Docosahexaenoic acid (DHA) is a long-chain (≥ 12 carbon (C) atoms), polyunsaturated (> 1 double bond) fatty acid (English : PUFAs, polyunsaturated fatty acids) belonging to the group of omega-3 fatty acids (n-3 FS, first double bond is present – as seen from the methyl (CH3) end of the fatty acid chain – at the third C-C bond) – C22:6; n-3. DHA can be supplied both through the diet, mainly through oils of fatty marine fish, such as mackerel, herring, eel and salmon, and synthesized (formed) in the human organism from the essential (vital) n-3 FS alpha-linolenic acid (C18:3). The relatively high content of DHA in the fat of many cold–water fish species comes directly from the food chain or from the precursor alpha-linolenic acid through intake of algae, such as spirulina, and krill (small crustaceans, shrimp-like invertebrates). Studies have shown that fish farm-raised fish, which lack natural dietary sources of omega-3 fatty acids, have significantly lower DHA concentrations than fish living under natural conditions.
Synthesis
Alpha-linolenic acid is the precursor (precursor) for the endogenous (body’s own) synthesis of DHA and enters the body exclusively through the diet, primarily through vegetable oils such as flax, walnut, canola, and soybean oils. Desaturation (insertion of double bonds, turning a saturated compound into an unsaturated one; in humans, this occurs only between already existing double bonds and the carboxyl (COOH) end of the fatty acid chain) and elongation (lengthening of the fatty acid chain by 2 C atoms at a time), alpha-linolenic acid is converted in the smooth endoplasmic reticulum (structurally rich cell organelle with a channel system of cavities surrounded by membranes) of leukocytes (white blood cells) and liver cells via the omega-3 fatty acid eicosapentaenoic acid (EPA; C20: 5) metabolized (metabolized) to DHA. The conversion of alpha-linolenic acid to DHA proceeds as follows:
- Alpha-linolenic acid (C18:3) → C18:4 by delta-6 desaturase (enzyme that inserts a double bond at the sixth C-C bond – as seen from the COOH end of the fatty acid chain – by transferring electrons).
- C18:4 → C20:4 by fatty acid elongase (enzyme that elongates fatty acids by a C2 body).
- C20:4 → eicosapentaenoic acid (C20:5) by delta-5 desaturase (enzyme that inserts a double bond at the fifth C-C bond – as seen from the COOH end of the fatty acid chain – by transferring electrons).
- C20:5 → docosapentaenoic acid (C22:5) → tetracosapentaenoic acid (C24:5) by fatty acid elongase.
- C24:5 → tetracosapentaenoic acid (C24:6) by the delta-6 desaturase.
- C24:6 → docosahexaenoic acid (C22:6) by ß-oxidation (oxidative shortening of fatty acids by 2 C atoms at a time) in peroxisomes (cell organelles in which fatty acids and other compounds are oxidatively degraded)
DHA in turn serves as a precursor for the endogenous synthesis of anti-inflammatory (anti-inflammatory) and neuroprotective (promoting the survival of nerve cells and nerve fibers) docosanoids, such as docosatrienes, D-series resolvins, and neuroprotectins, respectively, which occurs in cells of the immune system (→ neutrophils) and brain (→ glial cells) as well as in the retina, among others. Women exhibit more effective DHA synthesis from alpha-linolenic acid compared to men, which can be attributed to the effects of estrogen. Whereas healthy young women convert about 21% of alpha-linolenic acid supplied alimentarily (via food) to EPA and 9% to DHA, only about 8% of alpha-linolenic acid from food is converted to EPA and only 0-4% to DHA in healthy young men. To ensure endogenous synthesis of DHA, sufficient activity of both delta-6 and delta-5 desaturases is required. Both desaturases require certain micronutrients, particularly pyridoxine (vitamin B6), biotin, calcium, magnesium and zinc, to maintain their function. Deficiency of these micronutrients leads to a decrease in desaturase activity and subsequently to impaired DHA synthesis. In addition to micronutrient deficiency, delta-6 desaturase activity is also inhibited by the following factors:
- Increased intake of saturated and unsaturated fatty acids, such as oleic acid (C18:1; n-9-FS) and linoleic acid (C18:2; n-6-FS).
