On 2016 May 28, Simon Young commented:

This is an interesting article, but it contains a number of statements that are problematic.

1. The abstract concludes with the statement that the results support the hypothesis “higher tryptophan and vitamin B6 intake at breakfast could promote the synthesis of serotonin via light stimulation in the morning in children”. The first issue is whether higher dietary intake of tryptophan (Trp) can stimulate serotonin synthesis. Meals containing protein will increase plasma Trp but 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 a meal. The decline is small enough that there is unlikely to be any important decline in brain TRP and serotonin synthesis. Protein-containing meals certainly do not raise brain serotonin synthesis in humans Teff KL, 1989. In the pineal there is no blood-brain barrier, so meals containing Trp might cause a small increase in serotonin and melatonin. The only function for serotonin in the pineal is to act as a precursor of melatonin. In daytime melatonin levels are very low and any change in levels are unlikely to have any effect. Therefore, even if increased serotonin in the morning could enhance morning typology, and I am not aware of any such evidence, there would still be no valid rationale for the idea that the Trp content of breakfast will influence serotonin in any way that will promote morning typology.

2. The last paragraph of the article starts with the sentence “From these results, it might be suggested that a higher Trp and Vi-B6 intake may promote the synthesis of serotonin via light stimulation in the morning and have a natural sleep-inducing effect when converted to melatonin at night.” When plasma Trp rises due to Trp intake it is metabolized rapidly. For example, even after ingestion of pure Trp equivalent to several times the normal daily intake plasma levels are back to normal within 8 hours Yuwiler A, 1981. Plasma Trp varies throughout the day due to diurnal variation and the intake of meals during the day Fernstrom JD, 1979. Intake of Trp at breakfast will not influence melatonin at bedtime.

3. The last sentence of the first paragraph of the article states “The exposure to sunlight in the morning can be hypothesized to accelerate the synthesis of serotonin from Trp in the pineal gland [8]”. The reference cited, which is an abstract from a meeting, does not state that sunlight increases the synthesis of serotonin from Trp in the pineal. Bright light suppresses serotonin and melatonin synthesis in the pineal, a fact that has been known for more than 50 years, see e.g. WURTMAN RJ, 1964. However, it may increase serotonin synthesis in the brain. Sunlight is associated with an increase in serotonin synthesis in human brain Lambert GW, 2002, possibly due to a direct neural connection between the retina and the cell bodies of serotonin neurons in the raphe nuclei of the brainstem Ren C, 2013.

4. In the second paragraph of the Discussion the authors state “Among essential amino acids, the Trp content in food is quite small, and thus it is necessary to make a special effort to consume a sufficient amount in one’s diet”. Trp deficiency can occur in conjunction with protein deficiency, but otherwise is rare and related to genetic and other diseases Sainio EL, 1996. The requirement of adults for Trp is 4 mg/kg/day Elango R, 2009, and the daily requirement for 10-12-year-old children it at most 120 mg/day NAKAGAWA I, 1963, so the daily requirement for Trp of the 2-6-year-old children in this study will be small. This explains why billions of people in the world are able to obtain sufficient Trp in their diet even though they do not “make a special effort” to obtain enough Trp, due to lack of knowledge of Trp.

5. As stated in the first paragraph of the paper vitamin B6 (Vi B6) is “a coenzyme in the tryptophan-serotonin pathway”. It is the coenzyme of aromatic amino acid decarboxylase (AADC), the second enzyme on the pathway. The first enzyme on the pathway is Trp hydroxylase, which is the rate-limiting enzyme on the pathway. This, and the fact that Trp hydroxylase is not normally saturated with Trp, explains why increases in brain Trp will increase serotonin synthesis. Given that Trp hydroxylase is the rate-limiting enzyme on the pathway from Trp to serotonin, changes in AADC activity, except for very large decreases in activity, would not be expected to alter the rate of serotonin synthesis. I am not aware of any evidence that, in the absence of Vi B6 deficiency, variations in Vi B6 intake can alter the rate of serotonin synthesis in human brain.

6. This paper demonstrates an association between morning typology and intake of Trp plus exposure to light. An association does not necessarily imply causation. The authors do not consider any possible confounders that might explain both phenomena. They also do not consider reverse causation. For example, if the parents have morning typology they may be more likely to give their children a nutritionally balanced breakfast containing sufficient protein. More protein in a meal means that there will be more tryptophan in the meal. Also, if the parents have morning typology they may encourage their child to spend more time outside before arriving at nursery school or kindergarten. The timing of daily circadian behavior in humans is determined by both genetics and the environment Azzi A, 2014, so if the parents demonstrate morning typology the children are also likely to exhibit morning typology. Thus, a morning typology in the parents may cause the child to have a greater intake of Trp at breakfast and be exposed to more light in the morning. Furthermore, a morning typology in the parents could be responsible for a morning typology in the child, due to both genetic and environmental factors. This is one possible explanation for the results.

