Maintaining Optimal Cognitive Health in Older Age: The Role of Public Health Dietitians Free

Abstract from Puri S, Shaheen M, Grover B. Nutrition and cognitive health: A life course approach. Front Public Health. 2023;11:1023907.

Cognitive degeneration is a major cause of lost Disability-Adjusted Life Years (DALYs) in people aged over 65 years [1]. Addressing age-related degeneration in neural function is key to preserving the autonomy and overall well-being of older people. Maintaining good cognitive health involves both the development and preservation of the multidimensional cognitive structure which has important benefits to the individual. These benefits are numerous but may include, for example, the ability for older people to maintain social connectedness, to have an ongoing sense of purpose and the abilities to function independently, to be able to better cope with different life events and to enable functional recovery from illness or injury [2].

Evidence supporting the important role of good nutrition in preventing cognitive decline in older adults is now emerging. The scop­ing review presented by Puri, Shaheen & Grover [3] provides an excellent overview of how nutrition can affect cognitive health and considers the impact at different stages of the life-course. The brain develops from the foetal stage through infancy, childhood, and young adulthood, and diet influences early brain development, often being more important than the developmental environment to which a child is exposed [4]. Deficiencies of certain nutrients may significantly affect neurodevelopment, with lifelong implications. It is known that several nutrients play an essential role in brain development during pregnancy including protein, B vitamins, iron, copper, zinc, iodine, and omega-3 fatty acids [5]. During infancy, breastfeeding positively influences cognitive development through both mechanisms associated with the superiority of breast milk composition and breastfeeding experience [5]. Across all income levels, breastfeeding is associated with higher performance on intelligence tests in children and adolescents, and the cognitive benefits of breastfeeding are reported to continue into adulthood [6].

During adulthood, several nutrients are required to support neuroplasticity and brain performance and to reduce the adverse age-related effects on the brain that result from oxidative stress, subclinical inflammation, and free radical damage [7]. A high saturated fat, refined carbohydrate, energy-dense diet increases the risk of obesity, insulin resistance, impaired glucose metabolism, and type 2 diabetes mellitus which are recognised as risk factors for Alzheimer’s disease (AD) [8]. In contrast polyunsaturated fatty acids (PUFAs) have a role in maintaining cognitive function and preventing dementia due to their anti-thrombotic and anti-inflammatory properties and their influence on neural processes [9]. PUFAs regulate the structure and function of neurons, endothelial and glial cells in the brain. The omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), also modify neurotransmission, reduce neuroinflammation, and promote neuronal survival and neurogenesis [10]. Low dietary consumption of omega-3 PUFAs has been found to contribute to memory loss [11]. A cross-sectional study [12] found a positive association between protein intake from animal foods, meat, eggs, and legumes and cognition in adults aged 60 years or over.

Evidence for the beneficial effects of dietary fibre and probiotics through the management of gut microbes is emerging. Published research has shown the effect of intestinal dysbiosis caused by poor dietary habits on the brain’s cognitive functions [13], but the evidence supporting the use of probiotics as an intervention to improve cognition is currently provided by animal experiments [14].

The requirement for key micronutrients continues into later life as brain function preservation is an ongoing process. To date, studies assessing the association of B vitamins and cognition in older adults have been inconclusive [15, 16]. One US study showed a positive association between B vitamins (niacin, folate, B6, and B12) and better cognitive function in midlife [17]. In a separate study, in postmenopausal women free of mild cognitive impairment (MCI), a low folate intake was reported to increase the risk of MCI/dementia in later life [18]. The role of vitamin D in brain health and cognition is emerging with reporting of lower serum 25-hydroxyvitamin D (25(OH)D) levels in adults with impaired cognitive function and AD compared to healthy controls [19, 20]. In a separate longitudinal study, low vitamin D levels were found to increase the risk of AD 7 years later [21]. A systematic review and meta-analysis reported that poor outcomes in various cognitive tests was linked to a higher dementia incidence risk in older patients with low 25(OH)D levels [22]. There is also interest in the role of antioxidant vitamins (E and C). The use of vitamin E and C supplements resulted in a reduced risk of cognitive decline in a prospective cohort in Canada [23], and a cross-sectional US study found a positive association between vitamin E intake and verbal memory, immediate recall, and language/verbal fluency performance [24].

