What Function Does The Adipose Tissue Surrounding The Heart Serve

Cardiac adipose tissue, far from being an inert packing material, plays a surprisingly active and complex role in heart health. Understanding the function of this fat, specifically the epicardial and pericardial adipose tissues, is crucial for developing better strategies to prevent and treat heart disease.

The Two Main Types of Cardiac Adipose Tissue

There are two distinct depots of fat surrounding the heart:

• Epicardial Adipose Tissue (EAT): This is a visceral fat depot located between the myocardium (heart muscle) and the pericardium (the sac surrounding the heart). EAT shares the same microcirculation as the coronary arteries, meaning that substances secreted by EAT can directly affect the heart muscle and vessels.
• Pericardial Adipose Tissue (PAT): This fat depot lies outside the pericardium. While it's still close to the heart, its influence is generally considered less direct than EAT's. PAT is more closely associated with subcutaneous fat, the kind found under the skin, than with visceral fat.

Both EAT and PAT have distinct characteristics and contribute differently to heart function and disease.

Key Functions of Cardiac Adipose Tissue

Cardiac adipose tissue serves several important functions under normal physiological conditions:

• Energy Source: Adipocytes, the cells that make up adipose tissue, store energy in the form of triglycerides. The heart can utilize these fatty acids as fuel, particularly during times of increased energy demand, such as exercise.
• Mechanical Support and Protection: Adipose tissue provides a cushioning effect, protecting the heart from physical trauma and external pressures. It also helps to maintain the heart's shape and position within the chest cavity.
• Regulation of Inflammation: Adipose tissue secretes a variety of signaling molecules called adipokines, some of which have anti-inflammatory properties. These help to maintain a healthy balance within the heart and surrounding tissues.
• Angiogenesis: EAT promotes the formation of new blood vessels (angiogenesis) in the heart. This is crucial for maintaining adequate blood supply to the heart muscle, especially during growth and adaptation to increased workload.
• Thermogenesis: Brown adipose tissue (BAT), a specialized type of fat that generates heat, is sometimes found in small amounts around the heart, particularly in infants. BAT helps to maintain body temperature by burning calories.

The Dark Side: When Cardiac Adipose Tissue Becomes a Problem

While cardiac adipose tissue has beneficial functions, excessive accumulation or dysfunction can contribute to various heart problems. This is often driven by factors like obesity, insulin resistance, and chronic inflammation.

Here's how excess or unhealthy cardiac fat can harm the heart:

• Increased Inflammation: In obese individuals, cardiac adipose tissue becomes infiltrated with immune cells, leading to chronic inflammation. Adipocytes start secreting pro-inflammatory adipokines, such as TNF-α and IL-6, which damage the heart muscle and blood vessels.
• Lipid Overload and Lipotoxicity: When the heart is exposed to excessive amounts of fatty acids from surrounding adipose tissue, it can lead to lipid overload and lipotoxicity. This means that fat accumulates within the heart muscle cells, impairing their function and contributing to heart failure.
• Coronary Artery Disease (CAD): EAT is closely associated with the coronary arteries. Excessive EAT can lead to inflammation and plaque buildup in the arteries, increasing the risk of CAD, heart attack, and stroke.
• Atrial Fibrillation (AFib): Studies have shown a link between increased cardiac adipose tissue and atrial fibrillation, a common heart rhythm disorder. Inflammation and fibrosis (scarring) caused by excess fat can disrupt the electrical signals in the atria, leading to AFib.
• Heart Failure: Excess cardiac adipose tissue can contribute to both heart failure with preserved ejection fraction (HFpEF) and heart failure with reduced ejection fraction (HFrEF). Inflammation, lipotoxicity, and fibrosis can weaken the heart muscle and impair its ability to pump blood effectively.
• Pericardial Effusion and Constrictive Pericarditis: In rare cases, excessive pericardial fat can lead to pericardial effusion (fluid buildup around the heart) or constrictive pericarditis (thickening and scarring of the pericardium). These conditions can restrict the heart's ability to fill with blood and pump it out, leading to heart failure.

The Role of Adipokines in Cardiac Health

Adipokines, the signaling molecules secreted by adipose tissue, play a critical role in regulating cardiac function and inflammation. Some adipokines, like adiponectin, have beneficial effects, while others, like leptin and resistin, can be harmful in excess.

• Adiponectin: This is an anti-inflammatory adipokine that improves insulin sensitivity, reduces inflammation, and protects against atherosclerosis (plaque buildup in the arteries). Lower levels of adiponectin are associated with increased risk of heart disease.
• Leptin: This adipokine regulates appetite and energy expenditure. However, in obese individuals, leptin resistance can develop, leading to increased leptin levels. High leptin levels can promote inflammation, increase blood pressure, and contribute to heart disease.
• Resistin: This adipokine is thought to promote insulin resistance and inflammation. Higher levels of resistin are associated with increased risk of CAD and heart failure.
• TNF-α and IL-6: These are pro-inflammatory cytokines secreted by adipose tissue. They contribute to insulin resistance, inflammation, and damage to the heart muscle and blood vessels.

How to Measure Cardiac Adipose Tissue

Several imaging techniques can be used to measure cardiac adipose tissue:

• Echocardiography: This ultrasound-based technique can provide a rough estimate of pericardial fat thickness.
• Computed Tomography (CT) Scan: CT scans can accurately measure both epicardial and pericardial adipose tissue volume. However, CT scans involve exposure to radiation.
• Magnetic Resonance Imaging (MRI): MRI provides detailed images of the heart and surrounding tissues, allowing for accurate measurement of cardiac adipose tissue without radiation exposure.

