Understanding Aging Through Advanced Biomarkers: A Detailed Overview

Introduction

Chronological age—the number of years since our birth—does not always correspond with biological age, which reflects our body's physiological health. This means that the age listed on our identification documents, like driver’s licenses and passports, may not accurately represent our actual health or physical condition.

Many significant health conditions, such as stage four non-small cell lung cancer at age 60 or Type II diabetes at 45, often manifest unexpectedly later in life. In response, researchers have pinpointed a range of biomarkers—measurable indicators of biological states—that provide a more accurate assessment of our health. These biomarkers can offer crucial insights into our overall well-being. This article will explore various aging-associated biomarkers, discussing their importance in predicting health outcomes and their potential in guiding the development of therapeutic interventions.

The Hallmarks of Aging

Aging is a process characterized by a gradual decrease in functional and physical capabilities over time [1]. Broadly speaking, it encompasses a decline in an individual's health and vitality. To appreciate the importance of biomarkers in aging, it's crucial to first understand the key features, or hallmarks, of the aging process [2].

Genomic Instability: Our DNA contains the instructions that shape our identity and regulate the functions of our body. The entire collection of DNA in an individual is known as the genome. Typically, our body diligently protects and maintains the health of the genome. Yet, as we age, the genome becomes increasingly susceptible to damage. This deterioration can result in mutations, characterized by changes in the DNA sequence, and structural damage, which includes lesions and breaks in the DNA strands. Such damage can impair cellular function, potentially leading to various issues within the body.

Telomere Attrition: Telomeres are short stretches of DNA located at the ends of chromosomes [3]. Chromosomes, the structures within our cells that contain our DNA, are capped by these telomeres. They act as protective buffers, ensuring the stability and integrity of our genetic information. Over time, as cells divide, telomeres gradually become shorter. This shortening acts like a biological clock, influencing cellular aging and the overall lifespan of the cell. When telomeres shorten to a critical length, cells enter a state of 'senescence,' meaning they can no longer divide. This process is analogous to a burning candle: the wax represents the telomeres and the burning of the candle symbolizes cell division. As the candle burns down, the wax gradually diminishes. Once the wax is gone, the candle can no longer burn, just as a cell cannot divide once its telomeres are too short. In this senescent state, cells remain present 'like zombies,' releasing molecules that contribute to inflammation and aging.

Epigenetic Alterations: Epigenetics refers to the study of how genes are regulated and expressed in our bodies without altering the actual DNA sequence. Think of it as a set of molecular switches that can turn genes on or off. These switches are influenced by various factors such as environmental conditions, lifestyle choices, and personal experiences. Epigenetic changes occur through mechanisms like the addition of chemical tags to DNA or the histone proteins around which DNA is wound. These tags don't change the DNA sequence itself but can significantly impact which genes are active (expressed) and which are inactive (silenced) in different cell types at different times. As we age, our exposure to different environments and lifestyles can lead to alterations in these epigenetic tags, thereby affecting how our genes work. These changes can influence our cellular functions and are thought to be key contributors to the aging process, potentially impacting our health and susceptibility to various diseases.

Loss of Proteostasis: Proteostasis refers to the way our bodies regulate and maintain the proper balance of proteins within cells. This process ensures that proteins are correctly folded, assembled, and degraded as necessary. As we age, this process becomes less efficient, leading to the accumulation of misfolded or damaged proteins. These aggregated proteins are implicated in many age-related diseases, including neurodegenerative disorders like Alzheimer's and Parkinson's disease.

Deregulated Nutrient Sensing: Our bodies possess several signaling mechanisms that detect and respond to nutrient availability. These pathways are crucial in regulating metabolism, growth, and energy balance by monitoring our nutritional status. With aging, these pathways can become dysregulated, contributing to metabolic disorders such as diabetes and obesity, and impacting aging-related processes.

Mitochondrial Dysfunction: Mitochondria, vital structures within our body cells, are often described as the cells' energy powerhouses. They convert food into energy, fueling our daily activities. However, with aging, mitochondrial function can decline, leading to reduced energy production and increased production of reactive oxygen species (ROS). ROS are highly reactive molecules that can cause cellular damage. Without adequate control by the cell's antioxidant defenses, ROS can contribute significantly to aging.

Stem Cell Exhaustion: Stem cells are versatile cells capable of differentiating into various cell types, akin to the building blocks in a LEGO set. They have the potential to repair or replace damaged or aging tissues. As we age, however, these 'blank slates' lose their regenerative capacity, which leads to diminished tissue repair and maintenance.

