Cognitive dysfunction is defined as unusually poor mental function. This causes confusion, forgetfulness and difficulty concentrating.
Risk factors for cognitive impairment include:
- Old age
- Lack of exercise
- Diet high in animal fats
- Diet low in vegetables and fish
Unhealthy dietary habits also increase one’s risk of cardiovascular disease, hypertension and diabetes.
Diets high in fat and protein make the digestive tract more acidic, thereby increased the body’s absorption of heavy metals and aluminum. These toxins may generate free radicals in the brain. Free radicals destroy neurons and impair brain function.
Sunlight exposure and cognitive impairment risk
Ultraviolet-B (UVB) light is the primary source of vitamin D for most people. Studies have confirmed the positive effects of vitamin D on brain function, leading researchers to theorize that sun exposure may have a positive impact on cognition.
Although research regarding sun exposure and cognition is in its infancy, studies are beginning to link solar radiation to cognition. There are a few studies that highlight this relationship.
- In 2009, researchers conducted a study including 16,800 participants from a US national cohort, showing that lower levels of sun exposure resulted in a 2.58 times increased incidence of cognitive impairment.
- In 2013, the same researchers published findings from a 15-year study that evaluated the relationship between solar radiation and cognition among 19,896 adults (> 45 years) in the US. None of the participants experienced cognitive impairment at baseline. After 15 years, those who received below average sun exposure were 88% more likely to experience cognitive decline than those who received above average sun exposure. The researchers noted that possible mechanisms for cognitive decline included either lack of vitamin D or a mechanism involving circadian rhythm regulation, which manages sleep cycle.
- A 2015 study, including 65,277 individuals, found that people living in northern latitudes have a significantly increased risk for developing dementia compared to those living in southern latitudes.
Vitamin D and cognitive impairment
There is an abundance of research showcasing the role of vitamin D deficiency in cognitive impairment risk. Here are only a handful of the studies connecting vitamin D status to cognition:
- According to a 2017 systematic review and meta-analysis of 26 observational and 3 intervention studies including over 19,000 participants, low vitamin D status was associated with cognitive decline (OR: 1.26) and poorer cognitive performance (OR: 1.24) among participants without dementia. However, this study did not find a significant benefit of vitamin D supplementation on cognition.
- A study published in 2017 evaluated the role of vitamin D status in cognition among 369 individuals. The researchers found that those who were vitamin D deficient experienced a faster rate of cognitive decline. Additionally, vitamin D deficiency was associated with a nearly 3-fold increased risk of developing Alzheimer’s disease (HR: 2.85).
- A 2016 study found that vitamin D deficiency increased elderly Chinese individual’s risk of developing dementia by over twofold. In addition, dementia risk increased as vitamin D levels decreased.
- Research published in 2015 found that severe vitamin D deficiency was independently associated with future risk of mild cognitive impairment and dementia among elderly individuals. This was especially significant in those whose baseline cognitive function had decreased only modestly.
- In 2015, researchers from the Boston University School of Medicine discovered that vitamin D status is linked to poorer neurological skills but not dementia or Alzheimer’s disease.
- A study published in the Canadian Journal of Neurological Sciences found that both vitamin D insufficiency and seasonal decline of vitamin D levels are correlated with lower scores related to cognitive performance.
- A meta-analysis and review reported that lower vitamin D levels are associated with decreased cognitive function and an increased risk of Alzheimer’s disease.
- A 2015 study suggested cognitive ability of individuals with low 25(OH)D declined three times faster than that of healthy subjects with 25(OH)D above 20 ng/ml.
How vitamin D works
Vitamin D may protect the brain in the following ways:
- Reduce the risk of diseases that affect the brain (cardiovascular disease and hypertension)
- Provide antioxidative mechanisms
- Regulate calcium levels
- Regulate the immune system
- Enhance nerve conduction (signals)
- Helps rid body of toxins
Vitamin D levels above 40 ng/mL (100 nmol/L) may reduce the risk of cognitive impairment. To achieve these levels, most adults need to take at least 2000 to 5,000 international units/day of vitamin D3 (cholecalciferol). However, there is considerable variation from person to person. The vitamin D blood level should be measured before taking vitamin D supplements or increasing solar UVB exposure. Vitamin D levels should also be measured a few months after making any changes to supplementation or sun exposure routines.
Research has reported that vitamin D offered a treatment effect for cognitive impairment.
- A recent randomized, double-blind, placebo control trial found that vitamin D levels may play a crucial role in the mental health and cognitive function of young individuals.
- A RCT published in 2017 found that higher doses of vitamin D supplementation improved nonverbal (visual), specifically in individuals with inadequate baseline vitamin D levels (< 30 ng/ml; 75 nmol/l). However verbal memory and other cognitive domains did not appear to benefit significantly from supplementation.
- A 2017 animal study published by the International Journal of Obesity found that vitamin D supplementation reversed cognitive impairment induced by a high fat diet among obese rats.
