Fatty acid synthesis is the process by which cells build fat molecules from simpler carbon-containing precursors, primarily using sugars as the starting fuel. In humans, the liver is the main site where this happens, though other tissues pitch in under specific circumstances. The chemistry is surprisingly conserved across life: bacteria, yeast, plants, and animals all use essentially the same sequence of reactions, even though the molecular machinery they use to carry it out looks quite different from one kingdom to the next. Understanding how cells manufacture fat has become increasingly relevant to medicine, since overactive fatty acid synthesis contributes to fatty liver disease, obesity-related metabolic problems, and even tumor growth.
How Cells Build a Fat Molecule From Scratch
The raw ingredient for fatty acid synthesis is acetyl-CoA, a small two-carbon molecule that cells produce when they break down sugars, amino acids, or existing fats. But acetyl-CoA is made inside mitochondria, and fatty acid synthesis takes place out in the cell’s main compartment, the cytosol. Cells bridge this gap using a shuttle system: mitochondria first combine acetyl-CoA with oxaloacetate to form citrate, which a dedicated carrier protein then ferries across the inner mitochondrial membrane into the cytosol.1PubMed Central. Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology Once in the cytosol, an enzyme splits citrate back into acetyl-CoA and oxaloacetate, freeing up the two-carbon building block where it is needed. This citrate shuttle also supports other processes like protein modification and sugar metabolism.2PubMed. Inhibition of mitochondrial citrate shuttle alleviates metabolic syndromes induced by high-fat diet
The first committed step in fatty acid synthesis is carried out by acetyl-CoA carboxylase, or ACC. This enzyme tacks a carbon dioxide molecule onto acetyl-CoA to produce a three-carbon compound called malonyl-CoA. ACC is widely considered the rate-limiting enzyme of the entire pathway, meaning it acts as a bottleneck that determines how fast the whole process runs. The cell regulates ACC through several overlapping mechanisms, including feedback from downstream products and chemical modifications that switch the enzyme on or off.3PubMed. Short-term regulation of acetyl CoA carboxylase: is the key enzyme in long-chain fatty acid synthesis regulated by an existing physiological mechanism?
Once malonyl-CoA is available, the heavy lifting falls to fatty acid synthase, often abbreviated FASN. In humans, FASN is a large protein that works as a pair of identical subunits, each roughly 270 kilodaltons, and it carries out all the remaining steps of synthesis on a single molecular assembly line.4PubMed Central. Biochemistry, molecular biology, and pharmacology of fatty acid synthase, an emerging therapeutic target and diagnosis/prognosis marker The enzyme repeatedly adds two-carbon units from malonyl-CoA onto a growing chain, reducing the intermediates at each cycle. After seven rounds of elongation, the result is palmitate, a 16-carbon saturated fatty acid. The thioesterase domain at the end of the enzyme then clips the finished chain free.5PubMed Central. Human fatty acid synthase: structure and substrate selectivity of the thioesterase domain Palmitate is the default product of this system, though the body can modify it extensively afterward.
Where the Energy Comes From
Building fatty acids is energy-intensive. Each round of chain elongation requires reducing power in the form of NADPH, a molecule that donates electrons to convert intermediates into the fully saturated chain. The cell has more than one way to generate NADPH, and which pathway dominates depends on the tissue and organism. In many settings, the pentose phosphate pathway (a branch off glucose metabolism) is the main supplier. When fatty acid synthesis was blocked in rat fat cells, the pentose phosphate pathway slowed down in tandem, confirming a tight link between fat production and NADPH supply.6PubMed. The NADPH consumption regulates the NADPH-producing pathways (pentose phosphate cycle and malic enzyme) in rat adipocytes
A backup source of NADPH comes from malic enzyme, which converts malate to pyruvate while releasing carbon dioxide and NADPH. In one striking demonstration, when researchers knocked out the gene for the main pentose phosphate pathway enzyme in human cells, fatty acid synthesis continued at normal rates because malic enzyme ramped up to compensate.7PubMed Central. NADPH production by the oxidative pentose-phosphate pathway supports folate metabolism This redundancy makes sense from a survival standpoint: cells cannot afford to let fat production stall just because one supply line is disrupted. In the oleaginous yeast Yarrowia lipolytica, by contrast, the pentose phosphate pathway appears to be the dominant NADPH source, with malic enzyme contributing little, suggesting the balance between these suppliers varies across species.8PubMed. The oxidative pentose phosphate pathway is the primary source of NADPH for lipid overproduction from glucose in Yarrowia lipolytica
Beyond Palmitate: How the Body Makes Longer and Unsaturated Fats
Palmitate is just the starting point. Cells need a wide variety of fatty acids with different chain lengths and degrees of unsaturation to build cell membranes, produce signaling molecules, and store energy. Two major classes of enzymes handle these downstream modifications.
