GUIDE TO DIETARY FATS: REVIEW OF ESSENTIAL FATS (PART 2)
Medically Reviewed by Dr. Rosmy Barrios - September 17, 2024
The following article forms part 2 of a guide to dietary fats, covering essential dietary fats.
Part 1 covers fat classifications and the role fat plays in the body, part 3 covers non-essential dietary fats and part 4 offers useful tips for optimizing fat metabolism.
Review of Common Dietary Fatty Acids
The following review takes a look at the most commonly consumed fatty acids in the human diet. Each fat is discussed in accordance with what is currently known about its function and health effects, including potential benefits and side effects.
Fats have been divided into essential fats (covering polyunsaturated fats) and non-essential fats (covering monounsaturated and saturated fats). Where applicable, fatty acid isomers (chemical subtypes) are briefly reviewed to highlight their differences and shed light on the way in which processing alters the properties of fat.
NOTE: Food source examples are listed with fatty acid percentages where possible, however, the amounts are merely estimates. Just like any nutrient, the net fatty acid content of a food item depends upon the food’s variety, the way in which it was cultivated and the processing method applied.
Essential Dietary Fats
All essential fats happen to be polyunsaturated fats, deriving from omega-3 and omega-6 subclasses of fatty acid. The human body lacks the ability to produce these fats endogenously without dietary input. Due to their structural stability, they are required for many cellular processes in all cell types.
Only alpha-linolenic acid and linoleic acid have been classified as essential fats, as the body uses these from the diet to build more complex essential omega-3 and omega-6 fats (e.g. DHA and GLA).[1] However, dietetic science is challenging this definition, as this conversion is extremely inefficient, and dietary intake of these two fats alone is not enough to produce sufficient quantities of essential complex omegas. Therefore, a mixture of essential omega-3 and omega-6 fats in the diet should be considered for optimal health and well-being.
Omega-3 Fats
Derivatives of omega-3 fats form a ratio with the derivatives of omega-6 fats in the body. This ratio is absolutely essential for regulating many bodily functions. These include pain, inflammation, membrane integrity, metabolism, cellular adhesion and growth. Studies support a target Omega-6:3 ratio ranging from 1:1 to 5:1, with imbalances linked to issues such as increased inflammation, loss of membrane integrity, atherosclerosis, obesity, and diabetes.[2]
The majority of refined plant-based cooking oils are high in omega-6 and low in omega-3 fats. This has offset the ratio of omega-6:omega-3 in the diets of many who regularly consume refined fatty products, creating a need for omega-3 supplementation. Unfortunately, high doses of omega-3 fats do not necessarily abolish the problems associated with excessive omega-6 intake. Later research suggests that cutting down on refined foods and consuming natural whole food sources of these fats maintains the correct balance for optimal health.
Alpha-Linolenic Acid (ALA)
Alpha-linolenic Acid is an essential fat subtype of linolenic acid. It is the most abundant dietary omega-3, found in many plant foods. ALA is one of several dietary omega-3 fats used to produce EPA and DHA (complex omega-3s), as well as to generate anti-inflammatory eicosanoids[3]. Eicosanoids are lipid cellular signals derived from omega-3 and omega-6 fats that mediate pain, blood clotting, inflammation, and various other immune responses.
ALA derivatives form 15-HETE and PGE1, both of which serve to block inflammatory eicosanoids (derivatives of EPA, GLA and linoleic acid). These compounds also inhibit the conversion of arachidonic acid into leukotrienes, which effectively stifles associated pain and inflammation.[4]
The complex omega-3 fats are equally as essential for maintaining eicosanoid levels and for regulating immune function. Additionally, they are required for optimal neurological and reproductive function. A composite of omega-3s in the diet is required to maintain optimal eicosanoid and complex omega-3 levels. ALA is arguably one of the most important dietary omega fats as it provides a substrate for the body to produce its own EPA and DHA.
While uptake of ALA from the diet is typically under 1%, it bioaccumulates in fatty tissue and has been shown to single-handedly meet brain, kidney, blood and liver DHA requirements[5]. A diet high in ALA is known to increase EPA levels in animal studies and promote a balanced eicosanoid profile in general. However, conversion of ALA to either EPA or DHA differed in test subjects, with increases of either omega-3 noted to fall anywhere between 20 and 100%.[6] Thus, extra sources of EPA and DHA in the diet are required for optimal health, even in diets with a high ALA content.
Excessive dietary saturated fats or omega-6 fats substantially inhibit the conversion of ALA into EPA and DHA. Therefore, moderating the intake of these fats and boosting ALA intake are central to maintaining optimal omega-3 levels in the body. As omega-6s and saturated fats are found in high quantities in most animal products, animal products are not generally the best food sources for meeting omega-3 requirements. Fish is an exception, containing a much higher omega-3 content relative to omega-6.