Altered Intercellular Communication: Our bodies consist of cells and tissues that constantly interact to coordinate daily functions. Aging can disrupt these signaling processes, leading to altered systemic processes and impaired tissue maintenance.

These hallmarks are interconnected and play a role in aging and age-related diseases. To gauge aging and gain insights into your health status, theoretically, all of these hallmarks should be measured. However, this is not straightforward. Measuring aspects at the cellular and genetic level can be challenging because a) it may not always be feasible, and b) if feasible, it might be prohibitively expensive. Consequently, doctors and scientists have developed alternative methods to assess the hallmarks of aging, such as estimating biological age.

Biological Age

There are two commonly recognized types of aging: biological and chronological age. Biological age reflects a person's age in terms of their overall health, physical condition, and functional abilities. It takes into account factors like fitness level, organ function, and cellular health, which may not always align with the actual number of years a person has lived.

Chronological age, in contrast, refers to the number of years a person has lived. It's often used as a basic gauge of an individual's health status, but this measure can be misleading. There can be significant differences between a person's chronological age and their biological age. For instance, individuals with the same chronological age can have varying lifespans or life expectancies, influenced by genetic diversity and environmental factors. A person who is 50 years old chronologically might have the biological functions of someone who is 60.

This variation is even evident in identical twins, who share the same genetic makeup and chronological age. For example, a study involving 102 pairs of female Danish twins aged 59 to 81, along with 162 British females aged 45 to 75, revealed that factors like pigmented age spots, skin wrinkles, hair graying, hair recession, and lip height are influenced not just by genetics but also the environment. Environmental factors such as sun exposure, diet, and exercise were found to hold an influence on lifespan and health status. These findings highlight the importance of biological age over chronological age in determining our health and physical appearance [4].

Understanding Biomarkers

Now, biological age is one number that provides a broad description of our health, but to get more specific insights into our well-being, we often need to look at specific biomarkers. A biomarker is a measurable indicator that predicts a biological state or condition within the human body. It can be a molecule, gene, characteristic, or another quantifiable feature that provides insight into the state of a physical system, particularly in relation to health or disease. Biomarkers play a crucial role in medical research and clinical practice, aiding in the diagnosis of diseases, monitoring disease progression, assessing the effectiveness of treatments, and predicting health outcomes. [5]

To be classified as a biomarker, a feature must meet specific criteria. These criteria include:

• Measurability: Biomarkers must be objectively measurable and quantifiable using various techniques, such as laboratory tests, imaging, or other diagnostic tools.

• Specificity: Biomarkers should be specific to particular biological processes, conditions, or responses, thereby providing targeted information about the state of a physical system. For instance, amyloid-beta and tau proteins serve as biomarkers for Alzheimer's disease (AD). AD, a neurodegenerative disorder, is characterized by the abnormal aggregation of amyloid-beta into plaques and the misfolding of tau protein into tangles in the brain. These markers are typically measured in the cerebrospinal fluid, obtained through lumbar puncture, to assess AD severity. Elevated amyloid-beta levels may indicate increased amyloid plaque deposition, while higher tau levels are associated with neurodegeneration and neuronal injury, characteristic of AD progression.

• Sensitivity: Biomarkers should be sensitive enough to detect changes or abnormalities in the biological system, even at early stages or in response to treatment. An example is the prostate-specific antigen (PSA), a protein produced by prostate gland cells. It is normally found in semen, but a small amount is also present in the blood. The PSA test measures blood levels of PSA to detect prostate cancer early, facilitating timely intervention.

• Predictive Value: Biomarkers should provide information about the likelihood of a specific health outcome or response to treatment. For instance, oncologists use liquid biopsies to predict a patient's response to particular anti-cancer treatments. This helps in determining the suitability of a therapy based on its potential benefits and minimal adverse effects.

• Reproducibility: Measurements of biomarkers should be consistent and reproducible across different laboratories and testing conditions.

These characteristics render biomarkers invaluable tools in medical research, clinical diagnostics, and the field of personalized medicine.

To date, no singular biomarker has been universally accepted as robust enough to predict age-related outcomes. However, a combination of multiple biomarkers can provide valuable insights into an individual's current and future health. This network of collectively used biomarkers to assess an individual's biological or health status is known as a composite biomarker. [6]

The Predictive Power of Composite Biomarkers

The predictive power of composite biomarkers in aging assessments was demonstrated by Hastings et al. (2019) [7]. In their study, the researchers compared three different composite markers with a single biomarker (telomere length) to evaluate their association with age-related outcomes such as physical, cognitive, and perceptual functioning.

