Where do fatty acids come from? Your guide to synthesis and dietary sources

December 28, 2025 •

4 min read

Over 90% of the fat in the typical human diet is in the form of triglycerides, which are broken down into fatty acids during digestion. These crucial molecules are not only acquired from food but are also synthesized by the body itself, primarily from excess carbohydrates, to be used for energy and cell structure.

Quick Summary

This guide explains the complex processes of how the body synthesizes fatty acids from carbohydrates and breaks down dietary fats. It also details the importance of obtaining essential fatty acids, like omega-3s and omega-6s, directly from food sources and lists key animal and plant-based options.

Key Points

Endogenous Synthesis: The body synthesizes non-essential fatty acids from excess carbohydrates, primarily in the liver and fat tissue, through a process known as lipogenesis.
Dietary Intake: Essential fatty acids, which the body cannot produce, must be obtained from food sources like plant oils, nuts, and fish.
Source Diversity: Fatty acids come from both animal sources (like fish oil, meat, and dairy) and plant sources (like flaxseed, walnuts, and vegetable oils), with different types and proportions found in each.
Energy Storage: The body stores fatty acids as triglycerides in adipose tissue, serving as a concentrated long-term energy reserve.
Cellular Structure: Fatty acids are integral components of cell membranes (phospholipids) and are vital for cellular integrity, flexibility, and communication.
Hormone and Signaling Molecules: They act as precursors for various signaling molecules and hormones that regulate inflammation, blood pressure, and other physiological processes.
Brain Function: The brain is highly dependent on specific fatty acids, like DHA, for neurological development and proper cognitive function.

The Dual Origin of Fatty Acids

Fatty acids, the fundamental building blocks of lipids, have two main origins: they are either synthesized internally within the body or obtained externally from the foods we eat. This dual system ensures that the body always has an ample supply of these crucial molecules for energy, cell membranes, and hormone production. The balance between these two sources is vital for metabolic health and depends heavily on diet and lifestyle.

Endogenous Synthesis: Making Fatty Acids Internally

In animals, the process of creating fatty acids is known as lipogenesis and occurs predominantly in the liver and adipose (fat) tissue, particularly when there is an excess of carbohydrates. This complex biochemical pathway essentially converts glucose into fat for storage.

Here is a simplified step-by-step breakdown of how the body synthesizes fatty acids:

Carbohydrate Breakdown: Glucose, derived from carbohydrates, is first broken down through glycolysis into pyruvate.
Acetyl-CoA Formation: This pyruvate is then transported into the mitochondria, where it is converted into acetyl-CoA.
Transport to Cytosol: As fatty acid synthesis happens in the cytosol, the acetyl-CoA must be transported out of the mitochondria. It combines with oxaloacetate to form citrate, which can cross the mitochondrial membrane. In the cytosol, citrate is cleaved back into acetyl-CoA.
Malonyl-CoA Production: The cytosolic acetyl-CoA is converted to malonyl-CoA by the enzyme acetyl-CoA carboxylase, a crucial, rate-limiting step in fatty acid synthesis.
Elongation by Fatty Acid Synthase: The malonyl-CoA is then added repeatedly to a growing chain by the fatty acid synthase enzyme complex. Each addition lengthens the chain by two carbon atoms, a process that continues until the 16-carbon palmitic acid is formed.
Further Modification: After synthesis, enzymes can elongate the fatty acid chain further or introduce double bonds to create unsaturated fatty acids.

Dietary Intake: Obtaining Essential Fatty Acids

While the body is adept at producing many types of fatty acids, it cannot create certain essential ones, like linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). These must be acquired through diet. Once ingested, dietary fats (triglycerides) are broken down into fatty acids during digestion and absorbed into the bloodstream. They are then transported to cells throughout the body for use or storage.

Animal vs. Plant-Based Fatty Acid Sources

Fatty acids can be sourced from both animal and plant products, though the type and proportion of fatty acids differ significantly between them. The balance between these sources is important for maintaining a healthy nutritional profile.


