NCCIH Clinical Digest

for health professionals

Dietary Supplements and Cognitive Function, Dementia, and Alzheimer’s Disease: What the Science Says

November 2020

Clinical Guidelines, Scientific Literature, Info for Patients: 
Dietary Supplements and Cognitive Function, Dementia, and Alzheimer’s Disease

A senior Hispanic man in his 80s sitting in a wheelchair, talking to his adult daughter, who is in her 30s, pushing the wheelchair from behind. They are walking in a city park in fall, building out of focus in the background.

Ginkgo biloba

There’s no conclusive evidence that Ginkgo biloba is efficacious in preventing or slowing dementia or cognitive decline.

What Does the Research Show?

  • A large, placebo-controlled, randomized clinical trial studying the well-characterized ginkgo product EGb-761 in more than 3,000 older adults found it ineffective in lowering the overall incidence of dementia and Alzheimer’s disease. Further analysis of the same data also found ginkgo to be ineffective in slowing cognitive decline, lowering blood pressure, or reducing the incidence of hypertension.
  • However, a 2015 systematic review and meta-analysis involving a total of 2,561 participants concluded that Ginkgo biloba EGb-761 at 240 mg/day stabilized or slowed decline in cognition, function, behavior, and global change at 22 to 26 weeks in cognitive impairment and dementia, especially for people with neuropsychiatric symptoms.
  • Another 2016 systematic review and meta-analysis of 21 randomized controlled trials involving 2,608 participants concluded that Ginkgo biloba is potentially beneficial for improving cognitive function, activities of daily living, and global clinical assessment in people with mild cognitive impairment or Alzheimer’s disease; however, because of small sample size, inconsistent findings and methodological quality of studies included in the review, more research is need to confirm the effectiveness and safety of Ginkgo biloba for the treatment of cognitive impairment and Alzheimer’s disease.

Safety

  • Side effects of ginkgo supplements may include headache, nausea, gastrointestinal upset, diarrhea, dizziness, or allergic skin reactions. More severe allergic reactions have occasionally been reported. There are some data to suggest that ginkgo can increase bleeding risk.

Omega-3s

Among the nutritional and dietary factors studied to prevent cognitive decline in older adults, the most consistent positive research findings are for omega-3 fatty acids, often measured as how much fish is consumed in the diet. However, several high-quality reviews have found no convincing evidence for the efficacy of omega-3 polyunsaturated fatty acid supplements in the treatment of mild-to-moderate Alzheimer’s disease.

What Does the Research Show?

  • Among the nutritional and dietary factors studied to prevent cognitive decline in older adults, the most consistent positive research findings are for omega-3 fatty acids, often measured as how much fish people consumed. However, a 2016 Cochrane review of three randomized, placebo-controlled trials involving a total of 632 participants found no convincing evidence for the efficacy of omega-3 polyunsaturated fatty acid supplements in the treatment of mild-to-moderate Alzheimer’s disease. The result was consistent for all outcomes relevant for people with dementia.
  • Further, a 2012 Cochrane review of three randomized controlled trials involving more than 3,500 participants concluded that available evidence shows no benefit of omega-3 supplementation on the cognitive functioning of older people without dementia.

Safety

  • Omega-3 fatty acid supplements usually do not have negative side effects. When side effects do occur, they typically consist of minor gastrointestinal symptoms. Omega-3 supplements may extend bleeding time. People who take anticoagulants or nonsteroidal anti-inflammatory drugs should use caution.

Vitamin E

A 2017 Cochrane systematic review found no evidence that vitamin E given to people with mild cognitive impairment prevents progression to dementia, or that it improves cognitive function in people with mild cognitive impairment or dementia due to Alzheimer’s disease. However, there is moderate quality evidence from a single study that it may slow functional decline in Alzheimer’s disease.

What Does the Research Show?

Many laboratory and animal studies have investigated the role of vitamin E in the prevention and treatment of Alzheimer’s disease; evidence from human studies is much more limited.

