Heart Health
Q. I just got the results of my lipid profile. My cholesterol is high, but so are my triglycerides. What exactly are triglycerides, and how are they different from cholesterol?
A. Triglycerides are the most common form of fat in the bloodstream. They consist of three fatty acid chains linked by a molecule called glycerol.
When you eat food, enzymes in your gut break down fats into their component fatty acids, which are then reassembled to create triglyceride particles. These fatty particles can't move freely through the watery bloodstream. So they combine with cholesterol and protein to form lipoproteins. Your lipid profile lists two of these lipoproteins: low-density lipoprotein (better known as LDL cholesterol) and high-density lipoprotein (also called HDL cholesterol).
But there are actually several other types of lipoproteins, including some that contain mainly triglycerides. Triglycerides can provide energy to fuel your body, while the extras are deposited in fat tissue. After a very heavy, fatty meal, your bloodstream may contain so many triglyceride particles that a blood sample may have a milky tint. But within a few hours, they're mostly cleared out. When you need energy between meals, hormones release the stored triglycerides into the bloodstream.
Your liver also converts carbohydrates into triglycerides. When you eat extra calories — especially from carbohydrate-rich foods such as sweets and white bread — your liver churns out extra triglycerides. Other things that can raise blood levels of triglycerides include being overweight or obese, smoking, or heavy drinking. Certain medical conditions —including diseases affecting the liver, kidneys, or thyroid, and poorly controlled type 2 diabetes — can also lead to high triglycerides.
For healthy adults, normal triglyceride levels are under 150 milligrams per deciliter (mg/dL). Values of 151 to 200 mg/dL are considered borderline high; those 201 to 499 are high, and those 500 and higher are very high (and increase the risk of inflammation of the pancreas gland, called pancreatitis). Growing evidence suggests that above-normal triglyceride levels can raise the risk of cardiovascular disease independent of cholesterol levels.
To get your triglycerides back into a normal range, start with lifestyle changes. Aim for a healthy weight, get regular exercise, and eat more whole, unprocessed grains. Eat fewer refined carbohydrates, and cut back on saturated fat from meat. If you drink alcohol and aren't willing to stop, do not exceed moderate amounts (no more than a drink a day for women or two daily for men).
— by Deepak L. Bhatt, M.D., M.P.H.
Editor in Chief, Harvard Heart Letter
Image: Kiwis/Getty Images
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Triglycerides are lipid compounds composed of a glycerol esterified to 3 fatty acid chains of varying length and composition. These fatty acid chains can be saturated or unsaturated, and the chemical composition of each chain is different. Each chain consists of carbon and hydrogen atoms with varying single or double-bonded chains, depending on the degree of saturation or unsaturation. Triglycerides are formed of mixed chains, and the structural comparison between the chains is heterogenous in nature.
Triglyceride is the most abundant dietary lipid compound found throughout the diet and is the method with which energy is stored in the body. Initial digestion of dietary triglycerides takes place with pancreatic lipase, which hydrolyzes one fatty acid chain at a time to form 2 free fatty acid (FFA) chains and one 2-monoglyceride (2MG) compound per each triglyceride. Bile salts are released in the duodenum in response to cholecystokinin release occurring in the presence of lipid compounds within the ingesta. Bile salts aid in forming lipid micelles, which create a hydrophilic surface with a hydrophobic core of lipid molecules, including FFA.
Absorption of lipid compounds into the enterocyte for biochemical usage occurs through diffusion across the cellular membrane and also through lipid transporters that are located on the luminal side of the enterocyte. Once in the enterocyte, FFA chains and 2MG compounds are transported to the endoplasmic reticulum, where they are reformed into triglycerides and packaged into chylomicrons in the golgi apparatus to receive chylomicron specific apolipoproteins, namely apo B48, which is a marker for TG chylomicron. These newly formed chylomicrons are then released from the enterocyte and transported to circulation by the lymphatic system. [11]
Once in the circulation, the triglyceride-rich chylomicrons pass through the vasculature, where they undergo a complex process of protein exchange mediated by HDL and, based upon this protein exchange process, are either received in the liver for further metabolism and packaging or undergo delipidation at the vascular endothelial surface by lipoprotein lipase (LPL). [11] The largest proportion of chylomicrons containing dietary triglycerides undergo hepatic uptake, where triglycerides are packaged into very–low-density lipoprotein (VLDL) for transport to peripheral tissues.
VLDL is the major carrier of triglycerides and FFA in serum and is synthesized within the hepatocyte, while a smaller percentage of FFA travels in a unesterified form, which is complexed to albumin for transport. [12] Once the VLDL is release into serum, it travels to peripheral tissues where it undergoes a delipidation cascade, and triglyceride is removed by LPL at multiple LPL receptor sites along the endothelium. [11] Following delipidation, a VLDL remnant (IDL) is formed, which has released the bulk of triglyceride originally packaged and is cleared by the liver or transformed to LDL by serum protein exchange process.
