Scott TM, Rasmussen HM, Chen O, Johnson EJ. Avocado consumption increases macular pigment density in older adults: a randomized, controlled trial. Nutrients. 2017;9(9):E919.
To test the effects of avocado consumption on cognitive performance via macular pigment density (MPD) changes associated with lutein concentration in macular and brain tissue. A secondary objective was to evaluate potential antioxidation, antiinflammation, and lipoprotein profile changes attributable to avocado consumption.
Six-month randomized controlled trial
Participants in the avocado-consuming (AV) group (n=20) consumed 1 avocado per day (0.5 mg/d lutein), while the control group (n=20) consumed either 1 potato or 1 cup of chickpeas (0 mg/d lutein) per day.
Healthy men and women, average age of 63 years, who had not been taking any supplements 2 months prior to participation; 40 of the 48 original participants completed the study (83%). Criteria for study inclusion were low intakes of lutein-rich foods (less than 3 servings per week of green leafy vegetables, broccoli, eggs). Persons on the following medications were excluded: prescription steroids; antipsychotic, antimanic, or anti-inflammatory agents; monoamine inhibitors; medications that interfere with fat absorption; and dementia medications. Other exclusions included allergy to avocado, chickpeas, or potatoes; history of liver, kidney, or pancreatic disease; anemia; and active bowel disease or resection.
Study Parameters Assessed
The following parameters were evaluated at 0, 3, and 6 months: MPD (a biomarker of neural incorporation of lutein into brain tissue), serum lutein, markers of oxidative stress (oxidized low-density lipoprotein [LDL]), inflammatory markers (β-amyloid and C-reactive protein), and cognitive abilities, including memory, attention, working spatial memory, and problem-solving speed and efficiency.
Primary Outcome Measures
Change in serum lutein, MPD, and measures of cognitive ability from baseline to 6 months.
There were no changes in markers of oxidative stress or inflammation in either the control or test participants during the study period. Serum lutein concentration increased by more than 25% from baseline at both 3 and 6 months for the AV group (P= 0.001) vs 15% in the control group (P=0.030). The concentration of lutein was particularly high in neural tissue. The concentration of serum lutein in the AV group increased 0.93 nmol/L per mg lutein contained in avocado. At the conclusion of the 6-month period MPD increased by more than 25% from baseline in the AV group (P=0.001) vs no increase in the control group. This was related to improved working memory and efficiency in approaching a problem (P=0.036).
Evidence suggests the addition of oils or other sources of monounsaturated fats (oleic acid) to foods rich in lutein may increase its uptake into neural tissue.
There were no changes in serum zeaxanthin throughout the study period in the AV group. Triglycerides fell from baseline in the AV group (P=0.075) and rose from baseline in the control group (P=0.06); high-density lipoprotein (HDL) was correlated with the change in both serum lutein and zeaxanthin (Pearson r=0.43; P=0.058 and Pearson r=0.54; P=0.014, respectively) in the AV group only.
The authors of this paper previously reported on cognitive health and MPD which is also associated with higher lutein status as a marker for visual function and health.1-4 Lutein is the carotenoid most consistently linked to cognitive funtion. It is preferentially taken up into brain tissue.5 Although oxidative stress and inflammatory markers have been proposed as possible mechanisms for lutein’s beneficial effects on the brain, in this study they were not associated. Since these biomarkers were in normal ranges at the start of the study, changes may have been more difficult to detect.
As a possible explanation for lutein-induced cognitive benefits, the researchers referenced Carotenoids in Health and Disease, which hypothesizes that “the modulation of synaptic membranes, along with certain changes in the physiochemical and structural features of these membranes” may be at play.6 In other words, perhaps transport of beneficial nutrients like lutein into the brain requires certain types of fats to positively impact cognitive function.
Funding by the Haas Avocado Board aside, the evidence is compelling for much higher bioavailability of lutein in avocados.7 The amount of lutein contained in avocados is relatively small (0.5 mg/medium avocado); however, in the present study consumption of a single avocado per day effectively and substantially increased serum lutein, with a 0.93 nmol increase per mg lutein contained in avocado. By comparison, a previous study by the same authors found that lutein supplementation (12 mg/day for 4 months) increased serum lutein by only 0.22 nmol/L per mg lutein.8 A limitation of the study was the lack of comparison with more commonly consumed vegetables containing higher levels of luteins, particularly dark green leafy vegetables like spinach. The superior bioavailability of lutein contained in avocados is attributed to its lipid content (ie, monounsaturated fatty acids [MUFAs]). This was evidenced by earlier findings of the same team, which demonstrated lutein’s higher bioavailability in eggs vs spinach.9
The above evidence suggests the addition of oils or other sources of monounsaturated fats (oleic acid) to foods rich in lutein may increase its uptake into neural tissue. In addition to avocados, foods rich in MUFAs include olives, nuts, and some seeds. Those that offer the highest MUFA content include high-oleic sunflower or safflower oil and hazelnut, olive, canola, avocado, almond, peanut, sesame, rice bran, soybean, and cod liver oils. Adding these oils to salads makes perfect sense from this perspective.
The macula is a yellow spot near the center of the retina that is responsible for high-resolution vision. Macular pigment is composed of lutein, zeaxanthin, and meso-zeaxanthin. Meso-zeaxanthin is not available from the diet and must be made within the retina from dietary lutein. Macular pigment acts as a natural blue light filter and protects the eye from damage and prevention of age-related macular degeneration.10 There is concern that overexposure to blue light, all of which penetrates the cornea and lens to the retina, can lead to macular degeneration and ultimately vision loss. Most blue light originates from the sun, but there are many indoor sources of blue light that have eye care providers concerned, particularly blue light exposure from computer screens, smartphones, and other digital devices, which are also used in close proximity to the face and eyes. These frequent and extensive exposures might increase a person's risk of macular degeneration later in life. The use of blue light filters can help to reduce these type of exposures.11
However, some blue light is helpful for regulating circadian rhythms, which serve as our body clock. Research has shown that high-energy visible light boosts alertness, helps memory and cognitive function, and elevates mood, hence the popularity of full-spectrum light therapy.11 Nevertheless, research indicates there is an association between macular degeneration and cognitive decline.12 The present study supports consumption of lutein-rich foods in conjunction with MUFA oils to enhance natural blue light filtration within the macula. Given the evidence, it also makes sense to consider protection from excessive blue light.