Your Heart Is More Than A Pump

This article was originally published in Dr. Tenpenny’s substack, Eye on the Evidence.

Most of us learned in high school biology that the function of the heart is to circulate our blood. We were taught the heart is a rather static pump, moving the blood from the extremities through the lungs (to be oxygenated) and then pumping it back through our body. When the upper chambers, called the atria, contract, the blood is passed into the lower chambers called ventricles. The right ventricle pushes the blood into the lungs, and the powerful left ventricle launches the blood through the rest of the body. I have always thought of the heart as a box with doors (valves) that open and close in sequence to make this happen.

Even though it was first described in 1660 (not a typo!), I never thought the heart moved the blood forward in a spiral motion. The fibers of the heart muscle actually align in the form of a helix. When the heart muscle contracts, it moves like wringing water out of a saturated cloth. The bottom of the heart twists as it contracts in a counterclockwise direction while the top twists clockwise. Scientists call this the “left ventricular twist,” and it can be used as an indicator of heart health.

However, recent research has shown that the heart is much more than just a pump. It is also a metabolic and endocrine organ, secreting molecules that impact the body’s metabolism. This changes the way we view the heart and its role in metabolic diseases, particularly obesity and diabetes. As a metabolic organ, the heart has complex interactions with other tissues. Providing insight into these interactions will, hopefully, lead to more effective use of lifestyle modification, nutritional education, supplements, and collaboration between medical specialties (cardiology and endocrinology).  

Why is cardiac muscle unique?  

• Skeletal muscle comprises approximately 40% of total body weight and contains between 50-75% of all body proteins.

• Smooth muscle is present throughout the body. In the stomach and intestines, it helps with digestion and nutrient collection. In the urinary system, it helps filter toxins from the body and plays a vital role in electrolyte balance. Smooth muscle is present in arteries and veins, where it plays a critical role in the regulation of blood pressure and tissue oxygenation. Smooth muscle differs from skeletal muscle in a variety of ways; perhaps the most important difference is its ability to be controlled involuntarily and be stimulated by external means, especially medications. A person does not need to think about their blood pressure for it to adapt to increasing oxygen demands from exercise. The nervous system instead uses hormones, neurotransmitters, and various receptors to control smooth muscle spontaneously. 

• Cardiac muscle, unlike skeletal muscle, is completely under involuntary control. The heart is made up of three layers:

  • The myocardium (the muscle itself)

  • The endocardium, forming the inner lining of the heart chambers and valves

  • The pericardium is a fibrous sac surrounding the heart.

The myocardium is made of cells called cardiomyocytes, cells that are unique to the heart. The cells can use various substrates to generate energy (ATP), including lipids (fatty acids) and/or carbohydrates as the primary fuel and amino acids and ketones during times of extra stress. The heart cell’s ability to alternate energy sources is referred to as being “metabolically flexible.” Adipose tissue has this characteristic too. More on this below.

Cardiomyocytes secrete several hormones known as cardiokines. A relatively new discovery, the first references to cardiokines appeared in the medical literature around 2010. The term has since been applied to a growing set of proteins secreted from cardiac cells, including:

• ANP – (atrial natriuretic peptide) – a cardiokine primarily secreted by cells located in the upper part of the heart, called the atria. ANP can be significantly elevated in patients with left ventricular dysfunction independent of clinical symptoms.

• BNP (B-type natriuretic peptide) – a cardiokine primarily secreted by cells located in the left ventricle.  BNP plays a clinically significant predictive role in response to various CVDs, including heart failure, hypertension, and arrhythmias.

• Tumor necrosis factor-α (TNF-α) – excreted by myocardial cells under stress, it is a harmful cardiokine involved in atherosclerosis and is associated with severe congestive heart failure (CHF).

• GDF8/MSTN (Growth Differentiation Factor 8/Myostatin) is a signaling protein. When its value declines from normal (7-32 ng/ml), it has been correlated with cardiovascular events, even impending death.  GDF8/MSTN has broad physiological roles. It modulates skeletal muscle growth, assists with mineral density of bone, and plays a protective role in heart failure.  

It is well recognized that ANP and BNP are useful for the clinical diagnosis, treatment, and prognosis of cardiovascular disease (CVD).  Further research into ANP has shown it can also inhibit fat cell proliferation, trigger the breakdown of fat and increase fat cell metabolism.

