Fact or fiction: Research reveals the truth about common supplements

The Bottom Line

  • More and more adults are turning to dietary supplements, spurred on by a multitude of health-boosting claims.
  • Amidst these claims, it can be difficult to sort out fact from fiction. In certain cases, supplements may improve health; in others, they may do nothing at all, and in some, they may even cause unwanted side effects.
  • Talk to your health care provider about what dietary supplements may help improve your health.

Some say, “Without your health you have nothing”. This might explain why three-quarters of us take dietary supplements with the hope of boosting our health (1). Clearly, we are a captive market. The challenge comes in teasing out which health claims are true.

In some cases, such as in those with certain health conditions or a poor diet, supplements may be a beneficial strategy (2). Dietary supplements, however, are not closely regulated (3), and many have failed to live up to their promises of preventing or reversing chronic diseases. Aside from the costs, taking supplements can lead us to adopt a supplement regimen that is potentially dangerous or interferes with our prescribed medications (4).

What do we currently know about some of the most popular supplements? Click on the links to find out more about the research and the myth-busting results.

1. Probiotics for digestive problems?

If you’re on antibiotics, taking Lactobacillus rhamnosus, Lactobaccilus casei, and Bifidobacterium lactis—which are all probiotics—can reduce your risk of diarrhea from Clostridium difficile (5;6). Probiotics might also help treat stomach issues like irritable bowel syndrome and chronic constipation (7).

Digestive problems? Try probiotics

2. Prebiotics and probiotics to improve the flu shot?

If you plan to get your flu shot, you’re in luck—taking prebiotic and probiotic supplements for at least two weeks before you get your shot may give you an extra boost of immunity against certain strains of the flu virus. Healthy older adults who take the supplements for a longer period of time may benefit most (8).

Help flu shots stick with prebiotics and probiotics?

3. Vitamin C for the common cold?

Despite its popularity, taking vitamin C does not prevent colds, except maybe in people who do regular intense exercise in the cold like soldiers, marathon runners, or skiers. The good news? If you do get sick, taking vitamin C may knock a day off your cold and reduce the severity of your symptoms—but only if you take it regularly and not just when a cold strikes (9).

Does vitamin C help with the common cold?

4. Vitamin D for respiratory tract infections?

Hoping to avoid an acute respiratory tract infection, so things like the common cold, ear infections, bronchitis and pneumonia? Vitamin D3 supplements can help, particularly if you are vitamin D-deficient and take the supplements regularly. Unfortunately, if a respiratory infection has taken hold, vitamin D does not lower your chance of being hospitalized or visiting the emergency department (10).

Can vitamin D ward off acute respiratory tract infections?

5. Vitamin D for diabetes?

In people with type 2 diabetes, promising results show that vitamin D supplements can lower blood sugar levels. The optimal dosage and duration of vitamin D consumption seems to be taking ≥ 1000 IU/daily for more than 12 weeks. The caveat: supplements appear only effective in people who are not obese, and/or have a vitamin D deficiency (11).

Vitamin D: A possible ally in the fight against diabetes

6. Vitamin D and calcium for strong bones?

Is preventing broken bones top-of-mind? The combination of vitamin D plus calcium supplements helps lower the risk in older adults. However, the cons might outweigh the benefits in people with or at risk of kidney stones, kidney disease, high calcium levels, gastrointestinal disease, or heart disease (12).

Vitamin D and calcium: A dynamic duo in the maintenance of strong bones

7. Fish oil supplements for high blood pressure?

For people with high blood pressure, regularly taking fish-oil supplements can lower blood pressure by a small amount. With that said, supplements should not replace prescription blood pressure medications (13), instead they can be used as an additional strategy.

Fish-oil supplements: another weapon against high blood pressure

8. Fatty acid supplements for preventing heart disease?

In people who have been diagnosed with heart problems, taking fatty acid supplements like omega-3 or omega-6 will not lower the risk of heart attack. Omega-3’s may somewhat improve heart health in other ways, but more research is needed around this (14).

Don’t rely on fatty acid supplements for protection against heart disease

9. Antioxidant vitamins for eye health?

On this issue, the evidence is clear—antioxidant vitamins do not prevent eye diseases such as age-related cataracts or age-related macular degeneration (AMD) (15). However, they may slow down the progression of AMD in those who already have it (16).

Antioxidant vitamins for eye health? Research evidence provides clarity

10. Dietary supplements for cancer prevention?

Although diet plays an important role in our overall health and in reducing cancer risk, currently there is little to no quality evidence that supports the use of dietary supplements as a strategy for cancer prevention (17-19). Take for example, the mineral selenium. Over the years, selenium supplements have been promoted by some studies as cancer preventing. However, more recent evidence shows that selenium supplements are unlikely to lower your risk of cancer, and may even increase your chances of certain types of cancer and other chronic diseases (20).

