INTRO: This article documents the science that supplementing with Vitamin D can help protect us against the flu, recommendations for how much we should be supplementing, as well as excellent low carb dietary sources of this important fat-soluble vitamin.
The other day I shared an article on social media from a local Canadian news outlet about a 37 year old man from Ontario who decided to forgo his annual flu shot last year and ended up in hospital with pneumonia for two months, after he contracted the flu virus from his daughter. The article indicated that data from Statistics Canada indicates that only 1/3 of Canadians above the age of 12 get the flu shot every year, yet according to deputy chief public health officer for the Public Health Agency of Canada, Dr. Howard Njoo, more than 12,000 Canadians end up hospitalized for flu-related complications each year, and between 3,000-4,000 people die as a result. Dr. Njoo was quoted as saying;
“it makes common sense that you should get your flu shot.”
There was much heated discussion on Facebook whether there is “proof” that the flu shot prevents people from getting the flu.
A well-known epidemiologist said that “unlike childhood vaccinations (ie, diphtheria, tetanus, pertussis, polio, measles, mumps, rubella, etc), there is no conclusive proof that flu shots reduce risk of either getting the flu or its complications. There are no large randomized trials, which is the standard for establishing a causal connection. So all we really have are observational studies, which are subject to a lot of confounding“. The epidemiologist noted that there was a meta-analysis of 44 years’ worth of studies that was published in Lancet Infectious Diseases in 2011 which found only “moderate efficacy” of the flu vaccine.
In response, a GP referenced a 2012 review of 15 meta-analyses related to the effectiveness of the flu vaccine for adults and the elderly which found they provided what was described as ‘satisfactory immunogenicity’, but that data on inactivated vaccines for children was scarce.
One of the other people that commented about the CTV article brought up studies that I was previously unaware of that indicate that Vitamin D supplementation may reduce the risk of getting the flu — especially for those who are low in in this fat-soluble vitamin to begin with. Stats Can data from 2013 indicates that 1/3 of Canadians fall in this category . It was this fact that motivated me to write this article.
Studies Showing that Vitamin D Attenuates the Flu
There are two large-scale meta-analyses — one from 2013 and the other from 2017 that indicate that Vitamin D supplementation can reduce the risk of getting an upper respiratory infection (URI) including influenza (“the flu”).
The first study by Bergman et al  analyzed data from 11 placebo controlled trials that involved more than 5,600 subjects and found that those taking a daily dose of Vitamin D had half the risk of developing an upper respiratory infection (URI), including influenza (‘the flu”). This held true even though many of the studies used very low dose of supplementation.
The second of the two large-scale meta-analysis by Martineau et al  analyzed the data from 25 randomized controlled trials and involved more than 11,300 subjects. This study found that Vitamin D supplementation reduced the risk of developing an upper respiratory infection (URI), including the flu and those who were the most deficient experienced the most benefit. Even those subjects with very low Vitamin D status had 1/3 the risk when supplementing with Vitamin D, compared to those who did not take any.
Both meta-analysis found that daily dosing with Vitamin D was more effective than taking larger (bolus) doses once a week, or once a month.
There are numerous studies which indicate that people with lower levels of Vitamin D are more likely to get the flu and a 2010 study with healthy adults found that people with lower levels of were twice as likely to get the flu than people with high levels of Vitamin D .
Supplementing with Vitamin D to Reduce Risk of Flu
Health Canada’s recommended daily intake (RDAs) for Vitamin D (updated in 2011) are 600 International Units (IUs) for everyone aged one year old to 70 years old and 800 IU for adults over 70 years of age. Health Canada’s safe upper limit (UL) is listed as 4,000 IU per day, however recent scientific publications indicate that there was an error in the calculations used to determine them.
Two researchers from the School of Public Health at the University of Alberta published a paper in October 2014 which indicates that the Institute of Medicine (IOM) that develops the Recommended Dietary Allowances (RDAs) used by both Canadians and Americans made a serious error in their calculations in determining the RDAs for Vitamin D  and that rather than 600 IUs being needed to prevent deficiency in 97.5% of individuals, the actual amount is estimated to be 8895 IU of Vitamin D per day — which is above the Health Canada’s tolerable upper intake of 4000 IU per day.
On top of that, researchers from the University of California at San Diego and Creighton University in Omaha, Nebraska published a letter in the same online journal in March 2015 which said that they have confirmed the Institute of Medicine’s miscalculation that was noted by the Canadian investigators .
A press release published in Science News on March 17, 201 indicated that;
“The recommended intake of vitamin D specified by the IOM is 600 IU/day through age 70 years, and 800 IU/day for older ages. Calculations by us and other researchers have shown that these doses are only about one-tenth those needed to cut incidence of diseases related to vitamin D deficiency.“
How much Vitamin D should we supplement?
The Vitamin D Council (a US-based group) recommends adults take 5,000 to 10,000 IU/day, depending on body weight and recommend people have their levels checked to make sure it is > 40 ng/ml (100 nmol/l) and to maintain serum levels at 50 ng/ml (125 nmol/L). Since Vitamin D toxicity manifests as high levels of calcium in the blood and urine, the Vitamin D Council recommends monitoring via blood tests that serum levels don’t exceed 150 ng/ml (374 mmol/L).
Since Health Canada’s current upper limit is 4,000 IUs per day (which may be based on an error in calculation, as noted above), a prudence dosage for supplementation for a healthy adult would not exceed 4,000 IUs per day.