- Alcohol consumption in high doses and over a long period of time, chronic alcohol consumption.
- Increased cholesterol
- Insulin-dependent diabetes mellitus
- Viral infections
- Diseases, such as liver disease
- Stress – release of lipolytic hormones, such as adrenaline, which leads to the cleavage of triglycerides (TG, triple esters of the trivalent alcohol glycerol with three fatty acids) and release of saturated and unsaturated fatty acids through stimulation of triglyceride lipase.
- Aging
In addition to DHA synthesis from alpha-linolenic acid, delta-6 and delta-5 desaturase and fatty acid elongase are also responsible for the conversion of linoleic acid (C18:2; n-6-FS) to arachidonic acid (C20:4; n-6-FS) and docosapentaenoic acid (C22:5; n-6-FS) and oleic acid (C18:1; n-9-FS) to eicosatrienoic acid (C20:3; n-9-FS), respectively. Thus, alpha-linolenic acid and linoleic acid compete for the same enzyme systems in the synthesis of other biologically important polyunsaturated fatty acids, with alpha-linolenic acid having a higher affinity (binding strength) for delta-6 desaturase compared to linoleic acid. For example, if more linoleic acid than alpha-linolenic acid is supplied in the diet, there is increased endogenous synthesis of the proinflammatory (inflammation-promoting) omega-6 fatty acid arachidonic acid and decreased endogenous synthesis of the anti-inflammatory (anti-inflammatory) omega-3 fatty acids EPA and DHA. This illustrates the relevance of a quantitatively balanced ratio of linoleic acid to alpha-linolenic acid in the diet. According to the German Nutrition Society (DGE), the ratio of omega-6 to omega-3 fatty acids in the diet should be 5:1 in terms of a preventively effective composition. The excessively high intake of linoleic acid – in accordance with today’s diet (through cereal germ oils, sunflower oil, vegetable and diet margarine, etc.) and the suboptimal enzyme activity, especially of delta-6 desaturase due to frequently occurring micronutrient deficiencies, hormonal influences, interactions with fatty acids, etc., are the reason why DHA synthesis from alpha-linolenic acid in humans is very slow and at a low level, which is why DHA is considered an essential (vital) compound from today’s point of view. Consequently, consumption of DHA-rich cold–water fish, such as herring, salmon, trout, and mackerel, (2 fish meals/week, corresponding to 30-40 g fish/day) or direct administration of DHA through fish oil capsules is essential. Only a diet rich in DHA ensures optimal concentrations of this highly unsaturated fatty acid in the human body. The exogenous supply of DHA plays a crucial role especially during pregnancy and lactation, since neither the unborn nor the infant is able to synthesize sufficient amounts of the essential omega-3 fatty acid DHA by itself due to restricted enzymatic activities. DHA promotes the development of the brain, central nervous system and vision of the fetus while still pregnant, but also during breastfeeding and further fetal development. A study from Norway concluded that 4-year-old children of mothers who were supplemented with cod liver oil during pregnancy and during the first three months of breastfeeding (2 g EPA + DHA/day) performed significantly better on an IQ test than those 4-year-olds whose mothers did not receive cod liver oil supplementation. According to these findings, an undersupply of DHA during prenatal and early childhood growth can impair the child’s physical and mental development and lead to lower intelligence – reduced learning, memory, thinking, and concentration abilities – and poorer visual ability or acuity.