Do dietary supplements help reduce cancer risk?

Selenium and cancer: Friends or foes?

11. Over-the-counter vitamins and mineral supplements for dementia?

Unfortunately, promises that over-the-counter vitamin and mineral supplements can prevent cognitive decline have come up empty (21;22). There is currently no good evidence to support their use in people with or without cognitive impairment for this purpose.

Preventing dementia: Are vitamin and mineral supplements worth the money?

Ultimately, regardless of whether different dietary supplements are backed by evidence or not, always be sure to consult your health care provider or pharmacist before using them to tackle your health woes. These discussions will help you understand the potential positive and negative impacts on your health as an individual, especially since vitamins aren’t safe for everyone.

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  1. Council for Responsible Nutrition. 2017 CRN consumer survey on dietary supplements. [Internet] 2017. [cited October 2019]. Available from https://www.crnusa.org/resources/2017-crn-consumer-survey-dietary-supplements
  2. Fairfield KM. Vitamin supplementation in disease prevention. [Internet] 2019. [cited October 2019]. Available from https://www.uptodate.com/contents/vitamin-supplementation-in-disease-prevention 
  3. National Institutes of Health Office of Dietary Supplements. Dietary supplements: What you need to know. [Internet] 2011. [cited October 2019]. Available from  https://ods.od.nih.gov/HealthInformation/DS_WhatYouNeedToKnow.aspx
  4. Mayo Clinic. Herbal supplements: What to know before you buy. [Internet] 2017. [cited October 2019]. Available from https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/herbal-supplements/art-20046714
  5. Johnston BC, Ma S, Goldenberg JZ, et al. Probiotics for the prevention of Clostridium difficile-associated diarrhea: A systematic review and meta-analysis. Ann Intern Med. 2012; 157(2):878-888. 
  6. Hempel S, Newberry SJ, Maher AR, et al. Probiotics for the prevention and treatment of antibiotic-associated diarrhea: A systematic review and meta-analysis. JAMA. 2012; 307(18):1959-1969. doi: 10.1001/jama.2012.3507. 
  7. Ford AC, Quigley EM, Lacy BE, et al. Efficacy of prebiotics, probiotics, and synbiotics in irritable bowel syndrome and chronic idiopathic constipation: Systematic review and meta-analysis. Am J Gastroenterol. 2014; 109(10):1547-1561. doi: 10.1038/ajg.2014.202. 
  8. Lei WT, Shih PC, Liu SJ, et al. Effects of probiotics and prebiotics on immune response to influenza vaccination in adults: A systematic review and meta-analysis of randomized controlled trials. Nutrients. 2017; 9(11):E1175. doi: 10.3390/nu9111175. 
  9. Hemila H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;1:CD000980.  
  10. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: Systematic review and meta-analysis of individual participant data. BMJ. 2017; 356:i6583. doi: 10.1136/bmj.i6583. 
  11. Wu C, Qiu S, Zhu X, et al. Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systematic review and meta-analysis. Metabolism. 2017; 73:67-76. doi: 10.1016/j.metabol.2017.05.005. 
  12. Avenell A, Mak JC, O’Connell D. Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men. Cochrane Database of Syst Rev. 2014; 4:CD000227. doi: 10.1002/14651858.CD000227.pub4. 
  13. Campbell F, Dickinson HO, Critchley JA, et al. A systematic review of fish-oil supplements for the prevention and treatment of hypertension. Eur J Prev Cardiol. 2013; 20:107-120. 
  14. Chowdhury R, Warnakula S, Kunutsor S, et al. Association of dietary, circulating, and supplement fatty acids with coronary risk: A systematic review and meta-analysis. Ann Intern Med. 2014; 160:398-406. 
  15. Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for preventing age-related macular degeneration. Cochrane Database Syst Rev. 2017; 7:CD000253. doi: 10.1002/14651858.CD000253.pub4. 
  16. Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for slowing the progression of age-related macular degeneration. Cochrane Database Syst Rev. 2017; 7:CD000254. doi: 10.1002/14651858.CD000254.pub4. 
  17. Rutjes AW, Denton DA, Di Nision M, et al. Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life. Cochrane Database Syst Rev. 2018; 12:CD011906. doi: 10.1002/14651858.CD011906.pub2. 
  18. Butler M, Melson VA, Davila H, et al. Over-the-counter supplement interventions to prevent cognitive decline, mild cognitive impairment, and clinical Alzheimer-type dementia: A systematic review. Ann Intern Med. 2018; 168:52-62. doi: 10.7326/M17-1530. 

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