On 2016 May 28, Simon Young commented:

This is an interesting article, but it contains a number of statements that are problematic.

1. The abstract concludes with the statement that the results support the hypothesis “higher tryptophan and vitamin B6 intake at breakfast could promote the synthesis of serotonin via light stimulation in the morning in children”. The first issue is whether higher dietary intake of tryptophan (Trp) can stimulate serotonin synthesis. Meals containing protein will increase plasma Trp but 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 a meal. The decline is small enough that there is unlikely to be any important decline in brain TRP and serotonin synthesis. Protein-containing meals certainly do not raise brain serotonin synthesis in humans Teff KL, 1989. In the pineal there is no blood-brain barrier, so meals containing Trp might cause a small increase in serotonin and melatonin. The only function for serotonin in the pineal is to act as a precursor of melatonin. In daytime melatonin levels are very low and any change in levels are unlikely to have any effect. Therefore, even if increased serotonin in the morning could enhance morning typology, and I am not aware of any such evidence, there would still be no valid rationale for the idea that the Trp content of breakfast will influence serotonin in any way that will promote morning typology.

2. The last paragraph of the article starts with the sentence “From these results, it might be suggested that a higher Trp and Vi-B6 intake may promote the synthesis of serotonin via light stimulation in the morning and have a natural sleep-inducing effect when converted to melatonin at night.” When plasma Trp rises due to Trp intake it is metabolized rapidly. For example, even after ingestion of pure Trp equivalent to several times the normal daily intake plasma levels are back to normal within 8 hours Yuwiler A, 1981. Plasma Trp varies throughout the day due to diurnal variation and the intake of meals during the day Fernstrom JD, 1979. Intake of Trp at breakfast will not influence melatonin at bedtime.

3. The last sentence of the first paragraph of the article states “The exposure to sunlight in the morning can be hypothesized to accelerate the synthesis of serotonin from Trp in the pineal gland [8]”. The reference cited, which is an abstract from a meeting, does not state that sunlight increases the synthesis of serotonin from Trp in the pineal. Bright light suppresses serotonin and melatonin synthesis in the pineal, a fact that has been known for more than 50 years, see e.g. WURTMAN RJ, 1964. However, it may increase serotonin synthesis in the brain. Sunlight is associated with an increase in serotonin synthesis in human brain Lambert GW, 2002, possibly due to a direct neural connection between the retina and the cell bodies of serotonin neurons in the raphe nuclei of the brainstem Ren C, 2013.

4. In the second paragraph of the Discussion the authors state “Among essential amino acids, the Trp content in food is quite small, and thus it is necessary to make a special effort to consume a sufficient amount in one’s diet”. Trp deficiency can occur in conjunction with protein deficiency, but otherwise is rare and related to genetic and other diseases Sainio EL, 1996. The requirement of adults for Trp is 4 mg/kg/day Elango R, 2009, and the daily requirement for 10-12-year-old children it at most 120 mg/day NAKAGAWA I, 1963, so the daily requirement for Trp of the 2-6-year-old children in this study will be small. This explains why billions of people in the world are able to obtain sufficient Trp in their diet even though they do not “make a special effort” to obtain enough Trp, due to lack of knowledge of Trp.

5. As stated in the first paragraph of the paper vitamin B6 (Vi B6) is “a coenzyme in the tryptophan-serotonin pathway”. It is the coenzyme of aromatic amino acid decarboxylase (AADC), the second enzyme on the pathway. The first enzyme on the pathway is Trp hydroxylase, which is the rate-limiting enzyme on the pathway. This, and the fact that Trp hydroxylase is not normally saturated with Trp, explains why increases in brain Trp will increase serotonin synthesis. Given that Trp hydroxylase is the rate-limiting enzyme on the pathway from Trp to serotonin, changes in AADC activity, except for very large decreases in activity, would not be expected to alter the rate of serotonin synthesis. I am not aware of any evidence that, in the absence of Vi B6 deficiency, variations in Vi B6 intake can alter the rate of serotonin synthesis in human brain.

6. This paper demonstrates an association between morning typology and intake of Trp plus exposure to light. An association does not necessarily imply causation. The authors do not consider any possible confounders that might explain both phenomena. They also do not consider reverse causation. For example, if the parents have morning typology they may be more likely to give their children a nutritionally balanced breakfast containing sufficient protein. More protein in a meal means that there will be more tryptophan in the meal. Also, if the parents have morning typology they may encourage their child to spend more time outside before arriving at nursery school or kindergarten. The timing of daily circadian behavior in humans is determined by both genetics and the environment Azzi A, 2014, so if the parents demonstrate morning typology the children are also likely to exhibit morning typology. Thus, a morning typology in the parents may cause the child to have a greater intake of Trp at breakfast and be exposed to more light in the morning. Furthermore, a morning typology in the parents could be responsible for a morning typology in the child, due to both genetic and environmental factors. This is one possible explanation for the results.