Can fruit harm your brain?

Nutrition advice has constantly been changing and performing U-turns over the past hundred years. Thirty years ago, we were told to avoid fat, now we are told to eat more fat. Some people talk about the benefits of a vegetarian diet, whilst others recommend going back to the Paleolithic period and eating plenty of meat. Some ‘experts’ tell us to eat small meals frequently, whereas others are recommending we should incorporate fasting into our weekly lifestyles. What exactly do we mean by ‘healthy’ in light of such conflicting information?

What do we mean by ‘healthy’?

In reality, the definition of a ‘healthy diet’ is extremely complicated. We need to define exactly what we are targeting.

For instance, if we are obese and are suffering from various obesity-related health conditions, such as a high-blood pressure, diabetes, arthritis, etc. then any diet that helps us to lose weight successfully and be rid of all the obesity-associated illnesses can be called ‘healthy’ whether it involves eating nothing but bacon for a week or eating only fruit.

If we look at cancer, the story is completely different. If we are not obese and want to reduce the risk of suffering from cancer in the future, studies suggest we should be cutting down on red meat, especially processed red meat such as bacon. However, obesity is also linked to cancer as well as to a host of other illnesses, so if it is impossible for someone to lose weight without eating at least some bacon, then including bacon in the diet and losing body fat may be considered ‘healthier’ than eating five portions of fruit, not losing an ounce of body fat and suffering from a plethora of obesity-related illnesses including cancer.

Protein is another example. Protein increases levels of a growth factor known as IGF-1. IGF-1 encourages the growth of many kinds of cells, both ‘good’ and ‘bad’. If there is a small cancerous tumour growing somewhere in the body, then eating large amounts of protein may expedite its growth. On the other hand, a body-builder or athlete may want to stimulate IGF-1 to increase muscle mass. In the cancer scenario, a high protein diet may be ‘unhealthy’ but in the athlete scenario, it may be viewed as ‘healthy’.

One food group that is consistently and universally described as ‘healthy’ is fruit. This may all be set to change quite spectacularly. It seems that when it comes to the brain, fruit may not fulfill its ‘healthy’ reputation. We really need to go back to the drawing board and redefine exactly what we mean by ‘healthy’.

How the brain likes to ‘eat’

The brain is highly dependent on its energy supply. Every brain cell contains little ‘energy factories’ known as ‘mitochondria’. These mitochondria keep brain cells alive and ‘fed’. If the mitochondria stop working, brain cells struggle and die. We think that many of the most common age-related illnesses people suffer from, such as Alzheimer’s Disease and Parkinson’s Disease are at least partly caused by the mitochondria suffering years of ‘wear and tear’ damage. If we target mitochondrial health, we can keep brain cells supplied with plenty of energy.

If brain cells go through any kind of a challenge, for instance if they become injured through trauma or illness, then they might just be able to ‘pull through’ if they have healthy mitochondria. Struggling brain cells need all the energy they can get in order to be able to 'fight back'. If their mitochondria are not healthy, the brain cells may not survive. If the brain cells get tired and exhausted, their resilience will be severely compromised.

Glucose ends up as the energy ‘manufactured’ by mitochondria. Brain cells use glucose as a fuel source, but can also use alternative sources of fuel, such as a product known as a ‘ketone’, which our bodies can make from fat. It appears that ketones don’t need to go through all the stages of processing in mitochondria - they can can ‘leapfrog’ over some of the steps glucose must go through. This makes the workload ‘lighter’ when mitochondria process ketones and they might find it easier to process ketones rather than glucose.

This observation has been supported by some recent studies, which have revealed something extraordinary. Brain scan studies using a glucose tracer have shown that our brains start to use less and less glucose as fuel as we get older, despite needing more or less the same amount of energy. There is no such change in the use of ketones.

We don’t know exactly why parts of the brain start to use less glucose but as energy production lies at the hands of mitochondria, mitochondrial function may have something to do with it. As mitochondria age, they may become less efficient at processing glucose into energy and less glucose may get taken up by cells to be converted to energy. With less glucose providing energy, brain cells will be deprived of energy and will suffer. Starving brain cells of energy can lead to degeneration and death of brain cells, which we see in old age.

We don’t know at exactly what age this change for fuel preference occurs, but it could be far sooner than we think. A study on mice has shown that this may happen as early as the age at which females enter the menopause. At menopause, mouse brains seem to turn on a ‘switch’ where they start to actually prefer ketones over glucose.

Two astonishing studies are making us question everything

Two astonishing observations have put an even bigger spanner in the works. They both involved looking at brain cells that are struggling. Mice were made to undergo a traumatic brain injury before their brain cells were studied in detail.