Elongation happens in the endoplasmic reticulum, a network of membranes throughout the cell. A family of enzymes called ELOVLs (for “elongation of very long-chain fatty acids”) catalyzes the first and rate-limiting step in adding two-carbon units to existing fatty acid chains.9PubMed. The ELOVL proteins: Very and ultra long-chain fatty acids at the crossroads between metabolic and neurodegenerative disorders Humans have seven ELOVL family members, each with preferences for particular chain lengths. ELOVL7, for example, prefers chains around 18 carbons long but can handle substrates up to 20 carbons.10PubMed Central. The structural basis of fatty acid elongation by the ELOVL elongases Together, these elongases produce the very long-chain fatty acids found in skin, the brain, the retina, and sperm, where they play structural roles that shorter chains cannot fill. Mutations in ELOVL genes are linked to neurological and metabolic disorders.11PubMed. The ELOVL proteins: Very and ultra long-chain fatty acids at the crossroads between metabolic and neurodegenerative disorders
Desaturation introduces double bonds into the fatty acid chain, converting saturated fats into monounsaturated or polyunsaturated ones. The key enzyme for this in mammals is stearoyl-CoA desaturase (SCD), which inserts a double bond at a specific position in the chain. SCD’s preferred substrates are palmitoyl-CoA and stearoyl-CoA, converting them into palmitoleate and oleate, the two most abundant monounsaturated fatty acids in the human body.12PubMed Central. Biochemical and physiological function of stearoyl-CoA desaturase This step matters because the ratio of saturated to unsaturated fat in cell membranes affects their fluidity, which in turn influences how well membrane proteins function. SCD catalyzes the rate-limiting step in monounsaturated fat production, making it a natural control point for membrane composition.13PubMed. Development of a novel LC/MS method to quantitate cellular stearoyl-CoA desaturase activity
It is worth noting that humans cannot make certain polyunsaturated fatty acids from scratch. The omega-3 and omega-6 fatty acids are called “essential” specifically because we lack the desaturase enzymes needed to place double bonds in those particular positions on the carbon chain. We can elongate and further desaturate them once we have the dietary precursors, but the initial forms must come from food.
The Mammary Gland and Other Tissue-Specific Roles
In healthy adults, most tissues keep fatty acid synthesis at low levels, relying instead on fats absorbed from food. The liver is the major exception, and it becomes even more active after a carbohydrate-rich meal. But there is another tissue that dramatically ramps up fat production under the right circumstances: the lactating mammary gland.
During breastfeeding, the mammary gland becomes one of the most active fat-synthesizing tissues in the body. When researchers knocked out the fatty acid synthase gene specifically in mouse mammary cells, the glands failed to develop properly during lactation, underwent premature shrinkage, and produced milk with significantly reduced medium- and long-chain fatty acid content.14PubMed Central. Fatty acid synthase is required for mammary gland development and milk production during lactation This showed that dietary fat alone cannot substitute for the gland’s own synthesis, at least in mice.
In human lactation, the picture is consistent. Studies using isotope tracers in breastfeeding women found that the primary fatty acids being synthesized by the mammary gland were medium-chain saturated fats, specifically those with 10, 12, and 14 carbons.15PubMed. Human lactation. II: Endogenous fatty acid synthesis by the mammary gland These medium-chain fats are relatively uncommon in adult tissues but are abundant in breast milk, where they serve as an easily digestible energy source for infants. The mammary gland essentially runs a specialized version of the same fatty acid synthesis pathway, tuned to release chains earlier than the standard 16-carbon palmitate.
Fructose and Fatty Liver Disease
The liver’s capacity for fatty acid synthesis becomes a liability when it is chronically overstimulated. A growing body of evidence points to fructose as a particularly potent driver. Unlike glucose, which is metabolized throughout the body, fructose is absorbed through the portal vein and delivered to the liver at much higher concentrations than to other tissues.16PubMed Central. Role of Dietary Fructose and Hepatic De Novo Lipogenesis in Fatty Liver Disease Both human and animal studies indicate that fructose is a more potent trigger for hepatic fat synthesis than glucose.17PubMed Central. Fructose drives de novo lipogenesis affecting metabolic health
Several properties of fructose metabolism stack the deck toward fat accumulation. Fructose does not require insulin for its metabolism, so it continues to drive fat production even in insulin-resistant individuals. It increases the levels of essentially all the enzymes involved in the synthesis pathway. It also depletes the cell’s energy currency, ATP, which suppresses the normal burning of fatty acids in mitochondria, leaving the newly synthesized fats with nowhere to go but into storage. Fructose additionally triggers stress responses in the endoplasmic reticulum and boosts uric acid production, both of which further promote fat synthesis through insulin-independent pathways.18PubMed Central. Role of Dietary Fructose and Hepatic De Novo Lipogenesis in Fatty Liver Disease
The result is that diets high in fructose can substantially increase liver fat content, contributing to non-alcoholic fatty liver disease, a condition that now affects roughly a quarter of the global adult population. The fat synthesis pathway is increasingly viewed as the central metabolic abnormality in this disease, which has pushed researchers toward developing drugs that target its key enzymes.