The Hastings study first examined the Klemera-Doubal Method (KDM), a composite biomarker for calculating biological age. KDM incorporates measures of physical functions such as grip strength and walking speed, as well as biomarkers for inflammation, metabolism, and organ function. Additionally, it considers demographic variables like age and sex. By integrating these diverse parameters, KDM effectively predicts an individual's biological age.

The second composite biomarker evaluated was the Homeostatic Dysfunction Marker. To grasp this concept, one must first understand homeostasis, which is akin to a house's thermostat. Just as a thermostat maintains stable indoor temperatures regardless of external conditions, homeostasis ensures a stable internal environment in the body, despite external or internal changes. As we age, our capacity for maintaining this balance declines, leading to issues like chronic inflammation, hormonal imbalances, insulin resistance, and oxidative stress. The Homeostatic Dysfunction Marker aggregates these aspects to gauge overall bodily function.

The third biomarker analyzed in the study was the Levine-method (LM) Biological Age, a composite marker predicting mortality risk using a set of biological markers. These include albumin (regulating blood pressure and fluid balance), alkaline phosphatase (aiding in bone mineralization and liver function), creatinine (a waste product excreted by the kidneys), C-reactive protein (indicative of inflammation), and glucose, among others. The study used a complex statistical model to analyze these markers and chronological age, developing a "mortality score" which was then converted into a biological age metric

The study compared the predictive power of the above three composite measurements to a single biomarker, telomere length. Aging correlates with the shortening of telomeres at chromosome ends, with shorter telomeres often indicating more severe aging-related issues. Thus, telomere length is a significant indicator of biological aging and was utilized in this study. The researchers discovered that the composite markers were more predictive of biological age than telomere length alone.

In the Hastings study, participants who exhibited higher biological ages, as determined by the composite biomarkers, demonstrated reduced performance in physical, cognitive, and perceptual tests. Furthermore, these individuals reported greater challenges in conducting daily activities and assessed their overall health more negatively. In contrast, telomere length did not show the same level of correlation with physical and cognitive performance or overall health assessments. This disparity highlights the comprehensive nature of the composite biomarkers in reflecting biological age. Therefore, the study emphasizes the significance of using multiple biomarkers to assess biological age, providing a more accurate and holistic representation of an individual's health status compared to chronological age alone [7].

Routine Laboratory Biomarkers

There are a vast repertoire of biomarkers available to evaluate one's biological age, and overall health comprehensively. Many times, routine biomarkers can provide a lot of insight into health without needing advanced testing.

Routine biomarkers, commonly tested in accredited laboratories through standardized methods, are pivotal for diagnosing diseases, confirming or ruling out medical conditions, assessing treatment effectiveness, and forecasting a patient's future health status.

Key biomarkers in this category include interleukins (ILs) and C-reactive protein (CRP). ILs, a group of proteins produced by immune cells, are involved in the body's inflammatory response and immune function. Certain ILs, such as IL-6, IL-8, IL-15, and IL-1β, are linked to inflammation associated with aging. CRP, produced by the liver in response to inflammation, is measured in the blood to evaluate the risk for age-related diseases. Elevated CRP levels can indicate chronic inflammation, which is connected to various age-related health issues.

Additionally, biomarkers related to lipid metabolism are important. These include total cholesterol, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides. Cholesterol, essential for cell building and hormone production, can, in excess, heighten the risk of heart disease. Total cholesterol represents the sum of all cholesterol types in the blood. HDL cholesterol, often termed "good" cholesterol, aids in removing other cholesterol forms from the bloodstream. Conversely, LDL cholesterol, known as "bad" cholesterol, distributes cholesterol throughout the body, potentially leading to arterial buildup and increased heart disease risk. The balance of these lipid biomarkers shifts with age, influencing cardiovascular risk. Monitoring these markers helps in assessing cardiovascular health, managing fat levels, and reducing mortality and morbidity risks from cardiovascular complications.

Other important biomarkers assess organ function and integrity, including creatinine, cystatin C, urea, and albumin, which provide insights into renal and liver function changes with age. Blood glucose levels are also crucial for evaluating diabetes risk. These biomarkers collectively offer a comprehensive view of an individual's health and biological aging process. [5]

Advanced Laboratory Biomarkers

While routine biomarkers offer significant insights into overall health and well-being, they sometimes fall short of painting a complete picture of one's health status. Often, specialized tests are necessary for a deeper understanding, especially for early detection and management of potential health issues. Numerous advanced biomarkers exist to provide a deeper understanding of various health aspects. In this review, we will concentrate on a select few of these biomarkers [5].

