Since long-term controlled clinical studies on the effects of diet on human behavior are logistically difficult to conduct, most published studies are either anecdotal in nature or epidemiologic and plagued with poor design. Epidemiologic studies generally require subjects or sometimes an acquaintance of the subject to fill out questionnaires that involve recalling past events, a potential source of error. This is the major reason for inconsistencies in the conclusions of different reports. Despite these problems, a careful meta-analysis of such research often yields a fairly accurate idea of the role diet may play in human behavior. Let’s take a specific example, much has been written about the alleged relationship between natural sweeteners in the diet and hyperactivity in children and criminality in adults. Some of these studies suggest that excessive ingestion of simple natural sweeteners (i.e., sucrose and glucose that can be present in honey, mangoes, watermelon, raw chocolates) has profound effects on human physiology, behavior, and intellectual functions. Other studies have found no effect on blood glucose regulation, mood, bodily symptoms, or cognitive function in persons reporting two or more years of heavy consumption of natural sweeteners. The truth probably lies somewhere between these extremes. The behavioral sensitivity to natural sweeteners and other dietary constituents may vary from individual to individual, and the effects on behavior may be more subtle than many investigators realize or wish to acknowledge. 

Lifestyle Changes and Their Direct Effects

In contrast, dietary changes capable of having direct effects on brain neurotransmitter make-up may exert profound effects on human behavior. These include substances that act as neurotransmitter precursors (e.g., tyrosine, tryptophan, choline, and lecithin) or modulators of post-receptor events (e.g., caffeine). The different behavioral effects of diet and lifestyle on humans involve changes in sleep patterns, perception of pain, and mood, to name a few.


The amino acid composition of a single meal designed to affect brain tryptophan availability has been shown to modulate sleep behavior in newborns. Similarly, many studies conducted on adult humans receiving various dosages of tryptophan by different routes of administration (10-500 mg/kg, orally, to much lower intravenous doses) show that tryptophan increases drowsiness and feelings of fatigue and lethargy. In healthy female adult humans that consume good well balanced raw food, one week of daily consumption of a low-carbohydrate diet (50 g/day) caused a significant increase in REM sleep latency (66 � 8 to 111 � 38, P<0.05). In contrast, when the relationship between a 5-day self reported dietary history and the response to a sleep questionnaire was analyzed, no link between sleep and diet was found in adults. This suggests that it may be necessary to use high levels of a dietary lifestyle or to institute major dietary changes to see an effect on behavior.

Pain Perception

In a controlled double-blind crossover study, 50 mg/kg oral tryptophan reduced pain sensitivity to moderate but not to very mild or very painful stimuli. In a double-blind study, patients with chronic maxillofacial pain reported a 50% reduction in pain after 4 weeks of a high-carbohydrate diet and 3 g oral tryptophan daily; in contrast, the placebo group reported only a 20% reduction in pain. In another study, tryptophan-treated patients undergoing rhizotomy and cordotomy also reported a reduction in chronic pain. In a preliminary study, tryptophan has also been shown to potentiate electrically induced endorphin analgesia.


The association between food intake or eating patterns and mood is of great interest to scientists and lay persons alike. In addition to an abundance of anecdotal reports in the literature, there is considerable scientific evidence demonstrating that food influences mood and performance. It has been suggested that many patients consume large quantities of carbohydrate-rich food to elevate their mood; these include those diagnosed with conditions with a significant depressive component such as seasonal affective disorder (or winter depression), premenstrual stress syndrome, or nicotine withdrawal. Consumption of carbohydrate-rich food may elevate mood in such individuals by raising brain serotonin levels. Dexfenfluramine, an agent known to facilitate brain serotoninergic activity, is also known to elevate mood state in all of the above-mentioned conditions. Incidentally, vitamin D3 has been shown to enhance mood in healthy subjects prone to winter depression. It remains to be seen whether vitamin D3 acts by raising brain serotonin levels or through some other mechanism.

Concluding Thoughts

The relationship between food and mood in seemingly normal individuals is not a simple one but instead depends on the time of day, the type and macronutrient composition of food, the amount of food consumed, and the age and dietary history of the subject. For example, while skipping breakfast impairs cognitive performance, a larger-than-normal breakfast improves recall performance but impairs concentration. Furthermore, changes in the macronutrient composition of breakfast have differential effects on mood. For example, changes in mood have been produced most effectively by raising brain serotonin levels by administering tryptophan or by supplementing a carbohydrate-rich/protein-poor diet with tryptophan. I have discussed the theoretical basis for this approach in greater detail earlier. Please read other articles such as: Dietary Influences on Brain Chemistry for better understanding of this topic.

By TTS Cofounder Botanical Chef Omid Jaffari

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