Resorption
DHA can be present in the diet both in free form and bound in triglycerides (TG, triple esters of the trivalent alcohol glycerol with three fatty acids) and phospholipids (PL, phosphorus-containing, amphiphilic lipids as essential components of cell membranes), which are subject to mechanical and enzymatic degradation in the gastrointestinal (GI) tract.Mechanical dispersion – mastication, gastric and intestinal peristalsis – and the action of bile emulsify the dietary lipids and thus break them down into small oil droplets (0.1-0.2 µm) that can be attacked by lipases (enzymes that cleave free fatty acids (FFAs) from lipids → lipolysis). Pregastric and gastric (stomach) lipases initiate the cleavage of triglycerides and phospholipids (10-30% of dietary lipids). However, the main lipolysis (70-90% of lipids) occurs in the duodenum (duodenal) and jejunum (jejunum) under the action of esterases from the pancreas (pancreatic), such as pancreatic lipase, carboxylester lipase, and phospholipase, whose secretion (secretion) is stimulated by cholecystokinin (CCK, peptide hormone of the gastrointestinal tract). The monoglycerides (MG, glycerol esterified with a fatty acid, such as DHA), lyso-phospholipids (glycerol esterified with a phosphoric acid), and free fatty acids, including DHA, resulting from TG and PL cleavage combine in the small intestinal lumen together with other hydrolyzed lipids, such as cholesterol, and bile acids to form mixed micelles (spherical structures 3-10 nm in diameter, in which the lipid molecules are arranged so that the water-soluble molecule portions are turned outward and the water-insoluble molecule portions are turned inward) – micellar phase for solubilization (increase in solubility) of lipids – that allow uptake of lipophilic (fat-soluble) substances into the enterocytes (cells of the small intestinal epithelium) of the duodenum and jejunum. Diseases of the gastrointestinal tract associated with increased acid production, such as Zollinger-Ellison syndrome (increased synthesis of the hormone gastrin by tumors in the pancreas or upper small intestine), can lead to impaired absorption of lipid molecules and thus to steatorrhea (pathologically increased fat content in the stool), because the tendency to form micelles decreases with a decrease in pH in the intestinal lumen. Fat absorption under physiological conditions is between 85-95% and can occur by two mechanisms. On the one hand, MG, lyso-PL, cholesterol and free fatty acids, such as DHA, can pass through the phospholipid double membrane of enterocytes by means of passive diffusion due to their lipophilic nature, and on the other hand, by involvement of membrane proteins, such as FABPpm (fatty acid-binding protein of the plasma membrane) and FAT (fatty acid translocase), which are present in other tissues besides the small intestine, such as liver, kidney, adipose tissue – adipocytes (fat cells), heart and placenta, to allow lipid uptake into the cells. A high-fat diet stimulates intracellular (inside the cell) expression of FAT. In enterocytes, DHA, which has been incorporated (taken up) as a free fatty acid or in the form of monoglycerides and released under the influence of intracellular lipases, is bound to FABPc (fatty acid-binding protein in the cytosol), which has a higher affinity for unsaturated than for saturated long-chain fatty acids and is expressed (formed) particularly in the brush border of the jejunum. Subsequent activation of protein-bound DHA by adenosine triphosphate (ATP)-dependent acyl-coenzyme A (CoA) synthetase (→ DHA-CoA) and transfer of DHA-CoA to ACBP (acyl-CoA-binding protein), which serves as an intracellular pool and transporter of activated long-chain fatty acids (acyl-CoA), enables the resynthesis of triglycerides and phospholipids in the smooth endoplasmic reticulum (richly branched channel system of planar cavities enclosed by membranes) and thus – by removing lipid molecules from diffusion equilibrium – the incorporation of further lipophilic (fat-soluble) substances into enterocytes. This is followed by incorporation of DHA-containing TG and PL, respectively, into chylomicrons (CM, lipoproteins) composed of lipids-triglycerides, phospholipids, cholesterol and cholesterol esters-and apolipoproteins (protein portion of lipoproteins, function as structural scaffolds and/or recognition and docking molecules, for example, for membrane receptors), such as apo B48, AI, and AIV, and are responsible for the transport of dietary lipids absorbed in the intestine to peripheral tissues and the liver. Instead of being transported in chylomicrons, DHA-containing TGs and PLs, respectively, can also be transported to tissues incorporated into VLDLs (very low density lipoproteins). The removal of absorbed dietary lipids by VLDL occurs particularly in the starvation state.The re-esterification of lipids in enterocytes and their incorporation into chylomicrons can be impaired in certain diseases, such as Addison’s disease (adrenocortical insufficiency) and celiac disease (gluten-induced enteropathy; chronic disease of the mucosa of the small intestine due to gluten intolerance), which can result in reduced fat absorption and ultimately steatorrhea (pathologically increased fat content in the stool). Intestinal fat absorption may likewise be impaired in the presence of deficient bile acid and pancreatic juice secretion, for example, in cystic fibrosis (inborn error of metabolism associated with dysfunction of exocrine glands due to dysfunction of chloride channels), and in the presence of excessive intake of dietary fiber (indigestible food components that form insoluble complexes with fats, among others).
