GUIDE TO DIETARY FATS: TYPES, RATIOS, BEST USES AND BENEFITS EXPLAINED
Over the course of the 20th century, fat has been a prime point of debate with regard to health and nutrition. It has been long understood that impaired fat metabolism can drive the course of many health problems, especially cardiometabolic diseases. Despite recommendations to minimize consumption of “bad” fat in order to promote health, statistics for obesity, cardiovascular disease and other metabolic disorders have been steadily on the rise for several decades. Research from recent years has begun to challenge the older paradigm, pointing toward the importance of dietary fat for optimizing overall metabolism.
The below review aims to reveal the true effects of dietary fat intake on overall metabolism. Types of fat are discussed below in accordance with their relevance to bodily processes, highlighting the potential risks and benefits they pose towards health.
Part 1 of this review (below) covers fat classifications and the role fat plays in the body.
Part 2 covers essential dietary fats, part 3 covers non-essential dietary fats and part 4 offers useful tips for optimizing fat metabolism.
Classification of Fats
Fats are classified according to their chemical formula and structure. These factors denote the molecular functions of a fat at the cellular level and give insight into the way in which a fat is metabolized by the body.
Important fat classifications include:
Chain Length. Fats consist mainly of hydrogen and carbon atoms arranged in chains of carbon atoms (carbon chains). The chain length refers to how many carbons are in a fat molecule. Generally speaking, the shorter the length of the chain, the easier the fat is for the body to absorb and metabolize.
In this classification, fats are divided into:
- Short-chain fatty acids containing 6 or fewer carbons in a chain
- Medium-chain fatty acids containing 7 to 12 carbons in a chain
- Long chain fatty acids comprising 13 to 21 carbons in a chain
- Very long chain fatty acids consisting of 22 or more carbons in a chain
Saturation. Saturation of a fat refers to the degree of hydrogenation of the fat, which indeed refers to the hydrogen content. Hydrogen is bound to the carbon atoms that make up carbon chains in fat molecules. Carbon atoms can be joined together via single or double bonds, which influence the hydrogen content or saturation of the fat molecule, as well as the structure, function and bioavailability.
Fat types by saturation are divided into:
- Saturated fats which consist of only single carbon-to-carbon bonds and contain the most hydrogen atoms. The structure of saturated fats is linear, allowing for molecules to stack evenly and creating a solid fat at room temperature. These types of fat are also noted to be less efficiently absorbed by comparison to their unsaturated counterparts. Contrary to popular opinion, saturated fats are not necessarily unhealthy and typically offer health benefits when consumed in moderation.
- Monounsaturated fats (MUFAs) are those that contain only one double bond between carbon atoms in the entire molecule. Most monounsaturated fats are not as structurally stable as saturated fats and are therefore either liquid or semi-solid at room temperature.
- Polyunsaturated fats (PUFAs) contain more than one double bond between carbon atoms in a chain. The structure of polyunsaturated fats is non-linear due to their double bonds, which typically curve in what is referred to as a cis configuration. This ensures that they are liquid at room temperature. The more double bonds in unsaturated fat, the easier it is to absorb.
- Trans-fats are unsaturated fats that are classified as saturated fats due to their chemical structure and properties. These fats form when unsaturated fats become partially hydrogenated; a commercial process that involves exposing the fat to hydrogen at high pressure in the presence of a metal ion catalyst (e.g. nickel). This results in an unsaturated fat molecule with trans double bonds (instead of cis bonds). Trans double bonds are linear instead of curved, causing the unsaturated fat to act just like saturated fat. Therefore, trans-fats are solid or semi-solid at room temperature. Bacterial oxidation or heating of unsaturated fats also produces trans-fats in cooking oil, yet in amounts barely detectable . Industrial trans-fats are associated with negative health effects as the body is unable to recognize or metabolize them properly, while low quantities of naturally-produced trans-fats are not.
