On 2014 Feb 06, Simon Young commented:

This paper, demonstrating that giving humans tryptophan (TRP) in order to increase brain serotonin promotes interpersonal trust, is an interesting contribution to the literature on serotonin and social behavior. However, the authors’ interpretation of the results in relation to nutrition is problematic. They conclude their paper by stating:

“Food may thus act as a cognitive enhancer that modulates the way one thinks and perceives the physical and social world. In particular, TRP supplements, or TRP-containing diets, may promote interpersonal trust in inexpensive, efficient, and healthy ways.”

The word diet can refer to the food that a person habitually eats, or foods that are eaten for a special reason. In relation to the first definition of diet the Introduction of the paper mentions:

“TRP is an essential amino acid contained in food such as fish, soybeans, eggs, and spinach.”

TRP is a constituent of nearly all proteins and therefore all foods that contain protein contain TRP. Fish, soybeans and eggs are high in protein and are therefore relatively high in TRP, while spinach is low in protein and therefore TRP. Among the amino acids TRP is the least abundant in most proteins Heine W, 1995. While ingestion of normal foods containing protein will increase plasma TRP levels, it will not increase brain TRP or serotonin Sainio EL, 1996. This is because TRP is taken up into the brain from the blood by a transport system that is active towards all the large neutral amino acids (LNAA). The different amino acids compete with each other for the transport system, and as a rough approximation brain TRP levels will follow the plasma ratio of TRP to the other LNAA (TRP/LNAA). Because of the low abundance of TRP in most proteins this ratio will decline after ingestion of a normal meal. While the decline is small enough that there is unlikely to be any important decline in brain TRP and serotonin synthesis, normal protein-containing meals definitely do not raise brain serotonin Teff KL, 1989. Thus, normal meals would not promote trust by increasing serotonin. The statement in the paper that food may act as a cognitive enhancer may be true, but has nothing to do with TRP and serotonin. The acute improvement in memory after a meal is well known and is associated with an increase in blood glucose Smith MA, 2011. All macronutrients may have beneficial effects on cognition as protein, carbohydrate and fat meals all improve performance on different cognitive tests in humans Kaplan RJ, 2001. Among the dietary components that can potentially influence brain function acutely, in addition to TRP, are tyrosine and phenylalanine, which are precursors of the catecholamines, histidine, the precursor of histamine, choline, the precursor of acetylcholine, and threonine, after conversion into glycine Young SN, 1996. Furthermore, while real meals can have acute effects on cognition and mood these effects are influenced by context Sommer W, 2013. If normal meals were found to increase interpersonal trust such an effect would not be mediated by changes in TRP and serotonin. The second meaning of diet is foods, that may be specially prepared, that are eaten for a special purpose. One such specially prepared food that has been studied is alpha-lactalbumin, a protein that is a minor constituent of milk and has a higher than normal proportion of TRP Heine W, 1995. When purified alpha-lactalbumin is given to humans it increases the TRP/LNAA Markus CR, 2008 and can in some circumstances have effects on mood and cognition Markus CR, 2000. However, alpha-lactalbumin could not be considered a special diet that may “promote interpersonal trust in inexpensive, efficient, and healthy ways”. Colzato et al. gave the participants in their study 0.8g of TRP. When humans were given either 0.8g of pure TRP or 0.8g of TRP in alpha-lactalbumin (15g) the rise in TRP/ LNAA is much greater after the TRP than after the alpha-lactalbumin, because the alpha-lactalbumin contains the other LNAA Markus CR, 2008. Furthermore, the TRP/LNAA reached a peak 2 hours after ingestion of the alpha-lactalbumin and then declined. To increase serotonin synthesis sufficiently to promote interpersonal trust would presumably involve doses of alpha-lactalbumin greater than 15g, taken at relatively frequent intervals throughout the day. This would not be inexpensive and efficient as suggested by Colzato et al., and might not even be healthy given that the effects of frequent ingestion of a protein supplement that has calories but no minerals, vitamins or antioxidants could potentially have adverse effects on total dietary intake. In some countries TRP is sold as a dietary supplement (although in others such as Canada it is regulated as a prescription drug). This raises the question of whether TRP could be given long-term in pure form to promote interpersonal trust. When TRP is given for a few weeks it can help regulate behavior in pathologically aggressive patients Morand C, 1983, and promote more pro-social behavior in healthy people Moskowitz DS, 2001 aan het Rot M, 2006. However, taking TRP long-term to promote trust would be neither inexpensive, convenient, given that TRP has to be taken several time a day to ensure that brain serotonin remains elevated Moskowitz DS, 2001, or without risk Fernstrom JD, 2012. Barriers to obtaining more information on the effects of diet on mood and behavior include the complexities of the effects of dietary components on brain function, the fact that effects of food intake on brain function can be modulated by the setting, and the difficulties of carrying out randomized trials with altered diets. At this time research has not revealed any simple dietary strategy to increase brain serotonin, in spite of the potential benefits of such an approach.