Transport and distribution
Lipid-rich chylomicrons (consisting of 80-90% triglycerides) are secreted (secreted) into the interstitial spaces of enterocytes by exocytosis (transport of substances out of the cell) and transported away via the lymph. Via the truncus intestinalis (unpaired lymphatic collecting trunk of the abdominal cavity) and ductus thoracicus (lymphatic collecting trunk of the thoracic cavity), the chylomicrons enter the subclavian vein (subclavian vein) and jugular vein (jugular vein), respectively, which converge to form the brachiocephalic vein (left side) – angulus venosus (venous angle). The venae brachiocephalicae of both sides unite to form the unpaired superior vena cava (superior vena cava), which opens into the right atrium of the heart. By the pumping force of the heart, chylomicrons are introduced into the peripheral circulation, where they have a half-life (time in which a value that decreases exponentially with time is halved) of about 30 minutes. During transport to the liver, most of the triglycerides from the chylomicrons are cleaved into glycerol and free fatty acids, including DHA, under the action of lipoprotein lipase (LPL) located on the surface of endothelial cells of blood capillaries, which are taken up by peripheral tissues, such as muscle and adipose tissue, partly by passive diffusion, partly carrier-mediated – FABPpm; FAT. Through this process, chylomicrons are degraded to chylomicron remnants (CM-R, low-fat chylomicron remnant particles), which bind to specific receptors in the liver, mediated by apolipoprotein E (ApoE). Uptake of CM-R into the liver occurs via receptor-mediated endocytosis (invagination of the cell membrane → strangulation of CM-R-containing vesicles (endosomes, cell organelles) into the cell interior). The CM-R-rich endosomes fuse with lysosomes (cell organelles with hydrolyzing enzymes) in the cytosol of liver cells, resulting in the cleavage of free fatty acids, including DHA, from the lipids in CM-Rs. After binding of the released DHA to FABPc, its activation by ATP-dependent acyl-CoA synthetase and transfer of DHA-CoA to ACBP, reesterification of triglycerides and phospholipids occurs. The resynthesized lipids may be further metabolized (metabolized) in the liver and/or incorporated into VLDL (very low density lipoproteins) to pass through them via the bloodstream to extrahepatic (“outside the liver”) tissues. As VLDL circulating in the blood binds to peripheral cells, the triglycerides are cleaved by action of LPL and the fatty acids released, including DHA, are internalized by passive diffusion and transmembrane transport proteins, such as FABPpm and FAT, respectively. This results in the catabolism of VLDL to IDL (intermediate density lipoproteins). IDL particles can either be taken up by the liver in a receptor-mediated manner and degraded there or metabolized in the blood plasma by a triglyceride lipase to the cholesterol-rich LDL (low density lipoproteins), which supplies peripheral tissues with cholesterol. In the cells of tissues and organs, DHA is largely incorporated into the phospholipids, such as phosphatidylethanolamine, -choline, and -serine, of plasma membranes and the membranes of cell organelles, such as mitochondria (“energy powerhouses” of cells) and lysosomes (cell organelles with acidic pH and digestive enzymes).Particularly rich in DHA are the phospholipids of the synaptosomes (nerve terminals containing vesicles and numerous mitochondria) of the gray matter (areas of the central nervous system consisting mainly of nerve cell bodies) of the brain (→ cortex (cortex) of the cerebrum and cerebellum), making DHA essential for the normal development and function of the central nervous system, especially for nerve conduction (→ learning, memory, thinking, and concentration). The human brain is composed of 60% fatty acids, with DHA accounting for the largest proportion. Numerous studies have shown that the fatty acid pattern of phospholipids in cell membranes is strongly dependent on the fatty acid composition of the diet. Thus, a high DHA intake causes an increase in the proportion of DHA in the phospholipids of plasma membranes by displacing arachidonic acid and thus increasing membrane fluidity, which in turn affects activities of membrane-bound proteins (receptors, enzymes, transport proteins, ion channels), availability of neurotransmitters (messengers that transmit information from one neuron to another via their contact sites (synapses)), permeability (permeability), and intercellular interactions. High levels