Choline is a beta-hydroxyethyltrimethylammonium hydroxide. Pure choline is a colorless, viscous, strongly alkaline liquid that is notably hygroscopic. Choline is soluble in water, formaldehyde and alcohol, and has no definite melting or boiling point. The chloride salt of this compound, choline chloride, is produced by chemical synthesis for use in the feed industry, although there are other forms. Choline chloride consists of deliquescent white crystals, which are very soluble in water and alcohols. Aqueous solutions are almost pH neutral.
Choline is ubiquitously distributed in all plant and animal cells, mostly in the form of the phospholipids phosphatidylcholine (lecithin), lysophosphatidylcholine, choline plasmalogens and sphingomyelin–essential components of all membranes (Zeisel, 1990). Lecithin is the predominant phospholipid (>50%) in most mammalian membranes. In the lung, desaturated lecithin is the major active component of surfactant (Brown, 1964), lack of which results in a respiratory distress syndrome in premature infants. Choline is a precursor for the biosynthesis of the neurotransmitter acetylcholine. Glycerophosphocholine and phosphocholine are storage forms for choline within the cytosol and principal forms found in milk (Rohlfs et al., 1993).
Choline is present in the unsupplemented diet mainly in the form of lecithin, with less than 10% present either as the free base or as sphingomyelin. Choline is released from lecithin and sphingomyelin by digestive enzymes of the gastrointestinal tract, although 50% of ingested lecithin enters the thoracic duct intact (Chan, 1991). Choline is released from lecithin by hydrolysis in the intestinal lumen. Both pancreatic secretions and intestinal mucosal cells contain enzymes capable of hydrolyzing lecithin in the diet. Within the gut mucosal cell, phospholipase A1 cleaves the alpha-fatty acid, and phospholipase B cleaves both fatty acids. Quantitatively, digestion by pancreatic lipase is the most important process (Zeisel, 1990). The net result is that most ingested lecithin is absorbed as lysophosphatidylcholine.
Choline is absorbed in the jejunum and ileum mainly by an energy and sodium dependent carrier mechanism. Only one-third of ingested choline in monogastric diets appears to be absorbed intact. Absorbed choline is transported into the lymphatic circulation primarily in the form of lecithin bound to chylomicra; it is transported to the tissues predominantly as phospholipids associated with the plasma lipoproteins. The remaining two-thirds of choline is metabolized by intestinal microorganisms to trimethylamine, which is excreted in the urine between 6 and 12 hours after consumption (De La Huerga and Popper, 1952). In ruminants, dietary choline is rapidly and extensively degraded in the rumen from studies with both sheep (Neill et al., 1979) and cattle (Atkins et al., 1988; Sharma and Erdman, 1988). Estimates of rumen degradation have ranged from 85% to 99%. In in vivo studies with dairy cows, in which choline intake was increased up to 303 grams per day over controls, there was only a 1.3 grams per day increase in choline flow to the duodenum (Sharma and Erdman, 1988).
Work with sheep (Neill et al., 1979) and goats (Emmanual and Kennelly, 1984) suggests that ruminants must metabolize and utilize choline in a different manner than monogastric animals. Choline absorption must be very limited in all ruminants because of (1) almost complete degradation of dietary choline in the rumen, (2) only limited supplies from any rumen protozoa that might escape rumen degradation and (3) the complete absence of choline in rumen bacteria.