On 2014 Feb 06, Simon Young commented:

This paper, demonstrating that giving humans tryptophan (TRP) in order to increase brain serotonin promotes interpersonal trust, is an interesting contribution to the literature on serotonin and social behavior. However, the authors’ interpretation of the results in relation to nutrition is problematic. They conclude their paper by stating:

“Food may thus act as a cognitive enhancer that modulates the way one thinks and perceives the physical and social world. In particular, TRP supplements, or TRP-containing diets, may promote interpersonal trust in inexpensive, efficient, and healthy ways.”

The word diet can refer to the food that a person habitually eats, or foods that are eaten for a special reason. In relation to the first definition of diet the Introduction of the paper mentions:

“TRP is an essential amino acid contained in food such as fish, soybeans, eggs, and spinach.”

TRP is a constituent of nearly all proteins and therefore all foods that contain protein contain TRP. Fish, soybeans and eggs are high in protein and are therefore relatively high in TRP, while spinach is low in protein and therefore TRP. Among the amino acids TRP is the least abundant in most proteins Heine W, 1995. While ingestion of normal foods containing protein will increase plasma TRP levels, it will not increase brain TRP or serotonin Sainio EL, 1996. This is because TRP is taken up into the brain from the blood by a transport system that is active towards all the large neutral amino acids (LNAA). The different amino acids compete with each other for the transport system, and as a rough approximation brain TRP levels will follow the plasma ratio of TRP to the other LNAA (TRP/LNAA). Because of the low abundance of TRP in most proteins this ratio will decline after ingestion of a normal meal. While the decline is small enough that there is unlikely to be any important decline in brain TRP and serotonin synthesis, normal protein-containing meals definitely do not raise brain serotonin Teff KL, 1989. Thus, normal meals would not promote trust by increasing serotonin. The statement in the paper that food may act as a cognitive enhancer may be true, but has nothing to do with TRP and serotonin. The acute improvement in memory after a meal is well known and is associated with an increase in blood glucose Smith MA, 2011. All macronutrients may have beneficial effects on cognition as protein, carbohydrate and fat meals all improve performance on different cognitive tests in humans Kaplan RJ, 2001. Among the dietary components that can potentially influence brain function acutely, in addition to TRP, are tyrosine and phenylalanine, which are precursors of the catecholamines, histidine, the precursor of histamine, choline, the precursor of acetylcholine, and threonine, after conversion into glycine Young SN, 1996. Furthermore, while real meals can have acute effects on cognition and mood these effects are influenced by context Sommer W, 2013. If normal meals were found to increase interpersonal trust such an effect would not be mediated by changes in TRP and serotonin. The second meaning of diet is foods, that may be specially prepared, that are eaten for a special purpose. One such specially prepared food that has been studied is alpha-lactalbumin, a protein that is a minor constituent of milk and has a higher than normal proportion of TRP Heine W, 1995. When purified alpha-lactalbumin is given to humans it increases the TRP/LNAA Markus CR, 2008 and can in some circumstances have effects on mood and cognition Markus CR, 2000. However, alpha-lactalbumin could not be considered a special diet that may “promote interpersonal trust in inexpensive, efficient, and healthy ways”. Colzato et al. gave the participants in their study 0.8g of TRP. When humans were given either 0.8g of pure TRP or 0.8g of TRP in alpha-lactalbumin (15g) the rise in TRP/ LNAA is much greater after the TRP than after the alpha-lactalbumin, because the alpha-lactalbumin contains the other LNAA Markus CR, 2008. Furthermore, the TRP/LNAA reached a peak 2 hours after ingestion of the alpha-lactalbumin and then declined. To increase serotonin synthesis sufficiently to promote interpersonal trust would presumably involve doses of alpha-lactalbumin greater than 15g, taken at relatively frequent intervals throughout the day. This would not be inexpensive and efficient as suggested by Colzato et al., and might not even be healthy given that the effects of frequent ingestion of a protein supplement that has calories but no minerals, vitamins or antioxidants could potentially have adverse effects on total dietary intake. In some countries TRP is sold as a dietary supplement (although in others such as Canada it is regulated as a prescription drug). This raises the question of whether TRP could be given long-term in pure form to promote interpersonal trust. When TRP is given for a few weeks it can help regulate behavior in pathologically aggressive patients Morand C, 1983, and promote more pro-social behavior in healthy people Moskowitz DS, 2001 aan het Rot M, 2006. However, taking TRP long-term to promote trust would be neither inexpensive, convenient, given that TRP has to be taken several time a day to ensure that brain serotonin remains elevated Moskowitz DS, 2001, or without risk Fernstrom JD, 2012. Barriers to obtaining more information on the effects of diet on mood and behavior include the complexities of the effects of dietary components on brain function, the fact that effects of food intake on brain function can be modulated by the setting, and the difficulties of carrying out randomized trials with altered diets. At this time research has not revealed any simple dietary strategy to increase brain serotonin, in spite of the potential benefits of such an approach.