How fats we eat become energy in the body

The farther I get into organic and biochemistry the harder it is to find information that uses minimal jargon, helps me find the big picture amidst thousands of details, and that connects the biochemical concepts with what I’m learning in Anatomy & Physiology. So, I decided to post what I recently learned about how the fat we eat turns into energy in the body.

STEP 1: When you eat lipids (i.e. fats), an enzyme in your saliva, called Lingual Lipase, starts to break it down. (Lingual refers to your tongue).

STEP 2: When your food combines with saliva it’s called a bolus. When you swallow the bolus it moves through the esophagus and to the stomach. In the stomach an additional enzyme, called Gastric Lipase, breaks down fats from milk (it is specifically meant for breaking down breast milk in babies). When the bolus combines with gastric juice it’s called chyme.

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STEP 3: Your stomach slowly squirts chyme into the duodenum (the first part of your small intestines) where it meets bile from your liver. Most lipids haven’t been digested yet because they are big blobs of fat and the enzymes can’t get to them. Thankfully, bile contains bile salt which emulsifies any undigested lipids. Emulsifying basically means that the big blobs of fat get broken into little droplets.

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STEP 4: The little droplets of emulsified lipids are now ready for Pancreatic Lipase to break them down. For example, triglycerides are broken into 1 glycerol and 3 fatty acids molecules.

Triglyceride to Glycerol and 3 Fatty Acids

(The idea for this image came from this image. I used ChemAxon’s Marvin demo to create the molecule structures.)

STEP 5: Because fat and water don’t mix, fat needs help moving through the body. In the small intestines they organize themselves so that portion of the lipid molecules that hates water (“hydrophobic”) is protected by the portion of the lipid molecule that loves water (“hydrophilic”). This is called a micelle.

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STEP 6: The micelle moves through the wall of the intestines (called epithelial cells or absorptive cells) via diffusion. Diffusion means they move from a place where there are a lot of them (high concentration) to a place where there are few of them (low concentration).

How lipids move into absorptive epithelial cells

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STEP 7: In the absorptive cells 1 glycerol and 3 fatty acids are are recombined into triglycerides (reverse of step 4). The triglycerides are put into protein “packages” called chylomicrons.

STEP 8: Chylomicrons exit the absorptive cell via exocytosis and end up in the interstitial fluid (the fluid between cells)

STEP 9: From the interstitial fluid, the chylomicrons enter openings in lacteals. Lacteals are specific types of lymph vessels that transport fats.

Chylomicons in Lacteals

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STEP 10: The chylomicrons travel from lacteals to lymphatic vessels, to the Thoracic Duct and then are dumped into blood where the left Subclavian and left Internal Jugular Veins merge.

STEP 11: The chylomicrons in the blood are quickly captured by cells throughout the body using Lipoprotein Lipase.

STEP 12: If the cell needs energy it breaks the triglyceride into 1 glycerol and 3 fatty acids molecules (same as step 4.)

STEP 13: The fatty acid is activated by Coenzyme A (CoA) and forms Fatty Acyl-CoA (this is a substitution reaction).

Fatty Acid to Fatty Acyl-CoA

Beta  Oxidation CycleBeta Oxidation of Fatty Acid

STEP 14: Fatty Acyl-Coa is converted to Enoyl-CoA by the enzyme Acyl-CoA dehydrogenase.

STEP 15: The double bond in Enoly-CoA is hydrated to form an alcohol group (-OH).

STEP 16: The alcohol group (-OH) is oxidized to a carbonyl group (an Oxygen that is double bonded to a Carbon; C=O).

STEP 17: The acetyl-CoA portion of the molecule is cleaved off and enters to Citric Acid Cycle. The remaining portion cycles back through the beta oxidation cycle.

(image source. emphasis mine)


Citric Acid Cycle / Kreb’s Cycle / TCA Cycle

STEP 18: Acetyl-CoA enters the Citric Acid Cycle. Acetyl-CoA combines with Oxaloacetic Acid to form Citric Acid (also known as citrate).

STEP 19: Citric Acid undergoes a series of reactions that reduce FAD to FADH and FADH to NADH2. Reduce means the molecule accepts/gains H+ (a positively charged Hydrogen ion). FADH and NADH2 enter the Electron Transport Chain.

Krebs Cycle

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Electron Transport Chain.
The Electron Transport Chain takes place in the mitochondria of cells. This animated video and this Khan academy video help explain this process.


STEP 20: In the matrix of the mitochondria, FADH and NADH2 are oxidized. Oxidation means they lose a hydrogen ion (H+).

STEP 21:The hydrogen ions are moved across the Crista Membrane to the intermembrane space. As this happens, the intermembrane space becomes more positively charged than the matrix. As a result, the Hydrogen ion wants to move back to the matrix and this movement can be used to generate energy.

Electron Transport Chain

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STEP 22: The hydrogen ions get back to the matrix via the enzyme ATP-synthase.  This step is best shown in this video. As they move, the enzyme causes ADP and phosphate to form ATP (Adenosine Triphosphate).

ATP is the chemical that the body uses as an “energy currency”.

2 thoughts on “How fats we eat become energy in the body

  1. Krista, this is brilliant work. Thank you so much for sharing. I’m sitting for a rescheduled exam (due to ill health) on Nutritional Biochem on 6th Jan 2016 and what you’ve written is a big help.

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