Feeding the Mind: The Impact of Diet on Neurogenesis and Synaptic Plasticity in Aging

Introduction

While much of our body ceases to grow after a certain age, our brains are a remarkable exception. They constantly evolve, giving birth to new cells through neurogenesis and building and rearranging their flexible cellular connections, known as synaptic plasticity. Recent advances in neuroscience have increasingly focused on understanding how our brain's growth and development shift with age and what proactive steps we can take to ensure optimal cognitive health.

In this Research Review article, we aim to provide a more comprehensive and in-depth overview of the changes in our brains as we age. Additionally, we will explore the intriguing role of diet in maintaining brain health, illuminating how our food choices directly impact neurogenesis and synaptic plasticity in adulthood. This discussion offers insightful perspectives on potential dietary strategies to combat the aging process in the brain.

Overview of the Nervous System (NS)

The nervous system is an integral part of our body, comprising the brain, spinal cord, and nerves. It is responsible for communication, helping us notice and react to what's happening inside and outside our bodies.

The nerve cell (neuron) is the basic unit of the nervous system. Neurons have different shapes and functions depending on their location in the body. Each neuron has branching parts called dendrites and a long tail called an axon. Some axons have a protective covering called myelin, ensuring efficient transmission across the neuron [1].

Neurons connect at points called synapses, where they send messages to each other using chemical messengers called neurotransmitters. The process of sending messages across synapses via these special chemicals is known as neurotransmission [2]. You can think of neurotransmission as shipping. Goods (electrical signals) are delivered across a water body (neurotransmitters) from one port (first neuron) to the other (second neuron).

Development of the Nervous System (NS)

The development of the Nervous System proceeds in three phases that span our lifetime. In the first development phase, nerve cells are generated through cellular division. In the second phase, they form synapses (connections) with other remote cells to allow communication. The connections between nerve cells are refined and remodeled in the final stage. Let us dive into each step in greater detail [3].

Phase I: Birth of Nerve Cells

The process by which neurons are formed is known as neurogenesis. While most neurons are formed when an embryo develops, neurogenesis continues in certain brain parts after birth and throughout life. During neurogenesis, neurons are created with unique characteristics based on their location and birth time. Additionally, they are surrounded by glial cells, a type of cell that protects the neurons and creates space for them to do their vital work. 

Phase II: Formation of neuronal synapses

As neurons grow, they undergo morphogenesis, shaping themselves like a growing plant. Axons and dendrites extend from the cell body, like roots and stems sprouting from a seed. Dendrites go through a period of rapid growth so that by age two, a single neuron might have thousands of them. This growth process leads to synaptogenesis, where dendrites and axons connect with other parts of the nervous system. This phase, complete with dendritic expansion and connection, is called synaptic blooming.

Additionally, during morphogenesis, myelin is synthesized. This substance acts like insulation on a cable, coating the neuron's axon to speed up electrical signal transmission. Myelin development continues into adolescence but is most active during the first few years of life [1].

Phase III: Refining and remodeling synaptic connections

Connections are fine-tuned in the last phase of brain development through synaptic pruning. It's like a tree shedding dry leaves and branches to keep the healthy ones. Synaptic pruning enhances brain function and allows us to learn and carry out complex skills. Around 40% of early-formed connections are trimmed during this process.  This pruning process continues from childhood through adulthood in different brain areas.

Synaptic Plasticity (SP)

Synaptic Plasticity (SP), initially proposed by Canadian psychologist Donald Hebb in 1949, refers to alterations that occur between neurons and synapses due to external experiences. In response to environmental changes, our synapses may form or remove connections. The ability of the synapses to alter these connections is what makes them plastic.

Synaptic Plasticity is a crucial concept in understanding how neurons communicate in the brain. Imagine it like a conversation with varying volumes: some neurons whisper, others shout. The strength of these synaptic 'conversations' isn’t constant; it changes, referred to as short-term and long-term plasticity.

Short-term plasticity adjusts synaptic strength quickly, within seconds. Long-term plasticity, however, has lasting effects—from hours to years. It's crucial for forming long-term memories, like remembering names, addresses, and significant life events.