Food sources
- Chia seeds (17-18%)
- Canola oil (9-11%)
- Soybean oil (6-7%)
- Flaxseed oil (39-53%)
- Flaxseeds (22-23%)
- Walnut oil[7] (10-11%)
- Chloroplasts of green leafy vegetables and herbs
Benefits
- Antithrombotic and anti-inflammatory[8]
- Neuro-protective
- Associated with reduced risk of developing strokes[9], osteoporosis, cancer, metabolic syndrome, obesity and cardiovascular disease[10]
- Gut microbes transform ALA into EPA (see below) and EPA derivatives, which have proven to exert anti-inflammatory and anti-allergenic effects in the gut of mice[11]
- Improves mitochondrial function and synthesis in muscle tissue, promoting optimal muscle building and function[12]
Other benefits of ALA consumption pertain to its ability to increase DHA and EPA levels, which are discussed under the relevant omegas below.
Side Effects
- There is very little evidence regarding the toxicity of ALA. It’s safe to consume in dietary quantities.
Docosahexanoic Acid (DHA)
DHA is a vital omega-3 present throughout the whole body. It has critical regulating functions in multiple systems of the body, due in part to being a vital component of mitochondrial membranes.
The nervous system, reproductive system and the eyes especially rely on DHA for optimal functioning. DHA is also the precursor to several anti-inflammatory prostaglandins, including PGI3, PGD3 and PGE3[13].
During infancy and early childhood, the mother’s breast milk provides optimal DHA quantities for the child’s developing brain. The brain stores its bulk lifetime reserve of DHA during this phase of life, accounting for 9-18% of its total fat composition. Through the development and aging of the nervous system, stored neuronal membrane DHA is recycled and occasionally used up. Maintaining adequate brain levels throughout one’s life serves to reduce the risk of developing neurodegenerative disorders. It’s also a vital consideration for pregnant and breastfeeding mothers to ensure they provide optimal concentrations of DHA to the developing child.
A percentage of all cell membranes and membrane-bound eicosanoids comprise DHA and EPA. In body tissues where the ratio of DHA is lower than EPA, an increased tendency for inflammation can be seen. The opposite is true of having lower EPA levels and higher DHA levels. The conversion of dietary ALA into complex omega-3s is more efficient for EPA than for DHA. For these reasons, many researchers believe that increasing dietary DHA is more essential than increasing EPA for maintaining optimal well-being (unless EPA deficient). These two omega-3s are often found together in food sources in differing ratios.
The body can make DHA by converting ALA, which can be stored in fat tissue. Conversion of ALA is inhibited by excessive intake of saturated fats or omega-6 fats.[14] Aside from endogenous conversion, dietary sources of DHA or ALA can increase body levels. As little as 5% DHA is absorbed from dietary sources in healthy individuals, with roughly 0.5% of it entering the brain.
Food sources
- Shellfish
- Fish (0.1-1.8%; higher levels in deep sea fish, fatty fish and fish oil)
- Microalgae
- Krill oil
- Human breast milk (1-2%)
Benefits
- Reduces cellular inflammation while increasing cellular energy output when incorporated into mitochondrial membranes as a result of dietary intake[15] [16]
- Optimizes myelination of nerves and neurotransmission
- Enhances cell membrane receptor function, as well as membrane integrity and cell structure
- Lowers serum triglyceride concentrations, total cholesterol, and fat mass,[17] greatly improving cellular fat metabolism (specifically beta-oxidation)
- Regulates pancreatic beta cell activity and reduces diabetes risk
- Improves cognition, increases BDNF, regulates neuroinflammation, and promotes neurogenesis and neuroplasticity
- Deficiencies are associated with delayed cognitive development, increased pain, and visual impairment
- Breastfed infants and formula-fed infants on a DHA supplement have higher tissue DHA than formula-fed only infants (ALA without DHA is unable to meet newborn DHA requirements)[18]
- Promotes enhanced development of vision and physical coordination in the first year of life
- Supports healthy aging and protects against age-related cognitive decline
- Reduces the risk of developing neurodegenerative disorders and some types of cancer[19]
- Lowers diastolic blood pressure and platelet aggregation
Side Effects
- Dietary DHA consumption is regarded as safe, with no known side effects[20]
- High doses of DHA may reduce the expression of LDL receptors, resulting in an increase in serum LDL cholesterol levels[21] and LDL cholesterol turnover[22] (not seen when coupled with EPA).