  • 2017 Cochrane review of two randomized controlled trials involving a total of 820 participants with either mild cognitive impairment or Alzheimer’s disease found no evidence the alpha-tocopherol form of vitamin E given to people with mild cognitive impairment prevents progression to dementia, or that it improves cognitive function in people with mild cognitive impairment or dementia due to Alzheimer’s disease. However, there is moderate quality evidence from a single study that it may slow functional decline in Alzheimer’s disease. The researchers noted that these findings are still based on small numbers of trials and participants and further research is quite likely to affect the results.

Safety

  • Research has not found any adverse effects from consuming vitamin E in food. However, high doses of alpha-tocopherol supplements can cause hemorrhage and interrupt blood coagulation in animals, and in vitro data suggest that high doses inhibit platelet aggregation.
  • Recent results from the Selenium and Vitamin E Cancer Prevention (SELECT) Trial suggest that vitamin E supplements (400 IU/day) may increase the risk of prostate cancer. Followup studies exploring this finding are underway.
  • Vitamin E supplements have the potential to interact with several types of medications, including anticoagulant and antiplatelet medications, simvastatin and niacin, and chemotherapy and radiotherapy.

Curcumin

Preclinical studies have shown that curcumin targets inflammatory and antioxidant pathways as well as (directly) amyloid aggregation. However, the few clinical trials that have examined the effects of curcumin on Alzheimer’s disease are limited and inconsistent, making it difficult to draw conclusions about potential benefits.

What Does the Research Show?

A few preliminary clinical studies exploring the effects of curcumin on Alzheimer’s disease have been conducted.

  • A 2019 systematic review found that preclinical studies have demonstrated beneficial effects of curcumin on cognition in Alzheimer’s disease. However, results of clinical trials are less consistent. Preliminary evidence from clinical studies suggests that curcumin may prevent cognitive decline in healthy populations, but the reviewers stated that more research to improve curcumin’s bioavailability is crucial and more human trials examining curcumin’s cognitive-enhancing effect are necessary.
  • 2016 randomized, placebo-controlled trial involving 96 community-dwelling, cognitively healthy, older adults found that curcumin had limited influence on cognitive function, mood, or general quality of life over 12 months. The researchers concluded that additional longitudinal studies are warranted to determine whether curcumin can slow neurodegeneration leading to cognitive decline and Alzheimer’s disease.

Safety

  • Curcumin is considered safe for most adults, but high doses or long-term use may cause indigestion, nausea, or diarrhea.
  • In animals, very high doses of curcumin have caused liver problems. No cases of liver problems have been reported in people.

Vitamin B

Findings from short-term studies suggest that B-vitamin supplements (B12, B6, and folic acid) do not help cognitive functioning in adults age 50 or older with or without dementia.

What Does the Research Show?

  • A 2018 Cochrane systematic review of five trials involving 879 participants concluded that there is no evidence for beneficial effects on cognition of supplementation with B vitamins for 6 to 24 months. The reviewers also noted that evidence from one study of a reduced rate of brain atrophy in participants taking vitamin B and a beneficial effect of vitamin B on episodic memory in those with higher plasma homocysteine (tHcy) at baseline warrants attempted replication.

Safety

  • No adverse effects have been associated with excess vitamin B12 intake from food and supplements in healthy individuals. However, vitamin B12 has the potential to interact with certain medications. In addition, several types of medications might adversely affect vitamin B12 levels, including proton pump inhibitors, chloramphenicol, H2 receptor antagonists, and metformin.
  • High intakes of vitamin B6 from food sources have not been reported to cause adverse effects. However, chronic administration of 1–6 g oral pyridoxine per day for 12–40 months can cause severe and progressive sensory neuropathy characterized by ataxia (loss of control of bodily movements). Symptom severity appears to be dose dependent, and the symptoms usually stop if the patient discontinues the pyridoxine supplements as soon as the neurologic symptoms appear. Other effects of excessive vitamin B6 intakes include painful, disfiguring dermatological lesions; photosensitivity; and gastrointestinal symptoms, such as nausea and heartburn.

