Young adults who eat a heart-healthy diet may also be protecting their brain in middle age, a new study suggests.

It included more than 2,600 participants who were an average age of 25 at enrollment and followed for 30 years. They were asked about their eating habits at the beginning of the study and again seven and 20 years later.

They were grouped according to how closely they followed three heart-healthy diets: the Mediterranean diet, Dietary Approaches to Stop Hypertension (DASH), and a study-designed diet score called A Priori Diet Quality Score, or APDQS.

The Mediterranean diet emphasizes whole grains, fruits, vegetables, healthy unsaturated fats, nuts, legumes and fish, and limits red meat, poultry and full-fat dairy. The DASH diet emphasizes grains, vegetables, fruits, low-fat dairy, legumes and nuts, and limits meat, fish, poultry, total fat, saturated fat, sweets and sodium.

The APDQS diet emphasizes fruits, vegetables, legumes, low-fat dairy, fish and moderate alcohol, and limits fried foods, salty snacks, sweets, high-fat dairy and sugar-sweetened soft drinks.

The participants' thinking skills were assessed at about age 50 and again at around 55.

After adjusting for other factors -- education level, smoking, diabetes, physical activity levels -- which can affect thinking skills, the researchers found that people who closely followed the Mediterranean diet and the APDQS diet, but not the DASH diet, had less thinking decline.

Participants with high adherence to the Mediterranean diet were 46 percent less likely to have poor thinking skills than those with low adherence. Nine percent of those in the high group had poor thinking skills, compared to 29 percent of those in the low group.

Participants with high adherence to the APDQS diet were 52 percent less likely to have poor thinking skills than those with low adherence. Six percent of those in the high group had poor thinking skills, compared to 32 percent of those in the low group.

The study was published online March 6 in the journal Neurology.

"Our findings indicate that maintaining good dietary practices throughout adulthood can help to preserve brain health at midlife,..

The study does not prove that a heart-healthy diet causes better thinking skills; it only shows an association between the two..

It's unclear why the DASH diet was not associated with better thinking skills, but one "possibility is that DASH does not consider moderate alcohol intake as part of the dietary pattern, whereas the other two diets do..

"It's possible that moderate alcohol consumption as part of a healthy diet could be important for brain health in middle age, but further research is needed to confirm these findings..

"While we don't yet know the ideal dietary pattern for brain health, changing to a heart-healthy diet could be a relatively easy and effective way to reduce the risk for developing problems with thinking and memory as we age,