While these imaging techniques are valuable for research purposes, they are not routinely used in clinical practice to assess cardiac risk. However, as our understanding of cardiac adipose tissue grows, these measurements may become more common in the future.

Factors Influencing Cardiac Adipose Tissue

Several factors influence the amount and function of cardiac adipose tissue:

• Genetics: Genetic factors play a role in determining an individual's predisposition to accumulate cardiac fat.
• Age: Cardiac adipose tissue tends to increase with age.
• Sex: Men tend to have more visceral fat, including cardiac fat, than women.
• Diet: A diet high in calories, saturated fat, and processed foods can promote the accumulation of cardiac adipose tissue.
• Physical Activity: Regular exercise can help to reduce cardiac adipose tissue and improve its function.
• Hormones: Hormones like insulin, cortisol, and sex hormones can influence the development and function of cardiac adipose tissue.
• Inflammation: Chronic inflammation can promote the accumulation of cardiac adipose tissue and impair its function.

Strategies to Reduce Harmful Cardiac Adipose Tissue

Lifestyle modifications and medical interventions can help to reduce harmful cardiac adipose tissue and improve heart health:

• Weight Loss: Losing weight, especially visceral fat, can significantly reduce cardiac adipose tissue.
• Healthy Diet: A diet rich in fruits, vegetables, whole grains, and lean protein can help to reduce inflammation and promote healthy fat metabolism.
• Regular Exercise: Exercise helps to burn calories, reduce inflammation, and improve insulin sensitivity, all of which can reduce cardiac adipose tissue.
• Medications: Certain medications, such as statins and metformin, can help to reduce inflammation and improve insulin sensitivity, which may indirectly reduce cardiac adipose tissue.
• Surgery: In some cases, surgical removal of pericardial fat may be considered for patients with severe pericardial disease.

The Future of Cardiac Adipose Tissue Research

Research on cardiac adipose tissue is a rapidly growing field. Future research will likely focus on:

• Identifying specific adipokines that play a critical role in cardiac health and disease. This could lead to the development of targeted therapies to modulate adipokine signaling and prevent or treat heart disease.
• Developing new imaging techniques to better assess the function of cardiac adipose tissue. This could help to identify individuals at high risk of heart disease and guide treatment decisions.
• Investigating the role of cardiac adipose tissue in specific heart conditions, such as heart failure with preserved ejection fraction (HFpEF). This could lead to new strategies to prevent and treat this challenging condition.
• Understanding the interaction between cardiac adipose tissue and other organs, such as the liver and kidneys. This could provide a more comprehensive understanding of the metabolic and inflammatory processes that contribute to heart disease.

Scientific Explanation of Adipose Tissue Function

The function of adipose tissue, including that surrounding the heart, is governed by complex biological processes at the cellular and molecular levels. Here's a deeper look:

Cellular Composition

• Adipocytes: These are the primary cells in adipose tissue, responsible for storing and releasing triglycerides. They contain a large lipid droplet that occupies most of the cell volume.
• Pre-adipocytes: These are precursor cells that can differentiate into mature adipocytes.
• Macrophages: These immune cells reside in adipose tissue and play a role in inflammation and tissue remodeling.
• Other Immune Cells: T cells, B cells, and other immune cells are also found in adipose tissue and contribute to its inflammatory state.
• Fibroblasts: These cells produce collagen and other extracellular matrix components, providing structural support to the tissue.
• Endothelial Cells: These cells line the blood vessels within adipose tissue, facilitating nutrient and oxygen transport.

Molecular Mechanisms

• Lipogenesis: This is the process of synthesizing triglycerides from glucose and fatty acids. It is regulated by enzymes such as acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS).
• Lipolysis: This is the breakdown of triglycerides into fatty acids and glycerol. It is regulated by enzymes such as hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL).
• Adipokine Secretion: Adipocytes secrete a variety of adipokines, which are signaling molecules that influence metabolism, inflammation, and cardiovascular function. The production and secretion of adipokines are regulated by various factors, including nutrient availability, hormones, and inflammatory stimuli.
• Inflammation: In obese individuals, adipose tissue becomes inflamed, leading to the activation of immune cells and the production of pro-inflammatory cytokines. This inflammation is driven by factors such as excess nutrient availability, oxidative stress, and endoplasmic reticulum stress.
• Insulin Signaling: Insulin promotes glucose uptake and lipogenesis in adipocytes. However, in obese individuals, insulin resistance can develop, impairing insulin signaling and leading to increased lipolysis and inflammation.

Epicardial vs. Pericardial Adipose Tissue: A Deeper Dive

While both EAT and PAT are considered cardiac adipose tissue, they exhibit distinct characteristics and functions:

These differences highlight the importance of considering EAT and PAT as distinct entities when studying cardiac adipose tissue.

Conclusion

Cardiac adipose tissue is not just a passive fat depot; it's an active endocrine organ that plays a complex role in heart health. While it can provide energy, mechanical support, and regulate inflammation under normal conditions, excessive accumulation or dysfunction can contribute to various heart problems. Understanding the function of cardiac adipose tissue is crucial for developing better strategies to prevent and treat heart disease. Future research will likely focus on identifying specific adipokines that play a critical role in cardiac health and disease, developing new imaging techniques to assess the function of cardiac adipose tissue, and investigating the role of cardiac adipose tissue in specific heart conditions. By targeting cardiac adipose tissue, we may be able to improve heart health and reduce the burden of cardiovascular disease.