- Consuming excessive amounts of fish rich in DHA may result in mercury accumulation and eventual toxicity
- Omega-3 supplements are often overly oxidized during production, which can impair their efficacy and create trans-fats[23]
Isomers
- Conjugated DHA. In the brain and gut, conjugated DHA is easier for cells to absorb[24] and functions in a similar way to ordinary DHA[25]. In a mouse study, conjugated DHA proved to be a lot more neuroprotective than ordinary DHA[26]. The bacteria in the gut can produce conjugated DHA from dietary food sources.
Eicosapentaenoic Acid (EPA)
EPA is another essential omega-3 fat required by the body. The body is able to synthesize EPA from either DHA[27], ALA or DPA (docosapentaenoic acid, another dietary omega-3 fat). Like DHA, EPA is a vital component of cell membranes and a precursor for anti-inflammatory eicosanoids, which regulate the function of pro-inflammatory eicosanoids (omega-6 derived). Unlike DHA, it is the quickest complex omega-3 to bioaccumulate in tissues. A larger portion of the DHA we consume is converted into EPA than retained in tissue as DHA.
EPA is known to form a ratio with arachidonic acid (AA) in the bloodstream (AA is a substrate and catalyst for other inflammatory eicosanoids). When AA is lower than EPA, it promotes the lower production of thromboxane and reduces the risk for blood clots. Furthermore, it increases the production of prostaglandins (I3 and I2) that are known to block blood clotting factors, lessen vasoconstriction, protect against heart injury and promote vascular growth.[28] Studies show that an AA/EPA ratio between 1:1 to 5-10:1 may be beneficial for health.[29]
Food sources
- Shellfish
- Fish (0.1-1.2%[30]; higher levels in deep sea fish)
- Algae
- Krill oil
Benefits
- Reduces triglycerides and LDL cholesterol[31]
- Lowers systolic blood pressure[32]
- Regulates the hypercholesterolemic actions of DHA
- Inhibits the production of arachidonic acid, vascular inflammation, and blood clotting factors[33]
- Aids muscle building in a similar fashion to DHA and may reduce side effects of intensive exercise when administered with DHA[34]
- More effective than DHA at protecting against depression[35]
- Protected against stress-induced depression in rats through regulating glial cell function and preventing damage to the hippocampus[36]
- EPA was shown to slow down the formation of kidney stones and protect against developing them[37]
- High blood EPA (independent of DHA) has been linked with a reduced incidence of heart attacks, elevated blood pressure, blood clotting, heart disease and atherosclerosis
- EPA with DHA lowers inflammation and may reduce the risk for arthritis, kidney disease, cancer[38], mood disorders, ADHD, schizophrenia, autism, dyslexia, dyspraxia, cognitive decline and several neurodegenerative disorders[39]
Side Effects
- EPA and DHA supplements may cause nausea and belching, yet no adverse effects are associated with their use
- EPA/DHA supplements were shown to slightly elevate blood sugar and protein metabolism, as well as decrease hemoglobin
Isomers
- 17,18-epoxyeicostetraenoic acid. Generated in the gut microbiome from dietary EPA (and indirectly from ALA). Limited evidence suggests this fat is able to lower gut inflammation and allergic responses and protect against heart damage post-heart attack[40].
- Krill Oil. EPA and DHA are bound to phosphatidylcholine in krill oil and were proven superior to fish oil for lowering symptoms of PMS and dysmenorrhea.
Omega-6 Fats
Omega-6 fats have both a good and bad reputation in the health arena. This is largely due to the inflammatory effects of high omega-6, which are compounded by its inhibitory effect on the conversion of ALA to EPA or DHA (resulting in a 40-50% reduction[41]).
As omega-6 is equally as essential as omega-3, it’s important not to completely cut omega-6 from the diet. Instead, healthy unrefined sources of omega-6 are required, which naturally maintains a balanced ratio when consumed with adequate sources of omega-3.
Linoleic Acid
Linoleic acid is the most abundant omega-6 fat found in the diet[42], often considered as being complementary to ALA. Like ALA, it serves as a precursor for more complex omega-6 fats and/or eicosanoids. Complex omega-6 fats are a necessary component of cell membranes, and like omega-3 fats, they serve to regulate many functions at the cellular level.