Note: I also recommend people take 100 mcg of Vitamin K2 (menaquinone-4, or menaquinone-7) as Vitamin K2 plays a synergistic role with Vitamin D which regulates blood levels of calcium. Vitamin K prevents calcium from accumulating in soft tissues, such as the blood vessels (contributing to Coronary Artery Calcification). Put simply, Vitamin K helps ensure that calcium ends up in bone, not arteries.
NOTE: People taking Warfarin (Coumadin) or other anticoagulant medication should not supplement with Vitamin K2 except under the advice of the physician prescribing Warfarin.
If you are a healthy adult under 50 years old with no family risk of cancer* or osteoporosis, 1000 IU Vitamin D3 per day (plus 100 mcg of Vitamin K2) is probably sufficient. Be sure to choose the D3 form (not D2) as it is more efficient at raising serum levels. For adults under 50 with a family history of cancer or who are at risk for osteoporosis, a dosage of 2000 IU Vitamin D3 per day (plus 100 mcg of Vitamin K2) may be more appropriate.
*there are many studies indicating that supplementing Vitamin D levels for those in northern latitudes (such as Canada and the northern US) lowers the risk of certain types of cancer, especially breast, colon and prostate cancer.
I usually recommend that healthy adults over the age of 50 double the amounts above — so 2,000 IUs Vitamin D3 per day (plus 100 mcg of Vitamin K2) and for those with a family history of cancer to take 3,000 IUs Vitamin D3 per day (plus 100 mcg of Vitamin K2).
Remember though that Vitamin D is a fat soluble vitamin, so be sure to have your serum levels checked periodically as your body is able to stores for long periods of time. The best indicator of Vitamin D status is a routine blood test called 25-hydroxy vitamin D.
Best Low Carb Sources of Vitamin D to Help Our Bodies Fight the Flu
Fish and Seafood
Below is a list of foods that are naturally high in Vitamin D and that can be included on a low carb or ketogenic diet. The best source of all comes from the sea in the form of fish and seafood.
Wild-caught salmon has on average almost 1000 IU of Vitamin D (988 IU) vitamin D per 3.5-ounce (100-gram) serving and some studies have found up to 1,300 IU per serving .
Fresh Atlantic herring provides 1,628 IU Vitamin D per 3.5-ounce (100-gram) serving . That’s huge!
Halibut is an exquisitely delicious white-fleshed fish that provides 600 IU Vitamin D per per 3.5-ounce (100-gram) serving .
Mackerel is simply delicious grilled on the BBQ and provides 360 IU Vitamin D per 3.5-ounce (100-gram) serving .
Oysters contains 320 IU of Vitamin D in a 3.5-ounce (100-gram) serving .
Sardine is an inexpensive and pretty good source of vitamin D — with one serving containing 272 IU of Vitamin D .
Canned tuna is an easy and inexpensive way to incorporate more Vitamin D into the diet and contains 236 IU of Vitamin D in a 3.5-ounce (100-gram) serving .
Non-fish Sources of Vitamin D
Fish and seafood are not the only sources of Vitamin D. Whole egg is a good source, with most of the Vitamin D found in the yolk.
The average supermarket egg contains only 18–39 IU of vitamin D, but pastured eggs that roam outside and are exposed to sunlight produce eggs with 3 to 4 times that amount !
With the exception of fortified foods such as dairy, mushrooms are the only plant source of Vitamin D, however mushrooms only produce Vitamin D2, rather than Vitamin D3 which is not as effective at raising blood levels of Vitamin D. Even then, button mushrooms grown outside under sunlight contain as much as 2,300 IU per 3.5-ounce (100-gram) serving .
There is good evidence that adding Vitamin D3 supplementation to your daily routine may boost your ability to fight of upper respiratory infections, including the flu.
Supplementing your diet with Vitamin D and/or eating plenty of low carb foods rich in Vitamin D3 can help ensure adequate stores of this important nutrient.
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LEGAL NOTICE: The contents of this blog, including text, images and cited statistics as well as all other material contained here (the “content”) are for information purposes only. The content is not intended to be a substitute for professional advice, medical diagnosis and/or treatment and is not suitable for self-administration without the knowledge of your physician and regular monitoring by your physician. Do not disregard medical advice and always consult your physician with any questions you may have regarding a medical condition or before implementing anything you have read or heard in our content.
- CTV News, Canadian warns against vaccine apathy after flu sends him to hospital for two months, https://www.ctvnews.ca/health/canadian-warns-against-vaccine-apathy-after-flu-sends-him-to-hospital-for-two-months
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- Dairy Nutrition, Vitamin D status of Canadians – Results from the Canadian Health Measures Survey, https://www.dairynutrition.ca/nutrients-in-milk-products/vitamin-d/vitamin-d-status-of-canadians-results-from-the-canadian-health-measures-survey
- Vitamin D Council, Dr. John Cannell, MD, Influenza, https://www.vitamindcouncil.org/health-conditions/influenza/
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- Science News, Recommendation for vitamin D intake was miscalculated, is far too low, experts say, https://www.sciencedaily.com/releases/2015/03/150317122458.htm
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- Simona RR, Borzelleca JF, DeLuca HF, Safety assessment of the post-harvest treatment of button mushrooms (Agaricus bisporus) using ultraviolet light, Food and Chemical Toxicology, Volume 56, June 2013, Pages 278-289