Carotenoids are orange/yellow pigments found in a variety of foods, such as carrots, pumpkins, avocados, and salmon. While carotenoids are an extremely diverse group of organic pigments, with over 1,100 known carotenoids , this overview will focus on lutein and zeaxanthin, which are of particular interest due to their special roles in eye and brain health.
Carotenoids are one of the most common classes of bioactive compounds found in plants, along with alkaloids (e.g., berberine), organosulfur compounds (e.g., sulforaphane), and polyphenols (e.g., anthocyanins). Carotenoid molecules are characterized by long chains of carbon atoms connected to each other with double bonds. Because of this unique electron structure, carotenoids are able to donate multiple electrons to free radicals, quenching oxidative stress.
Yellow and orange carotenoids absorb light in the high-frequency blue/violet range, making them particularly well-suited for protecting the body from ultraviolet radiation, which creates hydroxyl and oxygen radicals that can damage DNA. In plants, carotenoids participate in photosynthesis by capturing energy from photons. Cleverly, the human body accumulates carotenoids in large quantities in the retina of the eye to protect from photodamage. Carotenoids also accumulate in the brain, where they protect tissues from other sources of oxidation.
A typical American diet contains between one and three milligrams per day of lutein and zeaxanthin from foods such as eggs, green leafy vegetables, and other vegetables; however, this amount may be suboptimal for disease prevention. Some researchers have proposed the creation of a daily reference intake (DRI) for lutein and zeaxanthin, believing that the existing research is sufficient to support a dose-dependent relationship between lutein intake and the risk of degenerative eye diseases. Among large clinical studies, lutein intake between three and five milligrams per day was most effective for reducing risk of age-related macular degeneration, a leading cause of vision loss in older adults. Eating more carotenoid-rich foods (e.g,, spinach, kale, parsley) and cooking carotenoid-rich foods with fat (e.g., olive oil, ghee, cream) are both effective in raising serum carotenoid levels, which are associated with lower macular degeneration risk
In addition to reducing the risk of macular degeneration, high serum carotenoid levels are also associated with a lower risk of obesity and insulin resistance, inflammation and cognitive dysfunction, and premature death from any cause.
Below is a selection of summaries from current research investigating the effects of carotenoids on health.
Cognitive function, including complex executive functions like working memory and basic functions like sensory processing, progressively declines with age. While executive function loss is highly variable and easily measurable in older adult populations, younger adults usually perform at a level consistent with their peers, making studying cognitive decline in younger adults difficult. In a 2014 report, researchers measured visual processing ability in young adults before and after supplementation with lutein and zeaxanthin.
Visual processing refers to the brain’s ability to utilize and interpret visual information. Because visual processing utilizes similar brain architecture as more complex tasks such as working memory, it is a useful measure in assessing brain health and cognitive decline.
Researchers measured the baseline visual processing speed and retinal concentration of lutein and zeaxanthin in healthy young adults (average age, 22 years). They assigned participants to consume a placebo, zeaxanthin only (20 milligrams), or a combination of zeaxanthin (26 milligrams), lutein (8 milligrams), and mixed omega-3 fatty acids (190 milligrams) per day for four months. They measured retina pigmentation and visual processing speed again following the intervention.
The authors reported a moderate, yet statistically significant, relationship between baseline retinal pigment levels and visual processing speed. Following the intervention, both supplement groups demonstrated a significant increase in retinal pigmentation compared to placebo. Finally, participants in the supplement groups also performed 12 percent better on the critical flicker fusion test and decreased visual motor reaction time by 10 percent, two measures of visual processing. The authors concluded that lutein and zeaxanthin supplementation may be an effective way to increase visual processing speed, even in young healthy adults.
Increasing "screen time," the amount of time a person spends using electronic screens (e.g., mobile phones, tablets, computers, TVs), is a public health concern. In addition to promoting sedentary activity, electronic screens emit bright blue light, which may have damaging effects on eye health.
In a randomized, double-blind trial, young adult participants with high screen use (six hours per day or more) took either a supplement with lutein and zeaxanthin (24 milligrams per day) or a placebo for six months. Carotenoid supplementation significantly increased macular pigment optical density, a measure of the intensity of yellow color and carotenoid concentration in the macula of the retina. Supplementation improved visual performance and sleep quality and reduced eye strain, eye fatigue, and headache frequency.
