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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.
Objective
To test the effects of avocado consumption on cognitive performance via changes in macular pigment density (MPD) associated with lutein concentration in macular and brain tissue. A secondary objective was to evaluate possible antioxidant, anti-inflammatory and lipoprotein profile changes attributable to avocado consumption.
Draft
Six-month randomized controlled trial
intervention
Participants in the avocado-consuming (AV) group (n=20) consumed 1 avocado per day (0.5 mg/day lutein), while the control group (n=20) consumed either 1 potato or 1 cup of chickpeas (0 mg/day). d lutein) per day.
Participant
Healthy men and women, mean age 63 years, who had not taken dietary supplements 2 months before participation; 40 of the 48 original participants completed the study (83%). Criteria for study inclusion were low intake of lutein-rich foods (less than 3 servings of green leafy vegetables, broccoli, eggs per week). Individuals taking 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, oxidative stress markers (oxidized low-density lipoprotein [LDL]), inflammatory markers (β-amyloid and C-reactive protein) and cognitive ability, including memory, attention, spatial working 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.
Key insights
There were no changes in markers of oxidative stress or inflammation in either control or test participants during the study period. Serum lutein concentrations increased by more than 25% from baseline in the AV group at both 3 and 6 months (P= 0.001) compared to 15% in the control group (P=0.030). The concentration of lutein in nerve tissue was particularly high. The concentration of serum lutein in the AV group increased by 0.93 nmol/L per mg of lutein contained in avocado. At the end of the 6-month period, MPD increased 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 a more efficient approach to a problem (P=0.036).
There is evidence that adding oils or other sources of monounsaturated fats (oleic acid) to lutein-rich foods may increase its uptake into nervous tissue.
There were no changes in serum zeaxanthin in the AV group during the study period. Triglycerides fell from baseline in the AV group (P=0.075) and increased compared to the baseline value in the control group (P=0.06); High-density lipoprotein (HDL) correlated with the change in both serum lutein and zeaxanthin (Pearsonr=0.43;P=0.058 and Pearsonr=0.54;P=0.014) only in the AV group.
Practice implications
The authors of this article previously reported cognitive health and MPD also being associated with higher lutein status as a marker of visual function and health.1-4Lutein is the carotenoid most consistently linked to cognitive function. It is preferentially absorbed into the brain tissue.5Although oxidative stress and inflammatory markers have been suggested as possible mechanisms for lutein's beneficial effects on the brain, they were not linked in this study. Because these biomarkers were within normal ranges at the start of the study, changes may have been more difficult to detect.
The researchers pointed to lutein-induced cognitive benefits as a possible explanationCarotenoids in health and diseasewhich hypothesizes that “modulation of synaptic membranes, along with certain changes in the physiochemical and structural features of these membranes” may play a role.6In other words, perhaps transporting beneficial nutrients like lutein to the brain requires certain types of fats to positively impact cognitive function.
Aside from funding from the Haas Avocado Board, the evidence for much higher bioavailability of lutein in avocados is compelling.7The 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 significantly increased serum lutein, with an increase of 0.93 nmol per mg of lutein contained in the avocado. In 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 of lutein.8One limitation of the study was the lack of comparison with more commonly consumed vegetables with higher lutein content, particularly dark green leafy vegetables such as spinach. The superior bioavailability of lutein contained in avocados is attributed to its lipid content (i.e. monounsaturated fatty acids [MUFAs]). This was supported by previous results from the same team showing the higher bioavailability of lutein in eggs compared to spinach.9
The above evidence suggests that adding oils or other sources of monounsaturated fats (oleic acid) to lutein-rich foods may increase its uptake into nervous 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, as well as hazelnut, olive, canola, avocado, almond, peanut, sesame, rice bran, soybean and cod liver oil. 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. The macular pigment consists of lutein, zeaxanthin and meso-zeaxanthin. Meso-Zeaxanthin is not available through food and must be made in the retina from dietary lutein. Macular pigment acts as a natural blue light filter and protects the eye from damage and prevents age-related macular degeneration.10There 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 comes from the sun, but there are many indoor sources of blue light that concern ophthalmologists, particularly blue light exposure from computer screens, smartphones and other digital devices that are also used in close proximity to the eyes. These frequent and extensive exposures can increase a person's risk of macular degeneration later in life. Using blue light filters can help reduce these types of exposures.11
However, some blue light is helpful in regulating the circadian rhythm, which serves as our internal clock. Research has shown that high-energy visible light increases alertness, supports memory and cognitive function, and elevates mood, hence the popularity of full-spectrum light therapy.11Nevertheless, research shows that there is a connection between macular degeneration and cognitive decline.12The present study supports the consumption of lutein-rich foods in conjunction with MUFA oils to improve natural blue light filtering in the macula. Given the evidence, it also makes sense to consider protection from excessive blue light.