Cancer Cells and Hijacked Fat Production
Fatty acid synthase is minimally expressed in most healthy adult tissues. Cancer cells, however, frequently crank up its production. Enhanced fat synthesis is now recognized as a hallmark of many tumor types, providing cancer cells with the raw materials for rapid membrane production, energy flexibility, and signaling molecules that support their growth and survival.19PubMed Central. Targeting fatty acid synthase for cancer drug discovery: Retrospective analyses and outlook
This overexpression is not just a side effect of being a fast-growing cell. FASN actively promotes cancer progression by reprogramming lipid metabolism, modulating growth-signaling pathways, and even contributing to drug resistance.20PubMed Central. Targeting fatty acid synthase for cancer drug discovery: Retrospective analyses and outlook The enzyme can rewire a tumor cell’s energy metabolism to allow greater flexibility in meeting its high fuel demands.21PubMed Central. Fatty Acid Synthase: An Emerging Target in Cancer Because normal adult tissues express so little FASN, blocking it should in principle harm tumor cells while sparing healthy ones, making the enzyme an appealing drug target. Several FASN inhibitors are in various stages of development, though translating promising lab results into approved therapies has proved challenging.
Drug Development Targeting the Pathway
The most advanced therapeutic efforts aim at ACC, the rate-limiting enzyme at the pathway’s entrance. The logic is straightforward: block the enzyme that makes malonyl-CoA, and you cut off the fuel supply for the entire downstream synthesis. Early clinical trials with an ACC inhibitor called PF-05175157 confirmed that the approach works in humans, producing robust reductions in liver fat synthesis. But there was a catch: the drug also inhibited ACC in bone marrow, causing platelet counts to drop.22PubMed. Optimizing the Benefit/Risk of Acetyl-CoA Carboxylase Inhibitors through Liver Targeting
Researchers responded by engineering a second-generation compound, PF-05221304, designed to be selectively taken up by the liver using the same transport proteins that pull bile acids from the blood. This liver-targeting strategy allowed the drug to suppress fat synthesis at the therapeutic site while maintaining substantial safety margins against platelet effects in primates.23PubMed. Optimizing the Benefit/Risk of Acetyl-CoA Carboxylase Inhibitors through Liver Targeting Phase 2a trials tested the compound at several doses in adults with non-alcoholic fatty liver disease, and a parallel trial explored combining it with an inhibitor of a different fat-metabolism enzyme.24Nature Medicine. ACC inhibitor alone or co-administered with a DGAT2 inhibitor in patients with non-alcoholic fatty liver disease: two parallel, placebo-controlled, randomized phase 2a trials
A persistent complication for ACC inhibitors is a paradoxical rise in blood triglycerides. When the liver cannot make new fat, it responds by reducing its production of polyunsaturated fatty acids, which triggers a compensatory increase in another enzyme involved in triglyceride assembly. A recent meta-analysis of randomized trials confirmed this pattern and highlighted the need for combination strategies or careful dose selection to manage the side effect.25PubMed Central. Acetyl-CoA Carboxylase Inhibitors for Nonalcoholic Fatty Liver Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials One approach being explored is pairing ACC inhibitors with drugs that block triglyceride synthesis downstream, addressing both the root cause and the compensatory response simultaneously.
Beyond ACC, SCD has also attracted attention as a target. In animal models, the ginger-derived compound 6-gingerol reduced fat accumulation in fructose-fed rats by specifically targeting SCD, suppressing the synthesis of monounsaturated fatty acids and reversing the rise in liver triglycerides.26PubMed Central. 6-Gingerol Inhibits De Novo Lipogenesis by Targeting Stearoyl-CoA Desaturase to Alleviate Fructose-Induced Hepatic Steatosis Whether natural SCD inhibitors can match the potency needed for clinical use remains to be seen, but the target itself is well validated.
