Trans fatty acids: the pathway in the body · Biomarker & Blood Analysis

This page shows the biochemical pathway behind the laboratory value Trans fatty acids: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Trans fatty acids are unsaturated fatty acids whose double bond is in the trans position; they arise from fat hardening and from rumen bacteria. The body absorbs them like other fatty acids, builds them into cell membranes and breaks them down by beta-oxidation.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Unsaturated fatty acid → Trans fatty acids Hydrogenation, rumen · Nickel (industrial), Rumen bacteria When fat is partially hydrogenated or converted by rumen bacteria, the double bond flips into the trans position. The chain becomes straighter, similar to a saturated fatty acid. Once built in, it occupies positions of cis fatty acids. Source 1, 2↓ depletes The straight chain packs more densely than a cis chain. Built into phospholipids, a trans fatty acid occupies positions that cis fatty acids would otherwise hold, and behaves there much like a saturated one. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If many trans fatty acids arrive, acyltransferases build more of them into phospholipids; there they occupy positions that cis fatty acids would otherwise hold.

   too little — If hardly any trans fatty acids arrive, cis and saturated fatty acids occupy these positions; the route continues in the same way with the other fatty acids.

   established physiology · Source 1, 4

2. Trans fatty acids → Chylomicrons Lipases · Bile acids In the gut, lipases split dietary fats. The fatty acids are reassembled in the gut wall and released as chylomicrons via the lymph into the blood. In this way the fatty acids reach the tissues. Source 1↑ supplies Chylomicrons carry the fatty acids from food, trans fatty acids included, via lymph and blood to the tissues that absorb them. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of fat arrives at once, more chylomicrons form, and more fatty acids reach the tissues at the same time.

   too little — If lipases split little fat, for instance when few bile acids are present, fewer chylomicrons form, and more fat stays in the gut.

   established physiology · Source 1

3. Chylomicrons → Membrane fatty acids Acyltransferases Acyltransferases build fatty acids into the phospholipids of cell membranes. Trans fatty acids end up there too; in the laboratory they are measured in the membrane of red blood cells. There they occupy positions of cis fatty acids. Source 4↓ depletes Acyltransferases build trans fatty acids into the phospholipids of plasma, red blood cells and platelets. There they occupy positions that cis fatty acids would otherwise hold. observed in studies Source 4
   ⚖ When the balance tips

   too much — If many trans fatty acids are built into the phospholipids, correspondingly fewer positions are occupied by cis fatty acids.

   too little — If few trans fatty acids are built in, cis and saturated fatty acids occupy the positions in the phospholipids.

   observed in studies · Source 4

4. Acyl-CoA → Rumenic acid (CLA) Δ9-desaturase · NADH, oxygen Δ9-desaturase inserts another double bond. In this way part of the vaccenic acid, the trans fatty acid from the rumen, becomes rumenic acid. With trans-12-18:1 this hardly happens. Source 3↑ supplies Δ9-desaturase converts part of the vaccenic acid (trans-11) into rumenic acid in the body; trans-12-18:1, by contrast, is hardly converted this way. The body thus makes part of its rumenic acid itself. observed in studies Source 3
   ⚖ When the balance tips

   too much — If a lot of vaccenic acid arrives, Δ9-desaturase converts more of it to rumenic acid.

   too little — If little vaccenic acid arrives, hardly any rumenic acid forms in the body; it then comes almost entirely from the fat of ruminants.

   observed in studies · Source 3, 2

5. Acyl-CoA → Beta-oxidation Carnitine shuttle · Carnitine The carnitine shuttle brings the fatty acid into the mitochondrion. There the chain is dismantled piece by piece. For elaidic acid this breakdown remains measurably incomplete. In rat mitochondria an intermediate builds up in the process. Source 5↑ supplies Beta-oxidation breaks fatty acids down into acetyl-CoA, FADH₂ and NADH. With elaidic acid, 5-trans-tetradecenoyl-CoA builds up in rat mitochondria; it is cleaved off or exported as a carnitine ester. observed in studies Source 5
   ⚖ When the balance tips

   too much — If a lot of elaidic acid arrives, more 5-trans-tetradecenoyl-CoA builds up in rat mitochondria; part of it leaves the mitochondrion incompletely broken down as a carnitine ester.

   too little — If little carnitine, FAD or NAD⁺ is available, beta-oxidation runs more slowly, and less fatty acid is broken down to acetyl-CoA.

   observed in studies · Source 5

6. Beta-oxidation → Acetyl-CoA β-oxidation enzymes · FAD, NAD⁺ Each round of beta-oxidation yields acetyl-CoA plus the electron carriers FADH₂ and NADH. Acetyl-CoA enters the citric acid cycle. FADH₂ and NADH supply electrons to the respiratory chain. Source 5↑ supplies Acetyl-CoA feeds the citric acid cycle. FADH₂ and NADH pass their electrons to the respiratory chain, which uses them to generate ATP. observed in studies Source 5
   ⚖ When the balance tips

   too much — If a lot of acetyl-CoA forms, the citric acid cycle sets how quickly it is broken down further.

