Tryptophan

Keith notes that serotonin is made from tryptophan and blames a veganism-induced tryptophan deficiency for her "emotional collapse" (p. 10). She specifically states that "there are no good plant sources of tryptophan." (p. 10).

While she's right that the body creates serotonin from tryptophan, the truth is that there are ample sources of tryptophan available to vegans and even vegan supplements to provide l-tryptophan and 5-HTP (the substance to which  tryptophan is converted before serotonin) available for those who think they might be deficient. (1)(2)

Suggested daily intake of tryptophan varies by organization and country but the range is generally between  3-5 mg per kg of body weight daily. The WHO recommends 4 mg minimum daily of tryptophan per kg of body weight.(3) This works out to about 252 mg for a person weighing 140 pounds. One cup of roasted soybeans contains 881mg of tryptophan (4). Keith expresses many concerns about soy, however, such as the phytate content, which will be addressed in other sections. Other good cources of tryptophan include the blue-green algae spirulina (929 mg of tryptophan per 100 grams), tempeh (322 mg per cup), pinto beans (185 mg per cup) as well as all other beans and lentils, tahini (104 mg per ounce), peanuts (peanut butter contains 145 mg in two tablespoons) , pumpkin seeds (347 mg per half cup) and sunflower seeds (295 mg per 100 grams of hulled dry roasted seeds) (4).

Another overlooked excellent source of tryptophan is protein powder. Powders can be economically purchased and easily added to a morning smoothie or shake. 30 grams of rice protein powder contains 400 mg of tryptophan which already exceeds the RDA.(5)

Keith misleads again when she implies that vegan diets, lower in protein (and hence lower in tryptophan) than her current meat-heavy diet, cause people to become "aggressive, rigid, on a hair trigger, and in a semi-constant state of rage. That's what happens to a human with a brain deprived of protein and fat." (p. 193). In fact, very high protein/low carbohydrate meals have been shown to prevent absorption of tryptophan and cause aggressiveness (6)(7)(12).This is due to the increased competition for transit between amino acids, the

relative scarcity of tryptophan in dietary protein and the greater molecular weight of tryptophan compared to its uptake competitors (7)(8)(12). Keith suggests that the ideal diet is one extremely low in carbohydrates (or free of them altogether) (p. 153). She says that diets rich in carbhohydrates cause brief "lacunas" of tryptophan absorption that lead to a cycle of carbohydrate craving:

"Eating sugar triggers a flood of insulin. Insulin moves through your bloodstream, sweeping up sugars, fats, and amino acids, and transporting them into your cells for storage. The only substance that insulin can’t lock onto is tryptophan. With all the other amino acids out of the way, tryptophan suddenly has no competition in crossing the blood-brain barrier. Hence, for a brief time, a serotonin-deprived brain gets some desperately needed tryptophan. It’s why depressed people crave sweets and starchy “comfort foods.” And it’s only during those short lacunas that a vegan’s brain feels normal." (p. 195)

While, it is true that some studies have shown that tryptophan deficiency can lead to an increased selection of high carbohydrate foods, (9) there have been other studies that suggest the effect is small to non-existant (10)(11). Some studies conclude with the suggestion of incorporating carbohydrates into the diet in order to enhance tryptophan uptake (8).Whatever the case, it is very difficult to find any research that supports Keith's assertion that a high protein low carbohydrate diet leads to increased brain serotonin levels. Even doctors who advocate for high protein/low carbohydrate diets admit that carbohydrate consumption results in increased serotonin levels. They just consider it a bad thing because tryptophan causes sleepiness (12).
Simon Young notes that there are four ways to increase serotonin levels in the brain, only one of which is diet. The other three are mood alteration (through mental exercise, increased social support or psychotherapy), phototherapy (exposure to bright light), and physical exercise. Young also confirms that animal protein on its own does nothing to increase serotonin levels:

"Although purified tryptophan increases brain serotonin, foods containing tryptophan do not. This is because tryptophan is transported into the brain by a transport system that is active toward all the large neutral amino acids and tryptophan is the least abundant amino acid in protein. There is competition between the various amino acids for the transport system, so after the ingestion of a meal containing protein, the rise in the plasma level of the other large neutral amino acids will prevent the rise in plasma tryptophan from increasing brain tryptophan. The idea, common in popular culture, that a high-protein food such as turkey will raise brain tryptophan and serotonin is, unfortunately,  false."(13)

Young adds that while increasing protein content overall does nothing for serotonin, humans have in the past and will likely continue to cultivate foods with a higher relative content of tryptophan than what's typically found in the wild. For instance, in a study of wild and domesticated chickpeas, Kerem and colleagues found something extraordinary. Through the benefit of agriculture, neolithic peoples were able to double the content of tryptophan relative to the other amino acids, not due to any overall increase in protein. Most of this tryptophan is in a readily absorped free form (separate from the protein)(13).

(1) http://www.betterhealthinternational.com/productDetails.asp?prodID=LX86609
(2) http://www.koshervitamins.com/shop/stores_app/Browse_Item_Details.asp?Item_ID=3197
(3) http://whqlibdoc.who.int/trs/WHO_TRS_935_eng.pdf
(4) http://www.nutritiondata.com
(5) https://www.trueprotein.com/Product_Details.aspx?cid=22&pid=6833#AminoAcids
(6) http://www.ncbi.nlm.nih.gov/pubmed/10953712
(7) http://www.ajcn.org/cgi/reprint/47/3/433.pdf
(8) http://www.ncbi.nlm.nih.gov/pubmed/2345536
(9) http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2883%2992877-5/fulltext
(10) http://www.ncbi.nlm.nih.gov/pubmed/2462018
(11) http://www.ncbi.nlm.nih.gov/pubmed/2623036
(12) http://www.second-opinions.co.uk/feed_brain.html
(13) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2077351/?tool=pmcentrez