Type I Versus Type II Systems
One of the more surprising facts about fatty acid synthesis is that while the core chemistry is universal, the physical organization of the enzymes differs dramatically across life. Organisms can be divided into two camps based on how they package their fat-making machinery. In animals and fungi, all the catalytic activities are fused into one or two giant multifunctional proteins, a setup called the type I system. In bacteria and plants, each reaction is carried out by a separate, stand-alone enzyme, known as the type II system.27Biochemical Journal. Current understanding of fatty acid biosynthesis and the acyl carrier protein
This difference is not just a molecular curiosity. It has practical consequences for antibiotic development. Several antibiotics work by targeting specific enzymes in the bacterial type II system, and because the human type I system is so structurally different, these drugs do not interfere with our own fat synthesis. The same architectural divide also matters for biotechnology. The fact that bacterial type II enzymes are discrete proteins makes them easier to manipulate individually through genetic engineering.
Evolutionary analysis reveals that the type I system arose through gene fusions that stitched together ancestral stand-alone enzymes into a single large gene. This is one of the clearest examples in biology of how new proteins can evolve by merging existing ones.28Trends in Biochemical Sciences. Fatty acid synthase — an example of protein evolution by gene fusion Fungal fatty acid synthase, which forms a massive barrel-shaped complex, appears to have acquired its structural scaffold from bacterial precursors, though exactly how remains an open question.29PubMed. Evolutionary origins of the multienzyme architecture of giant fungal fatty acid synthase
Engineering Microbes to Make Fats on Demand
The same fatty acid synthesis pathway that causes metabolic disease in overfed humans looks very different through the lens of industrial biotechnology. Researchers have been engineering microbes to overproduce fatty acids as renewable alternatives to petroleum-derived chemicals. E. coli, with its well-characterized type II system, has been the workhorse organism for many of these efforts.30PubMed Central. Fatty acid synthesis in Escherichia coli and its applications towards the production of fatty acid based biofuels The goal is to redirect carbon from sugar feedstocks into fatty acids and their derivatives, which can be converted into biodiesel, surfactants, lubricants, and other oleochemicals traditionally sourced from fossil fuels or palm oil.
Engineering strategies typically involve removing the cell’s natural feedback mechanisms that limit how much fat it makes, boosting the supply of acetyl-CoA and NADPH, and then channeling the fatty acids into specific products using tailored downstream enzymes. Recent advances have expanded beyond E. coli to include yeasts and cyanobacteria as production platforms.31PubMed. Engineering microbial fatty acid metabolism for biofuels and biochemicals The challenge is economic: microbial fats are still more expensive to produce than petroleum equivalents, but the gap is narrowing as yields improve and sustainability mandates grow more stringent.
A Separate Fat Factory Inside Mitochondria
While the cytosolic pathway described above handles the bulk of cellular fat production, mitochondria have their own independent fatty acid synthesis system, called mtFAS. This pathway uses the bacterial-style type II architecture, consistent with mitochondria’s evolutionary origin as engulfed bacteria. But mtFAS makes different products and serves different purposes than its cytosolic counterpart.
The best-understood product of mtFAS is octanoic acid, an eight-carbon fatty acid that serves as the precursor for lipoic acid, a cofactor essential for several mitochondrial enzymes involved in energy metabolism. When researchers disrupt mtFAS genes in yeast, the cells lose the ability to respire properly and develop small, malformed mitochondria.32PubMed Central. Mitochondrial fatty acid synthesis type II: more than just fatty acids This respiratory failure is a much more dramatic phenotype than you would expect from just losing a minor lipid-synthesis pathway, hinting that mtFAS has broader roles in mitochondrial biology.
Recent work in mammalian cells confirms this suspicion. Mitochondrial fatty acid synthesis also produces longer saturated fatty acids, though the exact identity of these products, where they end up in the cell, and what they do are all still active research questions. What is becoming clear is that mtFAS acts as a regulator of overall mitochondrial function, linking fat synthesis to the organelle’s capacity for oxidative metabolism.33PubMed Central. Mitochondrial fatty acid synthesis is an emergent central regulator of mammalian oxidative metabolism The field is still early, and mtFAS is one of those corners of cell biology where genuinely novel discoveries are likely in the coming years.
Watching Fat Synthesis in Real Time
A practical challenge in studying fatty acid synthesis has always been that fats are small, chemically similar molecules, making them hard to track inside living cells. Traditional approaches rely on radioactive or isotope-labeled tracers, which give precise measurements of synthesis rates but cannot show where in the cell the reactions are happening. New imaging techniques are beginning to change that.
One recent advance used optical photothermal infrared imaging to distinguish between different types of fatty acid chemical bonds in living cells. Researchers could identify metabolic intermediates of fatty acid processing concentrated in the endoplasmic reticulum and at contact sites between the ER and lipid droplets, the cell’s fat storage organelles.34PubMed Central. Optical photothermal infrared imaging of fatty acid esterification in the ER of living cells Being able to see these reactions spatially, rather than just measuring bulk rates in a cell extract, opens up new ways to understand how cells organize their fat metabolism and what goes wrong in disease states where lipid droplets accumulate abnormally.