On 2015 Jul 27, Simon Young commented:

While the aims of this article are good it has some problems: 1. The Abstract states “Because serotonin levels in the brain are dependent on the availability of the food-derived precursor tryptophan, foods such as chicken, soyabeans, cereals, tuna, nuts and bananas may serve as an alternative to improve mood and cognition”. However, as the authors state correctly in the Discussion “contrary to popular belief, most common foods high in Trp do not increase the plasma TRP:LNAA ratio enough to exert any positive effects on mood/cognition, because they also contain large amounts of other LNAA.” Ingestion of chicken, soyabeans, cereals, tuna and nuts would certainly not, as suggested in the Abstract, improve mood and cognition due to increased brain serotonin synthesis, as they would not increase brain synthesis. Why bananas are included in the list is not clear. Bananas contain high serotonin levels Feldman JM, 1985, but as mentioned in the article serotonin in food does not cross the blood-brain barrier. Apparently the high level of serotonin in bananas has resulted in the popular belief that bananas must contain high tryptophan levels. A search in Google using the key words banana and tryptophan finds numerous sites stating that bananas have high levels of serotonin. However, the United States Department of Agriculture Nutrient Database http://ndb.nal.usda.gov/ndb/foods gives the tryptophan content of bananas as 9mg per 100g, a very low level that would contribute a negligible fraction of the normal daily intake of tryptophan even if several bananas were eaten. 2. Another problem with the article is that the authors ignore that fact that food may act as a cognitive enhancer through mechanisms other than alterations in tryptophan and serotonin. The acute improvement in memory after a meal is well known and is, in whole or in part, associated with an increase in blood glucose, which may improve cognition through an increase in acetylcholine, but not serotonin 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. Thus, for example, the enhancement of memory due to a high carbohydrate diet mentioned in relation to citation 111 in the article may have nothing to do with changes in brain serotonin synthesis. 3. Hulsken et al discuss the effect of tryptophan supplementation using alpha-lactalbumin, a protein with high levels of tryptophan, and egg protein hydrolysate. They also mention some studies using pure tryptophan, but omit mention of many articles in which mood and/or cognition were measured after administration of pure tryptophan. The first such study was published more than 50 years ago SMITH B, 1962. Many of the missing studies on healthy participants are cited in a recent review Silber BY, 2010. Numerous studies in which tryptophan was given to vulnerable subjects were not cited by Hulsken et al. A Cochrane Database Systematic Review concluded that the available evidence suggests that tryptophan is better than placebo at alleviating depression Shaw K, 2002. Tryptophan has also been used in other conditions. For example, while the article mentions the beneficial effects of a high carbohydrate diet and alpha-lactalbumin on premenstrual mood it does not mention a clinical trial in which tryptophan (6g per day) was better than placebo in treating premenstrual dysphoria Steinberg S, 1999. Given the high dose of tryptophan given, studies such as this clinical trial need to be taken into account when assessing the authors’ suggestion that “Unphysiological high increases in brain Trp lead to negative effects on mood in both healthy and vulnerable subjects”.

On 2015 Jul 27, Simon Young commented:

While the aims of this article are good it has some problems: 1. The Abstract states “Because serotonin levels in the brain are dependent on the availability of the food-derived precursor tryptophan, foods such as chicken, soyabeans, cereals, tuna, nuts and bananas may serve as an alternative to improve mood and cognition”. However, as the authors state correctly in the Discussion “contrary to popular belief, most common foods high in Trp do not increase the plasma TRP:LNAA ratio enough to exert any positive effects on mood/cognition, because they also contain large amounts of other LNAA.” Ingestion of chicken, soyabeans, cereals, tuna and nuts would certainly not, as suggested in the Abstract, improve mood and cognition due to increased brain serotonin synthesis, as they would not increase brain synthesis. Why bananas are included in the list is not clear. Bananas contain high serotonin levels Feldman JM, 1985, but as mentioned in the article serotonin in food does not cross the blood-brain barrier. Apparently the high level of serotonin in bananas has resulted in the popular belief that bananas must contain high tryptophan levels. A search in Google using the key words banana and tryptophan finds numerous sites stating that bananas have high levels of serotonin. However, the United States Department of Agriculture Nutrient Database http://ndb.nal.usda.gov/ndb/foods gives the tryptophan content of bananas as 9mg per 100g, a very low level that would contribute a negligible fraction of the normal daily intake of tryptophan even if several bananas were eaten. 2. Another problem with the article is that the authors ignore that fact that food may act as a cognitive enhancer through mechanisms other than alterations in tryptophan and serotonin. The acute improvement in memory after a meal is well known and is, in whole or in part, associated with an increase in blood glucose, which may improve cognition through an increase in acetylcholine, but not serotonin 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. Thus, for example, the enhancement of memory due to a high carbohydrate diet mentioned in relation to citation 111 in the article may have nothing to do with changes in brain serotonin synthesis. 3. Hulsken et al discuss the effect of tryptophan supplementation using alpha-lactalbumin, a protein with high levels of tryptophan, and egg protein hydrolysate. They also mention some studies using pure tryptophan, but omit mention of many articles in which mood and/or cognition were measured after administration of pure tryptophan. The first such study was published more than 50 years ago SMITH B, 1962. Many of the missing studies on healthy participants are cited in a recent review Silber BY, 2010. Numerous studies in which tryptophan was given to vulnerable subjects were not cited by Hulsken et al. A Cochrane Database Systematic Review concluded that the available evidence suggests that tryptophan is better than placebo at alleviating depression Shaw K, 2002. Tryptophan has also been used in other conditions. For example, while the article mentions the beneficial effects of a high carbohydrate diet and alpha-lactalbumin on premenstrual mood it does not mention a clinical trial in which tryptophan (6g per day) was better than placebo in treating premenstrual dysphoria Steinberg S, 1999. Given the high dose of tryptophan given, studies such as this clinical trial need to be taken into account when assessing the authors’ suggestion that “Unphysiological high increases in brain Trp lead to negative effects on mood in both healthy and vulnerable subjects”.