gut health and immunity
Besides its role for protein deposition, threonine is particularly involved in maintenance processes, like the renewal of intestinal mucus and the synthesis of immune proteins.
Threonine: a key nutrient for the gut
Figure 1. Mucin amino acids composition (in % of crude mucins)
A major fraction of the dietary threonine is absorbed in the upper part of the small intestine - the ileum. The remaining fraction is recovered at the end of the ileum (indigestible threonine). The fraction that is absorbed by the ileum is not entirely delivered in the portal blood which collects the nutrients from the digestion process. Only 40% of the luminal threonine reaches the portal blood. A significant part of the digestible threonine is indeed uptaken by the digestive tract itself; the intestinal cells (enterocytes) use 60% of the threonine intake, which is twice more than what was found for lysine.
This important threonine uptake by the gut is consistent with the high threonine content of the digestive secretions, among which is mucus. The mucus gel layer, secreted by goblet cells scattered along the gut villi, covers the wall of the digestive tract. It is an important component of the non immune gut barrier that acts to protect the gut against digestive enzymes and physical damage by digesta. Mucus is mostly made of water (95%) and mucins (5%) which are large molecular weight glycoproteins very rich in threonine (Figure 1).
A significant part of the threonine intake is utilised by the gut itself and is used for the synthesis of endogenous secretions and particularly mucus. Considering the importance of digestive secretions for gut health and for the digestive process, an adequate dietary threonine level is the key to allow proper gut function.
Threonine and immune function
Figure 2. Immunoglobulin indispensable amino acids profile (%)
The humoral immunity implies the secretions of immunoglobulins (also named antibodies) by mature B lymphocytes in the blood. Immunoglobulins after reaching the site of the infection, recognise, bind and inactivate their antigens. Like mucins, they are globular glycoproteins which contain high amount of threonine (Figure 2).
Because of the high threonine content of immunoglobulins, dietary threonine deficiency may affect immunoglobulins production. Different authors have shown that sows fed added threonine during gestation have higher production of gamma immunoglobulin (IgG) in plasma and in milk at farrowing and for 10 days thereafter. In growing pigs, it has been described that animals receiving a threonine enriched diet had higher plasma IgG and specific antigen levels respectively after BSA (Bovine Serum Albumin) or ovalbumin injection.
Threonine being involved in the immune response, the health status may be a factor of variation of the Thr:Lys ratio requirement and AGP withdrawal may increase the demand for threonine for functions other than growth.
Figure 3 shows indeed that in the group of pigs fed without dietary AGP, increasing the SID Thr:Lys ratio up to 70% allows to reach the same performance as the group fed with dietary AGP. Moreover a threonine deficiency will penalize more severely the performance of pigs fed diets without AGP.
Figure 3. Effect of the standardized ileal digestible threonine to lysine ratio (SID Thr:Lys) on the average daily gain and on the feed conversion ratio in 25-45 kg pigs fed a diet with 0 ppm (- AGP) or 30 ppm of salinomycin (+ AGP).
Figure 4: Effect of the sanitary status (clean vs. dirty) on the standardized digestible threonine to lysine (SD Thr:Lys) requirement for body weight, feed efficiency, carcass weight, carcass yield and breast meat yield of 21-42 days old broilers.
A trial on broilers (21-42 days) studied the effect of litter status on Thr:Lys requirement. Birds were raised in floor pens that had either unused new litter (soft wood shaving) or used built up litter from 4 flocks. Optimal SD Thr:Lys ratio was always higher in dirty conditions than in clean conditions, either for growth performance or carcass traits (Figure 4).
For these two experiments, it is hypothesized that the higher threonine requirement raised in a "dirty" environment reflects the different microbial exposure levels to which these animals were faced and the resulting changes in the maintenance requirement for gastrointestinal functions and immunity.
Sanitary status and animals’ environment conditions are factors of variation of the Thr:Lys requirement; poor sanitary environment conditions lead to a higher Thr:Lys requirement for swine as well as for poultry. Ensuring an adequate amino acids dietary level allows therefore to maintain good performance under unfavourable health environment.