Newsletter #221 - Avocados for Eye and Brain Health 🥑
Hey friends, this week I thought I would take a look at research on avocados, more specifically at how they affect the health of the nervous system. A recent analysis of NHANES data from older adults found that people who consumed avocados generally had better scores on cognitive tests, after adjusting for relevant confounding factors. Why might that be?
Avocados generally have a lot going for them, from a nutritional standpoint. One whole avocado contains an estimated 26% of the daily value (DV) of vitamin K, 20% of the DV of folate, 17% of the DV of vitamin C, 14% of the DV of potassium, 14% of the DV of vitamin B5, 13% of the DV of vitamin B6, and 10% of the DV of vitamin E.
However, the most important bioactive compounds in avocados aren’t found on a nutrition label.
With respect to the brain, certain carotenoids found in avocados have emerged as being especially important. Among them, lutein and its isomers, zeaxanthin and meso-zeaxanthin, are the only carotenoids that cross the blood-brain barrier and form macular pigment in the retina. Macular pigment acts as an optical filter for blue light and provides antioxidant protection to the human retina, so it’s super important. Interestingly, macular pigment density is not just related to visual health, but it is also correlated to cognitive function, perhaps because lutein has been shown to also accumulate in the brain as well as the eyes. Supplementation with lutein, as well as higher dietary intake, has been shown to boost cognitive function in healthy individuals as well as clinical populations. And consuming it from avocado may be an even better bet, despite the fact that avocado contains a pretty modest amount of this carotenoid (see below for why).
This Week's Research Highlights
🧠 Lutein + zeaxanthin increases brain growth factors and improves cognition.
Fifty-nine young (18-25 years old) healthy subjects were randomly assigned to take either placebo or two different doses of the macular xanthophylls. After 6 months of this supplementation regimen, the researchers found that macular pigment density had increased, much as you might expect. Additionally, participants showed significantly higher serum antioxidant capacity, as well as lower levels of the pro-inflammatory cytokine IL-1β. But most compelling were the effects on their brainpower. Macular xanthophylls resulted in improvements in multiple aspects of cognitive performance, including composite memory, verbal memory, sustained attention, and processing speed. This was tied primarily to a rise in brain-derived neurotrophic factor (BDNF), which in turn appeared to be linked to the lower inflammation and higher antioxidant capacity. How come? Well, it has been observed that inducing inflammation experimentally can lower BDNF, and conditions associated with inflammation in the brain seem to coincide with lower levels of BDNF. The research team hypothesized that the macular xanthophylls were dampening an inflammatory cascade in these subjects, which facilitated greater production of BDNF and better cognitive performance.
👀 Higher dietary intake of lutein + zeaxanthin are linked to lower risk of developing age-related macular degeneration.
Researchers analyzed dietary data from two large US-based cohorts (n=102,046) who did not have diagnosed AMD at baseline. Predicted plasma carotenoid scores were calculated from repeated food frequency questionnaires, using models to account for bioavailability from different foods, and the researchers examined how these scores were associated with risk of developing AMD. Various carotenoids were linked to lower risk of developing AMD, but the strongest association was found for lutein + zeaxanthin. Compared to the lowest fifth of intake, those who consumed the most dietary lutein + zeaxanthin had 40% lower risk of advanced AMD
🥑 Avocados may increase serum lutein and macular pigment density more efficiently than supplementation.
Healthy older adults were randomly assigned to one of two groups: 1) one daily avocado (experimental), or 2) the same amount of calories in the form of one daily potato or cup of chickpeas (control). After six months, the avocado group showed a 25% increase in serum lutein (0.93 nmol/L), as well as significantly increased macular pigment density (0.101 optical density). They also showed significant improvements in memory and executive function.
But here is where things get a bit interesting. As I alluded to earlier, the lutein content in avocado is actually pretty pedestrian, especially compared to other edible plants - around 0.6 mg for a whole one.
