Bile, lipase and lecithin: what fat digestion really needs

Healthy state

Fat is the most demanding nutrient. It does not dissolve in water, so it first has to be broken down and then packed into tiny droplets. The stomach, the pancreas, bile and the gut wall work on this one after another. If one link fails, fat stays undigested — no matter how much the others contribute.

The pancreas does the main work. Its lipase splits fat into fatty acids and monoglycerides. In a study in healthy people, gastric lipase took on roughly every fourth to sixth split of fat, and the pancreas the rest. Lipase needs a helper, the protein colipase, and it needs space: after a meal, fat sits in large droplets whose surface the enzyme can barely attack.

This is exactly where bile comes in — but only as a washing-up liquid. Bile acids Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” split nothing. They attach themselves to the droplets with their water-loving and fat-loving ends, break them into the finest droplets and form micelles with the fatty acids, small transport packages that reach right up to the gut wall. Without bile, lipase can work as hard as it likes, but the split products barely reach the cells.

In the gut cell, the fat is reassembled and packaged. For this the cell needs phosphatidylcholine, a fat molecule with choline Choline A nutrient the body needs for phosphatidylcholine and for moving fat out of the liver. Found in eggs, fish, meat, milk, soy, legumes and broccoli. as its head group. Choline is a nutrient the body makes only to a limited extent itself; the reference values are between 400 milligrams a day in Europe and 550 milligrams in the USA (men). If you eat normally, you take in choline from eggs, fish, meat, milk, soy, legumes and broccoli. Choline acts in the liver and in fat metabolism, not in the gut.

Lecithin Lecithin A mixture of choline phospholipids, obtained from soy or sunflower. In bile it keeps cholesterol dissolved; as a capsule for fat digestion it is not proven. is the collective name for such choline phospholipids; soy and sunflower lecithin are the best known. In bile, lecithin matters: together with the bile acids it keeps cholesterol dissolved, which is why this trio also appears in the chapter on gallstones. As a capsule, though, lecithin does not arrive in the small intestine as lecithin, because a phospholipase from the pancreas already splits it there. And the share of phosphatidylcholine hardly differs between soy and sunflower: in purified form it is about 25 per cent in each.

Disrupted state: causes and effects

Fat is poorly digested when one link in the chain fails: too few enzymes from the pancreas (exocrine pancreatic insufficiency), too little bile, or a gut lining that no longer absorbs. Typical signs are fatty, pale, foul-smelling stool, weight loss and a lack of the fat-soluble vitamins A, D, E and K. How this can be investigated is described in the chapter on nutrient deficiency.

Direction decides everything. If bile is missing in the small intestine, fat stays undigested. If, by contrast, too much bile reaches the large intestine, it draws water in there and causes diarrhoea. After gallbladder removal, diarrhoea from too much bile is the more common consequence; a real lack of bile is rare. The treatments are opposite: bile acid binders take bile away, bile salt preparations add it. Anyone who takes both cancels them out.

Even so, bile salt, ox bile and TUDCA TUDCA Short for tauroursodeoxycholic acid, a bile acid. Not an approved medicine in Europe or the USA; sold as a capsule, but with no proven benefit for fat digestion. capsules are sold for fat digestion problems. TUDCA (tauroursodeoxycholic acid) is not an approved medicine in Europe or the USA. The largest study with it ran in the nerve disease ALS: 336 people, 2 grams a day, 18 months, 25 centres in seven countries. The result was negative, but the substance was well tolerated. In progressive multiple sclerosis it was likewise safe in a randomised study of 47 people, but with no measurable effect. After gallbladder removal, the only larger investigation (203 people, 1993, not randomised) showed a benefit only in the first month.

For ox bile there is not a single controlled study. What exists are two single case reports: in 1982 and 1999, liquid ox bile clearly improved fat absorption in one person each with short bowel syndrome, in one case by around 40 grams of fat a day. Both people had a confirmed bile acid deficiency because a piece of small intestine was missing. Nothing can be derived from this for capsules, for normal circumstances or for the time after gallbladder removal.

Ursodeoxycholic acid Ursodeoxycholic acid A bile acid used as a medicine, prescription-only in many countries. Approved among other things for dissolving gallstones and for certain liver diseases. , the related medicine, is a different matter. It is prescription-only in many countries and is used for gallstones and certain liver diseases. In cystic fibrosis with pancreatic insufficiency it improved fat digestion in a small study of 23 people. In severely malnourished children with an acute illness, by contrast, it brought no survival benefit in a randomised study of 429 children and was stopped as useless. It therefore has no established role against fat digestion disorders.

Choline and lecithin are a different pair. Choline is needed so that the liver can release fat as lipoprotein; if it is missing, the liver becomes fatty. This is shown above all from extreme situations: in trials with a choline-free diet, most men developed signs of fatty liver, and with artificial nutrition a fatty liver receded under choline. Whether choline plays a role with a normal diet is open. Choline does not act on fat absorption in the gut. Lecithin capsules are advertised as "supports fat digestion" — there is no human study for that. In ulcerative colitis small studies were positive, but the large independent study of 2024 with 466 patients found no effect.