Eating for good mental health
The human brain has come a long way over the last 1.9 million years. Evolutionary biology offers abundant evidence of the impact of diet on cognition. To understand the link between the brain and nutrition, we need to look at the reward systems in our brains, the mechanisms of macronutrients, and the case for ‘brain foods’.
Reward systems and amino acid transport
Your eyes are eating all the time - it’s hard to resist those glazed doughnuts! That’s biology at play. Genes evolved during our evolution on the African Savanna a few million years ago. They were well suited to extracting salt, sugar, fat, and protein from this nutritionally spartan environment. But the downside is that these same genes are often a liability in the modern “obesogenic” environment where an excess of these nutrients is more likely to contribute to our demise, not our survival.
You might have heard of serotonin, the ‘feel-good’ chemical produced in our brains. The gene that transports serotonin into the ‘mesolimbic reward centre’ of the human brain comes in two variants - short, which is less active, and long which is more active and normally dominant. Stressful life events can cause a switch from long to short and trigger mental health problems like depression and anxiety. A widely used treatment for these is to prolong the effect of serotonin in our brains using selective serotonin reuptake inhibitors (SSRIs - aka Prozac, Zoloft, Celapram, and others).
To produce this serotonin, our brain needs an amino acid called tryptophan, but tryptophan “competes” with other amino acids (particularly the branched chain amino acids (BCAAs) - leucine, isoleucine, and valine), so when these are abundant in the diet, they displace tryptophan from the transporter molecule resulting in lower levels of serotonin where it’s needed most - in the brain. Fortunately nature tends to dish up the right balance of amino acids in the main proteins we consume, so imbalances are rare, unless you happen to supplement BCAAs in a vain attempt to build muscle. For the record, salmon is a good source of tryptophan, as is poultry, eggs, spinach, nuts/seeds, milk, and soy products.
The brain is also dependent on glucose as its primary energy source. The brain can switch to ketones during starvation, but ketones are “neurotoxic” which explains why people on hunger strikes suffer devastating, sometimes irreversible brain damage. With all the hype surrounding 'low carb' diets, it’s interesting to look at the hard data - the science - in relation to carbs and health. As the graphic above shows, the lowest health risk occurs when around 50% of one's energy is derived from carbohydrates.¹ This is about average for most western diets, but it’s not carbs that are the issue… it’s refined carbs. So keep 'processed' cereals and grains to a minimum.
Why care about psychonutrition?
Not only can we damage our own brain health due to poor nutrition, we can pass this risk on to our children. Modern research in epigenetics is enhancing our understanding of the centuries-old Lamarck vs Darwin debate. Lamarck posited that changes in traits an organism undergoes during its lifetime can be passed on to its offspring. Darwin overthrew this concept - emphatically. Several new studies however are now pointing to the possibility that the effects of diet on mental health can be transmitted across generations.
Consider, for example, the 1944-45 Dutch famine during the German occupation in World War II. Studies during the famine have reported that women exposed to the famine during early gestation gave birth to smaller babies who had a predisposition to developing obesity later in life. But the unexpected longer term finding was that these changes persisted in future generations - the grandchildren of these Dutch women carry the legacy of that famine.² From an evolutionary perspective, this seems to make sense - having offspring that store fat increases survival during droughts and famines.
Getting it right
The benefits of good brain health are clear, most notably better concentration, improved retention span, and higher cognitive flexibility. But while our ancestors benefited from these by shifting to diets rich in ‘good fats’, the consumption of ‘bad fats’ has increased dramatically over the past century, while that of omega-3 fatty acids has decreased.³ This might shed light on the elevated incidence of major depression in countries such as the United States and Germany. The chart below shows the correlation between per capita fish consumption and the prevalence of depression.⁴ The trend is clear, but as the saying goes - “correlation is not causation.” Since our bodies cannot produce omega 3 fatty acids on their own, it is vital to include these in our diet and limit saturated fats found in foods like pork, fatty beef, and dairy products. The American Heart Association recommends limiting saturated fat to 5-6 percent of calories.⁵ Beware to not replace saturated fat with refined carbs though, since it can raise the ‘bad’ LDL cholesterol and lower the ‘good’ HDL cholesterol.⁶ The end result will be just as bad for cardiovascular health as consuming excess saturated fat.
The Psychological Health Explorer in SHAPE is entirely dedicated to mental health and wellbeing. It analyses an array of mental health problems in employees and offers guidelines for coping with them effectively.
¹Seidelmann, Sara B., Brian Claggett, Susan Cheng, Mir Henglin, Amil Shah, Lyn M. Steffen, Aaron R. Folsom, Eric B. Rimm, Walter C. Willett, and Scott D. Solomon. 2018. ‘Dietary Carbohydrate Intake and Mortality: A Prospective Cohort Study and Meta-Analysis’. The Lancet Public Health 3 (9): e419–28. https://doi.org/10.1016/S2468-2667(18)30135-X.
²Ravelli, Anita CJ, Jan HP van der Meulen, Clive Osmond, David JP Barker, and Otto P Bleker. 1999. ‘Obesity at the Age of 50 y in Men and Women Exposed to Famine Prenatally’. The American Journal of Clinical Nutrition 70 (5): 811–16. https://doi.org/10.1093/ajcn/70.5.811.
³Gómez-Pinilla, Fernando. 2008. ‘Brain Foods: The Effects of Nutrients on Brain Function’. Nature Reviews. Neuroscience 9 (7): 568–78. https://doi.org/10.1038/nrn2421.
⁴Hibbeln, Joseph R. 1998. ‘Fish Consumption and Major Depression’. The Lancet 351 (9110): 1213. https://doi.org/10.1016/S0140-6736(05)79168-6.
⁵‘Saturated Fat’. n.d. American Heart Association. Accessed 5 April 2021. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/saturated-fats.
⁶Boston, 677 Huntington Avenue, and Ma 02115 +1495‑1000. 2014. ‘Types of Fat’. The Nutrition Source. 9 June 2014. https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fats-and-cholesterol/types-of-fat/.