Let's begin by examining two advanced biomarkers that are crucial for understanding cardiovascular health. Cardiovascular health refers to the functionality and condition of the heart and blood vessels, encompassing factors such as the heart's efficiency in pumping blood, the condition of the blood vessels, and the circulation of blood through these vessels. While blood pressure and cholesterol levels are common indicators of cardiovascular health, there are additional biomarkers that offer more in-depth insights.

One important biomarker is the ApoA/ApoB Ratio. This blood test assesses the balance between two types of proteins: Apolipoprotein A (ApoA) and Apolipoprotein B (ApoB). ApoA is associated with HDL, or "good cholesterol," whereas ApoB is a component of LDL and VLDL, often referred to as "bad cholesterol." The ApoA/ApoB Ratio provides a comprehensive view of heart health by evaluating the balance between cholesterol carriers that are beneficial and potentially harmful in the bloodstream. This ratio offers a more detailed insight into cardiovascular health than merely analyzing HDL and LDL levels alone. A higher ApoA to ApoB ratio typically suggests a lower risk of cardiovascular disease, thereby aiding in the assessment of cardiovascular risk.

Another advanced biomarker for assessing cardiovascular health is homocysteine. Homocysteine is an amino acid produced in the body during protein digestion. Elevated levels of homocysteine can harm blood vessels and promote clot formation, substantially increasing the risk of heart issues. High homocysteine levels are often associated with a deficiency in B vitamins such as B12, B6, and folate. Monitoring homocysteine levels can provide valuable insights into potential dietary and supplement interventions needed to enhance cardiac health.

In addition to cardiac health, there are numerous biomarkers available for assessing hormonal health. Hormonal health involves the equilibrium and effective functioning of hormones within the body. Hormones, acting as chemical messengers, regulate a variety of bodily functions such as metabolism, growth and development, reproduction, mood, and sleep. Ensuring a balanced hormonal state is vital for overall health and well-being. While hormone levels are not commonly included in standard blood tests, measuring these levels can offer critical insights into various health conditions and challenges.

An important advanced biomarker in hormonal health is serum cortisol. Cortisol, a hormone that responds to stress, plays a vital role in regulating various body functions, including metabolism, immune response, and stress management. Serum cortisol levels typically exhibit a diurnal rhythm, meaning they vary throughout the day. Generally, cortisol levels peak in the early morning, shortly after waking, which helps jump-start the body's metabolism and energy for the day. As the day progresses, these levels gradually decrease, reaching their lowest at night to aid relaxation and sleep. Dysregulation of this cortisol balance can have adverse effects on energy levels, mood, and overall health. Recent studies have shown that imbalances in cortisol are associated with the development of cardiovascular disease, inflammation, and mental health disorders. Therefore, monitoring serum cortisol levels provides a means to detect such dysregulation early on, potentially preventing a range of problematic symptoms.

Outside of cardiovascular and hormonal health, several advanced biomarkers exist to measure a range of health fields including metabolic health, iron and copper regulation, thyroid health, vitamin and mineral deficiencies, and many more. Each of these advanced biomarkers goes deeper than what is seen in a standard lab panel and can provide more detailed information about health status.

Testosterone is another key hormonal biomarker offering deeper health insights. While primarily linked to male reproductive health, testosterone also plays significant roles in various bodily functions for both genders. In men, testosterone is crucial for the development of reproductive tissues and secondary sexual characteristics, such as facial hair growth, voice deepening, and muscle mass development. In both men and women, testosterone affects bone density, fat distribution, red blood cell production, and mood regulation.

Measuring testosterone levels is vital for diagnosing hormone imbalance-related conditions like hypogonadism (low testosterone) or hypergonadism (high testosterone). These imbalances can affect fertility, sexual function, mood, energy levels, and overall well-being. In women, assessing testosterone levels is particularly important for diagnosing conditions like polycystic ovary syndrome (PCOS), which can cause irregular menstrual cycles, infertility, and other health complications. Therefore, understanding testosterone levels is essential for gaining insights into an individual's hormonal health and informing suitable treatments and lifestyle changes to enhance overall well-being [5].

In this section, we've delved into a detailed examination of just a few advanced biomarkers, yet there are numerous others that can offer critical insights into health. Several clinicians will use a "composite calculation," which combines a broad spectrum of these advanced biomarkers. This holistic approach provides a comprehensive picture of an individual's health status, allowing for a more nuanced and complete understanding of various health aspects.