of DHA can also be found in the cell membranes of the photoreceptors (specialized, light-sensitive sensory cells) of the retina, where DHA is necessary for normal development and function, especially for the regeneration of rhodopsin (compound of the protein opsin and the vitamin A aldehyde retinal, which is critical for vision and sensitivity of the eye). Other tissues that contain DHA include gonads (gonads), sperm, skin, blood, cells of the immune system, and skeletal and cardiac muscles. Pregnant women are able to store DHA in the body through a complex mechanism and draw on this reserve when needed. As early as the 26th-40th week of pregnancy (SSW), during which the development of the central nervous system progresses rapidly – cerebralization phase, which extends into the first months after birth – DHA is incorporated into the brain tissue of the unborn, and the mother’s DHA status is crucial for the degree of accumulation. During the last trimester (28-40th SSW), DHA content increases threefold in the cortex (cortex) of the cerebrum and cerebellum of the fetus. In the last half of pregnancy, DHA is also increasingly deposited in the tissues of the retina – the period when the main development of the eye takes place. Preterm infants born before 32 weeks gestation have significantly lower DHA concentrations in the brain and score on average 15 points lower on an IQ test later in life than normally developing children. Accordingly, it is particularly important in preterm infants to compensate for the initial DHA deficiency with a DHA-rich diet. According to several studies, there is a positive correlation between maternal DHA intake and the DHA content of breast milk. DHA represents the dominant omega-3 fatty acid in breast milk. In contrast, infant formula foods, in which alpha-linolenic acid is the dominant omega-3 fatty acid, contain only small amounts or no DHA. When comparing the DHA concentration of breastfed infants and infants fed with infant formula, significantly higher levels were observed in the former. Whether fortification of infant formula foods with DHA promotes visual acuity and neuronal development in premature and normally developing infants or prevents deficiency symptoms remains unclear because of the controversial nature of the studies.
Degradation
Catabolism (breakdown) of fatty acids occurs in all body cells, especially liver and muscle cells, and is localized in mitochondria (“energy powerhouses” of cells). Exceptions are erythrocytes (red blood cells), which have no mitochondria, and nerve cells, which lack the enzymes that break down fatty acids. The reaction process of fatty acid catabolism is also called ß-oxidation, since oxidation occurs at the ß-C atom of the fatty acids.In ß-oxidation, the previously activated fatty acids (acyl-CoA) are oxidatively degraded to several acetyl-CoA (activated acetic acid consisting of 2 C atoms) in a cycle that is run through repeatedly. In this process, acyl-CoA is shortened by 2 C atoms – corresponding to one acetyl-CoA – per “run”. In contrast to saturated fatty acids, whose catabolism occurs according to the ß-oxidation spiral, unsaturated fatty acids, such as DHA, undergo several conversion reactions during their degradation – depending on the number of double bonds – because they are cis-configured in nature (both substituents are on the same side of the reference plane), but for ß-oxidation they must be in trans-configuration (both substituents are on opposite sides of the reference plane). In order to be made available for ß-oxidation, the DHA bound in triglycerides and phospholipids, respectively, must first be released by hormone-sensitive lipases. In starvation and stress situations, this process (→ lipolysis) is intensified due to increased release of lipolytic hormones such as adrenaline. The DHA released in the course of lipolysis reaches energy-consuming tissues, such as liver and muscles, via the bloodstream – bound to albumin (globular protein). In the cytosol of cells, DHA is activated by ATP-dependent acyl-CoA synthetase (→ DHA-CoA) and transported across the inner mitochondrial membrane into the mitochondrial matrix with the help of carnitine (3-hydroxy-4-trimethylaminobutyric acid, quaternary ammonium (NH4+) compound), a receptor molecule for activated long-chain fatty acids. In the mitochondrial matrix, DHA-CoA is introduced into ß-oxidation, the cycle of which is run once – as follows:
- Acyl-CoA → alpha-beta-trans-enoyl-CoA (unsaturated compound) → L-beta-hydroxyacyl-CoA → beta-ketoacyl-CoA → acyl-CoA (Cn-2).