A progressive loss of cognitive function is common with aging; however, previous research suggests that a higher intake of dietary carotenoids may preserve cognitive function in older adults. Findings of one report suggest that high carotenoid levels protect the brain from aging.
Researchers recruited 60 participants between the ages of 25 and 45 in order to determine the relationship between retinal carotenoid levels and cognitive function among young and middle-aged adults. They measured participants' macular pigment optical density (MPOD), a visual measurement of carotenoid concentration in the retina, and electrical data from electrodes worn by participants on their scalps as they completed computer tasks to measure multiple indices of attentional control (i.e.,selective attention, attentional inhibition, and response inhibition).
Younger participants scored better on attentional control tasks compared to older participants, as did participants with higher MPOD. Upon statistical analysis, MPOD was a better predictor of high attentional control than age. These findings suggest that maintaining higher carotenoid levels throughout adulthood may protect the aging brain.
Visceral fat is body fat that is stored in the abdominal cavity close to internal organs such as the liver, pancreas, and intestines. In contrast to subcutaneous fat, which is located under the skin, visceral fat plays a central role in the interrelationship between obesity and systemic inflammation through its secretion of proinflammatory cytokines. The accumulation of visceral fat is linked to type 2 diabetes, insulin resistance, inflammatory diseases, certain types of cancer, cardiovascular disease, and other obesity-related diseases.
For one randomized, double-blinded, controlled clinical trial, researchers recruited 28 males between the ages of 40 and 65 years who had overweight or obesity (BMI greater than 25). The researchers randomly assigned participants to consume a beverage containing one of four edible pastes that contained high lycopene/high lutein; high lycopene/low lutein; low lycopene/high lutein; or low lycopene/low lutein. The authors measured the levels of carotenoids in the participants' serum at the start of the study and after eight weeks.
The participants' carotenoid levels increased in every group and they experienced no adverse effects. Their visceral fat levels decreased for all groups, too, but waist circumference decreased only for the participants in the high lycopene/low lutein group. These findings suggest that high carotenoid intake can help with weight loss. They also support epidemiological data indicating that vegetable intake can play a positive role in modulating body weight.
Low-density lipoproteins (LDL) are formed in the liver and transport lipid molecules to cells. Often referred to as the “bad cholesterol,” LDL can drive cardiovascular disease if it becomes oxidized within the walls of arteries. LDL particles exist in different sizes, ranging from large, “fluffy” molecules to small, dense molecules. Scientific evidence suggests that small, dense LDL particles are more susceptible to oxidative modification. Findings from one clinical trial suggest that diets that include avocados may help reduce LDL oxidation.
Avocados are rich sources of monounsaturated fatty acids. They also contain polyphenols and lutein, a carotenoid compound that quenches and scavenges reactive oxygen species.
The randomized, controlled trial involved 45 men and women between the ages of 21 and 70 years. The participants, who had overweight or obesity and had elevated LDL cholesterol levels, followed three different diets for a period of five weeks each: a low-fat diet, a medium-fat diet with avocado, and a medium-fat diet with oleic acids (found in olive and canola oils).
After five weeks on the diet with avocado, the participants' levels of oxidized LDL cholesterol (especially the small, dense LDL cholesterol particles) were lower than their baseline levels or after completing the low- or moderate-fat diets. Concentrations of large, fluffy LDL particles were unchanged. Participants also had higher levels of lutein. These findings suggest that consuming avocados as part of an overall heart-healthy diet may reduce the risk of developing cardiovascular disease.
Q: Can antioxidant supplements suppress the benefits of strenuous exercise? A: Dose, timing, and mechanism all make a difference. The strongest data for supplements that suppress exercise benefits is for the vitamin E in the alpha-tocopherol form in doses greater than 400 International Units per day. Some studies have shown that vitamin C causes exercise suppression, but most of these studies combine vitamin C with alpha-tocopherol vitamin E. Vitamins C and E are direct antioxidants, meaning they directly donate electrons to free radicals. Other antioxidants work indirectly by activating the production of endogenous antioxidants, such as the carotenoid lycopene, which stimulates the Nrf2 pathway. These indirect antioxidants have a lower risk of suppressing exercise-induced adaptations. There is no evidence that consuming vitamin E-, vitamin C-, or antioxidant-rich foods blunts adaptations to exercise. On the contrary, in this study, astaxanthin-, β-carotene-, and resveratrol-rich foods supported resistance training-induced strength and metabolic adaptations.
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