Grain Brain to Today: The Evidence Against Sugar

As many of you are aware, I have been warning about the toxic effects of sugar on the brain for more than 15 years, ever since I wrote Grain Brain. Back then, the idea that sugar and carbohydrates could play a role in Alzheimer’s disease and other forms of cognitive decline was considered fringe, even radical. But over the years, the science has only become stronger, and today, study after study confirms that excess sugar is not just a threat to waistlines and insulin sensitivity, it is a direct assault on the health of the brain.

A powerful new study published in The Journal of Prevention of Alzheimer’s Disease provides fresh evidence that both the sugars we add to foods and drinks, and yes, even the naturally occurring sugars in fruits, vegetables, and dairy, increase the risk of dementia. Researchers examined more than 158,000 people from the UK Biobank, following them for nearly a decade, and the findings were sobering. Those who consumed the highest amounts of sugar had significantly greater risk of developing dementia compared to those who consumed the least.

Importantly, the study looked at both free sugars, those added to foods and found in juices and syrups, and non-free sugars, like those found naturally in whole foods. Both categories were linked to elevated dementia risk, with free sugars being particularly dangerous.

What makes this study stand out is that the scientists didn’t stop at dietary habits; they also factored in genetics. Using polygenic risk scores, looking at a suite of Alzheimer’s risk genes, they were able to determine how susceptible people were to the effects of sugar on the brain based on inherited factors and the genetic influence on their gut microbiome. The results showed that sugar’s impact was not uniform. Some individuals, because of their genetic makeup or the way their gut bacteria are programmed, were far more vulnerable.

In other words, the wrong genes and the wrong gut profile, when combined with high sugar consumption, created a perfect storm for dementia risk. For example, those with high genetic risk related to Alzheimer’s disease or with certain gut bacterial signatures saw their dementia risk climb dramatically when sugar intake was high.

This is an important reminder that brain health is never the product of a single factor; it is always the intersection of genetics, environment, diet, and lifestyle. What is fascinating is how sugar seems to amplify vulnerabilities that may already exist, accelerating pathways of brain degeneration that might otherwise take longer to manifest.

The mechanism makes sense when we step back and look at how sugar acts in the body. Free sugars, in particular, hit the bloodstream quickly, spiking glucose and insulin. Over time this leads to insulin resistance, which doesn’t just affect muscles and fat cells but also neurons and even our brain’s immune cells, the microglia. The brain is the most energy-demanding organ in the body, relying on glucose for its fuel. But when brain cells become resistant to insulin, they can no longer efficiently use glucose, setting the stage for dysfunction and eventually degeneration.

Animal studies show that diets high in refined sugars lead to increased amyloid plaque deposition, one of the hallmarks of Alzheimer’s disease. At the same time, sugar promotes inflammation and oxidative stress, both of which accelerate cellular aging and damage synapses.

This study also reinforces the importance of the gut–brain connection, a topic that was the central theme of my book, Brain Maker. We now know that the gut microbiome plays a profound role in shaping brain health. Excessive sugar intake damages this microbial community, lowering levels of beneficial bacteria that produce neuroprotective compounds and favoring microbes that promote inflammation.

The study identified two specific microbial groups, Oscillospira and Ruminococcaceae UCG-014, that appeared to interact with sugar consumption in ways that changed dementia risk. One group seemed to magnify the harm of sugar, while the other appeared to offer some protection. This underscores the reality that what we eat not only feeds us, but also feeds our microbes, and in doing so shapes the signals that travel from the gut to the brain.

One of the most striking aspects of the research is that even non-free sugars, the ones naturally present in whole foods like fruit and milk, were associated with risk when consumed in high amounts. This does not mean fruit and vegetables are the enemy; they are nutrient-dense and provide fiber, antioxidants, and countless brain-supportive compounds. But it does suggest that even natural sugars should be consumed with mindfulness, and that moderation is key.

A piece of fruit as part of a balanced diet is very different from drinking fruit juice, which strips away fiber and delivers a sugar rush similar to soda.

From a public health perspective, the implications are massive. Dementia currently affects more than 55 million people worldwide, and that number is expected to nearly triple by 2050. We do not yet have a cure, and the treatments available today offer almost zero benefits. Prevention remains our most powerful tool, and diet is one of the most accessible levers we can pull. If reducing sugar intake can meaningfully lower the risk of dementia, then it represents an intervention available to everyone, right now.

For individuals with a family history of dementia, this new research carries particular urgency. If your genes place you at higher risk, sugar can accelerate the process and bring on disease earlier. But the hopeful message is that lifestyle choices can modulate genetic risk. Even if you carry risk genes, you are not powerless. Choosing to reduce added sugars, limiting processed foods, focusing on whole foods rich in fiber and healthy fats, and supporting a healthy gut microbiome may tip the balance in your favor.

For years, people dismissed the idea that diet could shape brain destiny. That dismissal is no longer tenable. We now have robust, long-term human studies linking sugar to dementia, and we are beginning to understand the complex interplay of genes, microbes, and metabolism that explains why.

The takeaway is clear: sugar is not just empty calories; it is a brain toxin when consumed in excess. I argued this in Grain Brain and the evidence today is even stronger.

Protecting the brain is not just about puzzles or memory games; it is about daily choices at the dinner table. Every teaspoon of sugar is not only a challenge to your pancreas but a potential threat to your neurons.

With dementia on the rise, it is time we take this truth seriously, cut back on sugar, and give our brains the best chance to thrive in the years ahead.