Both ANP and BNP can inhibit the release of ghrelin, a hormone that signals the drive to eat (makes you feel hungry). Blood levels of ghrelin are highest before meals, returning to lower levels after mealtimes. Further, there is new evidence suggesting that ANP and BNP may block the proliferation of various cancers.

Taken together, these findings show the multifaceted role of hormones released by the heart to regulate energy and metabolism, having an impact on the development of metabolic diseases.

What is metabolic flexibility?

Metabolic flexibility describes the ability to respond or adapt according to changes in energy demand and caloric need. For example, exercise requires a large amount of available fuel to match the huge increased energy demand. Vigorous exercise can increase energy expenditure 25-fold compared to resting metabolic rate. Thus, exercise requires tremendous metabolic flexibility to increase energy supply from all sources to meet the demand. In fact, there is evidence that exercise training can improve metabolic flexibility. This is one of the many reasons why exercise not only improves strength and cardiac muscle function; it also helps to eliminate obesity and type 2 diabetes. Other research shows exercise can slow aging.

More on Obesity and CVD

Leptin was the first hormone identified (1990) to be released from fat tissue. Leptin is secreted in response to food intake. It inhibits appetite, stimulates satiety, promotes lipid oxidation (fat burning), and accelerates metabolism, inducing weight loss. However, it has been established that obese patients often have high levels of circulating leptin and are leptin-resistant.

In the cardiovascular system, leptin regulates cellular metabolism and maintains homeostasis. Leptin has been linked with cardiovascular complications resulting from obesity, such as hypertension and heart disease.

In fact, there is a condition named “obesity cardiomyopathy,” which refers to cardiac abnormalities solely related to obesity. This form of cardiomyopathy occurs in persons with severe and long-standing obesity. It can lead to progressive congestive heart failure and, frequently, sudden cardiac death. Another unusual and relatively rare condition called myocardial steatosis, the intracellular accumulation of fat within the heart muscle that can lead to disruption of the heart’s electrical system and heart failure. Clinical and population studies have correlated high levels of circulating leptin with the development of cardiac hypertrophy in obesity. 
 

Adiponectin, another hormone secreted by adipose tissue, exerts anti-obesity and antidiabetic effects by reducing insulin resistance. It also has anti-inflammatory properties. Reduced adiponectin levels have been associated with an increased load on the heart, leading to left ventricular hypertrophy (wall thickening). Weight loss or caloric restriction increases adiponectin levels, and this increases insulin sensitivity. Therefore, adiponectin appears to improve glucose and lipid metabolism.

Review of BMI

I wrote about this in my substack on Ozempic, but it’s worth reviewing again.

Obesity is an epidemic in the US. In a National Health Statistics Report from June 2021, among children and adolescents aged 2–19 years, the prevalence of obesity is around 19%, and the prevalence of untreated or restored dental caries in one or more primary or permanent teeth was 46%, another marker for poor and poor nutrition. Among adults aged 20 and over, the age-adjusted prevalence of obesity was about 41%, severe obesity was 9%.  From 1999 through March 2020, US obesity increased from 30.5% to 41.9%. During the same time, the prevalence of severe obesity increased from 4.7% to 9.2%.
 

Although these statistics paint a dismal picture of health (really, the lack of health) in our country, the numbers may be somewhat skewed because a person is labeled as obese primarily by a measurement called the BMI, or Body Mass Index. The BMI measurement was introduced around 1830 by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician, astronomer, and statistician, not a physician. He proposed the idea of the BMI to quantify his idea of the “average man” even though the concept was developed by using the heights and weights only collected on random French and Scottish participants. Apparently, he had no particular interest in the study of obesity. Also in his many papers, Quetelet apparently never advocated for his ratio be used as a general measure of ‘body build’ or amount of fat.

It took almost 150 years for Quetelet’s original index to be popularized by researcher Ancel Keys and colleagues, claiming it was the most superior and the most direct assessment of obesity that could be used.) In his 1972 paper, Keys and colleagues proposed that Quetelet’s ratio be termed body mass index and for it to become the standard population-based form of measurement. A good review of this info can be found here.

Mentioning all of these hormones further demonstrates the interaction between adipose (fat) tissue and the heart. The link between obesity and cardiovascular disease (CVD) is particularly noteworthy since CVD accounts for more than two-thirds of the deaths in those with a high body mass index (BMI). The close connection between obesity and CVD underscores the importance of addressing obesity for preventing and managing cardiovascular health.