Other Natural Products

  • Research has shown that taking a multivitamin or high doses of individual vitamins, such as selenium, B vitamins, and calcium and vitamin D, doesn’t have any clear benefits for cognition in well-nourished people.
    • Excessive selenium intake, or selenosis, may result in hair and nail loss or brittleness, lesions of the skin and nervous system, nausea, diarrhea, skin rashes, mottled teeth, fatigue, irritability, and nervous system abnormalities.
    • Excessively high levels of calcium in the blood known as hypercalcemia can cause renal insufficiency, vascular and soft tissue calcification, hypercalciuria (high levels of calcium in the urine), and kidney stones. High calcium intake can cause constipation.
    • Excess amounts of vitamin D are toxic. Because vitamin D increases calcium absorption in the gastrointestinal tract, vitamin D toxicity results in marked hypercalcemia, hypercalciuria, and high serum 25(OH)D levels.
  • Although melatonin may help promote sleep in healthy people, a 2014 Cochrane review of two studies found no evidence that melatonin, either immediate- or slow-release, improved any major sleep outcome in patients with Alzheimer’s disease.
    • For melatonin supplements, particularly at doses higher than what the body normally produces, there’s not enough information yet about possible side effects to have a clear picture of overall safety. Short-term use of melatonin supplements appears to be safe for most people, but information on the long-term safety of supplementing with melatonin is lacking.
  • Coconut oil to prevent or treat Alzheimer’s disease is of interest to researchers; however, only small clinical trials have been conducted to date, and the use of coconut oil for Alzheimer’s disease is not supported by any large, rigorous clinical data.
    • Coconut oil supplements appear to be safe when used in the short term (doses of 10 mL two or three times daily for up to 12 weeks).

References

  • Burckhardt M, Herke M, Wustmann T, et al. Omega-3 fatty acids for the treatment of dementia. Cochrane Database Syst Rev. 2016;4:CD009002.
  • DeKosky ST, Williamson JD, Fitzpatrick AL, et al. Ginkgo biloba for prevention of dementia: a randomized controlled trial. JAMA. 2008;300(19):2253-2262.
  • Farina N, Llewellyn D, Isaac MG, et al. Vitamin E for Alzheimer’s dementia and mild cognitive impairment. Cochrane Database Syst Rev. 2017;1:CD002854.
  • Fernando WM, Martins IJ, Goozee KG, et al. The role of dietary coconut for the prevention and treatment of Alzheimer’s disease: potential mechanisms of action. Br J Nutr. 114(1):1-14.
  • Klein EA, Thompson IM Jr, Tangen CM, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549–1556.
  • McCleery J, Cohen DA, Sharpley AL. Pharmacotherapies for sleep disturbances in Alzheimer’s disease. Cochrane Database Syst Rev. 2014;3:CD009178.
  • McCleery J, Cohen DA, Sharpley AL. Pharmacotherapies for sleep disturbances in Alzheimer’s disease. Cochrane Database Syst Rev. 2014;3:CD009178.
  • Rainey-Smith SR, Brown BM, Sohrabi HR, et al. Curcumin and cognition: a randomized, placebo-controlled, double-blind study of community dwelling older adults. Br J Nutr. 115(12):2106-2113.
  • Snitz BE, O’Meara ES. Carlson MC, et al. Ginkgo biloba for preventing cognitive decline in older adults: a randomized trial. JAMA. 2009;302(24):2662-2670.
  • Sydenham E, Dangour AD, Lim W-S. Omega 3 fatty acid for the prevention of cognitive decline and dementia. Cochrane Database Syst Rev. 2012:6:CD005379.
  • Tan MS, Yu JT, Tan CC, et al. Efficacy and adverse effects of ginkgo biloba for cognitive impairment and dementia: a systematic review and meta-analysis. J Alzheimers Dis. 2015;43(2):589-603.
  • Voulgaropoulou SD, van Amelsvoort TAMJ, Prickaerts J, et al. The effect of curcumin on cognition in Alzheimer’s disease and healthy aging: a systematic review of pre-clinical and clinical studies. Brain Res. 2019;1725:146476.
  • Yang G, Wang Y, Sun J, et al. Ginkgo biloba for mild cognitive impairment and Alzheimer’s disease: a systematic review and meta-analysis of randomized controlled trials. Curr Top Med Chem. 2016;16(5):520-528.

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