Feature	Animal-Based Fatty Acids	Plant-Based Fatty Acids
Primary Form	Stored as triglycerides in adipose tissue and milk fat. Also found in egg yolks and fish fat.	Stored as triglycerides in seeds and fruits, such as in vegetable oils, nuts, and seeds.
Common Saturated Fats	Often higher in saturated fats, like palmitic and stearic acids, which are solid at room temperature.	Generally lower in saturated fats than animal products.
Essential Fatty Acids	Provides long-chain omega-3s, such as EPA and DHA, primarily from fatty fish (e.g., salmon, mackerel).	Provides short-chain omega-3 (ALA) from sources like flaxseed, chia seeds, and walnuts. Plant sources also offer omega-6 (linoleic acid).
Natural Trans Fats	Some naturally occurring trans fats are found in the meat and milk of ruminant animals like cattle and sheep.	Very few naturally occurring trans fats; industrial hydrogenation is the primary source of trans fats in many processed plant oils.

The Role of Fatty Acids in the Body

Regardless of their origin, fatty acids serve multiple vital functions:

Energy Storage: When the body has excess energy, fatty acids are stored as triglycerides in adipose tissue, forming a compact and efficient fuel reserve.
Structural Components: They are crucial components of phospholipids, which form the lipid bilayers of all cell membranes, ensuring proper cell function and communication.
Hormone Production: Fatty acids are precursors to hormones and other signaling molecules like eicosanoids and prostaglandins, which play roles in inflammation and blood clotting.
Brain Health: Certain polyunsaturated fatty acids, especially DHA, are highly concentrated in the brain and are essential for neurological health and cognitive function.
Carrier Molecules: In the bloodstream, free fatty acids are transported bound to plasma albumin, allowing these insoluble molecules to travel throughout the body to where they are needed.

Conclusion

Fatty acids are fundamental to our biological makeup, with their origins tied both to the food we consume and the metabolic processes within our bodies. The endogenous synthesis pathway provides a robust mechanism to produce many non-essential fatty acids from excess carbohydrates, particularly when energy intake is high. At the same time, a healthy diet rich in specific plant-based and marine foods is essential for supplying the body with crucial omega-3 and omega-6 fatty acids that it cannot produce on its own. By understanding these diverse origins, we can make informed nutritional choices that support overall health, energy regulation, and cellular function.

For more in-depth information on the complex biochemistry of fatty acids and their synthesis, refer to the detailed analysis provided by Wikipedia's article on fatty acid synthesis.

Frequently Asked Questions

Essential fatty acids, such as omega-3 and omega-6, cannot be made by the body and must be acquired through your diet. Non-essential fatty acids can be synthesized internally by the body from other nutrients, like excess carbohydrates.

Yes, when you consume more carbohydrates than your body needs for immediate energy, the excess is converted into acetyl-CoA through glycolysis. This acetyl-CoA is then used to synthesize fatty acids in the liver and adipose tissue.

Fatty acids are primarily stored as triglycerides within specialized cells called adipocytes, which make up the body's adipose or fat tissue. They are released for energy when needed.

During digestion, enzymes called lipases break down dietary triglycerides into monoglycerides and free fatty acids. These are then absorbed into the cells lining the small intestine, re-packaged, and transported via the bloodstream.

The terms omega-3 and omega-6 refer to the position of the first double bond in the fatty acid's chemical structure, counting from the omega (methyl) end of the molecule. This structural difference determines their biological function.

No, fats vary significantly in their chemical makeup, from saturated to unsaturated and in chain length. For instance, saturated fats are typically solid at room temperature, while unsaturated fats are liquid. These differences influence their effect on the body and cell membranes.

Yes, plants synthesize fatty acids within their chloroplasts and plastids. They also use them to form complex lipids for cell membranes and store them as triglycerides in seeds and fruits.