The result is a DHA shortened by 2 C atoms, which must be enzymatically trans-configured at its cis double bond before entering the next reaction cycle. Since the first double bond of DHA – as seen from the COOH end of the fatty acid chain – is located on an even-numbered C atom (→ alpha-beta-cis-enoyl-CoA), it occurs under the influence of a hydratase (enzyme, which stores H2O in a molecule), alpha-beta-cis-enoyl-CoA is converted to D-beta-hydroxyacyl-CoA and then, under the influence of an epimerase (enzyme that changes the asymmetric arrangement of a C atom in a molecule), is isomerized to L-beta-hydroxyacyl-CoA, which is an intermediate product of ß-oxidation. After ß-oxidation has been run once more and the fatty acid chain has been shortened by a further C2 body, the trans configuration of the next cis-double bond of DHA takes place, which – viewed from the COOH end of the fatty acid chain – is localized on an odd-numbered C atom (→ beta-gamma-cis-enoyl-CoA). For this purpose, beta-gamma-cis-enoyl-CoA is isomerized under the action of an isomerase to alpha-beta-trans-enoyl-CoA, which is introduced directly into its reaction cycle as an intermediate of ß-oxidation. Until the activated DHA is completely degraded to acetyl-CoA, 4 further conversion reactions (2 isomerase reactions, 2 hydratase-epimerase reactions) and 8 further ß-oxidation cycles are necessary, so that in total the ß-oxidation is run through 10 times, 6 conversion reactions (3 isomerase, 3 hydratase-epimerase reactions) – corresponding to 6 existing cis-double bonds – take place and 11 acetyl-CoA as well as reduced coenzymes (10 NADH2 and 4 FADH2) are formed. The acetyl-CoA resulting from DHA catabolism are introduced into the citrate cycle, in which oxidative degradation of organic matter occurs for the purpose of obtaining reduced coenzymes, such as NADH2 and FADH2, which together with the reduced coenzymes from ß-oxidation in the respiratory chain are used to synthesize ATP (adenosine triphosphate, universal form of immediately available energy). Although unsaturated fatty acids require conversion reactions (cis → trans) during ß-oxidation, whole-body analyses in fat-free fed rats revealed that labeled unsaturated fatty acids exhibit similar rapid degradation as saturated fatty acids.
Excretion
Under physiological conditions, fat excretion in feces should not exceed 7% at a fat intake of 100 g/day because of the high absorption rate (85-95%).Malassimilation syndrome (impaired nutrient utilization due to reduced breakdown and/or absorption), for example due to deficient bile acid and pancreatic juice secretion in cystic fibrosis (inborn error of metabolism, associated with dysfunction of exocrine glands due to dysfunction of chloride channels) or diseases of the small intestine, such as celiac disease (chronic disease of the mucosa of the small intestine due to gluten intolerance), can lead to reduction of intestinal fat absorption and thus to steatorrhea (pathologically increased fat content (> 7%) in the stool).
