Eating well to maintain cognitive health

As with all the organs of the human body, the functional capacities of the brain gradually decline during aging. This decline is usually manifested by a decrease in learning and memory skills, decision-making speed, sensory perceptions (hearing, vision, smell, taste), motor coordination as well as in higher cognitive processes such as abstraction, judgment or problem solving.

The decrease in cognitive performance during aging is associated with structural changes in the brain, in particular with a reduction in total brain volume and sometimes more pronounced in certain regions such as the hippocampus (the seat of memory). A decrease in the integrity of white and gray matter, more evident in the frontal regions, has also been associated with the decrease in higher cognitive processes with advancing age. These phenomena seem to accelerate after the age of 50 and could therefore contribute to the increased risk of neurodegenerative diseases observed in people who reach an advanced age. From the age of 65, for example, the risk of developing Alzheimer’s disease doubles every 5 years, reaching nearly 50% after age 85. As the world’s population ages, it is estimated that the number of people affected by Alzheimer’s disease could triple over the next few years, increasing from 50 million to approximately 130 million people in 2050.

The importance of prevention
However, this situation is not irreversible. It has been known for several years that the adoption of a healthy lifestyle (good nutrition, maintenance of normal body weight, regular physical activity, not smoking) makes it possible to prevent a significant proportion of several chronic diseases, whether cardiovascular disease, type 2 diabetes, or several types of cancer. Several studies carried out in recent years suggest that these good lifestyle habits would also have a preventive impact on the risk of cognitive decline and, ultimately, neurodegenerative diseases. For example, according to a study that examined the influence of certain modifiable risk factors (overweight, hypertension, type 2 diabetes, hypercholesterolemia, smoking, and low educational level) on the risk of Alzheimer’s disease, the complete elimination of these modifiable risk factors would make it possible to prevent approximately one third of neurodegenerative diseases. This protection would be particularly effective in individuals who have a low or moderate genetic risk of developing Alzheimer’s disease. In the absence of effective drugs capable of reversing cognitive decline or even simply slowing its progression, the implementation of a preventive approach based on the improvement of the lifestyle clearly represents the best strategy currently available to preserve functional capabilities during aging.

The impact of food
Several observations suggest that the nature of the diet represents one of these lifestyle factors that can influence cognitive health. As we recently highlighted, studies show that pro-inflammatory diets, especially those containing a high proportion of ultra-processed industrial foods, are associated with an increased risk of dementia. Conversely, a high intake of fruits and vegetables is associated with a slowing of cognitive decline, this protective effect being particularly pronounced for plants rich in certain polyphenols of the flavonoid class, in particular those of the flavonol subclass. Some studies have also noted an association between the consumption of plants rich in carotenoids (lutein, zeaxanthin and β-cryptoxanthin) and a reduced risk of dementia. It therefore seems likely that a plant-based diet (fruits, vegetables, legumes, nuts, whole grains), combined with a reduction in the intake of ultra-processed foods, could represent a valid strategy for reducing the risk of cognitive decline.

The Mediterranean-style diet
The Mediterranean diet is without a doubt one of the plant-rich diets that has been most studied in recent decades, particularly for its positive impact on the prevention of cardiovascular disease, both in the general population (primary prevention) and in people at high risk due to a history of cardiovascular events (secondary prevention).

This diet is characterized by a high content of unprocessed plant foods (whole grains, vegetables, fruits, legumes, nuts and extra-virgin olive oil), a moderate intake of fish/shellfish and a low intake of red meat and deli meats, fats of animal origin, and products containing simple sugars (desserts, pastries, snacks). It is therefore an exemplary diet, where extra-virgin olive oil is the main source of fat (these fats are unsaturated and play several essential roles in maintaining good health), and in which complex carbohydrates come mainly from fibre and whole grains and where legumes, fish and poultry are the main sources of protein.

Studies in recent years suggest that the Mediterranean diet may also have a positive impact on cognitive health. For example, it has been observed that higher adherence to the Mediterranean diet is associated with better cognitive performance, as well as a reduced risk of mild cognitive decline and neurodegeneration. Although these studies are very heterogeneous (age of the populations studied, measurement of adherence to the Mediterranean diet, types and duration of the studies, etc.), a meta-analysis shows that the majority of them report a statistically significant reduction in the risk of cognitive decline. This is supported by the results of a recent large study, carried out with 60,298 participants followed for almost 10 years, which showed a 23% reduction in the risk of dementia in people who adhered most closely to a Mediterranean diet.

The MIND diet
It seems that the positive impact of the Mediterranean diet on cognitive functions could even be increased by adapting a variant of this diet, namely the MIND diet (Mediterranean-DASH Intervention of Neurodegenerative Delay). As its name suggests, this diet is a hybrid between a Mediterranean-style diet and the DASH diet (Dietary Approach to Systolic Hypertension), which is low in salt and fat, and which has also been associated with improved cognitive performance.

The Mediterranean and MIND diets are very similar (see Table 1), but a specific feature of the MIND diet is the addition of two specific classes of foods to the diet that have been linked to improved brain function in epidemiological studies, i.e., leafy green vegetables and berries. It is proposed that the higher intake of certain phytochemicals and antioxidants provided by the addition of these plants could enhance the effects of the Mediterranean diet on cognitive health.

Table I. Comparison of the Mediterranean and MIND diets. Note that beyond some minor differences (more fish and legumes for the Mediterranean diet, more whole grains and nuts for the MIND diet), the main distinction between the two diets is the presence of leafy green vegetables and berries in the MIND diet. *Cabbage, spinach, lettuce, Swiss chard, watercress, arugula; **Strawberries, blueberries, raspberries.

This is suggested by the results of a recent study which examined the postmortem presence of pathological markers of Alzheimer’s disease (amyloid plaques) in the brains of elderly people (average age 91). These patients (581 people) had been followed for about ten years as part of the Rush Memory and Aging Project, which allowed the researchers to use information collected on their eating habits during this period to determine whether these habits could influence the level of Alzheimer’s disease markers detected at autopsy. They observed that this was indeed the case for people who adhered to the Mediterranean and MIND diets, with an approximately 40% reduction in the risk of Alzheimer’s disease diagnosis at death, this reduction being particularly evident in the heaviest consumers of leafy green vegetables. Specifically, the researchers noted that the numbers of amyloid plaques present in the brains of people who ate 7 or more servings per week of green vegetables corresponded to those typically present in the brains of people 19 years younger. These observations are in agreement with several studies showing that the MIND diet appears to be superior to the Mediterranean diet in terms of improving cognitive performance and preventing cognitive decline.

In sum, while much work remains to be done to clarify the link between diet and cognitive health, currently available evidence suggests that a Mediterranean-style diet enriched with leafy green vegetables is an excellent starting point to improve brain health and preserve cognitive functions during aging.