What is L-Tryptophan?
L-tryptophan is an essential amino acid, meaning it cannot be produced naturally by the body and must be obtained from the diet. It is one of the tryptophan isomers, the other being D-tryptophan. It is unique among amino acids as it contains an indole ring, a structure that contributes to producing various bioactive molecules in the body [1]. These include serotonin, melatonin and vitamin B6 [2].
Increasing levels of l-tryptophan, for example, using dietary supplements, can increase serotonin levels with minimal risk of side effects, including serotonin syndrome. This has led to the use of l-tryptophan supplements as a natural antidepressant and aid in improving sleep quality [9, [1].
L-tryptophan is widely used in the food, medicine, and feed sectors. It can be produced through fermentation and engineered bacteria [3].
L-Tryptophan in Dogs
L-tryptophan, an essential amino acid, plays a significant role in dogs' health and behaviour. It is a precursor to important molecules such as serotonin, melatonin, and niacin, which are crucial for physiological functions and canine health [4].
The requirement for tryptophan in dogs' diets varies with age and breed. For instance, the tryptophan requirement for maximal weight gain in dogs aged 6 to 14 weeks was found to be 0.15% [5].
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L-Tryptophan and Food Intake in Dogs
One key area where l-tryptophan has been studied in dogs is its impact on food intake. A study found that dogs supplemented with tryptophan at 1g/day tended to have higher food intake than those not taking tryptophan supplements [4].
This was based on studies in pigs that found the amino acid tryptophan increased food intake and weight gain in farmed animals [6]. Tryptophan did not increase levels of ghrelin, the hunger hormone, so the mechanism through which tryptophan increases appetite in dogs is to be determined.
L-Tryptophan and Metabolism in Dogs
The metabolism of l-tryptophan in dogs has also been studied. One study found that the d-isomer of tryptophan was metabolised less efficiently compared to l-tryptophan, making l-tryptophan a more efficient tryptophan supplement [7]
Whether dietary tryptophan enters the brain also depends on the ratio of neutral amino acids. A high concentration of neutral amino acids in the diet can impair tryptophan permeation across the blood-brain barrier, lowering its bioavailability [4, 8].
Additionally, a single meal with a high carbohydrate level significantly increased the TRP/5LNAAs ratio, affecting l-tryptophan's bioavailability [9]. Therefore, it is not only supplements but also diet composition that can modulate l-tryptophan in dogs.
L-tryptophan and Behaviour in Dogs
Some studies suggest that dietary modifications involving l-tryptophan could potentially influence aggressive behaviour in dogs. Adding tryptophan to high-protein diets or switching to a low-protein diet may reduce dominance aggression in dogs [10]. Imagine transforming your dog from a snarling, teeth-bared companion to a joyful, tail-wagging friend simply by tweaking their diet to include more tryptophan.
However, it's important to note that the relationship between l-tryptophan and dog behaviour is complex, where factors outside of tryptophan intake or tryptophan supplementation likely influence behaviour more. For instance, several studies found no significant difference in serum tryptophan or serotonin concentrations among dogs exhibiting different behavioural responses to a potentially stressful procedure [8, 11].
In conclusion, l-tryptophan plays a crucial role in dogs' health, metabolism, and potentially their behaviour. However, more research is needed to fully understand the complex interactions between l-tryptophan, diet, and dog behaviour. Furthermore, excess tryptophan consumption can have potential side effects, which warrant caution.