Way back when I first started Healthy Emily, I wrote a blog post about how eating different foods could affect our brains’ production of neurotransmitters that affect mood. This post was, and still is, one of the most popular posts on this site. About two years after I wrote it, I took a Food Science class that asked us to write a term paper on a topic relating to, as you might assume, food and science. Since the topic of nutrition and mood stability still fascinated me, I decided to turn the idea for that post into my term paper.
Since the post was so popular, I thought I would share with all of you the final results of my paper, with the actual, scientific research that answers the question:
Can dietary changes positively (or negatively) affect mood at a neurological level?
This is part two of the paper – read part one here!
Nutrition’s Effects on Mood
by Emily Star
written May 2015
Tryptophan and Tyrosine
The amino acid tryptophan is used by the body to produce serotonin, while tyrosine is one of the amino acids that goes into excitatory neurotransmitters.2 On a basic level, it would seem that simply increasing dietary intake of these two amino acids would allow the body to produce more mood-enhancing neurotransmitters, and therefore improve mood. However, before these nutrients can become mood food, they must first cross the blood-brain barrier.2 The brain only allows a certain amount of amino acids through the blood-brain barrier, and they move in on a concentration gradient.2 Part of what inhibits the brain from absorbing tryptophan and tyrosine is the amount of other amino acids in the bloodstream.2 A meal heavy on protein will increase both tryptophan and tyrosine, but it will also increase other amino acids, thus preventing extra tryptophan and tyrosine from being absorbed and thus improving mood.2,3
This issue can be prevented by eating a meal that, rather than being protein heavy, includes carbohydrates. “Carbohydrate ingestion causes insulin secretion that lowers blood levels of …[competing]… amino acids while not affecting blood tryptophan levels – providing a competitive advantage in terms of uptake into the brain.”2 Therefore, increasing dietary tryptophan and tyrosine will not increase mood enhancing neurotransmitter production; adequate carbohydrates must also be on the menu.2,3
Additionally, Choi et al., in their study on protein ingestion effects on serotonin synthesis in rat brains, found that in general, tyrosine levels and excitatory neurotransmitter synthesis were unaffected when dietary protein was increased; however, diets of 10% protein increased both tryptophan levels in the brain and serotonin synthesis when compared with diets of 2% protein in rats.4 Their findings that ingested protein can affect tryptophan/serotonin levels, but not tyrosine/dopamine levels, is in contrast to most of the studies reviewed by Parker and Brotchie.2 While they find that in general, dietary adjustments do not affect neurotransmitter production, they found slightly more evidence that it can affect levels in “depressed patients, especially those with melancholic depression,” since they tend to have low tryptophan levels in the first place.2 This evidence largely suggests that increasing ingested tryptophan will help these patients because they are already deficient; patients with mild depression most likely already have normal tryptophan levels and therefore will not benefit from increased ingestion.2
So increasing dietary tryptophan, in combination with carbohydrate heavy meals, may affect moods in patients already prone to or suffering from depression.
Carbohydrates and Fat
According to Moin et al., who studied rats on protein rich diets, carbohydrate rich diets, and fat rich diets, and then coupled those diets with stress, “restrained stress in sugar rich diet fed rats did not alter TRP, 5-HT and 5-HIAA* levels,” while the protein rich and fat rich diet rats were highly affected by stress – “Carbohydrate consumption … increased serotonin release whereas protein [and fat] intake lacked this affect.”3
In the rat study done by Kirac, et al., similar results were found. Their goal was to study the “effect of high-fat diet on the stress-related alterations in brain biogenic amine turnover.”5 They studied four groups of rats: on a standard diet without stress, on a standard diet with stress, on a high fat diet without stress, and on a high fat diet with stress. They found that “5-HIAA levels … were lower in both high fat and high fat + stress groups compared to the stress group.”5 This was not a slight decrease in 5-HIAA levels, either – it reached a level of significance (F=4.689; P<0.05).5 Additionally, they hypothesized that the lower levels of serotonin under stress might provoke aggressive responses to stressors, which could mean that a high fat diet combined with stress does not just make a patient more depressed, but also more anxious and angry.5
As in all things, balance and moderation are the key. Having these three neurotransmitters in the correct balance is what will positively affect mood.1 As shown in various studies, it is unlikely that increasing the dietary amount of amino acids used in neurotransmitter production will actually cause excess serotonin production and reuptake.2,3 This is largely because it is not the amino acids alone that lead to neurotransmitter production – the process requires the correct sugars and the correct balance of other amino acids as well to maintain the most helpful concentration gradient in the blood-brain barrier.2,3 There is some evidence to show that a lack of these amino acids may negatively affect moods,2 and that a high fat diet may do the same.5 However, it is typically only people with a predisposition to severe depression (via genetics, past history, etc.) that have these low tryptophan and tyrosine levels.2 Therefore, while a healthy diet is always a good recommendation, dietary changes are unlikely to affect neurotransmitter production and uptake in healthy patients.
- VanPutte, CL, Regan, JL, et al. Seeley’s Anatomy and Physiology. New York: McGraw Hill; 2014:384-399.
- Parker G, Brotchie H. Mood effects of the amino acids tryptophan and tyrosine: ‘Food for Thought’ III. Acta Psychiatrica Scandinavica [serial online]. December 2011;124(6):417-426. Available from: MEDLINE with Full Text, Ipswich, MA. Accessed April 23, 2015
- Moin S, Haider S, Khaliq S, Tabassum S, Haleem D. Behavioral and Neurochemical Studies in Stressed and Unstressed Rats Fed on Protein, Carbohydrate and Fat Rich Diet. Pakistan Veterinary Journal [serial online]. June 2012;32(2):260-264. Available from: Food Science Source, Ipswich, MA. Accessed April 23, 2015.
- Choi S, DiSilvio B, Fernstrom M, Fernstrom J. Effect of chronic protein ingestion on tyrosine and tryptophan levels and catecholamine and serotonin synthesis in rat brain. Nutritional Neuroscience [serial online]. November 2011;14(6):260-267. Available from: Food Science Source, Ipswich, MA. Accessed April 23, 2015.
- Kirac D, Ozden I, Yildirim A, Genç E. Effect of high-fat intake on motor activity, homovanillic acid and 5-hydroxyindoleacetic acid levels in striatum and cortex of rats exposed to stress. Nutritional Neuroscience [serial online]. April 2009;12(2):89-94. Available from: Food Science Source, Ipswich, MA. Accessed April 23, 2015.
** Where TRP stands for tryptophan, 5-HT refers to serotonin blood levels, and as previously mentioned, 5-HIAA refers to a byproduct of serotonin breakdown