   too little — If beta-oxidation runs incompletely, as described for elaidic acid in rat mitochondria, less acetyl-CoA forms per fatty acid.

   observed in studies · Source 5

Further stations

• Unsaturated fatty acid — cis form, e.g. linoleic acid Unsaturated fatty acids have a double bond. In the cis form, the two chain segments lie on the same side, which gives the chain a kink. The kink keeps membranes loosely packed. Source 1↑ supplies The kink of the cis chain keeps the fatty acids in the phospholipids loosely packed, so the cell membrane stays flexible. Linoleic acid, one such cis fatty acid, is also the starting material for longer fatty acids. established physiology Source 1
  ⚖ When the balance tips

  too much — If many cis-unsaturated fatty acids arrive, more of them are built into phospholipids, and the membrane becomes more loosely packed.

  too little — If little of them is available, saturated and trans fatty acids fill more positions in the phospholipids; the membrane becomes more densely packed.

  established physiology · Source 1

• Acyl-CoA — activated fatty acid Before a fatty acid can be converted, the cell attaches coenzyme A. Two routes lead on from this activated form. Only as acyl-CoA does a fatty acid reach the mitochondrion via the carnitine shuttle. Source 5↑ supplies Only as acyl-CoA can the carnitine shuttle bring a fatty acid into the mitochondrion or Δ9-desaturase convert it. Activation therefore decides whether a fatty acid is processed further at all. established physiology Source 5
  ⚖ When the balance tips

  too much — If a lot of acyl-CoA forms, the carnitine shuttle sets how much of it moves on into the mitochondrion.

  too little — If little coenzyme A is available, less fatty acid is activated, and both routes receive less supply.

  established physiology · Source 5

Cofactors in this pathway

• Carnitine — Carries the activated fatty acid into the mitochondrion via the carnitine shuttle; without carnitine, no breakdown there Source 5In the ORY catalogue as a laboratory value: Carnitin
• Riboflavin (vitamin B2) — As FAD, cofactor of the acyl-CoA dehydrogenases of beta-oxidation, which carry out the first step of each round Source 5
• Niacin (NAD⁺) — NAD⁺ accepts electrons in every round of beta-oxidation and passes them on to the respiratory chain Source 5In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Pantothenic acid (coenzyme A) — Building block of coenzyme A, which activates the fatty acid before conversion; only then does breakdown start Source 5
• Iron — Di-iron centre of Δ9-desaturase, which inserts a double bond Source 6In the ORY catalogue as a laboratory value: Eisen
• Bile acids — Disperse dietary fat into fine droplets on which lipases act; only then are the fats split Source 1In the ORY catalogue as a laboratory value: Gallensäuren (Stuhl)
• Magnesium — Forms the complex with ATP that acyl-CoA synthetase uses for activation; without it, no activation Source 7In the ORY catalogue as a laboratory value: Magnesium

Sources

1. Tvrzicka E, Kremmyda LS, Stankova B et al. Fatty acids as biocompounds: their role in human metabolism, health and disease — a review. Part 1: classification, dietary sources and biological functions. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub 2011 · PubMed 21804620
2. Or-Rashid MM, Wright TC, McBride BW. Microbial fatty acid conversion within the rumen and the subsequent utilization of these fatty acids to improve the healthfulness of ruminant food products. Appl Microbiol Biotechnol 2009 · PubMed 19685048
3. Kuhnt K, Kraft J, Moeckel P et al. Trans-11-18:1 is effectively Delta9-desaturated compared with trans-12-18:1 in humans. Br J Nutr 2006 · PubMed 16571155
4. Emken EA, Rohwedder WK, Dutton HJ et al. Incorporation of deuterium-labeled cis- and trans-9-octadecenoic acids in humans: plasma, erythrocyte, and platelet phospholipids. Lipids 1979 · PubMed 459721
5. Yu W, Liang X, Ensenauer RE et al. Leaky beta-oxidation of a trans-fatty acid: incomplete beta-oxidation of elaidic acid is due to the accumulation of 5-trans-tetradecenoyl-CoA and its hydrolysis and conversion to 5-trans-tetradecenoylcarnitine in the matrix of rat mitochondria. J Biol Chem 2004 · PubMed 15466478
6. Shen J, Wu G, Pierce BS et al. Free ferrous ions sustain activity of mammalian stearoyl-CoA desaturase-1. J Biol Chem 2023 · PubMed 37290533
7. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev 2015 · PubMed 25540137

Related pathways

• APOE — chylomicrons
• Carnitine — acyl-coa
• Cholesterol and lipoproteins — chylomicrons
• Triglycerides — chylomicrons
• Vitamin A — chylomicrons

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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