Often contained in plant-based cooking oils, linoleic acid is more commonly available than ALA. Unfortunately, a diet with too much linoleic acid can inhibit the conversion of ALA (and other omega-3s) into complex omega-3s that are vital for optimal health. This results in increased inflammation, nutrient deficits, and metabolic issues. Increased dietary consumption of LA may also promote the formation of arachidonic acid and pro-inflammatory eicosanoids; however, evidence is lacking to support this theory.[43]
Epidemiological studies confirm that diets with moderate linoleic acid intakes (1-2%) are mostly associated with beneficial health outcomes[44]. It is theorized that the ratio of omega-6:omega-3 in the diet ought to be 1-3:1-2. Consuming minimally processed whole foods and a variety of healthy cooking fats and oils are able to maintain this ratio naturally. Refined foods found in the standard Western diet often obscure this ratio to anywhere between 15-16.7 omega-6: 1 omega-3.[45]
Food sources
- Grapeseed oil (65-75%)[46] [47]
- Safflower oil (60%)
- Sunflower oil (44-75%)[48]
- Olive oil (3.5-21%)
- Canola oil (17-26%)
- Corn oil (50-60%)
- Palm oil (10%)
- Sesame oil (28-29%)[49]
- Almond oil (14-30%)[50]
- Avocado oil (10-55%)
- Animal products[51]
Benefits
- Decreased cellular fatty acid oxidation of saturated fats[52], thereby reducing a source of inflammation
- Increases HDL cholesterol levels relative to LDL cholesterol
- Associated with a reduced risk of developing cardiovascular disease, diabetes[53] and cancer[54]
- Linoleic acid was shown to modestly lower the risk for all-cause mortality
- Is a precursor for GLA and DGLA, which can be used to produce anti-inflammatory eicosanoids
Side Effects
- Decreases all forms of cholesterol[55], which has been shown to shorten lifespan if consumed in excessive amounts over a long time[56]
- Limits the use of enzymes that increase endogenous DHA and EPA production, potentially promoting deficiencies in excessive amounts
- Can increase the production of arachidonic acid, which contributes to inflammation and blood platelet aggregation
- May increase the risk for blood clotting disorders, rheumatoid arthritis and other pain-related disorders
- Proven to be toxic to lymphocytes in vitro[57] [58]
- Excessive levels consumed in unnatural quantities may increase the risk for certain reproductive cancers, however, studies have been contradictory
- High LA intake by pregnant or breastfeeding mothers may slow down neurological development in exposed offspring[59]
Isomers
- Conjugated linoleic acid (CLA; cis-9, trans-11) has been shown to lower the risk for cancer and atherosclerosis[60], reduce blood cholesterol, and have no short-term effect on blood pressure.[61] Studies reveal that it has the potential to reduce fat mass in some obese individuals but not others. Nonetheless, in vitro studies show that CLA aids mitochondrial synthesis and metabolism, which enhances muscle building and function[62]. In healthy women, CLA reduced insulin levels in combination with other common dietary fats (oleic, palmitic, stearic and linoleic acids).[63] Found in meat and dairy products.
- Linolelaidic acid (trans-fat isomer of linoleic acid) is associated with an increased risk for coronary heart disease.
Gamma-Linolenic Acid (GLA)
Gamma-linolenic acid is an omega-6 derivative of linoleic acid. It can be produced endogenously from linoleic acid or absorbed from dietary consumption. GLA converts into dihomo-gamma-linolenic acid (DGLA), which is associated with both inflammatory and anti-inflammatory properties in the body.
DGLA can be converted into either arachidonic acid or anti-inflammatory prostaglandins. When made into anti-inflammatory prostaglandins, it also competes with some of the enzymes that transform arachidonic acid into pro-inflammatory eicosanoids.[64]
While GLA is generally considered less inflammatory than linoleic acid, it may still increase the risk of inflammation. Consuming GLA with EPA was proven to lower conversion to arachidonic acid and increase anti-inflammatory eicosanoid levels.[65]
Food sources
- Borage oil (21%)
- Blackcurrant seed oil (17%)
- Evening primrose oil (9%)
Benefits
- Is more potent than EPA at inhibiting platelet aggregation
- Competes with enzymes that transform arachidonic acid into leukotrienes, thus lowering inflammatory eicosanoid production
- DGLA helps to regulate PGE1 and 2, working synergistically with ALA and other omega-3 fats to regulate inflammation
- Protects against neuronal excitotoxicity and neuroinflammation
- Lowers the risk of developing cancer
- May help to reduce breast pain[66]
- Deficits are associated with PMS[67] and supplementation may be able to lower menstrual inflammation in the endometrium[68]
- DGLA may reduce blood pressure, dilate the blood vessels and lower the risk for atherosclerosis[69]
- Animal studies support that DGLA increases HDL cholesterol and reduces triglycerides and VLDL cholesterol.[70]
- Capable of suppressing the activation of T lymphocytes, which may be beneficial for those with autoimmune disorders
Side Effects
- Can increase pro-inflammatory compounds when consumed in excess alongside other omega-6 fats
- Generally regarded as safe from dietary sources[71] and proven to be non-toxic in animal studies[72]
Non-Essential Dietary Fats
To be continued in part 3 (non-essential dietary fats).
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