- Conjugated Fats. Conjugated fats refer to unsaturated fats that have been conjugated either through bacterial transformation or a chemical reaction. This results in an isomer of the fat molecule in which the double bonds alternate with single carbon bonds. Conjugated fats can contain cis bonds, trans bonds or a combination of the two types. More research is required before conclusions can be drawn, however, conjugated fats appear to be easier to absorb with more potent health effects than non-conjugated fats. Conjugated (natural) trans-fats also appear to have health benefits in contrast to their industrial counterparts. These can be found in animal products and fermented foods.
Carboxyl Reference. Both chain length and saturation are referred to in short-hand as the carboxyl reference or the C:D (Carbons : Double carbon bonds) of a fat. For example, oleic acid has 18 carbons with 1 double bond, written as 18:1. This instantly reveals that it is a long chain monounsaturated fat. This is often coupled with the omega reference system (see below).
Omega Reference System. When a fat is named as an omega number, it refers to the position of a double carbon bond and therefore only applies to unsaturated fats. This information gives insight into the shape and structure of an unsaturated fat, which underpins how it interacts with cells and, therefore, its functionality in a living organism. Omega (ω) refers to the last carbon in the chain that is farthest from the carboxyl group (-COOH). The number refers to the number of carbons away from the omega carbon that holds the double bond. For example, essential omega-3 (ω-3) and omega-6 (ω-6) fats maintain a double bond at 3 and 6 carbons from the omega carbon, respectively.
The Role of Fats in the Body
Fat is largely renowned for being a highly efficient energy source. However, it plays many diverse roles in the body that make it critical for optimal health and well-being. These include:
- Influencing cell membrane structure, fluidity and other physical properties
- Chaperoning hormones, fat-soluble vitamins and other vital molecules
- Providing insulation for organs and nerves
- Forming the basic building blocks for numerous cellular signals, including neurotransmitters, immune signals and hormones
- Serving as a cell signal for insulin secretion and pancreatic activity
- Modulating cell signaling in general
Naturally, the forms of fat consumed in the diet determines what fatty acid molecules are stored in the body and contributes towards the overall ability of cells to regenerate, signal and function.
A diet devoid of essential fats in the correct ratios is known to promote numerous health problems pertaining to dysfunctional cellular growth and signaling. These can manifest in a variety of ways, from neurological disorders to kidney damage to infertility. Excessive fat consumption often increases the risk for essential fat deficiencies by offsetting vital ratios and altering fat metabolism. Moreover, fat metabolism differs from person to person due to genetics, making some more susceptible than others to certain health conditions pertaining to dysmetabolism.
Fats are stored in the body in the form of triglycerides, which are molecules that contain three fatty acid chains. Triglycerides are broken down by various enzymes into their fatty acid building blocks, which are used as required in cellular processes. Delta5 and delta6 desaturases are examples of enzymes that promote the synthesis of polyunsaturated fats, including AA and DHA. Delta9 desaturase promotes the synthesis of monounsaturated fats, such as oleic acid.
Fat molecules, such as those referred to above, are further converted into a variety of different fatty acid derivatives that fulfill niche cellular functions. In the liver, many triglycerides are converted into cholesterol which is used to transport hormones around the body. In other cell types, fats are converted into cellular signaling molecules and often used to mediate immune processes.
Cholesterol is an essential type of lipoprotein that serves many vital functions in the body. Every cell is able to produce cholesterol if required; however, the liver is the main site of production. It is a component of cell membranes, chaperones fat reserves and hormones around the body, is a component of bile salt, and can be used as a substrate for all bodily hormones.
There are several types of cholesterol, each of which has been classified according to its size and function. All types of cholesterol are required to maintain optimal health, as they regulate the functions of one another.
- Low Density Lipoprotein (LDL) cholesterol comprises larger molecules and up to 60% of all cholesterol in the body. It is the main substrate for hormones (including vitamin D3) and the main chaperone for fat molecules (triglycerides) around the body. LDL cholesterol consists of fat particles that are more solid than HDL cholesterol, which allows for it to be used as a ‘lipid raft’ when chaperoning molecules through cell membranes. It may also be required for optimal muscle building. Elevated LDL cholesterol is known to increase the risk for some cardiovascular diseases and blood clotting. Men tend to produce more LDL cholesterol than women on average and are known to be at an increased risk for LDL-related cardiovascular diseases.
