But it is not only those environmental factors that cause higher free radicals, it can be also caused by high caloric, fatty diets and overeating, as well as cigarette smoking. Vice versa, a balanced diet and physical activity/exercise increases our levels of antioxidants and hence decreases our oxidative stress. Since the free radicals can damage anything in our cells, from DNA to proteins, it should come not as a surprise now that oxidative stress can cause a whole range of diseases, including cancer, cardiovascular disease, diabetes and most importantly for us dementia.
Cigarette smoking has been shown to increase our oxidative stress significantly, by raising the levels of free radicals. Many smokers at the same time do little physical activity/exercise, creating therefore the perfect storm of oxidative stress with high levels of free radicals and low levels of antioxidants. This literally turbocharges the cell damage, including in our brain. Evidence has shown that the increased oxidative stress caused by smoking directly affects the accumulation of proteins typical for dementia. Both amyloid and tau accumulation levels are directly linked to the effects of cigarette smoking on oxidative stress in our cells. This explains, why smoking increases our risk for dementia, since it directly increases our dementia protein levels, making it more likely for us to develop dementia in the future.
How about nicotine?
Nicotine
Nicotine is one of the components of cigarette smoking and its most addictive component. It is really the nicotine we are craving when wanting to smoke as nicotine gives us a ‘feel good’ feeling. The reason is that nicotine mimics another molecule in our brain (dopamine) which is important for our pleasure feelings. We already know now that cigarette smoking is bad for our brain health and increases our dementia protein accumulation but the scientific evidence is less clear for nicotine. Studies have found variable findings as to whether nicotine influences levels of the protein amyloid in our brain – these variable findings, were the target of the tobacco industry to falsely claim that cigarette smoking does not increase the risk for dementia. However, more recent studies have shown that despite nicotine not potentially increasing levels of amyloid, it does increase significantly levels of the protein tau which is also important for the development of many dementias.
These nicotine findings also have significant implications for the new shift from cigarette smoking towards vaping. Vaping consists, in essence, of the vaporising of a nicotine-containing liquid in an electric cigarette. The electric cigarette uses electricity to heat up and vaporise the liquid so that we can breathe in nicotine and other flavours added to the liquid. Research has shown that vaping is much better for our health and many public health systems in the world, are suggesting to people to switch from cigarette smoking to vaping. The main reason for the better health outcomes is that vaping does not include all the other, nasty substances in cigarettes. So, switching to vaping allows us to maintain our nicotine addiction, but in a healthier way. Healthier is the keyword here. Obviously, the healthiest option is to stop smoking or vaping completely but as with any addiction, steering people to a healthier option improves their health outcomes and gives them a greater chance to quit their addiction.
How about the evidence for dementia risk due to vaping?
Vaping
To my knowledge, there are no existing studies that have investigated the risk for dementia due to vaping – although I am pretty sure that someone is currently conducting this study somewhere, as it is such an obvious question. However, based on what we know from cigarette smoking we can speculate on the effects of vaping on dementia risk. Since, vaping consists mostly of inhaling nicotine, the risk for dementia should be not as high as for cigarette smokers. However, we also know that nicotine can affect the accumulation of the protein tau in our brain, which means that vaping might still cause a greater risk of dementia compared to non-smokers – albeit the risk smaller than cigarette smoking. Just to re-emphasise, this conclusion is at this stage a ‘best guess’ and needs to be confirmed by scientific studies in the future.
But what has been scientifically established is that vaping still causes free radicals in our body and that the levels of free radicals are dependent on the flavours added to the vaping liquid. Studies have found that certain flavours increase free radical production significantly, such as lemon (46%), grape (56%), coffee (58%), ‘real honey’ (67%), raspberry (72%) and bubblegum (76%) – percentages in brackets gives the levels of increase in free radicals. However, certain flavours bases on a molecule called linalool, which is often used for cinnamon flavours, can increase free radical levels by up to 105%, which is staggering. But not all flavours are bad, for example, vanilla flavours, based on the molecule ethyl vanillin actually reduces free radical production by up to 42%. So, the flavour you choose for your vaping can make a significant difference to your oxidative stress levels and consequently health outcomes. But we have to wait to find out whether this also influences our overall risk for dementia.
Summary
In summary, there is very strong evidence that smoking increases our risk for dementia by at least 30%. This is likely an underestimate due to the survivor bias in studies, meaning that many smokers will have died before developing dementia and hence the risk is based on the ‘healthiest smokers’. Besides having a significant impact on our cardiovascular health and associated mini-strokes, smoking also increases our oxidative stress by generating higher levels of free radicals in our cells. Oxidative stress is a key process to increase the protein accumulation for dementia with higher oxidative stress being related to a higher risk of dementia. Nicotine itself seems to only affect the accumulation of tau, with evidence for the other key dementia protein – amyloid – not clear at this moment. There are no existing scientific findings for vaping and dementia risk but since vaping still contains nicotine, we can for now only speculate that it still slightly increases the risk for dementia compared to non-smokers, but to a lesser degree than smokers. However, an interesting finding has been that the different flavours in vaping can increase or decrease our free radical levels and might therefore affect our risk for dementia as well. As always, the best evidence is to stop smoking/vaping completely but if we cannot manage that then switching from smoking to vaping is the best option for us to reduce our risk for dementia and many other health conditions.
Links
- https://en.wikipedia.org/wiki/Merchants_of_Doubt
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3490543/