Polyphenols are secondary metabolites of plants, with flavonoids being the most well-known. They are present abundantly in colourful fruits (tomatoes, berries, and grapes), vegetables, cocoa, tea, spices, herbs, and olive oil. They contribute to brain health in a similar way to antioxidants by regulating oxidative stress and facilitating anti-inflammatory mechanisms [25].

Iron, as a constituent of haemoglobin, is essential for transport of oxygen to the brain, and iron deficiency anaemia (IDA) is a risk factor for short- and long-term cognitive impairment [26]. Iron deficiency in the brain is associated with disordered neurophysiological mechanisms that compromise motor and cognitive development negatively affecting coordination, executive function, attention, and memory [27]. However, excessive iron levels in the brain impair neurophysiological mechanisms via increased oxidative stress and neuronal cell death and are also associated with decline in motor and cognitive function [28].

Many older people are at risk of magnesium (Mg) deficiency for multiple reasons [29], and yet Mg concentrations in the brain influence biochemical processes involved in cognitive functions. In healthy adult men followed for 8 years, higher dietary Mg intakes were associated with a reduced risk of developing MCI [30]. In a separate cohort study including more than 1,000 community-dwelling participants aged over 60 years and studied for 17 years, it was found that those consuming more than 200 mg/day of Mg had a 37% less chance of developing any type of dementia and 74% reduced chance of developing vascular dementia [31].

It is preferred that these important nutrients for maintaining a healthy level of cognitive function are obtained from a balanced diet. Specific dietary patterns that have been found to be valuable include the Mediterranean diet, the Nordic Diet, the DASH Diet, and more recently the MIND diet. The key features of the Mediterranean diet are the unrefined carbohydrates consumed in large amounts along with vegetables and fruit, cheese and yoghurt, and olive oil. Chicken, fish, and eggs are consumed a few times per week and red meat less frequently. A PREDIMED study assessed cognitive performance at baseline and after 4 years. There was improvement in cognitive function in adults on the Mediterranean diet, supplemented with either extra olive oil or nuts, and a decline in cognitive function in those on the control diet [32]. There is also evidence of a moderate protective effect of the Mediterranean diet against cognitive decline and AD based on large longitudinal observational studies [33, 34]. A systematic review [35] found that greater adherence to a Mediterranean diet is associated with less cognitive decline, dementia, or AD.

The emphasis of the Nordic diet is on foods such as fruits and vegetables, fish, canola (rapeseed) oil, and several types of meat [36]. A study investigating the associations between adherence to the Nordic Diet and cognitive function in over 1,000 men and women with normal cognition found that those who followed the guidelines of the Nordic Diet had enhanced levels of cognitive functioning at 4-years follow-up [37].

The Dietary Approaches to Stop Hypertension (DASH) diet was designed to reduce cardiovascular risk and focuses on vegetables, fruits, and wholegrains. It includes low-fat dairy, fish, poultry, legumes, beans, and nuts and has a low sodium content whilst being rich in potassium, calcium, and magnesium. The MIND or Mediterranean-Dietary Approaches to Stop Hypertension (DASH) Intervention for Neurogenerative Delay combines the benefits of the Mediterranean and DASH diets with emphasis on foods with antioxidant properties and oily fish due to the high levels of EPA and DHA [38]. It is an approach being recommended by the WHO to reduce cognitive decline and dementia risk alongside the importance of weight management including physical activity [39].

To conclude, preventative strategies to reduce cognitive decline that adopt a life-course approach, from cradle onwards, are required. There needs to be more emphasis on the middle years before people develop dementia so that older people can maintain a healthy independence as long as possible. A dietary approach with a macronutrient and micronutrient intake that has protective effects against cardiovascular disease is likely also to be effective against neurodegenerative disorders. Public health dietitians are well-placed to help lower the global burden of cognitive decline by increasing the awareness of the importance of a high-quality diet that provides plenty of vegetables, fruit, unrefined grains, adequate amounts of protein from a variety of sources, a lower fat diet but that provides adequate omega-3 fatty acids, and key micronutrients. Public health dietitians can translate these recommendations into diets that are culturally acceptable and accessible to people from a diverse range of backgrounds thus helping to reduce health inequalities.

I hereby declare that there are no conflicts of interest with regard to this commentary.