Where do the fats we eat go?
Fats are cut up in the gut by bile and lipase. Most of them (butter, olive oil, omega 3) are repackaged into chylomicrons that travel through the lymph before the blood. MCTs, medium-chain fats, take a shortcut: the portal vein, straight to the liver.
Choose a fat: at the top, the molecule is cut up; at the bottom, follow its route through the body.
Simplified diagram. Durations are orders of magnitude: they change with the size and richness of the meal.
How are fats cut up?
A dietary fat is mostly a triglyceride: a small glycerol carrying three fatty acid chains. Bile disperses it into fine droplets, then pancreatic lipase cuts off two of the three chains. What remains are two free fatty acids and a monoglyceride, small enough to enter the cells of the gut (Mu, 2004).
Why go through the lymph?
In the gut cell, long chains (16 to 22 carbons) are rebuilt into triglycerides and packed into big balls, the chylomicrons. Too big to enter blood capillaries, they go into the lymphatic vessels, then join the blood near the neck, through the subclavian vein. Muscles and fat tissue help themselves along the way; the leftovers end up in the liver.
Why do MCTs go straight to the liver?
Medium chains (8 to 10 carbons) dissolve fairly well in water. They cross the gut cell without being repackaged and leave through the portal vein, straight to the liver, which burns them quickly or turns them into ketones (Bach, 1982). Coconut oil contains mainly lauric acid (12 carbons), which largely takes the chylomicron route: about 80% in rats (Mu and Høy, 2000).
| Fat | Main chain | Main route |
|---|---|---|
| Butter | Palmitic, 16 carbons, saturated | Chylomicrons, lymph |
| Olive oil | Oleic, 18 carbons, one double bond | Chylomicrons, lymph |
| Fish omega 3 | EPA and DHA, 20 and 22 carbons | Chylomicrons, lymph |
| MCT oil | Caprylic and capric, 8 and 10 carbons | Portal vein, liver |
Do omega 3s have a special role?
Yes. EPA and DHA settle into cell walls, especially in the brain and retina, and are used to make messengers that calm inflammation (Calder, 2018).
Frequently asked questions
Why does a fatty meal make the blood milky?
A few hours after a fatty meal, chylomicrons are so numerous that a tube of blood left to stand shows a whitish layer on top.
Do MCTs help you lose weight?
They are burned faster and stored a bit less than other fats, but the effect on weight remains modest according to a review of trials (Mumme and Stonehouse, 2015). Taken in large amounts, they can cause digestive trouble.
What happens without a gallbladder?
The liver keeps making bile, which flows continuously instead of arriving all at once. A very fatty meal may then be digested less well.
How long do fats stay in the blood?
Blood triglycerides, carried by chylomicrons, rise for 3 to 4 hours after the meal, then fall back within a few hours (Mihas, 2011).
See also
Lymph in motion A meal and its hormones The pancreas live
Sources
- Bach AC, Babayan VK. Medium-chain triglycerides: an update. Am J Clin Nutr. 1982;36(5):950-62. PMID 6814231
- Mu H, Høy CE. The digestion of dietary triacylglycerols. Prog Lipid Res. 2004;43(2):105-33. PMID 14654090
- Calder PC. Very long-chain n-3 fatty acids and human health: fact, fiction and the future. Proc Nutr Soc. 2018;77(1):52-72. PMID 29039280
- Mu H, Høy CE. Effects of different medium-chain fatty acids on intestinal absorption of structured triacylglycerols. Lipids. 2000;35(1):83-9. PMID 10695928
- Mumme K, Stonehouse W. Effects of medium-chain triglycerides on weight loss and body composition: a meta-analysis of randomized controlled trials. J Acad Nutr Diet. 2015;115(2):249-63. DOI 10.1016/j.jand.2014.10.022
- Mihas C, et al. Diagnostic value of postprandial triglyceride testing in healthy subjects: a meta-analysis. Curr Vasc Pharmacol. 2011;9(3):271-80. DOI 10.2174/157016111795495530