Nucleic-acid-based biomarkers

In addition to the standard and advanced biomarkers discussed earlier, another category of biomarkers to consider is nucleic acid-based biomarkers. Nucleic acids, which carry genetic information and are essential for cellular function, are akin to a recipe book providing instructions for building and maintaining life. This category includes DNA, which holds genetic information determining an organism's traits, and RNA, which plays a role in translating DNA instructions to synthesize proteins. Telomere length, as previously mentioned, is an example of a nucleic acid-based biomarker used in aging assessments.

Another nucleic acid-based biomarker is the extent of DNA damage. Over time, our DNA accumulates damage, contributing to the development of age-related pathologies like cancer and inflammation. Within cells, mitochondria – often referred to as the 'energy powerhouses' – convert food into energy and contain their own DNA. This mitochondrial DNA is also susceptible to age-related damage. Damage to mitochondrial DNA impairs mitochondrial function, leading to increased production of reactive oxygen species (ROS). ROS are oxygen-rich molecules that naturally arise during energy production in cells. Elevated levels of ROS, associated with mitochondrial dysfunction, can disrupt protein synthesis, folding, trafficking, and degradation, and further contribute to DNA damage and inflammation.

Growth differentiation factor 15 (GDF15) is another biomarker related to mitochondrial damage. When mitochondrial damage occurs, it often triggers the production of GDF15, which acts like a defensive mechanism against the consequences of this damage, such as inflammation. By measuring GDF15 levels, one can gauge the extent of mitochondrial damage: a higher GDF15 level indicates more severe damage. Therefore, GDF15 is another valuable nucleic acid-based biomarker for assessing the body's functional integrity and, consequently, biological age.

While nucleic acid-based biomarkers, such as the extent of DNA damage, mitochondrial DNA integrity, and levels of GDF15, offer valuable insights into biological aging and overall health, they are typically more challenging to measure compared to standard and advanced biomarkers.

The complexity of measuring these nucleic acid-based biomarkers arises from several factors. Firstly, the procedures required to assess DNA damage or mitochondrial integrity are often intricate and require highly specialized laboratory techniques, such as PCR (polymerase chain reaction), next-generation sequencing, or advanced microscopy. These techniques demand precise handling and interpretation by skilled professionals.

Secondly, the quantification of DNA damage or the assessment of mitochondrial function involves analyzing minute changes at the molecular level, which requires not only sophisticated equipment but also a deeper understanding of molecular biology and genetics. This complexity can make these tests more time-consuming and expensive compared to routine blood tests.

Furthermore, interpreting the results of nucleic acid-based biomarkers can be challenging. The implications of DNA damage or mitochondrial dysfunction are not always straightforward and often require correlation with other clinical and laboratory data to draw meaningful conclusions about an individual's health or biological age.

Lastly, unlike standard biomarkers that have established reference ranges and are widely used in clinical practice, the reference ranges and clinical implications of many nucleic acid-based biomarkers are still under research and may not be as well-defined. This can make it difficult to apply these biomarkers in routine clinical settings.

Therefore, while nucleic acid-based biomarkers are extremely valuable for in-depth health assessments and aging research, their measurement and interpretation are more complex and challenging compared to standard and advanced biomarkers. [6]

Conclusion

In this review, we have provided a detailed overview of the role that biomarkers play in understanding our aging and health. We've seen that chronological age, the simple count of years since birth, doesn't always match up with biological age, which is a more accurate reflection of health.

To understand our biological age and the various aspects of our health that it measures, we need to delve into specific biomarkers. These biomarkers, ranging from simple blood tests to more complex genetic markers, give us deeper insights into our biological age.

Through these biomarkers, we can assess different facets of health, such as cardiovascular function, hormonal balance, and metabolic processes. They help us detect early signs of aging-related diseases, enabling preventative measures and timely interventions. Furthermore, advanced biomarkers, including nucleic acid-based markers, provide a window into the cellular and genetic changes associated with aging. While more complex and challenging to measure, these advanced markers offer valuable information about our body's response to aging at a molecular level.

In essence, biomarkers serve as a bridge between our external age and internal health, offering a more comprehensive understanding of our body's aging process. This knowledge not only enhances our ability to manage and improve health as we age but also paves the way for personalized medical treatments. As research in this area continues to evolve, it is likely that our understanding and application of biomarkers will become even more sophisticated, further refining our approach to health and aging.