Choline - An Essential Nutrient with New Perspectives


What is Choline?

Natural choline sources occur at meaningful levels in most feed ingredients, thus choline is not considered a vitamin in the classic sense.  That said, the feed industry has learned from experience that there are many feeding conditions where choline becomes inadequate for optimum animal health and performance.  As such, target choline levels are considered in several stages of swine production.  Choline (and related metabolites) have three broad physiological roles in the animal: building and maintaining cellular structures; mobilizing and metabolism of lipids in support of liver function; and neurotransmission maintenance. Choline flows through the animal system quickly and is metabolically altered for specific biological purposes by oxidation, phosphorylation, and acetylation pathways (Chen et al., 2024).  Choline naturally occurs in bulk ingredients, such as soybean meal, primarily in the form of phosphatidylcholine 

Why do we feed it?

There are several production benefits associated with choline supplementation, particularly in sows and piglets. Choline supplementation in sows has been shown to increase sow litter size, improve conception rate, and leads to more born alive and weaned piglets (K-State Animal Science, 2022). Choline supplementation in gestating and lactating diets has a positive influence on sow milk composition, bodily choline concentrations, and piglet performance (Mudd et al., 2016; Getty and Dilger, 2015). Choline is often not supplemented in the grow/finish phase, as choline requirements are met by intrinsic (natural) choline forms from bulk ingredients. However, supplementation of choline during this phase has been shown to positively alter the gut microbiota and can lead to improvements in body weight gain (Jiao et al., 2018; Xie et al., 2023). 

Classic choline deficiency signs are not common in swine production systems, as most requirements are met by well-formulated diets with external choline chloride supplementation. However, with omission of supplemental choline or a possible choline antagonist, signs of choline deficiency can include reduced growth and reproductive performance, as well as increased fat accumulation around te liver and mis formation of cartilage and tendons. 

Recommendations

According to the NRC (2012), choline requirements are 600-1250 ppm in gestation and lactating sows diets, and 400-600 ppm in starter, grower, and finisher diets. Practical diets that are high in intrinsic choline can often meet the pig’s requirements for choline. In piglets, the choline requirement is met through milk. Pigs are also able to meet some of their dietary choline needs through internal conversion of the amino acid methionine.

While most US based diets theoretically have enough intrinsic choline to meet requirements, the uniform bioavailability of feed choline is questionable. Little research work has been done examining choline bioavailability in common feedstuffs, and the research that has been completed was published over 25 years ago. Previous work done by the NRC in 1994 shows that soybean meal has a choline level of around 2300 mg/kg and DDGS has a choline level of around 2100 mg/kg, with almost all of that being in the form of phosphatidylcholine. In the last 25 years, there have been numerous changes to how we feed pigs, pig genetics, and how ingredients are processed and sourced.  It is uncertain if past research properly represents the current status of intrinsic choline supply and bioavailability.  

Traditional Choline Supplementation

External choline supplementation has traditionally been in the form of dry or liquid choline chloride. The amount of active choline in these products typically runs about 60% for dry, carrier-based choline chloride and 70-75% for liquid choline chloride. Many operators and nutritionists opt for the liquid form due to the hydroscopic and hyper-reactive/destructive properties of dry choline chloride.  Because of its hygroscopicity, dry choline chloride can have flowability and clumping issues that can lead to feed blending problems. Additionally, choline chloride should not be put into a premix due to the likelihood of degrading vitamins and minerals. Choline chloride also has the potential to shorten the life of feed plant equipment.  For unresearched reasons, only about 1/3 of dry choline chloride is able to be absorbed and utilized by the pig. Tri-methyl amine (TMA) is also used in the synthesis of dry choline chloride, and has been known to cause off flavored and poorer quality meat, with pork having high concentration of TMA being described as having an off putting, fishy smelling odor (Hamid et al., 2014) Although feed-grade choline chloride is an inexpensive way to boost dietary choline levels, this method of choline supplementation can lead to potential concerns within feed application and equipment. 

Metabolic derivatives of Choline

As mentioned earlier, choline can be metabolized by three different metabolic pathways depending on the physiological need of the pig. The oxidation pathway yields betaine, which has several useful physiological roles in the pig, especially related to methyl group metabolism.  Betaine itself is also a common feed ingredient that is sometimes included in swine diets. Direct supplementation of betaine in swine diets has been shown to have positive effects on carcass and meat quality due to its positive effects on fat metabolism. Betaine can assist in the metabolic synthesis of carnitine and phosphatidylcholine, both of which are involved in fat transport and fat oxidation. Betaine can also convert homocysteine to methionine by donating one of its three methyl groups and therefore increases methionine concentration in the body. Another unique property of betaine is its cellular protective capabilities as an osmolyte. Betaine is commonly included in diets as a heat stress aid.  However, because betaine cannot conversely be converted to choline, betaine can only reduce (spare) the choline requirement and is not a choline source as such. 

As in plants, the phosphorylation pathway of choline creates phosphatidylcholine, which is the most abundant form of choline found in the body. Phosphatidylcholine is involved in many functions in the body, being a large component of many cells structure and highly involved in metabolizing and mobilizing lipids from intestinal absorption.  Phosphatidylcholine is essentially a fat-soluble form of choline and is indirectly involved in all pathways for choline in the body. Crude phosphatidylcholine that is extracted from oilseeds or other plant materials is commonly referred to as lecithin.  Historically, lecithin has been valued more for its physical fat emulsifying properties than for its choline contribution, as such.  Interestingly, a new plant-based product from Barentz Animal Nutrition, Kolin Plus, is based on highly available phosphatidylcholine and four different bioactive phytogenic materials.  This material has been shown in research to be a natural alternative to synthetic feed-grade choline chloride but at a lower use rate. Kolin Plus is non-hydroscopic and chemically non-reactive, thus being more suitable for premixes and other blending applications for swine.