Yet a previous study from the same research team found that directly supplementing lutein at a dose of 12 mg/day only increased serum lutein by 0.22 nmol/L and macular pigment density by 0.041 optical density. Therefore, even though avocados aren’t a particularly impressive source of lutein, and contained a mere fraction of the amount of lutein found in a supplement, they were shown to be far more effective at increasing serum lutein and macular pigment density. Good example of how it’s not just what you take in, but what you absorb, that matters for diet outcomes.
🥑 Avocados may enhance absorption of lutein and zeaxanthin in the nervous system through their monounsaturated fatty acids.
Researchers recruited eleven healthy subjects and put them through two experiments. In Study 1, participants visited the lab on two different occasions. On one, they consumed chunky salsa with 150 grams of avocado, and on the other they had the salsa without avocado (or any other fat source). In Study 2, the format was similar except they had a salad with carrots and spinach that was served either with or without avocado/avocado oil. When the participants consumed the meals with avocado or avocado oil, absorption of carotenoids (determined by subsequent bloodwork) was increased up to 15-fold, and absorption of lutein specifically was increased by more than 5-fold. This may be because avocados are extraordinarily rich in monounsaturated fatty acids, which have been shown to dramatically increase uptake of carotenoids, even more than polyunsaturated fats or saturated fats. Why this is the case is not entirely clear. Monounsaturated fats do tend to have more favorable effects on HDL, and it is thought that HDL is the preferred carrier of macular carotenoids to the nervous system.
🧠 A daily avocado may boost aspects of cognitive performance outside of lutein content.
Researchers recruited 84 adults with overweight/obesity and randomly assigned them to consume a daily meal with fresh avocado, or a meal with the same number of calories but without avocado (control). After 12 weeks, participants in the avocado group showed increases in serum lutein levels, as well as improvements in a cognitive test meant to assess attentional control. However, further analysis showed that improvements in cognition were independent of changes in lutein concentrations, suggesting that other components of the avocado may have mediated its influence on cognitive function. This isn’t necessarily surprising. Whole plants like avocados contain hundreds of bioactive compounds, some of which work synergistically. That is one reason why supplements will probably never be a perfect substitute for whole foods, even as our understanding of underlying mechanisms gets better and better.
Random Trivia & Weird News
🔪 Avocado-related knife injuries, referred to medically as “Avocado Hand,” have risen dramatically over the past 25 years.
An analysis of emergency room data found that from 1998-2002, there were an estimated 3,143 avocado-related knife injuries. From 2013-2017, that number had skyrocketed to 27,059.
Injuries were usually to the left hand, likely because most people cut using their right hand, and sometimes incurred long-term damage to tendons. You don’t want to do a google image search on this.
As someone who had to pay a visit to the ER a few years ago due to this sort of thing (in my case it was a fennel bulb), I would strongly recommend caution when involved with food prep.
Podcasts We Loved This Week
- Michael Gershon: The gut-brain connection. Via The Drive.
- Thomas Insel: Why mental illness is a medical problem that requires social solutions. Via Ezra Klein.
Products We Are Enjoying
Precision Digital Kitchen Weight Scale
I've tested out a bunch of different kitchen scales and this is by far the best. It is accurate to a tenth of a gram, making it ideal for baking or for measuring powder supplements like creatine, and it is reasonably priced and easy to use.
humanOS Catalog Feature of the Week
What Components Make the Mediterranean Diet So Healthy?
This week, we’d like to highlight one of the courses from our Mediterranean Diet Program. In this course, we examine specific components that are prominently featured in the Mediterranean diet, and explain potential mechanisms through which these dietary components might enhance health and lower risk of disease. Obviously, one of the hallmarks of the Mediterranean diet is olive oil (and olives themselves), which is a major source of monounsaturated fat, and thus is rather similar to avocados. Check out the course if you want to get a little more insight into how these fatty acids work.
Thanks for reading, enjoy the weekend, and we'll see y'all next week!