In the first study (on young mice) recovery from traumatic brain injury was improved if their brain cells were given ketones instead of glucose as fuel.

In a second study, giving mice with traumatic brain injury 'fructose' was shown to actually impede recovery from brain injury by as much as one-third! Although the study describes the fructose as ‘processed’, the compound 'fructose' is still 'fructose'. If the molecule 'fructose' really does impede brain recovery, this poses some extremely vital questions.

All fruit contains fructose, as do sweeteners such as honey and agave nectar.

If this study really does show that fructose makes brain cells struggle even more when under duress and actually impairs their ability to heal, the implications of the study are enormous. Fruit could potentially become an 'unhealthy' food for the brain, at least in some specific contexts.

Before we assume anything, this study needs to be replicated many times and the effect of fructose on human brains also needs to be evaluated. If - and that is a big 'if' - the results of this study are confirmed, we can try to fit the observations in with what we already know.

How does this fit in with our idea of fruit being ‘healthy’?

Fruit contains anti-oxidants which is why we are unanimously advised to eat more fruit. What do anti-oxidants do? Remember, mitochondria are the little energy factories within our cells. Like all factories, mitochondria produce ‘pollutant’ by-products during energy production. These pollutants are usually removed by anti-oxidants within the mitochondria. Over time, the ability of the mitochondria to mop these up diminishes. As these pollutants accumulate, they end up damaging the mitochondria, which results in ‘wear and tear damage’. If mitochondria can no longer do the mopping up unassisted, we think we may be able to help mitochondria mop up free radicals by eating food with anti-oxidants, such as fruit.

Fruit contains fructose and anti-oxidants. The beneficial effects of fruit anti-oxidants (mopping up free radicals to reduce wear and tear damage) may take place slowly, over the longer term. If brain cells are acutely struggling then according to the results of the mouse study, the fructose in fruit could make that acute struggle worse. The negative effect of fructose may outweigh the positive effect of anti-oxidants if brain cells are struggling.

We could in theory take this one step further. If all brain cells in 'older' people (again, at what age can we call ourselves ‘older’ ... at 40? At 30?) have undergone some wear and tear damage and are struggling to work at their best, then could eating fruit make that struggle worse?

  • Could it be that fruit is overwhelmingly 'healthy' for the brain only until the wear and tear damage has occurred?

  • Could it be that once there is wear and tear damage in mitochondria that compromises energy production and makes brain cells struggle, the fructose in fruit may actually be hindering those struggling brain cells?

  • Could this play a role in Alzheimer's Disease? Or even in ageing, in general?

We don’t know. We need more studies.

Enter the ketogenic diet

There is a special way of eating known as the ‘ketogenic diet’ where bringing carbohydrate intake extremely low through eliminating fruit and all starches while at the same time eating a large proportion of fat (sometimes 75% of total calories) results in the body producing lots of ketones. If carbohydrate intake is not cut this drastically, and protein is not kept at 'moderate' levels, the body does not produce ketones. Simply adding fat to your diet will not make your body produce ketones (although coconut oil may be an exception).

Miraculously, the ketogenic diet seems to improve the health and numbers of mitochondria. In light of the above findings that show how a struggling or an ageing brain may start to prefer ketones instead of glucose, a ketogenic diet is now being investigated for its potential in treating neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

Such scientific evidence goes beyond observation-based studies and is making us re-examine exactly what makes a diet or particular food group ‘healthy’. If the fructose in fruit harms the brain’s ability to repair itself, then is fruit really ‘healthy’ when it comes to the brain?

The Future

The future is following a very tortuous and unpredictable path, making us overturn and question things we have taken for granted for years and years. The traditional and popular definitions of ‘healthy’ are undergoing a revolution when it comes to brain health. We really need to ‘watch this space’. Who knows, the ‘cure’ to age-related diseases may simply lie in a holy grail that drastically overhauls how and what we eat!


This site is for discussion only and should not be used as a source of medical information. Please consult your medical doctor before making any changes to your diet, lifestyle or medications.


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Yin F, Yao J, Sancheti H, Feng T, Melcangi RC, Morgan TE, Finch CE, Pike CJ, Mack WJ, Cadenas E, Brinton RD. The perimenopausal aging transition in the female rat brain: decline in bioenergetic systems and synaptic plasticity. Neurobiol Aging. 2015 Jul;36(7):2282-95.

Chételat G, Landeau B, Salmon E, Yakushev I, Bahri MA, Mézenge F, Perrotin A, Bastin C, Manrique A, Scheurich A, Scheckenberger M, Desgranges B, Eustache F, Fellgiebel A. Relationships between brain metabolism decrease in normal aging and changes in structural and functional connectivity. Neuroimage. 2013 Aug


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