- High Density Lipoprotein (HDL) cholesterol “mops up” the bloodstream of excess fatty molecules, returning them to the liver for re-use or excretion. Chronic excessive levels of HDL cholesterol are known to promote the risk for atherosclerosis due to being deposited into the lining of blood vessel walls over time. Women have higher levels of HDL cholesterol than men on average and are known to be at a higher risk for HDL-related cardiovascular diseases and microbleeds.
- Very LDL (VLDL) cholesterol takes a much longer time to be processed and stands a much higher chance of clogging up blood vessels due to being larger in size than normal cholesterol. Trans-fats have the propensity to increase VLDL, which battles to be utilized and metabolized by the body properly.
The quality, quantity and type of cholesterol depends on the dietary fats one consumes. Recommendations for health often point towards consuming a variety of fats in balanced ratios for optimal results.
Phospholipids and Other Functional Fats
Fats that are not stored are often converted into phospholipids or other cellular fats (sphingolipids, cerebrosides, oxylipins, etc) for the purpose of building cell membranes. Phospholipids are generated in every cell, with the endoplasmic reticulum being the major cellular organelle responsible. All fats stored in the membranes of cells and their organelles are used as substrates for a wide variety of signaling molecules. These include hormones, neurotransmitters, and signals of pain and inflammation.
In the nervous system, fats are additionally used to generate the myelin sheath that insulates large neurons and enhances their conductivity.
Lipids as Disease Biomarkers
Due to their extensive role in every compartment of the body, fats are being ever more recognized for their role in promoting both health and disease. Just like markers of inflammation, specific configurations, concentrations, and distributions of various types of lipids are able to serve as biomarkers for states of disease. While this is not new for cardiovascular and metabolic conditions, breakthroughs in this field are now highlighting biomarkers for neurological conditions, such as Alzheimer’s disease, autoimmune conditions such as multiple sclerosis, polycystic kidney disease, and several types of cancer .
Intestinal Fat Absorption and Uptake
Fats are most often found stored in the form of triglycerides in food sources. During digestion, they are mixed together before being broken down by digestive enzymes (namely, pancreatic lipases) at the top portion of the digestive tract. Simultaneously, bile acids are also secreted into the digestive tract. These contain bile salt micelles which bind to the free fat molecules and incorporate them into their structure. Once bound, fat-laden micelles can easily diffuse in water and cross enterocyte cell membranes in the small intestine.
Short Chain Unsaturated Fat Absorption. Fats with fewer carbons (shorter chain fats) and more double carbon bonds (unsaturated fats) are easier for the gut to absorb. These properties make for easier incorporation into micelles and better diffusion across the gut membrane. As a result, short-chain fatty acids pass directly through enterocytes into portal circulation (the bloodstream connecting the gut and liver), and are metabolized very quickly.
Long Chain Saturated Fat Absorption. Saturated longer chain fats require more processing than short chain fats and take longer to metabolize. Enterocytes re-esterify long chain fats into triglycerides which are then used to produce chylomicrons prior to their secretion into portal circulation. Chylomicrons are a type of lipoprotein molecule rich in triglycerides. Other lipoprotein molecules consist of the three types of cholesterol (see cholesterol above). HDL cholesterol and apolipoproteins serve to activate lipoprotein lipase, which frees longer chain fats from the triglycerides present in chylomicrons for cellular use. Freed fats are used for energy production in muscle and fat tissue or stored for later use as triglycerides. 
Colonic Fat Absorption. Short chain and medium chain fats are also absorbed through passive diffusion in the large intestine (colon). Gut bacteria typically produce short chain fats as by-products of fermentation, which are immediately absorbed in the colon . They are also able to transform non-absorbable long chain fats into readily absorbed medium and short chain fats. Bacterial conjugation of fats additionally increases their uptake in the colon.
Review of Common Dietary Fatty Acids
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