Beyond memory, SP plays a vital role in brain recovery post-injury or disease. It allows neurons to reorganize, helping to mitigate damage and restore lost skills. This concept underpins the “practice makes perfect” approach in rehabilitation. Our brain operates on a "use-it-or-lose-it" principle. Active signaling (or practice) strengthens neural connections, whereas lack of activity weakens them.

Post-injury, lost neuronal connections can be rebuilt through increased signaling. For instance, structured exercises in stroke rehabilitation help relearn lost skills by promoting this neural rewiring. Physical therapy assists in regaining movement, while speech therapy aids in restoring speech functions.

However, the effects of SP aren't solely beneficial. Plasticity is also linked to mental and emotional disorders like Major Depressive Disorder (MDD) and Post-Traumatic Stress Disorder (PTSD). MDD manifests as persistent low mood, loss of interest, and concentration issues, sometimes leading to suicidal thoughts. PTSD, triggered by traumatic events, causes emotional and physical distress that impacts overall well-being. These experiences can range from natural disasters to violent incidents like terrorism or assault. Both MDD and PTSD are associated with synaptic loss in the dorsolateral prefrontal cortex (DLPFC), a brain area linked to emotions and mental processing. Hence, changes in neuronal connections during SP can benefit and harm our mental health and cognitive functions.

Impact of Diet on Neurogenesis

In the adult brain, the hippocampus is a region where neurogenesis is actively carried out. Like a hard drive that stores your computer's memory, your hippocampus is your body's 'hard drive.' It is principally involved in keeping long-term memories and making them resistant to forgetting. Our diet is crucial in regulating hippocampal neurogenesis [9] and our capacity to remember things as we age.

Studies in rats have demonstrated that diets high in fats and sugars can negatively impact neurogenesis, the process by which new neurons are formed in the brain. These diets may decrease the production of new neurons, potentially affecting cognitive functions and brain health.

Conversely, diets rich in polyunsaturated fatty acids (like omega-3 and omega-6) and polyphenols, found in foods like fish, nuts, and certain fruits and vegetables, have been shown to support neurogenesis. These nutrients not only promote the growth of new neurons but also aid in maintaining the health and functionality of existing brain cells, contributing to overall brain health and potentially reducing the risk of neurodegenerative diseases.

How we eat also matters—intermittent fasting or fewer calories can boost neurogenesis [9]. A recent study in mice showed that cutting calorie intake by 30-40% for three months enhanced neurogenesis in the hippocampus. By using a labeling compound called BrdU, researchers tracked new cell growth. After four weeks, mice on restricted diets showed better survival of these labeled cells than those with unrestricted diets.

The restricted diet group also had higher brain-derived neurotrophic factor (BDNF) levels, a protein supporting neuron growth and survival. The results of this study highlight how caloric restriction positively impacts adult brain cell growth [10]. Even the timing between meals can impact neurogenesis, independent of calorie intake. Studies suggest that longer intervals between meals can boost adult brain cell growth in the hippocampus, also affecting genes linked to mood and cognition. Surprisingly, the texture of food matters, too. Older adults are often put on soft diets due to dental issues, but these diets can slow down brain cell growth, potentially contributing to cognitive decline [9]. In a study on rats, those on a soft diet showed a more significant decrease in brain cell growth in the hippocampus than those on a regular solid diet over time, highlighting how even food texture can affect brain health and cell growth [11].

The intricate relationship between our diet and neurogenesis in the hippocampus underscores the profound impact of what we eat on our brain health and cognitive abilities. From the quality of our diet—favoring polyunsaturated fats and polyphenols over high fat and sugar content—to the quantity and timing of our food intake, each aspect plays a significant role in maintaining and enhancing hippocampal function.

The emerging evidence on the influence of food texture further adds a layer of complexity, suggesting that engaging in physical activities like chewing harder foods can also contribute to neurogenesis. These insights deepen our understanding of neurogenesis and open up potential dietary and lifestyle strategies for preserving cognitive function and preventing neurodegenerative diseases, especially in aging populations.

However, the challenge remains in adapting these findings into practical nutritional recommendations that are accessible and feasible for people at different stages of life, particularly for older adults with specific dietary needs.

Impact of Diet on Synaptic Plasticity

Our diet doesn't just impact neuron creation; it also affects synaptic plasticity, reshaping connections that boost memory and thinking.

Foods abundant in omega-3 fatty acids and curcumin, such as fish and turmeric, play a pivotal role in enhancing brain and spinal cord functions, crucial for supporting overall brain function and plasticity.

