Our brains shrink as we grow older, but that doesn’t mean you can wait until old age to worry about it—neurodegeneration starts 15–20 years before the onset of symptoms of mild cognitive impairment, dementia, or Alzheimer’s disease.

Certain brain structures, like the hippocampus, are particularly vulnerable, accumulating neurofibrillary tangles, undergoing a reduction in synapses, and losing neurons.

The good news is that we can intervene in the process, and one of the most important ways to do this is to keep blood glucose levels in check.

Glucose levels that fall within the high-normal range are associated with an atrophying of the hippocampus.^[1] It gets even worse for people with metabolic dysfunction or diabetes, who have higher levels of brain atrophy—hence why Alzheimer’s disease is sometimes referred to as “type 3 diabetes.” Glucose control is a modifiable risk factor for cognitive decline.

How can one bolster metabolic health in an attempt to stave off brain aging? Two strategies are high-intensity exercise and time-restricted eating. But a diet that’s high in polyphenols might also be effective.

Polyphenols improve metabolic function

Polyphenols are plant-derived compounds that have potent antioxidant and anti-inflammatory properties. They also improve brain function and are neuroprotective. A great source of polyphenols is blueberries—consuming blueberries (which are rich in a class of polyphenols called anthocyanins) improves cognitive function, while at the same time, leads to better insulin and glucose levels. Metabolic and brain function are intertwined, and polyphenols might benefit both!

One reason for the neuroprotective effects of polyphenols is likely better glucose control.

Polyphenols may benefit glucose control by:^[2]

• Inhibiting digestive enzymes that break down starch into glucose
• Slowing carbohydrate digestion and absorption
• Exhibiting antioxidant and anti-inflammatory effects
• Reducing the formation of advanced-glycation end products (AGEs)
• Improving pancreatic beta-cell function
• Stimulating insulin secretion
• Improving insulin sensitivity
• Promoting the growth of beneficial gut bacteria

Improvements in metabolic function mean less long-term complications caused by elevated glucose levels—that includes brain damage that plays a major role in cognitive decline but also “brain fog” caused by acute spikes and drops in blood glucose.

In light of this evidence, it makes a lot of sense to include several polyphenol-rich foods in our diets. As we’ll see, it could make a massive difference for brain and metabolic health.

A polyphenol-rich diet slows brain aging

To determine the effects of a polyphenol-rich Mediterranean diet on metabolic and brain health, the participants in one study consumed one of three different calorie-restricted diets for 18 months.^[3]

Mediterranean diet: Rich in vegetables and low in red meat. The main sources of added fat were olive oil and a handful of walnuts each day.

Polyphenol-rich Mediterranean diet: The Mediterranean diet with the addition of 3–4 cups of green tea per day and a polyphenol-rich green shake at dinner. This shake was composed of Mankai, a duckweed-based product with more than 200 polyphenols including luteolin and apigenin. The green tea provided polyphenols including green tea catechins like EGCG.

Healthy control diet: The participants were provided with guidance on how to eat a healthy calorie-restricted diet that was based on dietary guidelines.

The participants who consumed the polyphenol-rich Mediterranean diet experienced less brain aging (assessed using MRI of the hippocampus) than the other groups—their brains underwent less changes than would be expected after 18 months of “normal” aging.

The polyphenol-rich diet also led to greater improvements in glycemic control—58% of participants in the polyphenol-rich diet group had improved glycemic control compared to 32% and 29% of participants in the Mediterranean diet and healthy diet groups, respectively.

The addition of polyphenols to an already healthy Mediterranean diet led to further improvements in glucose control and brain health. This is supported by the findings that the green shake and green tea showed independent beneficial effects. The participants who reported consuming the shake more than 3 times per week and green tea 2 or more times per week had the greatest improvements in glucose control biomarkers (e.g., fasting glucose and HbA1c). More importantly, consuming more of these polyphenol-rich dietary components was associated with a more potent slowing of brain aging trajectories during the study.

Glucose control has independent effects on brain aging

At baseline before the diets began, participants with a lower body weight, systolic and diastolic blood pressure, insulin, and HbA1c had a younger brain age than would be expected based on their chronological age. They were already aging healthier! To use one example, more people with an obese BMI had hippocampal volumes that were smaller than normal compared to people with a BMI less than 30.

But the real changes happened when the participants started the intervention.

Regardless of diet, improvements in glucose control biomarkers and inflammation (C-reactive protein) were linked to less brain aging in the hippocampus.

Less brain aging was also observed in the participants whose HbA1c levels (HbA1c is a marker of long-term glucose control) went from prediabetic or diabetic levels (above 5.7%) to normal (below 5.7%). Those who were prediabetic or diabetic at the start of the study but then were classified as having normal glucose regulation at the end of the study received a much greater benefit than those who maintained their glucose levels or got worse during the study.

The conclusion? The “reduction in glycemic biomarkers has an independent effect on the neuroprotective benefits of diet and may play a major role in attenuating neurodegeneration."