- A RCT published in 2017 found vitamin D, combined with leucine and medium chain triglycerides, improved cognition among elderly adults.
- Jacqueline Petterson, a neuroscientist at the University of British Columbia, discovered that verbal fluency peaks when an individual’s vitamin D level is between 40 and 50 ng/ml (100 – 125 nmol/l).
- A pilot RCT found that vitamin D may help decrease plaque build-up in the brain in Alzheimer’s patients over the age of 60 years.
- Researchers recently published a study in Proceedings of the National Academy of Sciences of the United States of America that found supplementation with vitamin D improved hippocampal-dependent learning and memory as well as hippocampal synaptic functioning in rats.
- In a 2014 mouse model of Alzheimer’s disease (AD), supplementation with activated vitamin D reduces plaques in the brain and improves cognition.
What does this mean for me?
Research suggests vitamin D deficiency is highly prevalent among patients with cognitive impairment, and that low vitamin D status may negatively impact cognition.
Evidence supports maintaining healthy vitamin D levels (40 – 60 ng/ml; 100 – 150 nmol/l) may help improve cognition among adults and elderly individuals.
Although supplementing with vitamin D is safe for those with cognitive impairment to take, there is no guarantee that individuals will see any improvement in their symptoms.
Vitamin D should not be taken in place of other medications for your condition. Talk to your doctor about taking vitamin D or any other supplement.
How much vitamin D to take?
For adults, the Vitamin D Council recommends supplementing with 5,000 IU of vitamin D3 on days one is unable to receive safe, sensible sun exposure when their shadow is shorter than they are tall. This dosage enables the average 150-pound adult to reach 40-60 ng/ml; however, there is no evidence of additional benefits on cognition at levels above 60 ng/ml.
The only way to determine whether optimal vitamin D status is met is to have your 25(OH)D levels tested. This can be accomplished through your doctor’s office or from the comfort of your home by purchasing the Vitamin D Council’s in-home vitamin D test kit. If you are unable to reach healthy vitamin D status through daily supplementation, a higher dosage may be required. The federal government states that taking up to 10,000 IU of vitamin D a day is unlikely to do any harm. Therefore, to minimize your risk of vitamin D toxicity, the Vitamin D Council recommends avoiding daily supplementation above 10,000 IU.
Sturges, M., Grant, W.B. & Cannell, J.J. Health Condition: Vitamin D and cognitive impairment. The Vitamin D Council Blog & Newsletter. September, 2017.
 Kent ST, McClure LA, Crosson WL, Arnett DK, Wadley VG, Sathiakumar N. Effect of sunlight exposure on cognitive function among depressed and non-depressed participants: a REGARDS cross-sectional study. Environ Health. 2009 Jul 28;8:34
 Kent ST, Kabagambe EK, Wadley VG, Howard VJ, Crosson WL, Al-Hamdan MZ, Judd SE, Peace F, McClure LA. The relationship between long-term sunlight radiation and cognitive decline in the REGARDS cohort study. Int J Biometeorol. 2014 Apr;58(3):361-70.
 Matchar DB, Chei CL, Yin ZX, Koh V, Chakraborty B, Shi XM, Zeng Y. Vitamin D Levels and the Risk of Cognitive Decline in Chinese Elderly People: the Chinese Longitudinal Healthy Longevity Survey. The Gerontological Society of America, 2016.
 Miller JW, Harvey DJ, Beckett LA, Green R, Farias ST, Reed BR, Olichney JM, Mungas DM, DeCarli C. Vitamin D Status and Rates of Cognitive Decline in a Multiethnic Cohort of Older Adults. JAMA Neurol. 2015.
 BJØRN, G. et al. Linking vitamin D status, executive functioning and self-perceived mental health in adolescents through multivariate analysis: A randomized double-blind placebo control trial. Scandinavian Journal of Psychology, 2017.
 G Hajiluian, G Nameni, P Shahabi, M Mesgari-Abbasi, S Sadigh-Eteghad, M A Farhangi, Vitamin D administration, cognitive function, BBB permeability and neuro-inflammatory factors in high-fat diet induced obese rats, International Journal of Obesity, 2017.
 Sakiko Abe, Osamu Ezaki, and Motohisa Suzuki. Medium-Chain Triglycerides in Combination with Leucine and Vitamin D Benefit Cognition in Frail Elderly Adults: A Randomized Controlled Trial. 133 J Nutr Sci Vitaminol, 2017
 Miller Bj, Whisner Cm, Johnston Cs. Vitamin D Supplementation Appears To Increase Plasma Aβ40 In Vitamin D Insufficient Older Adults: A Pilot Randomized Controlled Trial. J Alzheimers Dis. 2016 Mar 31.
 Latimer, C. et al. Vitamin D prevents cognitive decline and enhances hippocampal synaptic function in aging rat. Proceedings of the National Academy of Sciences of the United States of America, 2014.
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