Not All Carbs Are Equal. A 13-Year Study Of 202,000 Adults Linked Carb Quality To Dementia Risk.

By Paul E. Bendheim, MD — Founder of BrainSavers; board-certified neurologist and Clinical Professor of Neurology, University of Arizona College of Medicine – Phoenix; author of The Brain Training Revolution: A Proven Workout for Healthy Brain Aging

Novau-Ferré N, Mateu-Fabregat J, Papagiannopoulos CK, Chalitsios CV, Panisello L, Markozannes G, Tsilidis KK, Bulló M, Papandreou C. "Glycemic index, glycemic load, and risk of dementia: a prospective analysis within the UK Biobank cohort." International Journal of Epidemiology, volume 54, issue 6, article dyaf182, October 14, 2025.

When I teach about carbohydrates, I spend a few minutes on two terms that sound more technical than they are: the glycemic index and the glycemic load. I also admit something you don't often hear from a doctor: The research tying the glycemic index to disease has been, in my words, "somewhat inconclusive." So I told you not to get too caught up in the numbers, and to focus instead on eating real food.

A large study published last fall adds something new to that picture, and it speaks directly to the aging brain.

A Quick Refresher

The glycemic index ranks a food by how much its carbohydrate raises your blood sugar. White bread and potatoes are high. Most fruits, legumes, and whole grains are low.

The glycemic load adds a second question: how much of that carbohydrate are you actually eating? Watermelon is my favorite example. It tastes sweet and has a fairly high glycemic index, but a normal serving is mostly water, so its effect on your blood sugar is modest.

In short, the index describes the quality of a carbohydrate and the load describes the quantity. This new study looked at both.

What The Researchers Did

A team from Rovira i Virgili University in Spain, working with colleagues in Greece and at Imperial College London, turned to the UK Biobank, a long-running study of British adults. They included 202,302 people who did not have dementia when they joined. Each person filled out an online questionnaire describing everything they had eaten over the previous 24 hours. From those answers, the researchers calculated the average glycemic index and glycemic load of each person's diet.

Then they followed everyone for an average of about 13 years. According to the university, 2,362 people developed dementia over that time.

What They Found

A lower glycemic index was linked to lower risk. People whose diets had a lower average glycemic index, below about 49 on the scale the researchers used, had about a 16% lower risk of dementia than people above that point.

A higher glycemic load was linked to higher risk. People whose daily glycemic load was above about 111 had about a 15% higher risk of dementia.

The same pattern held for Alzheimer's disease and vascular dementia, the two most common forms.

The relationships weren't straight lines. The researchers found turning points in the data, which is why the results are described as "below" or "above" a particular value. The practical message is simpler than the statistics. Both the kind of carbohydrate you eat and the amount you eat appeared to matter.

Dr. Mònica Bulló, one of the senior authors, summarized it this way in the university's announcement: "These results indicate that following a diet rich in low-glycaemic-index foods, such as fruit, legumes or whole grains, could decrease the risk of cognitive decline, Alzheimer's and other types of dementia."

Why Blood Sugar Matters To Your Brain

I like to call insulin "the doorman." It ushers sugar out of your blood and into your cells. A diet full of simple sugars, year after year, can wear that system down and lead to insulin resistance and type 2 diabetes. The authors point to the same chain of events. High insulin levels, chronic inflammation, and diabetes are all recognized contributors to dementia risk, and diets that send blood sugar up and down again and again may feed into them.

That doesn't prove that a single bagel harms your brain. It does suggest that the steady pattern of what you eat, meal after meal, adds up.

What This Study Does Not Say

This was an observational study. It shows an association between diet and dementia. It can't prove that the diet caused the difference. People who eat more whole grains and legumes may differ in other ways, such as income, exercise, or other health habits, even after the researchers adjusted for many of them.

The diet information came from 24-hour questionnaires, which capture what people remember eating over one day or a few days, not a lifetime of meals. And UK Biobank volunteers tend to be healthier than the general population, so the results may not apply equally to everyone.

A 16% difference is also modest. Diet is one pillar among six, and no single food choice decides your future.

What I Take From It

For years, the glycemic index was an interesting tool with mixed evidence behind it. This study doesn't settle the debate, but it points in the direction we already teach. And it adds weight to the idea behind the glycemic load: quantity matters along with quality.

The advice I give about carbohydrates follows exactly this logic. Choose whole grains over refined ones. Choose whole fruit over juice. Add beans, lentils, and chickpeas when you can. Add nuts, a little olive oil, or a vinaigrette, all of which help lower the glycemic load of a meal. None of this requires a calculator.

For BrainSavers Participants

If you already "eat the rainbow," filling your plate with colorful vegetables, beans, and a little olive oil, you are already putting this study into practice. Keep going. And if you have diabetes or prediabetes, or take medication that affects your blood sugar, talk with your healthcare provider or a registered dietitian before making big changes to how you eat.

Three Things You Can Do This Week

• Swap one refined grain for a whole one. Try whole grain bread instead of white, brown rice instead of white rice, or steel-cut oats instead of a sugary cereal.

• Add a legume to one meal a day. A handful of chickpeas on a salad, lentils in a soup, or hummus with vegetables all count.

• Read one label. Pick a food you buy often and look for added sugars, including names like fruit juice concentrate, cane juice, or rice syrup. If sugar is near the top of the list, look for a better choice.

You are not simply a passenger on the journey of your own brain aging. What you put on your plate today is one of the choices that shapes it.

This article is for general educational purposes and is not medical advice. Talk with your own healthcare provider before making changes to your diet, especially if you have diabetes or take medication that affects your blood sugar.