Triglyceride is the major high-energy compound for energy storage supplying 9 Kcal/g of FFA. Those lipids that are intended for storage are recognized by and are removed from VLDL by LPL as well as storage specific transmembrane proteins that aid in a process of lipid droplet formation within adipocytes and muscle tissue for use later as an energy source. [11, 13] Liberation of triglycerides from lipid stores begins under metabolic stressors when circulating systemic nutrient supply is not sufficient to meet metabolic energy demand.
Regulation of enzymes needed for lipolysis occur through cyclic adenosine monophosphate (cAMP)–mediated and cAMP-independent pathways that activate adipose triglyceride lipase, hormone-sensitive lipase, and monoacylglycerol lipase, which hydrolyzes the ester bonds of stored triglyceride producing glycerol and FFA chains. [14] Glycerol undergoes cellular removal through transcellular aquaporins, and FFAs are either moved to serum, esterified or metabolized into signaling molecules. [14]
Once FFAs have been liberated from adipocytes for use in energy production, they are transported and received by cells for metabolism and mobilized to intracellular mitochondria and peroxisomes for use. These lipid compounds undergo fatty acid oxidation, providing acetyl-CoA for hepatic ketogenesis and substrates for energy production through oxidative phosphorylation. [12]
Triglycerides and FFA have been implicated in playing a role in atherosclerotic disease formation. High triglyceride is a marker for elevated levels of atherogenic lipoproteins that contain triglyceride and FFA. [15] As mentioned previously, elevation of triglycerides can indicate insulin resistance in the setting of low levels of HDL and elevated LDL. [4, 15] Patients with this lipid profile typically have elevated VLDLs, small LDL and HDL particles, and have elevated levels of circulating chylomicrons and places patients at risk for coronary heart disease. [15] Hypertriglyceridemia is a clinical risk factor for coronary artery disease (CAD), especially when low HDL is present, and should be considered a continued risk factor despite adequate control of LDL cholesterol. [4, 5, 15]
The USPSTF recommends lipid screening in both men and women who are at increased risk for coronary heart disease, men aged 35 years and older, or women aged 45 years and older. [16] AHRQ has similar recommendations to start routine screening at age 35 years or in those patients who have heart disease risk factors and aged 20-35 years. [17] Currently, no guidelines support routine screening of lipids in young adults aged 20-35 years without risks of coronary heart disease. Likewise, supporting evidence of routine lipid screening in children is lacking, and, therefore, no recommendation exists. The NCEP Report of the Expert Panel on Blood Cholesterol levels in Children and Adolescents recommends screening lipid panels as routine health care in children of families in which premature heart disease is evident or familial dyslipidemias are established. [18]
Secondary causes for hypertriglyceridemia can be a source for abnormal triglyceride on screening lipid panel, and clinical investigation should aim at discovering suspected secondary causes and treating appropriately. Possible etiologies of secondary hypertriglyceridemia include disease states such as uncontrolled diabetes, nephrotic syndrome, end-stage renal disease, hypothyroidism, and HIV. [19]
Common substances and medications that may be responsible for elevation of triglycerides include but are not limited to ethanol, corticosteroids, noncardioselective beta-blockers, thiazide diuretics, bile-binding resins, oral estrogens, progestins, and tamoxifen, as well as antiretroviral therapy. [19, 20] Patients on medications or ingesting substances known to raise triglycerides who are at risk for developing secondary hypertriglyceridemia in the setting of CAD, diabetes, disease states secondarily elevating triglycerides, or other coronary artery disease–equivalent states should receive routine screening. Additionally, surveillance lipid profiles should be considered at appropriate intervals with attention to removal of secondary causes of hypertriglyceridemia or institution of triglyceride-lowering therapy as needed.
Very-high levels of triglyceride can place patients at risk for the development of pancreatitis and work-up of a new diagnosis of pancreatitis should include a baseline lipid panel to investigate triglyceride levels. [9] Patients who present with extraordinarily high lipids levels or patients that have pancreatitis due to hypertriglyceridemia should have institution of triglyceride-lowering therapy, investigation of factors causing secondary hypertiglyceridemia, and consideration of concomitant familial dyslipidemia. [6, 10]
ACC/AHA cholesterol guidelines have not provided guideline specifics pertaining to triglycerides, but a 2011 scientific statement describes the screening processes of special populations, [9, 21] which is beyond the scope of this article. The clinical applicability of the lipid panel for analyzing triglyceride levels rests on the clinician's understanding of patient risk factors and use of those risk factors as guides to screening patients for hypertriglyceridemia and other dyslipidemias.
Once lipid-lowering therapy has been initiated, surveillance of lipid levels should continue until lipid levels are at recommended goal levels based upon the patient’s risk factor profile, with yearly checks thereafter. Continued surveillance of lipid panels should persist if any adjustments in therapy occur or if poor compliance to therapy is suspected. Screening lipid panel can show falsely positive high-triglyceride levels and occurs in patients who have consumed a meal high in lipid compounds and have not fasted for 8 hours prior to venipuncture. Also, patients who have recently consumed ethanol can have elevation of triglycerides that may not necessarily be indicative of baseline levels.