These nutrient-rich foods boost the levels of brain-derived neurotrophic factor (BDNF). This vital protein fosters neurons' growth, survival, and interconnectivity, facilitating improved cognitive functions and thought processes. These compounds are also instrumental in preserving and maintaining neural networks associated with movement, commonly impacted by aging. This protective and nurturing effect on the brain's structure and function underscores the significant impact of diet on maintaining mental agility and motor skills, especially as we age [12].

Egg yolks have recently emerged as one food that can significantly support synaptic plasticity. Often misunderstood due to their cholesterol content, egg yolks actually contain choline, a vital nutrient in the production of acetylcholine, a neurotransmitter pivotal for memory and learning. In a recent study, people who took a choline supplement from egg yolks showed better verbal memory than those who didn't. The choline seemed to boost acetylcholine levels in their blood, supporting brain networks linked to learning and memory [13].

When prepared healthily and consumed in moderation, lean red meat has also surfaced as a valuable dietary component for enhancing synaptic plasticity. Notably, when lean red meat is not over-processed or cooked in excessive fats, it is a significant source of DHA (docosahexaenoic acid), an omega-3 fatty acid fundamental for neurons' structural integrity and functionality.

Since our bodies are not highly efficient at synthesizing DHA, incorporating it through diet becomes crucial. DHA plays a critical role in the rewiring and regeneration of neurons, thereby supporting cognitive functions and overall brain health. Therefore, alongside egg yolk, responsibly sourced and prepared lean red meat stands out as a beneficial addition to a diet to boost brain health and enhance synaptic plasticity [14].

It's crucial to be aware that not all popular foods and additives benefit brain health. Aspartame, a widely used artificial sweetener in diet products and sugar-free alternatives aimed at weight loss or diabetes management is a case in point. Research involving mice has revealed the concerning effects of prolonged aspartame consumption. These studies have shown that extended intake of aspartame can reduce the brain networks associated with acetylcholine, a key neurotransmitter in memory and cognitive processes.

The implications of these findings are significant, as the diminished acetylcholine networks resulted in noticeable deficits in memory and overall cognitive function. Moreover, these adverse effects were not limited to the individual consuming aspartame; they were also observed to be passed down to their offspring, thereby impacting the memory capabilities of the next generation. This evidence underscores the need to consider aspartame and similar additives carefully in our diets, given their potential long-term effects on brain health and cognitive functions [15]. The insights gathered from the above discussion unequivocally emphasize our dietary choices' profound influence on cognition and memory through synaptic plasticity (SP). Foods traditionally labeled as “bad” for health, such as egg yolks and red meat, actually harbor surprising benefits for memory enhancement. They provide essential nutrients that serve as building blocks for neurotransmitters and neurons, thereby supporting brain function.

Conversely, certain products commonly perceived as “healthy” alternatives, like aspartame, have potentially been shown to induce memory deficits. This paradox highlights the complex relationship between diet and brain health, underscoring the importance of making informed dietary choices.

Selecting the right foods is crucial for maintaining general health and plays a pivotal role in safeguarding against age-related memory loss and cognitive decline. It's imperative to look beyond conventional dietary labels and understand the specific impacts of various foods on brain health to make choices that genuinely benefit cognitive function.

Conclusion

Our nervous system operates like an intricate and bustling phone network, undergoing significant development mainly during early life but also continuing to generate neurons in specific brain areas throughout adulthood. In this dynamic system, neurogenesis and synaptic plasticity allow our brains to continue adapting to our external environments.

The role of diet in this neurological landscape is paramount: nutritious diets rich in polyunsaturated fats found in lean red meat, choline present in egg yolks, polyphenols abundant in fruits, and curcumin from turmeric, all contribute significantly to brain growth and development. These essential nutrients not only enhance learning and memory capabilities but also aid in maintaining and improving movement functions. Conversely, diets laden with high sugar and saturated fats can impede these neurological processes.

Therefore, adopting smart dietary choices and practices, such as incorporating intermittent fasting and extending the intervals between meals, is vital in supporting brain health. Such habits are instrumental in maintaining mental acuity and ensuring that memory and cognitive functions remain sharp as we age, reinforcing the connection between our dietary habits and overall brain health.