A review of the evidence demonstrates a need for nuance
By Austin Perlmutter, MD
Every few years, there’s a new diet making headlines. From the South Beach to the “cabbage soup” diet (popular in the 1950’s), diets rise and fall in popularity. In the last decades we’ve seen diet claims become more specific to macronutrient intake. In the 1960’s, we were told to avoid dietary fat to lose weight and protect our hearts (side note—this didn’t work). Now, many popular diets advocate for low carbohydrate intake (e.g., “keto”) for weight loss and overall health benefits, including boosting brainpower and protecting our brains from dementia. So, what’s the deal with carbohydrates and brain health? Is avoiding carbs just another fad, or is it a good long-term plan for dementia prevention?
What are carbohydrates (carbs) and how might they influence dementia?
Carbohydrates (carbs) are a diverse group of macronutrients in our diet. They are often classified into three major groups, sugars, starches, and fibers. The impact of carbohydrates on brain function is likely to differ from person to person for several reasons, but there are some consistent findings worthy of all our attention. Here are the major sources of carbs and what we know about their potential impact on dementia.
1.Sugars
Sugars, (sometimes called “simple carbohydrates”) are made up of one or two sugar units. Common sugars include fructose and, galactose and glucose, as well as sucrose and lactose. Sugars are found naturally in fruits and vegetables and dairy, but they’re also added to most foods and beverages to increase palatability.
What does the research say about sugar intake and dementia?
By and large, the best data connecting dietary sugar with dementia risk relates specifically to added sugar in beverages. A recent review paper that looked at nearly 190,000 people traced for over 10 years found a linear relationship with added sugar in beverages and risk for dementia. This included sugary sodas, fruit drinks, milk-based drinks and juice.
Another recent study found that consuming 2 or more daily beverages with added sugar was linked to a 34% increased risk for dementia over a 9.5 year window. While there is less data to support the link between added sugars in foods and dementia, it’s clear that people who eat a diet lowest in processed foods rich in added sugar (e.g., the Mediterranean diet) are at lower risk for developing conditions like Alzheimer’s disease. It’s also important to understand that eating a diet rich in added sugars of any form may increase risk for metabolic dysfunction, oxidative stress and chronic inflammation, none of which are optimal for brain health. Of all the carbs that may damage the brain, sugars, especially those in beverages, seem the most important to avoid.
2.Starches
Starches (also called “complex carbohydrates,” are molecules made up of long chains of sugar molecules linked together. These can be broken down in our gut and turned into simple sugars that can be rapidly digested. Starches are found in fruits, vegetables and dairy but are also highly prevalent in processed foods. Particularly “starchy” foods include potatoes, bread, rice, pasta and cereal.
What does the data say about starch and dementia?
There’s currently not much published data specific to starches and risk for dementia. But because starches are generally part of a higher carbohydrate diet, there may be benefit to looking at larger scale analyses of dietary carbs and dementia outcomes. In a 2023 study looking at older adults, people who ate a higher carbohydrate to fat ratio had evidence for worse cognition. One reason for the potential link is due to the rapid metabolism of starch to sugars within our body.
By definition, starches are carbs that can be broken down and convert into simple sugar on digestion. For people with trouble managing health blood sugar, a big dose of sugar from starchy food isn’t likely to help things. Additionally, eating more starchy foods has been found to relate to increased weight gain, and obesity is a known risk factor for developing dementia. Based on this data, consumption of excess starchy foods is unlikely to do our brains any favors.
3. Fiber
Fiber is another form of complex carbohydrate. The difference is that unlike starch, fiber can’t be broken down and converted to simple sugar. This means that fiber stays in the gut, where it helps to feed the bacterial microbiome and contributes to healthy digestion. Fiber is found naturally in whole fruits, vegetables, seeds and nuts, grains, and legumes, among other foods. Fiber intake is correlated with better blood sugar regulation and immune balance, longevity and supports a healthy gut microbiome.
What does the data say about fiber and dementia?
As it relates to the brain, some data suggest that eating more fiber may have a dramatic benefit in preventing dementia. A recent smaller study found that people who at the most fiber were at a 26% decreased risk for developing disabling dementia over a 20 year period. All of this suggests that fiber is a carb we really should be eating more of to help protect our brains (likely around 30 grams a day).
Putting it together:
Like most things in the world of nutrition science, many people have jumped to an oversimplified understanding of the topic of carbs for brain health and dementia. Yes, most people are likely to benefit from cutting back on foods and especially beverages with added sugars. Yes, there’s probably not a lot of benefit (and potentially some downside) to eating foods rich in starches. But eating more fiber may confer significant advantages to our brain health and help prevent dementia. Though some data now suggest that a low carb (aka ketogenic) diet may be beneficial to some, the sum of the existing data indicate that eating a minimally processed diet rich in whole foods that includes lots of fiber rich plants is the best overall plan for long-term brain health and dementia prevention.